In the big bang theory there is no outside, so it isn't a lump. Indeed, it's exactly the opposite. In a true "big bang" theory the universe is totally smooth and featureless, and evolving. It's built on "homogeneous and isotropic surfaces". The main observational motivation for this is the microwave background, which to one part in 1000 is identical everywhere we look. That 1/1000 discrepency is a pure dipole -- nothing but a Doppler shift. What *causes* that is mildly debatable, but the effect has to mimic the Earth's motion with respect to the microwave background so closely that an alternative is liable to fall to Occam's razor. In any event, no matter what its source, we know how to remove pure dipoles, so we remove it. And we're left with something that is identical everywhere we look to one part in ten thousand!
So the microwave background is "isotropic" around Earth - everywhere we look it is identical, for all practical purposes. Any model of cosmology has to be able to explain that, and as a bonus also explain what those tiny fluctuations are doing on there and where they came from, and predict their statistical nature. (The big bang theory, plus inflation, does this as perfectly as we could ever ask. No-one seriously suggests that inflation is other than, at best, an effective field theory that describes a more fundamental underlying theory. Well, no-one except people who believe they can boil a moduli inflation out of one string theory or another, but those are still somewhat contrived. But the success of inflation tells us something that acted exactly like it had to happen. (The answer is easy: so-called R^2 inflation. The first inflationary model is believed in the West to be due to Alan Guth, of MIT. This isn't, strictly speaking, true, and Guth would never claim it was. Guth - and Tye - presented the first quantum field theoretical model of inflation, which they based on the Higgs. The first actual inflation came a few years earlier, behind the Iron Curtain, and due to Starobinsky who is a big name in cosmology but deserves to be bigger. Starobinsky was examining what happens when you look at the 'low-energy' limits of a wide variety of modified gravities. General relativity can be described by the equation L=R. Here L is the "Lagrangian density" from which the equations of the theory can be derived while R is the "Ricci scalar" which describes the curvature of spacetime; for comparison, the Lagrangian of normal classical mechanics is L=K-V where K is the kinetic energy and V the potential energy. I'm brushing over the difference between a Lagrangian and a Lagrangian density but it's exactly what it sounds like... Anyway, Starobinsky started from the observation that virtually any modification of gravity will end up reducing, at energies beginning to approach sanity, to something of the form L=R + alpha * R^2 +... where the dots include a wide variety of grotesquely ugly terms alongside the expected R^3. The interesting thing here is that when R gets very large, as would happen in the very early universe, the Lagrangian becomes L=alpha R^2. Solve this and you find you have an exponentially growing universe -- inflation. Study it in more detail, and you find it acts exactly like a more normal inflation (with a potential V proportional to phi^2, I think; it may be phi^4, I forget which), including exactly predicting the form of the perturabtions on the CMB. Actually, if you look at the recent Planck results, R^2 inflation is still stubbornly by *far* the best result... if you judge by eye. Its nearest widely-known competitor is only excluded at the one sigma level, which you'd be laughed at if you seriously tried to say that excluded it, but R^2 lies slap in the middle of every contour and will never be budged from there as long as we live, unless there is a significant detection of cosmological gravitational waves.)
Anyway, I digress.
There are two conclusions we can draw from the CMB:
1) The Earth is at the centre of the Universe. I don't know why religious crazies ne
Yes, of course. Thing is that we can't apply that logic here. We know that very large structures are extremely unlikely - in a brute force, frequentist approach which is the nearest analogy to flipping a coin a hundred times, we can make a hundred simulations. If in those hundred simulations we do not see a structure that size, we know that the likelihood of it occuring *within the confines of the model tested* - and that is a very important proviso; we can only assign probabilities in this manner with respect to a given concrete model - is less than 1 in 100. If we do 10,000 simulations nad a structure of that size occurs in 1 of them, we know the probability is approximately of the order of 1/10,000. I simplify, of course, but in this manner we can gain a feeling on the likelihood that such a large structure would just happen to crop up in our particular realisation of the universe.
I use the word "realisation" statistically. In cosmology we have knowledge of the statistical nature of hte universe. What we have no knowledge of, a priori, is the actual distribution of matter in the universe. Instead, we have the assumption that fields were initially Gaussian (strongly borne out by CMB observations, and also currently borne out by large-scale structure, although non-linear evolution does introduce a measure of non-Gaussianity), and we have their power spectrum. From this we can make a realisation of the statistical field. The simulations I'm referring to are just such realisations. In the most complex case (such as n-body simulations like the surprisingly old Millenium Simulation) the realisations are made when the universe was a tiny fraction of its current size when all available data suggests linear theory held, and then evolved forwards. That's a punishingly slow procedure, so various other approaches can be used. In all cases, it's about trying to see what the probabiiity of, in this context, a given structure appearing in our universe is.
Again, cosmology is not a science of people entirely ignorant of statistics. In many ways, cosmology is one of the most statistical of the natural sciences, since by its very nature we can *only* ever know statistics, or else have an entirely descriptive and non-predictive survey of the sky. That isn't to say that all cosmologists are master statisticians - I'm very willing to admit that my own statistics aren't the strongest of the cosmologists I know - but at least a passing knowledge of statistics is a prerequisite.
Yeah, I was just meaning that the CfA Great Wall was superceded by the Sloan Great Wall. If this current structure turns out to genuinely be a structure, it supercedes the Sloan Great Wall by some considerable size.
Thank God we have people on Slashdot to tell us things like this. Where would we have been if generations of cosmologists were entirely ignorant of statistics or gravitational physics? The mind boggles!
Sorry, but the problem isn't that there are lumps - if there weren't our existence would be a bit suspect since we live on the edge of a reasonably large lump (the Virgo supercluster) ourselves. The problem (if you want to call it a problem; it's more an interesting question) concerns the *size* of the lumps. We can predict with reasonable certainty the probability of a bound structure of such and such a size appearing in the universe. That's quite straightforward in principle. And structures this big are pushing the bounds of the standard cosmological model quite hard; basically, they shouldn't really be there. I don't know the actual probability but it's extremely low, and low enough that we would not expect to see it. That there are now three structures that are rather too large (this one, if it comes to be accepted as a genuine structure; the Sloan great wall, if it turns out to actually be a structure; and the CfA great wall) is getting interesting.
This is what the "relativity" in "special relativity" and "general relativity" is referring to, yes:)
Thing is that the model we use, there *is* no centre -- honestly. It's probably best viewed as a four-dimensional stack of three-dimensional, infinite, flat surfaces, and the expansion has no centre because there is no centre of an infinite plane. Anywhere the observer is, that looks like the centre. The usual analogy is to paint galaxies on a balloon and then inflate it. If you then visualise the surface of the balloon as being all there is, then you can quickly see that both there is no centre to the expansion (remember, the inside of the balloon doens't exist and the surface is all there is) and that from every point it looks like you're in the centre.
It's not a perfect analogy because of that third dimension, but it's close.
"An event happened within our light cone but due to the expansion of the universe, the cause of this event has moved beyond our horizon faster than the speed of light. In effect, we are affected by something over the horizon. No?"
It's a bit more subtle than that. If you think of things in terms of wavelengths, the expansion of the universe means that more and more wavelengths are suddenly smaller than our horizon -- which means they can start interacting with us. Anything with a wavelength larger than the horizon is not yet going to be able to interact, but will eventually. (This is known as "horizon-crossing" in the jargon, and you'll see it used pretty much ubiquitously.)
The difference comes if the expansion is *accelerating*. An inflationary period -- which looks likely to have happened even if all the current models are not entirely convincing -- takes an extremely small area and blows it up to a ridiculously large volume. In this situation, things are reversed, and wavelengths are *leaving* the horizon. A different way of viewing the same thing is that, yes, if we assume an inflationary epoch occurred then we are affected by things that we used to be in causal contact with but which inflation rapidly pushed far, far away from us.
Since in the standard model the universe is currently accelerating we actually have things right now leaving our horizon, which does indeed mean that, in principle, we have been affected by thigns that we are no longer in causal contact with. If this is the genuine setup in the universe, the future looks pretty bleak -- eventually you can expect to see the local group, and perhaps the local supercluster, with everything else swept way outside our horizon. The universe will look pretty empty in that case, and we'll have lived in about the only period where we could tell there *is* a cosmology at all, which makes us rather lucky... (Lawrence Krauss once gave a good talk on that premise, so I credit him with that statement.)
I won't lie to you - I'm extremely sceptical about it. But wading through the maths, at a genuinely fundamental level, is a prerequisite, as is testing any theory of gravity first and foremost on Solar System scales, and then against the battery of tests that GR has passed. (You can only avoid this if you explicitly state that you're working with an effective field theory valid on particular energies or particular scales, but that takes some careful setting-up to pull off without it looking basically unmotivated.)
A lot of astronomers miss it too, chiefly because it's so easy to assume that Newtonian gravity works in these regimes, and without wanting to sound too critical astronomers (and increasingly cosmologists) are increasingly trained within their own field. I more and more feel I was fortunate to have done my Masters' in condensed matter theory before doing a PhD in cosmology. You get a more... physical view of things, and a strong appreciation for complex emergent behaviours which are practically inevitable when you look at systems composed of large numbers of particles. And a galaxy, and an entire universe, definitely fit that definition. (Interestingly, a galaxy cluster much less so, you're probably best modeling that as a system of about 100 or 200 weakly-interacting, loosely-bound bodies. Statistical physics is unlikely to be of much help there.)
"The matter's gravitational influence is the only thing being detected because the object is going faster than light away from earth."
Citation needed. That appears to suggest a model that cannot possibly work. How can galaxies be held together by dark matter if it's always moving away from Earth? Is Earth ejecting vast quantities of dark matter? If not, why don't we see the stuff coming *towards* us? Though we should be glad we don't because it would be blue-shifted to oblivion (probably ours).
"Light and gravity do not travel at the same speed."
Citation needed. The "speed of gravity" is a pretty nonsensical statement in GR, but what we do have is the speed of a *change* in gravity, and that is the speed of light in vacuum. If you've a proof otherwise you have to cite it.
"I think this bothered Einstein and (I heard) he tried to solve that problem all the way to his final days."
Citation needed. Einstein worked on a lot of things to his final days, including the paradoxes of quantum mechanics and the unification of gravity with electromagnetism.
"Anything traveling faster than light away from earth is not going to be "seen" because any light from that (gravitational) mass is not reaching us; but, the gravity is."
Citation needed. See the first point.
And so on. At the minute you're making assertions that flatly contradict a model that, no matter what you may think, has been staggeringly successful. They also flatly contradict the theory that underlies that model. When you do this you have to have a good reason, a good argument, and supporting evidence. What you have at the minute doesn't make a large amount of sense. We can send things faster than light in a medium -- we get radiation from that while the object slows down. We can't send things faster than light in a vacuum and we've tried. If we had something moving faster than light then you get some extremely odd things happening, due chiefly to gross violations of causality. I'm not a fan of the idea of being hit in the back of the head by something emitted (evidently from Earth) in three weeks' time.
"We know gravity as at least as fast as light but it may not be equally as fast."
So you're saying the "speed of gravity" (whatever that means; at present the best we know is if you disturb a system the change in the gravity propagates out at the speed of light) is greater than or equal to the speed of light.
"According to big bang theory, the mass of the viewable universe (read: that part of the matter ratio traveling within one light speed of one another) can be traced back to within a small percentage of it's age."
I don't even know what you're saying here.
"Mass ejected from the point of origin had to compel mass faster than the speed of light (in relation to earth) away from point of origin."
There was no point of origin, and Earth's position is irrelevant. You're also not going to make much headway with a cosmology that places the Earth at the centre of the universe.
Look, I've said this kind of thing before but if you want to convince people you've got to use more than words and suggestions -- these are ultimately vague and worthless, no matter *who* is suggesting them. Einstein was a genius for thought experiments, but in the end they boiled down to justifications for a mathematical framework in which to work. The actual work then has to be with the mathematical model, and in managing to make it hang together coherently. If you can do that, excellent, but it has to pass, and keep passing, every observational test thrown at it, and any ad-hoc additions you make have to be justified. It may surprise you to learn that dark energy was actually postulated ten years before it was broadly accepted; Wetterich derived a model from high-energy physics, and at the same time (within weeks) Ratra and Peebles put out more phenomenological but broader models they'd been working on at the same time. This was in 1987. Dark energy was only really acc
"Actually - what has been detected is wrong by 3 or 4 orders of magnitude - in some derivative measurements - compared to what was previewed under teh big bang theory."
?
In the 60s it was predicted that there would be a uniform bath of microwaves around us with a temperature around 5-10K. Given that they were working on no data, getting it within a factor of two to four is pretty impressive, and by no means "three or four orders of magnitude", which would have involved them predicting a bath of radiation at between 3000K and 30,000K, which would be presently entertaining itself roasting the Moon, knocking our satellites out of commission and probably destroying our atmosphere. I think someone's got confused somewhere.
I think the only way a person outside academia can present a rigorous mathematical model is to ensure it really is as rigorous as it can be. It's unfair, I know, but getting something published from outside the system is extraordinarily difficult, which is not least a result of the number of submissions that are, well, crackpot. (The arXiv moved to an endorsement system about ten years ago now to help combat that. Endorsement doesn't mean anything other than "This is in a field that is probably worth looking at", but has to come from someone who has posted published work to their part of the arXiv, and it works as a good filter. We no longer get papers claiming that ball lightning is formed from primordial black holes.)
So I'd basically recommend start a mathematical education in the time you have. Start with vector calculus again, and electromagnetism and fluid dynamics, then move onto tensor analysis and then tensor calculus. Move to the phrasing of electromagnetism and fluid dynamics in terms of tensor calculus. It's also worth spending time learning linear algebra, complex analysis (at an applied level in both cases; the Schaum's Outline series are good for this), and then stepping on to first a modern approach to special relativity, which will phrase things in the language of tensor analysis (SR is based in Minkowski space, which is a generalisation of Euclidean space with a non-positive definite, err, Pythagoras theorem will do for now). This will include special relativistic fluid dynamics and electromagnetism. Then you can finally start looking at general relativity. Start with an old-fashioned, metric-based approach -- it's a lot clearer that way. Work through the derivation of the Einstein equations, and apply them to Minkowksi space, then to Schwarzschild and Robertson-Walker. Move onto more advanced topics, and particularly the Lagrangian formulation of GR. (A Lagrangian is a scalar that encapsulates a theory, and the Lagrangian density of general relativity is the Ricci scalar. If you're not clear on Lagrangians, get "Classical Mechanics" by Landau and Lifshitz and work through the first few chapters, and then jump to the final few chapters to fill in on the Hamiltonian theory of classical mechanics too.) You can start looking at the covariant approach to gravity, a name which sounds redundant but which involves separating the field equations according to how an observer is moving; the Weyl tensor emerges as rather fundamental in this approach.
Then you can have fun starting picking apart the theory. The thing is that to do so requires an understanding of the theory in the first place, and how it can be modified, the caveats of modifying it in a particular way, and an awareness and understanding of the criticisms absolutely any modification are going to receive. Oh, I forgot, look at linearised gravity and Solar System tests, and then the parameterised post-Newtonian approximation since that can characterise a wide sweep of modified models, and every one of them has to pass Solar System bounds to be viable. And when you come to put something on paper, make sure you've been reading the literature, write in that style, cite appropriately, be aware of who's also put out similar work in the field. At that point you may find a conference that you can register for, which is probably the best way to turn up and chat with experts in the field. You may or may not get someone interested this way, but if you act professionally and not over-eager you'll at least get something out of it.
I know it sounds a hell of a lot of work and it is - if you do it in the requisite detail, that's about three years of study, or more if your maths isn't too hot right now. But none of it is impossible, and it's not an entire degree plus PhD because it skips out all the quantum, solid state, condensed matter etc.
" I'd still quibble a bit here - you seem to be conflating "observational evidence" with "experimental evidence"."
To some degree I probably am, yes. Even so, if we have a dinosaur bone in front of us, we have that bone. We can poke it, we can hit our colleague with it, and we can scrape bits off and drop them in each other's tea. If we're excruciatingly lucky, we can dig out traces of organic material inside it and piss off a generation of paleontologists in the process. (So far as I'm aware, that's been done - not enough to do much with it, alas, but still organic.) The difference here is we can't do that with dark matter. The only evidence we have is far more indirect, and can be explained through a wide variety of mechanisms, few or none of which are amenable to direct testing, meaning something we ourselves can actively do. In the dinosaur case, we can at least dig up more bones, or find previous bones, scrape bits off those, and drop them in someone else's tea.
I think there is a very big distinction between these, though I do see your point.
"I haven't heard of a "gravity is different" theory that made accurate quantitative predictions of the CMBR data, where the dark matter theory did. Maybe I just didn't hear about it?"
Probably. If nothing else, you can get the CMB without too much issue from Brans-Dicke, from f(R) (no surprise there; it's effectively Brans-Dicke with a weird parameter), and if you play enough absurd games with an absurd theory you can even get things out of TeVeS which is a contrived relativistic form of MOND. If you write down a vaguely sensible bimetric theory -- and TeVeS is not really very sensible, but others are -- it seems likely we can get the CMB out of those, too. Braneworld theories give it happily, and Turok and Steinhardt's somewhat... eccentric ekpyrotic universe where two branes repeatedly slam together like cymbals with each slam kicking off a big bang, can also do it.
On a weaker scale, you can take GR but change the metric. Cosmology is built on the (Friedman-Lemaitre-)Robertson-Walker geometry, which is the second-most simple solution of GR there is. The simplest is Euclidean space. An FLRW geometry is a whole bunch of 3D Euclidean spaces stacked one on top of the other. It's slightly more complicated than that, since you can get closed FLRW (effectively a bunch of concentric, ultra-smooth spheres) and open FLRW (basically a load of saddles piled one on top of the other). There is good observational support for FLRW, but the same support can be given to a variety of particularly Bianchi universes, which are like an FLRW but slightly anisotropic. Control that anisotropy, and you've got a perfectly valid universe, with a slight directionality (which, intriguingly, Planck has seen in the sky -- though the form of anisotropy is actually not that easy to reconcile with simple Bianchi models). Until recently you could play games with Lemaitre-Tolman-Bondi metrics, which are like FLRW but lack the homogeneity, so that while around Earth everything looks spherical, away from us it is distinctly less so. In reality you still can use LTB models, but you have to be careful, and their main use (the observable effects of dark energy without having to introduce a physical dark energy or accelerating the universe) has been pretty comprehensively rubbished. (It's still not certain, since we haven't yet finished working out the perturbation theory properly, and without it any claims to be genuinely looking at the CMB and the oscillations in the large-scale structure should be taken with a bit of salt but, realistically... it's a very small bit of salt.)
And then we can assume gravity is the same but the problem is simply coming because even on galactic scales we're working with averaged motions (or, more concretely, a statistical mechanical system). On cosmological scales we can view things in three ways: a spatial average, an average across observations (these are distinct; one is the average of, say, the angular distance to objects of eq
Not necessarily. I just posted a reply to someone else addressing a similar question. This kind of system - the Bullet Cluster is the most famous example - was originally touted as a "proof" of dark matter. It isn't, though it is another sign that if you want to beat dark matter (for whatever that means) you have to be able to predict what that model does. Basically, a modification to general relativity will almost certainly change the reaction of the Newtonian potential (which pops up in weak fields as the time-time perturbation to the metric) and the scalar spatial curvature (ditto, on the space-space component) to the presence of matter, and can do so in surprising ways. One of those surprises is that if you're reasonably careful how you choose your modified theory of gravity, you can get peaks in the lensing potential quite displaced from your matter distribution, without introducing any exotic forms of matter. (The cost is an exotic form of gravity, of course... but we actually know more about the fundmental nature of matter than we do about the form of gravity on supergalactic scales, so as daft as it may sound I'm a lot more comfortable this way.)
Of course, it makes the model look a bit more contrived, particularly compared with dark matter which has effectively three assumptions: there is a non-interacting species, with a density roughly five times high than standard-model matter, and it is pressureless. However, to do calcualtions you have to make a vast array of other assumptions on the form of the distribution, which introduce a wide number of parameters and arbitrary functions, which makes the whole thing a lot less clean than it initially appears.
Even so, yes, interacting galaxies form a powerful testbed for this kind of theory. But they're not the killer they were originally touted to be - merely a strong discriminant. (Which is excellent, we need that. This isn't -- or shouldn't be, but I know people who've made it into one -- an ego game. It's meant to be about finding better explanations for things, in a field where I think increasingly people are going to realise they're not going to be able to link all the way back to fundamental physics. Or they will if the education level stays up, but I'm concerned about that when I see cosmologists coming in who at most have been taught a bit of GR, enough to do cosmology, and nothing more. That is a bit alarming; there is a breed of modern cosmologist who doesn't seem to know, or care, about what underpins the theory, and for whom Robertson-Walker metrics are the be-all and end-all. Worse, *linear* Robertson-Walker. Even people I respect hugely -- naming no names though -- have published papers ascribing effects to "modified gravity" that are far more easily, and physically, explained through, err, physics, by looking at second-order perturbations. But that's a different topic...)
"Correct, they only detected gravity, which we currently assume is only caused by the mass of matter."
Well, to be pedantic, they didn't detect gravity at all. A Nobel prize waits for that one, too. The assumption that it is "only" caused by mass is one of GR, certainly, but there are a plethora of theories in which this isn't the case, such as Brans-Dicke theories, generalised Brans-Dicke, f(R), f(G), scalar-vector-tensor, bimetric, etc. etc. etc.
"They've recently tested relativistic gravity by measuring red shifts and have some to the conclusion that gravity at galactic scale is working the same as solar-system scale."
Really? Could you provide a link to that one?
"And why does everyone keep bringing up only the rations of galaxies, what about gravitational lensing in dust-less gas-less empty space?"
A good question. Proponents of particulate dark matter theories have enjoyed bringing up gravitational lensing for some time now - and rightly so. They've enjoyed even more bringing up the Bullet Cluster, in which the gravitational lensing (presumably tracing the dominant mass) is in a very different location to the X-ray emission -- and rightly so. The thing is that you can actually get very similar results with, say, a bimetric theory. Even TeVeS can fit the Bullet Cluster if you're really careful about the massive neutrinos you add in. Massive neutrinos are not at all controversial, and sterile neutrinos aren't particularly so either, and a blend of those two can fit the Bullet Cluster with no problem -- and no vast quantities of dark matter.
What's more, if you take the physics to a genuine level (rather than one-dimensional, linearised systems) and start considering three-dimensional distributions of, say, a scalar/tensor theory, you find some extremely interesting interactions going on -- such as domain walls between areas where the scalar field is negative and areas where it is positive, followed by a sudden collapse and a dramatic ringing of oscillations through the universe. In this type of model, gravitational lensing is... different. It may not account for the entire lensing, it may not account for any, or it may account for the lot - I don't know, and to be honest neither does anyone else.
(I'd also like to point out that if we're talking properly, lensing does not actually trace mass. Except very near to a black hole or neutron star where the description breaks down, lensing traces the sum of the Newtonian potential and the scalar spatial curvature. In vanilla GR that "lensing potential" is certainly set by mass. In a slightly different theory, it can be set by a wide variety of things. The simplest, by a long way, is massive particles, and that's one reason dark matter is currently the most favoured explanation, but it is not the only way.)
I'm not wanting to argue, as such -- dark matter is by far the most accepted model, for a good reason. It explains the vast bulk of the observations more simply than any other model. Same with dark energy. Any replacement model has to replicate observations predicted by a dark energy/dark matter model, practically perfectly. But there are fewer fundamental reasons to believe it is literally true, particularly in the form currently presented, than is often believed (even by professionals).
"They use a model of a galaxy as if the mass was in the center like the star in a solar system, and wonder why it then doesn't match"
That's because there's a theory in Newtonian gravity that the force you experience is the same as if it were all concentrated at the centre of mass. For *spherical* systems a similar theory holds in general relativity. It wasn't ad-hoc, it was people applying Newtonian gravity to galaxies, and other than a few oddballs like me, most people do not question that relativistic effects in galaxies are entirely subdominant and that we may as well just use Newtonian theory. And hell, they may very well be right; this is a totally open question.
I think a lot of the issue here is actually your "they" vs "we" position. This isn't the case! All we really have -- and I speak as basically an insider -- is a "educated" vs "layman". I'm honestly, honestly not wanting to sound offensive or smug when I say that, just that the directions that physics goes in might seem nonsensical but there is almost always a very good reason to do it. If nothing else, it's starting from a theory that the researchers know is probably phenomenological (meaning "not fundamentally true", "grounded in observation and nothing more") but is at least self-consistent, coherent, complete, capable of taking in a situation and making concrete predictions, and then it's pushing that theory a bit further. There is absolutely no reluctance to introducing different theories, no matter what the popular conception is. My own field is cosmology, and I stopped bothering counting the endless variations on gravity, or the entirely different approaches to cosmology, or the weird shit coming in from the high-energy physicists, or the ways of producing inflatons or dark energy from different (reasonably well-motivated or batshit insane) multidimensional theories. Every single one of them was introduced for a solid, concrete reason, and I don't think *anyone* has ever attempted to state that this or that is fundamental reality, unless they had a very clear reason for doing so.
At the heart of everything is the knowledge that one is working from a theory -- a particular set of equations, true on a certain scale or in a certain energy range. I wouldn't try and use general relativity when describing gluons; it really doesn't work. I would try and use quantum chromodynamics when describing the orbits of galaxies in clusters; that would be ridiclous. I wouldn't even use GR for calculations within the solar system, for the most part, because it's way too complicated and the errors in using Newtonian gravity in that situation are so small.
The planet thing -- yes, true. But any explanation anyone comes up with has to fit with known physics, or challenge it in ways that leads, quantitatively, to further predictions that are then borne out by observation. If it can't do that, or if a suggestion fails on some fatal grounds (such as predicting planets orbiting close to stars, but failing to account for distant gas giants, or what have you) then it will definitely die. If something explains things perfectly but is controversial, it's probably already been written down and published by a professional astronomer...
You accuse astrophysicists of making up crap to fit new things into broken theories. OK, in some cases, fair enough - but my point is that some of that made up crap (such as braneworlds; what a load of bullshit) was introduced for very specific reasons, to answer very specific, and very pertinent questions. I've got a lot of contempt for braneworlds, but those theories were not introduced to stroke someone's ego or to give someone something to do; they were introduced as a way of examining the gross cosmological features one would expect to see in a world described by string theories. Braneworld theories themselves were pretty specious things, but that was the point, and so far as that goes, all power to them. My opposition comes entirely from the undue attention (and money) thrown at them, not for their introduction, and not for the reasons for their introduction, and not for the fact that the universe is obviously not a 3+1D brane suspended in a 4+1D universe. Because it isn't, such a suggestion has never seriously been made.
In the case of dark matter and dark energy, there have been alternatives, and plenty of them. I've gone on record - on Slashdot as well as in publications - stating that the "answer" to the dark matter problem is very likely to be an ugly mixture of every solution we have yet proposed: massive neutrinos (they are massive, but extremely warm), sterile neutrinos (likely cold), a lightest supersymmetric particle or two, unforeseen effects of general relativity
"This is the Electro-Gravitic theory of space and provides a clear explanation for dark matter, dark energy without resorting to anything we have not already proven experimentally or incredibly complex math that defies human understanding."
That's the kind of comment that always makes me extremely wary, particularly as practically the only place I've ever seen it is following a wall of text that builds -- on relatively specious assumptions and assertions -- without any actual concrete theory. The problem is entirely that, no matter what philosophy we can dig out of it, physics is about algorithms. We *have* to have numbers we can put into our algorithms, which typify the scenario we wish to consider, and we *have* to have numbers out, which are what, according to this theory, we expect to see coming out.
The problem with the theories I've seen where people promise "clear explanations" for dark matter, dark energy, and frequently cheap or free energy, is that they fail the first step -- they do not provide a robust and self-consistent mathematical framework.
Anyway, with that little rant out of the way, I'd be cautious. What Einstein demonstrated is that a theory of gravity that is both far simpler and frankly better experimentally supported than Newtonian gravity is general relativity. In Newtonian gravity, the gravitational field is a three-dimensional field, instantaneously sourced by two or more objects of positive gravitational charge. (This is commonly dubbed "mass", and in this context is normally called "gravitational mass" in modern physics.) One enormous issue with this concept is that if you take Newton's law of acceleration (F=ma) and equate it with Newton's law of gravitation (F=GMm/r^2), you cancel the ms and get a=GM/r^2. OK, brilliant. Nothing there... except that Newton's law of acceleration has sweet FA to do with gravity. That mass is an "intertial mass"; it describes the response of a body to a force, and there is no a priori reason to link it with a gravitational mass at all. I don't think -- though I may be very wrong, of course -- that Newton was aware of this subtlety, but by the early 20th century it was very well known, and things like Eotvos experiments were set up to try and tell whether these masses were actually different at all.
The point here is that if the intertial and gravitational masses are the same, every object reacts to a gravitational force with the same acceleration. Try and think of the last time you saw that. I would put a vast amount of money on it being the last time you were in a vehicle that was turning a corner. (If you think slightly more subtly, it was the last time you wondered why weather patterns on Earth run the way they do.) It is well known that centrifugal and coriolis forces are artificial ("fake") forces, caused by observing in an accelerating frame of reference, such as a car going round a corner, or on the surface of a spinning planet. However, they feel very real to the objects that are in those frames, as anyone going round a corner, or being flung from a roundabout, can verify. The hallmark of a fictional force is that every object experiencing them moves with the same acceleration. In this context this is obvious: the "force" is entirely due to the acceleration of the frame of reference, so it's bloody obvious that the force felt is going to be the same acceleration by every body. The thing is that there is no "natural" force that does this: any force where all bodies feel the same acceleration is probably fictional, and that "probably" is only there in case someone cooks up a bizarre theory where they can get this any other way.
That's the soul of GR, and as soon as you try and work through from there (known as the "weak equivalence principle": that the intertial and gravitational masses *are* equivalent, not merely that they're close to it) you're lead straight to a four-dimensional theory of gravity. It also leads to issues of causality, of the propagation of gravitational radiation at what we link with the
Yes, in a sense. I have a strong suspicion that if we were able to do a proper statistical mechanical analysis of the situation we'd see some odd emergent behaviour -- a galaxy is, after all, a rarified gas of about 10^9 interacting, confined bodies. We'd get different behaviour in a cluster, and different on cosmological scales.
Of course, I may be wrong and what we'd get out would be effectively pressureless dust, which is what we currently put in. Thep roblem is that at the minute we can't do a proper statistical mechanical analysis. We don't even have a full theory to work with, though there's progress here, too.
Yeah I tried to go through some of that stuff years back, and it was distinctly unconvincing, sketchily-laid out, and in a far weaker state than the author(s) would wish you to believe. Ultimately, if they feel they have a truly viable theory they have to apply it, in as much detail as the current LCDM model has been applied. That means they have to start off in the early universe (or the distant past, if you prefer; we don't *have* to assume a Big Bang), then justify in some way the existence of both the cosmic microwave background, and the exact spectrum of perturbations on it; then in the same, self-consistent coherent model, they have to account for structure formation and the presence of a wave imprinted on the largest scales of galactic structure which just happens to have a wavelength that perfectly matches that on the CMB... if the universe evolved as predicted by a Lambda CDM model; they have to include a form of nucleosynthesis to explain the ratio of elements we see in the oldest stars; they have to explain why old stars tend to be metal poor and young stars are metal rich; they have to explain the collapse of shards in clusters to form galaxies; and so on and so on.
Do that, and people might just start paying attention... but they have to do it at a level of rigour that is equivalent to that employed in professional cosmology. If they can't, they don't have a theory, they have words, and words are extremely cheap. It has to be couched in a mathematical language, and that's because it has to have a surmise and make a testable prediction. It has to be directly testable. I am very definitely not a fan of Lambda CDM, and a hunt back through my posts on/. that relate to cosmology would probably make that quite clear, but I've spent many years looking at it and its perturbations anyway. In my view, Lambda CDM has one absolute killer of a prediction: the wavelength which it predicted, from that on the CMB, was imprinted on the large-scale structure, and which was later found, exactly where it said. That wavelength, and the amplitude of the wave, is exquisitely sensitive to any change in the evolution of the perturbations, which is itself exquisitely sensitive to a change in the background spacetime. Lambda CDM got it right; any successor model -- and I hope to God there is one, because Lambda CDM is not satisfactory -- also has to.
The last that I knew, the Electric Universe stuff doesn't do any of this. (I would emphasise again that to gain acceptance it is not enough to posit a model -- and it's not even enough to present some back-of-the-envelope calculations. Frankly, the absolute minimum is a full analysis of possible backgrounds -- containing at least photons, neutrinos and standard model matter -- before you can even think of putting a paper out. That would then need to be followed up with an analysis of the perturbations, which we are all after all made from. Effectively, a version of the CAMB code, or one of its competitors, is necessary. Without it, you don't really have a viable model, just yet another model that can recreate something with observables matching the background Lambda CDM, and those come ten a penny. And so on. This is not an easy job, which is why we have no answers yet -- but it sure as shit isn't because the people working in the field are purblind idiots devoid of imagination or soul. Well, certainly not all of them;) )
The difference here is that whereas normally the "indirect" signals we receive are photons directly from a particle, or indeed a measurable and reproducible influence on known quantities in a laboratory setting (which includes the tracks of known particles through accelerators), dark matter is not easily amenable to such tests. We only see it (interpreting "it" loosely -- the way I use the words, 'dark matter' should be interpreted as 'the fact that galaxies, clusters and the universe as a whole act as though there is more matter than we observe', which is probably infuriatingly vague:( ) through its gravitational effects, and by the sheer weakness of gravity and the impractical idea of creating, well, galaxies in a laboratory setting it is never going to be directly detectable that way.
The Higgs boson, on the other hand, was seen in reproducible experiments. I do agree that we can quibble on whether it was a direct detection, or whether it was indirect, given that its existence was ultimately deduced from the pattern of particles around it - but there are big differences. For one thing, a (relatively) quick analysis of the shrapnel from a collision that produced a Higgs will point to a particle of a particular mass and nature. That can then be reproduced (albeit at a low likelihood, given the nature of the experiment), and has been. We only even saw announcements from CERN when two independent experiments both reported an excess at the same mass. (In particle physics these certainly used to be called "resonances" -- when you find that collisions with a particular energy change nature dramatically, you can be pretty certain there's a particle there. For all I know, they're still called resonances, but my particle physics is second-hand through textbooks and therefore about 25 or 30 years out of date.)
It basically comes down to a detection on local scales, under conditions we can control, through a force other than gravity. We can't examine anything through gravity - it's uselessly weak, and impossible to control. That's a "direct detection", and can be through interactions with photons, or the influence of the new particle on the particles we observe coming out of its interactions and annihilations, or anything along those lines that can be seen, influenced, reproduced, observed. We can't do that with the evidence for dark matter. All we have is that galaxies rotate faster than they should (and they do, unequivocably), and that clusters should not really be bound (but they are, equally unequivocably), and that we cannot account for this with our current theories of gravity. The easiest solution is at least one particulate dark matter, certainly -- but if that exists it *is* amenable to production in a lab, even if to actually observe it we would have to wade through ten times more data than the LHC pours out, or a billion times more. But that isn't the only solution, because the only evidence we have is through gravity, and there is absolutely no reason at all (and it would be a mild form of intellectual blindeness) to prematurely declare that "dark matter" is definitely particulate and not, say, a sign that gravity does not behave on kpc scales the way it does on AU scales, let alone on Mpc and Gpc.
It's always enlightening to see how it looks to people who have had occasional glimpses from the outside but never bothered looking any further.
No-one is so wedded, philosophically, to the idea of CDM as is. Everyone knows its an approximation. The arguments over what it *is*. Mirage, particle, multiple particles, modifications to gravity, unanticipated effects of relativity on large scales, unanticipated effects of *averaging* observations across large scales, or a combination of the lot of them. And I can guarantee that practically no-one has been arrogant enough to stand up in a room and declare that we know what dark matter is.
I saw one person - who shall remain nameless - say something along these lines. He said to a room full of distinguished cosmologists (and me, I'm not distinguished at all), and I paraphrase since this was a few years back, "We can be absolutely certain that supersymmetry exists". That quite took my breath away. Firstly: no we can't be. Secondly: lol. Thirdly: winning that prize obviously turned you into an even bigger prick than you already were. I can't remember if anyone made these points to him because his talk was so stultifyingly boring, and so overlong, that I was comatose long before the end. Anyway, the corollary of his flabbergastingly inaccurate statement is that he also believes firmly that there is a single species of particulate dark matter, since this is more or less a prediction of general supersymmetric theories.
He's wrong, anyway. There may very well be supersymmetry, but we can in no way be certain that it exists.
Same goes for "dark matter", whatever you want to call it. The only thing you can't do is deny that the problem is there, and that the simplest explanation, which basically works all the way from galactic scales up to cosmological scales, is that it is composed of massive, weakly-interacting particles.
I'm a professional cosmologist, and I have to take issue with your first statement. The instruments did not, and categorically have not, detected the presence of something that is matter. If they had, that would be a direct detection of dark matter, and a Nobel prize would already be sitting on their desk. What they have detected are indirect signals of dark matter. It is very hard to reproduce the observations - particularly the cosmological observations - without adding at least one component of dark matter. So the observations are typically interpreted in terms of dark matter.
But this is very much not, strictly speaking, necessary. What we have is something that has an effect which, when viewed through a Robertson-Walker model, looks for all the world like a species of massive, weakly-interacting particle (or two or three such species - no-one ever said there has to be only one). On smaller scales, we have what for all the world appears to be a large amount of mass that can't be seen.
Any of this could be down to a modification of gravity. We know the nature of gravity roughly up to the position of the Voyager craft -- call it 300AU to be generous. We are extrapolating that a thousand times to get to galactic scales, a million times to get to cluster scales, and a thousand million times to get to cosmological scales, all without evidence. Of course, without a better theory to replace relativity, it's the best we can do, so we do it - but don't try and claim that instruments have detected that it is matter (they haven't), nor that we are wedded to particulate dark matter (with caveats, we aren't; the caveats are firstly that neutrinos have a mass and are therefore a rather warm dark matter, and secondly that it seems rather unlikely that there isn't at least one species of weakly interacting matter which would act as CDM, but maybe not in sufficient abundance to answer our woes).
Slashdot Mirror
In the big bang theory there is no outside, so it isn't a lump. Indeed, it's exactly the opposite. In a true "big bang" theory the universe is totally smooth and featureless, and evolving. It's built on "homogeneous and isotropic surfaces". The main observational motivation for this is the microwave background, which to one part in 1000 is identical everywhere we look. That 1/1000 discrepency is a pure dipole -- nothing but a Doppler shift. What *causes* that is mildly debatable, but the effect has to mimic the Earth's motion with respect to the microwave background so closely that an alternative is liable to fall to Occam's razor. In any event, no matter what its source, we know how to remove pure dipoles, so we remove it. And we're left with something that is identical everywhere we look to one part in ten thousand!
So the microwave background is "isotropic" around Earth - everywhere we look it is identical, for all practical purposes. Any model of cosmology has to be able to explain that, and as a bonus also explain what those tiny fluctuations are doing on there and where they came from, and predict their statistical nature. (The big bang theory, plus inflation, does this as perfectly as we could ever ask. No-one seriously suggests that inflation is other than, at best, an effective field theory that describes a more fundamental underlying theory. Well, no-one except people who believe they can boil a moduli inflation out of one string theory or another, but those are still somewhat contrived. But the success of inflation tells us something that acted exactly like it had to happen. (The answer is easy: so-called R^2 inflation. The first inflationary model is believed in the West to be due to Alan Guth, of MIT. This isn't, strictly speaking, true, and Guth would never claim it was. Guth - and Tye - presented the first quantum field theoretical model of inflation, which they based on the Higgs. The first actual inflation came a few years earlier, behind the Iron Curtain, and due to Starobinsky who is a big name in cosmology but deserves to be bigger. Starobinsky was examining what happens when you look at the 'low-energy' limits of a wide variety of modified gravities. General relativity can be described by the equation L=R. Here L is the "Lagrangian density" from which the equations of the theory can be derived while R is the "Ricci scalar" which describes the curvature of spacetime; for comparison, the Lagrangian of normal classical mechanics is L=K-V where K is the kinetic energy and V the potential energy. I'm brushing over the difference between a Lagrangian and a Lagrangian density but it's exactly what it sounds like... Anyway, Starobinsky started from the observation that virtually any modification of gravity will end up reducing, at energies beginning to approach sanity, to something of the form L=R + alpha * R^2 +... where the dots include a wide variety of grotesquely ugly terms alongside the expected R^3. The interesting thing here is that when R gets very large, as would happen in the very early universe, the Lagrangian becomes L=alpha R^2. Solve this and you find you have an exponentially growing universe -- inflation. Study it in more detail, and you find it acts exactly like a more normal inflation (with a potential V proportional to phi^2, I think; it may be phi^4, I forget which), including exactly predicting the form of the perturabtions on the CMB. Actually, if you look at the recent Planck results, R^2 inflation is still stubbornly by *far* the best result... if you judge by eye. Its nearest widely-known competitor is only excluded at the one sigma level, which you'd be laughed at if you seriously tried to say that excluded it, but R^2 lies slap in the middle of every contour and will never be budged from there as long as we live, unless there is a significant detection of cosmological gravitational waves.)
Anyway, I digress.
There are two conclusions we can draw from the CMB:
1) The Earth is at the centre of the Universe. I don't know why religious crazies ne
Yes, of course. Thing is that we can't apply that logic here. We know that very large structures are extremely unlikely - in a brute force, frequentist approach which is the nearest analogy to flipping a coin a hundred times, we can make a hundred simulations. If in those hundred simulations we do not see a structure that size, we know that the likelihood of it occuring *within the confines of the model tested* - and that is a very important proviso; we can only assign probabilities in this manner with respect to a given concrete model - is less than 1 in 100. If we do 10,000 simulations nad a structure of that size occurs in 1 of them, we know the probability is approximately of the order of 1/10,000. I simplify, of course, but in this manner we can gain a feeling on the likelihood that such a large structure would just happen to crop up in our particular realisation of the universe.
I use the word "realisation" statistically. In cosmology we have knowledge of the statistical nature of hte universe. What we have no knowledge of, a priori, is the actual distribution of matter in the universe. Instead, we have the assumption that fields were initially Gaussian (strongly borne out by CMB observations, and also currently borne out by large-scale structure, although non-linear evolution does introduce a measure of non-Gaussianity), and we have their power spectrum. From this we can make a realisation of the statistical field. The simulations I'm referring to are just such realisations. In the most complex case (such as n-body simulations like the surprisingly old Millenium Simulation) the realisations are made when the universe was a tiny fraction of its current size when all available data suggests linear theory held, and then evolved forwards. That's a punishingly slow procedure, so various other approaches can be used. In all cases, it's about trying to see what the probabiiity of, in this context, a given structure appearing in our universe is.
Again, cosmology is not a science of people entirely ignorant of statistics. In many ways, cosmology is one of the most statistical of the natural sciences, since by its very nature we can *only* ever know statistics, or else have an entirely descriptive and non-predictive survey of the sky. That isn't to say that all cosmologists are master statisticians - I'm very willing to admit that my own statistics aren't the strongest of the cosmologists I know - but at least a passing knowledge of statistics is a prerequisite.
Yeah, I was just meaning that the CfA Great Wall was superceded by the Sloan Great Wall. If this current structure turns out to genuinely be a structure, it supercedes the Sloan Great Wall by some considerable size.
Thank God we have people on Slashdot to tell us things like this. Where would we have been if generations of cosmologists were entirely ignorant of statistics or gravitational physics? The mind boggles!
Sorry, but the problem isn't that there are lumps - if there weren't our existence would be a bit suspect since we live on the edge of a reasonably large lump (the Virgo supercluster) ourselves. The problem (if you want to call it a problem; it's more an interesting question) concerns the *size* of the lumps. We can predict with reasonable certainty the probability of a bound structure of such and such a size appearing in the universe. That's quite straightforward in principle. And structures this big are pushing the bounds of the standard cosmological model quite hard; basically, they shouldn't really be there. I don't know the actual probability but it's extremely low, and low enough that we would not expect to see it. That there are now three structures that are rather too large (this one, if it comes to be accepted as a genuine structure; the Sloan great wall, if it turns out to actually be a structure; and the CfA great wall) is getting interesting.
That was superceded 14 years later by the Sloan Great Wall: http://en.wikipedia.org/wiki/Sloan_Great_Wall
This is what the "relativity" in "special relativity" and "general relativity" is referring to, yes :)
Thing is that the model we use, there *is* no centre -- honestly. It's probably best viewed as a four-dimensional stack of three-dimensional, infinite, flat surfaces, and the expansion has no centre because there is no centre of an infinite plane. Anywhere the observer is, that looks like the centre. The usual analogy is to paint galaxies on a balloon and then inflate it. If you then visualise the surface of the balloon as being all there is, then you can quickly see that both there is no centre to the expansion (remember, the inside of the balloon doens't exist and the surface is all there is) and that from every point it looks like you're in the centre.
It's not a perfect analogy because of that third dimension, but it's close.
"An event happened within our light cone but due to the expansion of the universe, the cause of this event has moved beyond our horizon faster than the speed of light. In effect, we are affected by something over the horizon. No?"
It's a bit more subtle than that. If you think of things in terms of wavelengths, the expansion of the universe means that more and more wavelengths are suddenly smaller than our horizon -- which means they can start interacting with us. Anything with a wavelength larger than the horizon is not yet going to be able to interact, but will eventually. (This is known as "horizon-crossing" in the jargon, and you'll see it used pretty much ubiquitously.)
The difference comes if the expansion is *accelerating*. An inflationary period -- which looks likely to have happened even if all the current models are not entirely convincing -- takes an extremely small area and blows it up to a ridiculously large volume. In this situation, things are reversed, and wavelengths are *leaving* the horizon. A different way of viewing the same thing is that, yes, if we assume an inflationary epoch occurred then we are affected by things that we used to be in causal contact with but which inflation rapidly pushed far, far away from us.
Since in the standard model the universe is currently accelerating we actually have things right now leaving our horizon, which does indeed mean that, in principle, we have been affected by thigns that we are no longer in causal contact with. If this is the genuine setup in the universe, the future looks pretty bleak -- eventually you can expect to see the local group, and perhaps the local supercluster, with everything else swept way outside our horizon. The universe will look pretty empty in that case, and we'll have lived in about the only period where we could tell there *is* a cosmology at all, which makes us rather lucky... (Lawrence Krauss once gave a good talk on that premise, so I credit him with that statement.)
I won't lie to you - I'm extremely sceptical about it. But wading through the maths, at a genuinely fundamental level, is a prerequisite, as is testing any theory of gravity first and foremost on Solar System scales, and then against the battery of tests that GR has passed. (You can only avoid this if you explicitly state that you're working with an effective field theory valid on particular energies or particular scales, but that takes some careful setting-up to pull off without it looking basically unmotivated.)
A lot of astronomers miss it too, chiefly because it's so easy to assume that Newtonian gravity works in these regimes, and without wanting to sound too critical astronomers (and increasingly cosmologists) are increasingly trained within their own field. I more and more feel I was fortunate to have done my Masters' in condensed matter theory before doing a PhD in cosmology. You get a more... physical view of things, and a strong appreciation for complex emergent behaviours which are practically inevitable when you look at systems composed of large numbers of particles. And a galaxy, and an entire universe, definitely fit that definition. (Interestingly, a galaxy cluster much less so, you're probably best modeling that as a system of about 100 or 200 weakly-interacting, loosely-bound bodies. Statistical physics is unlikely to be of much help there.)
OK, to be annoyingly facetious:
"The matter's gravitational influence is the only thing being detected because the object is going faster than light away from earth."
Citation needed. That appears to suggest a model that cannot possibly work. How can galaxies be held together by dark matter if it's always moving away from Earth? Is Earth ejecting vast quantities of dark matter? If not, why don't we see the stuff coming *towards* us? Though we should be glad we don't because it would be blue-shifted to oblivion (probably ours).
"Light and gravity do not travel at the same speed."
Citation needed. The "speed of gravity" is a pretty nonsensical statement in GR, but what we do have is the speed of a *change* in gravity, and that is the speed of light in vacuum. If you've a proof otherwise you have to cite it.
"I think this bothered Einstein and (I heard) he tried to solve that problem all the way to his final days."
Citation needed. Einstein worked on a lot of things to his final days, including the paradoxes of quantum mechanics and the unification of gravity with electromagnetism.
"Anything traveling faster than light away from earth is not going to be "seen" because any light from that (gravitational) mass is not reaching us; but, the gravity is."
Citation needed. See the first point.
And so on. At the minute you're making assertions that flatly contradict a model that, no matter what you may think, has been staggeringly successful. They also flatly contradict the theory that underlies that model. When you do this you have to have a good reason, a good argument, and supporting evidence. What you have at the minute doesn't make a large amount of sense. We can send things faster than light in a medium -- we get radiation from that while the object slows down. We can't send things faster than light in a vacuum and we've tried. If we had something moving faster than light then you get some extremely odd things happening, due chiefly to gross violations of causality. I'm not a fan of the idea of being hit in the back of the head by something emitted (evidently from Earth) in three weeks' time.
"We know gravity as at least as fast as light but it may not be equally as fast."
So you're saying the "speed of gravity" (whatever that means; at present the best we know is if you disturb a system the change in the gravity propagates out at the speed of light) is greater than or equal to the speed of light.
"According to big bang theory, the mass of the viewable universe (read: that part of the matter ratio traveling within one light speed of one another) can be traced back to within a small percentage of it's age."
I don't even know what you're saying here.
"Mass ejected from the point of origin had to compel mass faster than the speed of light (in relation to earth) away from point of origin."
There was no point of origin, and Earth's position is irrelevant. You're also not going to make much headway with a cosmology that places the Earth at the centre of the universe.
Look, I've said this kind of thing before but if you want to convince people you've got to use more than words and suggestions -- these are ultimately vague and worthless, no matter *who* is suggesting them. Einstein was a genius for thought experiments, but in the end they boiled down to justifications for a mathematical framework in which to work. The actual work then has to be with the mathematical model, and in managing to make it hang together coherently. If you can do that, excellent, but it has to pass, and keep passing, every observational test thrown at it, and any ad-hoc additions you make have to be justified. It may surprise you to learn that dark energy was actually postulated ten years before it was broadly accepted; Wetterich derived a model from high-energy physics, and at the same time (within weeks) Ratra and Peebles put out more phenomenological but broader models they'd been working on at the same time. This was in 1987. Dark energy was only really acc
"Actually - what has been detected is wrong by 3 or 4 orders of magnitude - in some derivative measurements - compared to what was previewed under teh big bang theory."
?
In the 60s it was predicted that there would be a uniform bath of microwaves around us with a temperature around 5-10K. Given that they were working on no data, getting it within a factor of two to four is pretty impressive, and by no means "three or four orders of magnitude", which would have involved them predicting a bath of radiation at between 3000K and 30,000K, which would be presently entertaining itself roasting the Moon, knocking our satellites out of commission and probably destroying our atmosphere. I think someone's got confused somewhere.
I think the only way a person outside academia can present a rigorous mathematical model is to ensure it really is as rigorous as it can be. It's unfair, I know, but getting something published from outside the system is extraordinarily difficult, which is not least a result of the number of submissions that are, well, crackpot. (The arXiv moved to an endorsement system about ten years ago now to help combat that. Endorsement doesn't mean anything other than "This is in a field that is probably worth looking at", but has to come from someone who has posted published work to their part of the arXiv, and it works as a good filter. We no longer get papers claiming that ball lightning is formed from primordial black holes.)
So I'd basically recommend start a mathematical education in the time you have. Start with vector calculus again, and electromagnetism and fluid dynamics, then move onto tensor analysis and then tensor calculus. Move to the phrasing of electromagnetism and fluid dynamics in terms of tensor calculus. It's also worth spending time learning linear algebra, complex analysis (at an applied level in both cases; the Schaum's Outline series are good for this), and then stepping on to first a modern approach to special relativity, which will phrase things in the language of tensor analysis (SR is based in Minkowski space, which is a generalisation of Euclidean space with a non-positive definite, err, Pythagoras theorem will do for now). This will include special relativistic fluid dynamics and electromagnetism. Then you can finally start looking at general relativity. Start with an old-fashioned, metric-based approach -- it's a lot clearer that way. Work through the derivation of the Einstein equations, and apply them to Minkowksi space, then to Schwarzschild and Robertson-Walker. Move onto more advanced topics, and particularly the Lagrangian formulation of GR. (A Lagrangian is a scalar that encapsulates a theory, and the Lagrangian density of general relativity is the Ricci scalar. If you're not clear on Lagrangians, get "Classical Mechanics" by Landau and Lifshitz and work through the first few chapters, and then jump to the final few chapters to fill in on the Hamiltonian theory of classical mechanics too.) You can start looking at the covariant approach to gravity, a name which sounds redundant but which involves separating the field equations according to how an observer is moving; the Weyl tensor emerges as rather fundamental in this approach.
Then you can have fun starting picking apart the theory. The thing is that to do so requires an understanding of the theory in the first place, and how it can be modified, the caveats of modifying it in a particular way, and an awareness and understanding of the criticisms absolutely any modification are going to receive. Oh, I forgot, look at linearised gravity and Solar System tests, and then the parameterised post-Newtonian approximation since that can characterise a wide sweep of modified models, and every one of them has to pass Solar System bounds to be viable. And when you come to put something on paper, make sure you've been reading the literature, write in that style, cite appropriately, be aware of who's also put out similar work in the field. At that point you may find a conference that you can register for, which is probably the best way to turn up and chat with experts in the field. You may or may not get someone interested this way, but if you act professionally and not over-eager you'll at least get something out of it.
I know it sounds a hell of a lot of work and it is - if you do it in the requisite detail, that's about three years of study, or more if your maths isn't too hot right now. But none of it is impossible, and it's not an entire degree plus PhD because it skips out all the quantum, solid state, condensed matter etc.
" I'd still quibble a bit here - you seem to be conflating "observational evidence" with "experimental evidence"."
To some degree I probably am, yes. Even so, if we have a dinosaur bone in front of us, we have that bone. We can poke it, we can hit our colleague with it, and we can scrape bits off and drop them in each other's tea. If we're excruciatingly lucky, we can dig out traces of organic material inside it and piss off a generation of paleontologists in the process. (So far as I'm aware, that's been done - not enough to do much with it, alas, but still organic.) The difference here is we can't do that with dark matter. The only evidence we have is far more indirect, and can be explained through a wide variety of mechanisms, few or none of which are amenable to direct testing, meaning something we ourselves can actively do. In the dinosaur case, we can at least dig up more bones, or find previous bones, scrape bits off those, and drop them in someone else's tea.
I think there is a very big distinction between these, though I do see your point.
"I haven't heard of a "gravity is different" theory that made accurate quantitative predictions of the CMBR data, where the dark matter theory did. Maybe I just didn't hear about it?"
Probably. If nothing else, you can get the CMB without too much issue from Brans-Dicke, from f(R) (no surprise there; it's effectively Brans-Dicke with a weird parameter), and if you play enough absurd games with an absurd theory you can even get things out of TeVeS which is a contrived relativistic form of MOND. If you write down a vaguely sensible bimetric theory -- and TeVeS is not really very sensible, but others are -- it seems likely we can get the CMB out of those, too. Braneworld theories give it happily, and Turok and Steinhardt's somewhat... eccentric ekpyrotic universe where two branes repeatedly slam together like cymbals with each slam kicking off a big bang, can also do it.
On a weaker scale, you can take GR but change the metric. Cosmology is built on the (Friedman-Lemaitre-)Robertson-Walker geometry, which is the second-most simple solution of GR there is. The simplest is Euclidean space. An FLRW geometry is a whole bunch of 3D Euclidean spaces stacked one on top of the other. It's slightly more complicated than that, since you can get closed FLRW (effectively a bunch of concentric, ultra-smooth spheres) and open FLRW (basically a load of saddles piled one on top of the other). There is good observational support for FLRW, but the same support can be given to a variety of particularly Bianchi universes, which are like an FLRW but slightly anisotropic. Control that anisotropy, and you've got a perfectly valid universe, with a slight directionality (which, intriguingly, Planck has seen in the sky -- though the form of anisotropy is actually not that easy to reconcile with simple Bianchi models). Until recently you could play games with Lemaitre-Tolman-Bondi metrics, which are like FLRW but lack the homogeneity, so that while around Earth everything looks spherical, away from us it is distinctly less so. In reality you still can use LTB models, but you have to be careful, and their main use (the observable effects of dark energy without having to introduce a physical dark energy or accelerating the universe) has been pretty comprehensively rubbished. (It's still not certain, since we haven't yet finished working out the perturbation theory properly, and without it any claims to be genuinely looking at the CMB and the oscillations in the large-scale structure should be taken with a bit of salt but, realistically... it's a very small bit of salt.)
And then we can assume gravity is the same but the problem is simply coming because even on galactic scales we're working with averaged motions (or, more concretely, a statistical mechanical system). On cosmological scales we can view things in three ways: a spatial average, an average across observations (these are distinct; one is the average of, say, the angular distance to objects of eq
Not necessarily. I just posted a reply to someone else addressing a similar question. This kind of system - the Bullet Cluster is the most famous example - was originally touted as a "proof" of dark matter. It isn't, though it is another sign that if you want to beat dark matter (for whatever that means) you have to be able to predict what that model does. Basically, a modification to general relativity will almost certainly change the reaction of the Newtonian potential (which pops up in weak fields as the time-time perturbation to the metric) and the scalar spatial curvature (ditto, on the space-space component) to the presence of matter, and can do so in surprising ways. One of those surprises is that if you're reasonably careful how you choose your modified theory of gravity, you can get peaks in the lensing potential quite displaced from your matter distribution, without introducing any exotic forms of matter. (The cost is an exotic form of gravity, of course... but we actually know more about the fundmental nature of matter than we do about the form of gravity on supergalactic scales, so as daft as it may sound I'm a lot more comfortable this way.)
Of course, it makes the model look a bit more contrived, particularly compared with dark matter which has effectively three assumptions: there is a non-interacting species, with a density roughly five times high than standard-model matter, and it is pressureless. However, to do calcualtions you have to make a vast array of other assumptions on the form of the distribution, which introduce a wide number of parameters and arbitrary functions, which makes the whole thing a lot less clean than it initially appears.
Even so, yes, interacting galaxies form a powerful testbed for this kind of theory. But they're not the killer they were originally touted to be - merely a strong discriminant. (Which is excellent, we need that. This isn't -- or shouldn't be, but I know people who've made it into one -- an ego game. It's meant to be about finding better explanations for things, in a field where I think increasingly people are going to realise they're not going to be able to link all the way back to fundamental physics. Or they will if the education level stays up, but I'm concerned about that when I see cosmologists coming in who at most have been taught a bit of GR, enough to do cosmology, and nothing more. That is a bit alarming; there is a breed of modern cosmologist who doesn't seem to know, or care, about what underpins the theory, and for whom Robertson-Walker metrics are the be-all and end-all. Worse, *linear* Robertson-Walker. Even people I respect hugely -- naming no names though -- have published papers ascribing effects to "modified gravity" that are far more easily, and physically, explained through, err, physics, by looking at second-order perturbations. But that's a different topic...)
"Correct, they only detected gravity, which we currently assume is only caused by the mass of matter."
Well, to be pedantic, they didn't detect gravity at all. A Nobel prize waits for that one, too. The assumption that it is "only" caused by mass is one of GR, certainly, but there are a plethora of theories in which this isn't the case, such as Brans-Dicke theories, generalised Brans-Dicke, f(R), f(G), scalar-vector-tensor, bimetric, etc. etc. etc.
"They've recently tested relativistic gravity by measuring red shifts and have some to the conclusion that gravity at galactic scale is working the same as solar-system scale."
Really? Could you provide a link to that one?
"And why does everyone keep bringing up only the rations of galaxies, what about gravitational lensing in dust-less gas-less empty space?"
A good question. Proponents of particulate dark matter theories have enjoyed bringing up gravitational lensing for some time now - and rightly so. They've enjoyed even more bringing up the Bullet Cluster, in which the gravitational lensing (presumably tracing the dominant mass) is in a very different location to the X-ray emission -- and rightly so. The thing is that you can actually get very similar results with, say, a bimetric theory. Even TeVeS can fit the Bullet Cluster if you're really careful about the massive neutrinos you add in. Massive neutrinos are not at all controversial, and sterile neutrinos aren't particularly so either, and a blend of those two can fit the Bullet Cluster with no problem -- and no vast quantities of dark matter.
What's more, if you take the physics to a genuine level (rather than one-dimensional, linearised systems) and start considering three-dimensional distributions of, say, a scalar/tensor theory, you find some extremely interesting interactions going on -- such as domain walls between areas where the scalar field is negative and areas where it is positive, followed by a sudden collapse and a dramatic ringing of oscillations through the universe. In this type of model, gravitational lensing is... different. It may not account for the entire lensing, it may not account for any, or it may account for the lot - I don't know, and to be honest neither does anyone else.
(I'd also like to point out that if we're talking properly, lensing does not actually trace mass. Except very near to a black hole or neutron star where the description breaks down, lensing traces the sum of the Newtonian potential and the scalar spatial curvature. In vanilla GR that "lensing potential" is certainly set by mass. In a slightly different theory, it can be set by a wide variety of things. The simplest, by a long way, is massive particles, and that's one reason dark matter is currently the most favoured explanation, but it is not the only way.)
I'm not wanting to argue, as such -- dark matter is by far the most accepted model, for a good reason. It explains the vast bulk of the observations more simply than any other model. Same with dark energy. Any replacement model has to replicate observations predicted by a dark energy/dark matter model, practically perfectly. But there are fewer fundamental reasons to believe it is literally true, particularly in the form currently presented, than is often believed (even by professionals).
"They use a model of a galaxy as if the mass was in the center like the star in a solar system, and wonder why it then doesn't match"
That's because there's a theory in Newtonian gravity that the force you experience is the same as if it were all concentrated at the centre of mass. For *spherical* systems a similar theory holds in general relativity. It wasn't ad-hoc, it was people applying Newtonian gravity to galaxies, and other than a few oddballs like me, most people do not question that relativistic effects in galaxies are entirely subdominant and that we may as well just use Newtonian theory. And hell, they may very well be right; this is a totally open question.
I think a lot of the issue here is actually your "they" vs "we" position. This isn't the case! All we really have -- and I speak as basically an insider -- is a "educated" vs "layman". I'm honestly, honestly not wanting to sound offensive or smug when I say that, just that the directions that physics goes in might seem nonsensical but there is almost always a very good reason to do it. If nothing else, it's starting from a theory that the researchers know is probably phenomenological (meaning "not fundamentally true", "grounded in observation and nothing more") but is at least self-consistent, coherent, complete, capable of taking in a situation and making concrete predictions, and then it's pushing that theory a bit further. There is absolutely no reluctance to introducing different theories, no matter what the popular conception is. My own field is cosmology, and I stopped bothering counting the endless variations on gravity, or the entirely different approaches to cosmology, or the weird shit coming in from the high-energy physicists, or the ways of producing inflatons or dark energy from different (reasonably well-motivated or batshit insane) multidimensional theories. Every single one of them was introduced for a solid, concrete reason, and I don't think *anyone* has ever attempted to state that this or that is fundamental reality, unless they had a very clear reason for doing so.
At the heart of everything is the knowledge that one is working from a theory -- a particular set of equations, true on a certain scale or in a certain energy range. I wouldn't try and use general relativity when describing gluons; it really doesn't work. I would try and use quantum chromodynamics when describing the orbits of galaxies in clusters; that would be ridiclous. I wouldn't even use GR for calculations within the solar system, for the most part, because it's way too complicated and the errors in using Newtonian gravity in that situation are so small.
The planet thing -- yes, true. But any explanation anyone comes up with has to fit with known physics, or challenge it in ways that leads, quantitatively, to further predictions that are then borne out by observation. If it can't do that, or if a suggestion fails on some fatal grounds (such as predicting planets orbiting close to stars, but failing to account for distant gas giants, or what have you) then it will definitely die. If something explains things perfectly but is controversial, it's probably already been written down and published by a professional astronomer...
You accuse astrophysicists of making up crap to fit new things into broken theories. OK, in some cases, fair enough - but my point is that some of that made up crap (such as braneworlds; what a load of bullshit) was introduced for very specific reasons, to answer very specific, and very pertinent questions. I've got a lot of contempt for braneworlds, but those theories were not introduced to stroke someone's ego or to give someone something to do; they were introduced as a way of examining the gross cosmological features one would expect to see in a world described by string theories. Braneworld theories themselves were pretty specious things, but that was the point, and so far as that goes, all power to them. My opposition comes entirely from the undue attention (and money) thrown at them, not for their introduction, and not for the reasons for their introduction, and not for the fact that the universe is obviously not a 3+1D brane suspended in a 4+1D universe. Because it isn't, such a suggestion has never seriously been made.
In the case of dark matter and dark energy, there have been alternatives, and plenty of them. I've gone on record - on Slashdot as well as in publications - stating that the "answer" to the dark matter problem is very likely to be an ugly mixture of every solution we have yet proposed: massive neutrinos (they are massive, but extremely warm), sterile neutrinos (likely cold), a lightest supersymmetric particle or two, unforeseen effects of general relativity
"This is the Electro-Gravitic theory of space and provides a clear explanation for dark matter, dark energy without resorting to anything we have not already proven experimentally or incredibly complex math that defies human understanding."
That's the kind of comment that always makes me extremely wary, particularly as practically the only place I've ever seen it is following a wall of text that builds -- on relatively specious assumptions and assertions -- without any actual concrete theory. The problem is entirely that, no matter what philosophy we can dig out of it, physics is about algorithms. We *have* to have numbers we can put into our algorithms, which typify the scenario we wish to consider, and we *have* to have numbers out, which are what, according to this theory, we expect to see coming out.
The problem with the theories I've seen where people promise "clear explanations" for dark matter, dark energy, and frequently cheap or free energy, is that they fail the first step -- they do not provide a robust and self-consistent mathematical framework.
Anyway, with that little rant out of the way, I'd be cautious. What Einstein demonstrated is that a theory of gravity that is both far simpler and frankly better experimentally supported than Newtonian gravity is general relativity. In Newtonian gravity, the gravitational field is a three-dimensional field, instantaneously sourced by two or more objects of positive gravitational charge. (This is commonly dubbed "mass", and in this context is normally called "gravitational mass" in modern physics.) One enormous issue with this concept is that if you take Newton's law of acceleration (F=ma) and equate it with Newton's law of gravitation (F=GMm/r^2), you cancel the ms and get a=GM/r^2. OK, brilliant. Nothing there... except that Newton's law of acceleration has sweet FA to do with gravity. That mass is an "intertial mass"; it describes the response of a body to a force, and there is no a priori reason to link it with a gravitational mass at all. I don't think -- though I may be very wrong, of course -- that Newton was aware of this subtlety, but by the early 20th century it was very well known, and things like Eotvos experiments were set up to try and tell whether these masses were actually different at all.
The point here is that if the intertial and gravitational masses are the same, every object reacts to a gravitational force with the same acceleration. Try and think of the last time you saw that. I would put a vast amount of money on it being the last time you were in a vehicle that was turning a corner. (If you think slightly more subtly, it was the last time you wondered why weather patterns on Earth run the way they do.) It is well known that centrifugal and coriolis forces are artificial ("fake") forces, caused by observing in an accelerating frame of reference, such as a car going round a corner, or on the surface of a spinning planet. However, they feel very real to the objects that are in those frames, as anyone going round a corner, or being flung from a roundabout, can verify. The hallmark of a fictional force is that every object experiencing them moves with the same acceleration. In this context this is obvious: the "force" is entirely due to the acceleration of the frame of reference, so it's bloody obvious that the force felt is going to be the same acceleration by every body. The thing is that there is no "natural" force that does this: any force where all bodies feel the same acceleration is probably fictional, and that "probably" is only there in case someone cooks up a bizarre theory where they can get this any other way.
That's the soul of GR, and as soon as you try and work through from there (known as the "weak equivalence principle": that the intertial and gravitational masses *are* equivalent, not merely that they're close to it) you're lead straight to a four-dimensional theory of gravity. It also leads to issues of causality, of the propagation of gravitational radiation at what we link with the
Yes, in a sense. I have a strong suspicion that if we were able to do a proper statistical mechanical analysis of the situation we'd see some odd emergent behaviour -- a galaxy is, after all, a rarified gas of about 10^9 interacting, confined bodies. We'd get different behaviour in a cluster, and different on cosmological scales.
Of course, I may be wrong and what we'd get out would be effectively pressureless dust, which is what we currently put in. Thep roblem is that at the minute we can't do a proper statistical mechanical analysis. We don't even have a full theory to work with, though there's progress here, too.
No, I can assure you it wasn't Smoot. I've never encountered him but I've never heard anything bad.
Yeah I tried to go through some of that stuff years back, and it was distinctly unconvincing, sketchily-laid out, and in a far weaker state than the author(s) would wish you to believe. Ultimately, if they feel they have a truly viable theory they have to apply it, in as much detail as the current LCDM model has been applied. That means they have to start off in the early universe (or the distant past, if you prefer; we don't *have* to assume a Big Bang), then justify in some way the existence of both the cosmic microwave background, and the exact spectrum of perturbations on it; then in the same, self-consistent coherent model, they have to account for structure formation and the presence of a wave imprinted on the largest scales of galactic structure which just happens to have a wavelength that perfectly matches that on the CMB... if the universe evolved as predicted by a Lambda CDM model; they have to include a form of nucleosynthesis to explain the ratio of elements we see in the oldest stars; they have to explain why old stars tend to be metal poor and young stars are metal rich; they have to explain the collapse of shards in clusters to form galaxies; and so on and so on.
Do that, and people might just start paying attention... but they have to do it at a level of rigour that is equivalent to that employed in professional cosmology. If they can't, they don't have a theory, they have words, and words are extremely cheap. It has to be couched in a mathematical language, and that's because it has to have a surmise and make a testable prediction. It has to be directly testable. I am very definitely not a fan of Lambda CDM, and a hunt back through my posts on /. that relate to cosmology would probably make that quite clear, but I've spent many years looking at it and its perturbations anyway. In my view, Lambda CDM has one absolute killer of a prediction: the wavelength which it predicted, from that on the CMB, was imprinted on the large-scale structure, and which was later found, exactly where it said. That wavelength, and the amplitude of the wave, is exquisitely sensitive to any change in the evolution of the perturbations, which is itself exquisitely sensitive to a change in the background spacetime. Lambda CDM got it right; any successor model -- and I hope to God there is one, because Lambda CDM is not satisfactory -- also has to.
The last that I knew, the Electric Universe stuff doesn't do any of this. (I would emphasise again that to gain acceptance it is not enough to posit a model -- and it's not even enough to present some back-of-the-envelope calculations. Frankly, the absolute minimum is a full analysis of possible backgrounds -- containing at least photons, neutrinos and standard model matter -- before you can even think of putting a paper out. That would then need to be followed up with an analysis of the perturbations, which we are all after all made from. Effectively, a version of the CAMB code, or one of its competitors, is necessary. Without it, you don't really have a viable model, just yet another model that can recreate something with observables matching the background Lambda CDM, and those come ten a penny. And so on. This is not an easy job, which is why we have no answers yet -- but it sure as shit isn't because the people working in the field are purblind idiots devoid of imagination or soul. Well, certainly not all of them ;) )
Sloppiness :( Sorry, I'll try and do that a lot more often in the future.
The difference here is that whereas normally the "indirect" signals we receive are photons directly from a particle, or indeed a measurable and reproducible influence on known quantities in a laboratory setting (which includes the tracks of known particles through accelerators), dark matter is not easily amenable to such tests. We only see it (interpreting "it" loosely -- the way I use the words, 'dark matter' should be interpreted as 'the fact that galaxies, clusters and the universe as a whole act as though there is more matter than we observe', which is probably infuriatingly vague :( ) through its gravitational effects, and by the sheer weakness of gravity and the impractical idea of creating, well, galaxies in a laboratory setting it is never going to be directly detectable that way.
The Higgs boson, on the other hand, was seen in reproducible experiments. I do agree that we can quibble on whether it was a direct detection, or whether it was indirect, given that its existence was ultimately deduced from the pattern of particles around it - but there are big differences. For one thing, a (relatively) quick analysis of the shrapnel from a collision that produced a Higgs will point to a particle of a particular mass and nature. That can then be reproduced (albeit at a low likelihood, given the nature of the experiment), and has been. We only even saw announcements from CERN when two independent experiments both reported an excess at the same mass. (In particle physics these certainly used to be called "resonances" -- when you find that collisions with a particular energy change nature dramatically, you can be pretty certain there's a particle there. For all I know, they're still called resonances, but my particle physics is second-hand through textbooks and therefore about 25 or 30 years out of date.)
It basically comes down to a detection on local scales, under conditions we can control, through a force other than gravity. We can't examine anything through gravity - it's uselessly weak, and impossible to control. That's a "direct detection", and can be through interactions with photons, or the influence of the new particle on the particles we observe coming out of its interactions and annihilations, or anything along those lines that can be seen, influenced, reproduced, observed. We can't do that with the evidence for dark matter. All we have is that galaxies rotate faster than they should (and they do, unequivocably), and that clusters should not really be bound (but they are, equally unequivocably), and that we cannot account for this with our current theories of gravity. The easiest solution is at least one particulate dark matter, certainly -- but if that exists it *is* amenable to production in a lab, even if to actually observe it we would have to wade through ten times more data than the LHC pours out, or a billion times more. But that isn't the only solution, because the only evidence we have is through gravity, and there is absolutely no reason at all (and it would be a mild form of intellectual blindeness) to prematurely declare that "dark matter" is definitely particulate and not, say, a sign that gravity does not behave on kpc scales the way it does on AU scales, let alone on Mpc and Gpc.
It's always enlightening to see how it looks to people who have had occasional glimpses from the outside but never bothered looking any further.
No-one is so wedded, philosophically, to the idea of CDM as is. Everyone knows its an approximation. The arguments over what it *is*. Mirage, particle, multiple particles, modifications to gravity, unanticipated effects of relativity on large scales, unanticipated effects of *averaging* observations across large scales, or a combination of the lot of them. And I can guarantee that practically no-one has been arrogant enough to stand up in a room and declare that we know what dark matter is.
I saw one person - who shall remain nameless - say something along these lines. He said to a room full of distinguished cosmologists (and me, I'm not distinguished at all), and I paraphrase since this was a few years back, "We can be absolutely certain that supersymmetry exists". That quite took my breath away. Firstly: no we can't be. Secondly: lol. Thirdly: winning that prize obviously turned you into an even bigger prick than you already were. I can't remember if anyone made these points to him because his talk was so stultifyingly boring, and so overlong, that I was comatose long before the end. Anyway, the corollary of his flabbergastingly inaccurate statement is that he also believes firmly that there is a single species of particulate dark matter, since this is more or less a prediction of general supersymmetric theories.
He's wrong, anyway. There may very well be supersymmetry, but we can in no way be certain that it exists.
Same goes for "dark matter", whatever you want to call it. The only thing you can't do is deny that the problem is there, and that the simplest explanation, which basically works all the way from galactic scales up to cosmological scales, is that it is composed of massive, weakly-interacting particles.
I'm a professional cosmologist, and I have to take issue with your first statement. The instruments did not, and categorically have not, detected the presence of something that is matter. If they had, that would be a direct detection of dark matter, and a Nobel prize would already be sitting on their desk. What they have detected are indirect signals of dark matter. It is very hard to reproduce the observations - particularly the cosmological observations - without adding at least one component of dark matter. So the observations are typically interpreted in terms of dark matter.
But this is very much not, strictly speaking, necessary. What we have is something that has an effect which, when viewed through a Robertson-Walker model, looks for all the world like a species of massive, weakly-interacting particle (or two or three such species - no-one ever said there has to be only one). On smaller scales, we have what for all the world appears to be a large amount of mass that can't be seen.
Any of this could be down to a modification of gravity. We know the nature of gravity roughly up to the position of the Voyager craft -- call it 300AU to be generous. We are extrapolating that a thousand times to get to galactic scales, a million times to get to cluster scales, and a thousand million times to get to cosmological scales, all without evidence. Of course, without a better theory to replace relativity, it's the best we can do, so we do it - but don't try and claim that instruments have detected that it is matter (they haven't), nor that we are wedded to particulate dark matter (with caveats, we aren't; the caveats are firstly that neutrinos have a mass and are therefore a rather warm dark matter, and secondly that it seems rather unlikely that there isn't at least one species of weakly interacting matter which would act as CDM, but maybe not in sufficient abundance to answer our woes).