Hear fucking hear. I'm no supporter of string theory in the slightest (relativist turned cosmologist here) but this summary bears no resemblance to the actual story. All I take from this is "that uppermost tip of braneworld theories that people used because it wasn't yet ruled out has now been ruled out to the surprise of absolutely nobody since nobody seriously treated any of these as anything but toy models".
I wish you'd not posted AC because maybe a few more people would have read your comment and it would now be +5 Informative and have spawned a lot of discussion. I'm impressed by/. that it's at +4 Interesting even though it's AC.
The CMB dipole isn't a "universal velocity reference", it's a dipole in the CMB. If one wishes one can attribute it to our velocity with respect to the CMB (although it could be other things; for example, we could live near the centre of a large void in which case the dipole would be related to our displacement from the void centre, or it could be a sign of an intrinsic anisotropy in the universe from a magnetic field with a coherence length greater than the horizon scale, or a super-horizon cosmic string, or even just an imprint from anisotropic inflation), but that doesn't say that the dipole is a "universal velocity reference". It says that the *CMB* is a "universal velocity reference" in that the existence of the CMB provides us with a fundamental observer with which to define cosmology. (In that we can do a spacetime split parallel to the CMB velocity, which acts as a "time", and onto its rest-frame, perpendicular to the velocity.)
But as someone else also pointed out, that doesn't even begin to go against one of the "tenets" of relativity (either special or general). It just says that the CMB -- the monopole, not the dipole -- is a convenient reference frame. If we really wanted, and we often do, we can pick a reference frame defined with respect to cold dark matter instead. The benefit of the CMB is we actually observe it, while CDM is a parameter in the equations. The benefit of the CDM is that it makes all the actual calculations easier.
The joy of relativity isn't that the CMB destroys a central tenet, it's that we can describe the same physics from a reference frame attached to the CMB, a reference frame attached to (possibly but probably not fictional dark matter), a reference frame attached to hydrogen atoms or a reference frame attached to people who post on Slashdot.
I think one of the main issues you have is you criticise relativity for being an ad hoc mathematical model. I won't argue that -- it is. The problem is *so is all of physics*. Physics is a collection of algorithms that are postulated to explain the behaviour of a part of nature and applied to another part of nature to test their validity. We can make good, persuasive arguments (and there are many that very convincingly argue that what we see as "gravity" is, at least on large scales, metric in nature -- that is, that gravity is a fictional force related to the curvature of a four or more dimensional spactime) but ultimately that's all they are. Arguments to describe nature.
Yes, we can instead try and quantise "space and time" (time being the particularly difficult part of that). But what does that *mean*? It means that we're simply applying a different collection of totally ad hoc mathematical models to gravity. There's nothing particularly fundamental about quantum mechanics, especially not in its first quantised form. After all, when Heisenberg first formulated matrix mechanics, which is ultimately the more conceptually pleasing formulation of quantum mechanics given its extremely close relationship with classical mechanics, what he did was collect all the observables he could think of together and fucked around until he got a set of mathematical rules that fitted experiment. Totally arbitrary! It was only afterwards that someone (Jordan, I think) told him that he'd rediscovered matrices and helped him formulate it as matrix mechanics instead of just a collection of equations. Still totally arbitrary.
And what about Schroedinger? He took a classical energy equation, postulated a form for a "wavefunction" (which had no meaning; and people still debate what the "wavefunction" actually means even if the Copenhagen interpretation is taught in universities) which lead to an operator that describes momentum, and lumped them together. What came out was the Schroedinger equation. Totally arbitrary.
Second quantisation? Equally arbitrary. Successful? Yes, absolutely. QED is I think still the best-tested theory we possess and it's astonishingly accurate, to one part in 10^12 or something like that these days. Th
Unfortunately such reasoning is certainly suspect in general -- Aristotle did it by postulating that nature was designed with niches to fit the various animals perfectly because he was unaware of the theory of evolution which has animals evolving into those niches -- but often quite successful in particle physics. The neutrino was first postulated totally arbitrarily to help explain weak decays and wasn't discovered for decades after, but people generally accepted it existed because the theory worked nicely. Much the same happened with things like W and Z bosons which weren't detected for ten or fifteen years after they were postulated to fit the symmetries of an apparently arbitrary theory. The current example is the Higgs' boson which we've *still* not discovered but which will cause ructions across all of high-energy physics if it doesn't exist.
That doesn't mean I *like* it. I get a distinct flavour of epicycles from a lot of this kind of thing too, but I can't deny it works.
(For the record, I hope the Higgs' doesn't exist. We'll have to junk about 50 years worth of theory and I quite like the idea of rebuilding without basing the whole lot on ideas from QED and group theory.)
You're missing the firt-order effects. At the "background level" -- so on average -- it's exactly as you've said. But the universe is by nature quantum-mechanical. If we assume inflation is roughly an accurate theory then we've got an inflaton field, which is a scalar field that comes into operation at extremely high energies, and "on average" it's totally smooth and drives an exponential expansion. But since it's a quantum field it also has random fluctuations all over it. These fluctuations themselves seed fluctuations in matter (and radiation) and in the spacetime curvature. Earlier in inflation these random ripples just get wiped out by the expansion but at the end of inflation they don't. So the ripples that were there at the end of inflation (which are about 4,500,000,000 times smaller than the background field) are allowed to evolve on and eventually became galaxy clusters and galaxies and so on.
Gravity is weak but it's extremely pervasive. If there is the slightest ripple at the start of the universe it will collapse under its own weight. Have a random distribution of these (of the right form) and you get the massive network of globules and filaments and voids that we see around us, or can simulate in things like the Millennium simulation.
It's one of the most impressive results of inflationary theory that it predicts the right kind of initial fluctuations when it wasn't designed to. True, you can also build inflationary models that don't work at all, but in general they work very well.
There are other ways of dealing with the problems inflation was invented to solve (flatness, horizon, defects) but all of them have to have a way of seeding these perturbations and get them to look about right. And they all do, generally quantum mechanical in nature. (Though I remember years back seeing Joao Magueijo giving a talk where he generated the perturbations through thermal effects. Last I knew, which was a couple of years back when I last met him, he was still working on that and it wasn't published.)
I'm actually a cosmologist by trade. I've been steeped in this stuff for about ten years now...:) Kaku's generally very good actually. I'm not a fan of when he's interviewed on TV but I like his books a lot. They're really clearly (and cleanly) written.
Glad to be of service, if it actually makes sense. When you've steeped yourself in something for years it's hard to realise whether it makes sense to normal people or not. Or even to slashdotters...
"Big Bang theory borrowed the idea of calculating the evolution of a system according to GR from black hole theory."
That may even be true - I'm not sure. But what's actually done is very different; at most, the motivation of looking at the evolution of a system with GR would be there. And that I'm seriously unconvinced by but I'm happy to be proven wrong. The thing is that the actual solutions bear no real resemblance to one-another. Schwarzschild is an isotropic but inhomogeneous, spherically-symmetric solution. FLRW is an isotropic and homogeneous solution. That makes an enormous difference. (Totally off-topic but McVittie metrics, amongst others, are exact solutions which are Schwarzschild embedded in FLRW.)
So yeah, the motivation for Friedman, or Lemaitre, or Robertson and Walker, might have been from looking at people modelling the formation of black holes. I've never heard that but it doesn't mean it's not true. Obviously. Even my ego isn't *that* big...
Yep, not long after the CMB anisotropies were first predicted, too.
It's a bit of a crime that there were no Nobel prizes forthcoming for any of this, except for Penzias and Wilson, who originally thought it might be pigeon shit in their antennae. (To be fair to them they had to rule out all possible sources of noise -- but the people who initially predicted the existence of the thing such as Gamow and his ilk, and then the structure of it, were totally overlooked. It seems seriously unfair. I still feel that Jim Peebles should get a very belated Nobel for his services to statistical cosmology.)
i'm not knowledgeable about philosophy, unfortunately:( but i wouldn't say it implies eternal recurrence as i understand it -- such that everything that happens repeats. the problem is that the things that are repeating are cosmological, so it's only things on the very largest scales. that means that the bulk properties of hte universe would repeat in a cyclic model (although entropy is an issue in that), but it doesn't say that anything on smaller scales will. each time it's extremely likely that the actual distribution of matter will be different since gravitational collapse is an extremely complicated process (and can be chaotic), and that means that even the massive networks of galaxy clusters will look different on each cycle, let alone the suns, planets and sentient beings.
this is because the initial perturbations, in almost every model, are quantum in nature and seeded in the first microseconds (or before) -- being quantum in nature they're also random in nature. so each time through the universe as a whole will be identical, but you'll get different ripples on top of it and a different "micro-structure", if i can use "micro-structure" to describe objects as large as superclusters of galaxies.
not sure if that answers your question though, unfortunately.
"classic", yeah, but not sure about "best". but i last read it 15 years back before i started doing any of this stuff properly so don't quote me on that:)
No offence meant, btw. For some reason I just felt like having a go at people linking religion and cosmology and kind of got off-topic from your post:)
Thank you. Your permission means a lot to me. From my side, please continue to walk around talking shit about ancient religions having anything pertinent to say on physical cosmology. When you can get out a prediction for the CMB sky please get back to me.
Caveat: don't misunderstand me, I couldn't care the slightest about ancient cosmological models one way or the other; they're absolutely fine by me. People can have any religion they want and that's also fine by me. I know some superb cosmologists, much better than I am, who are devoutly religious. But pretending that you can get a feasible cosmological model out of a religion is sheer delusion. A cosmological model is about predictive power -- basically, it involves numbers. No religion and particularly not ancient religions, are built on that premise. They're not about physics. Pretty obviously, they're about religion. And that's a good thing and quite how it should be. Physics killed my own belief in religion but that's my problem. Basically, physics is about how the world behaves and *nothing more*. It's algorithms. Set up a scenario, run your algorithm, and get out a prediction. That's not at all what religions are set up to do. I can sit there and dig in religious texts and support an argument if I like, but attempting to pin any scientific meaning to it is both missing the point and is, in all reality, grossly offensive to the believers of that religion while at the same time saying nothing of value to science.
You might not like that answer, but science is just about numbers. Religion has nothing to say about that. I'm a cosmologist, meaning ultimately I care about the CMB and the distribution of galaxy clusters. Until your vaunted Buddhist cosmology can give me a concrete prediction about the CMB and galaxy clusters I'm going to (rightly) dismiss it, because it has zero predictive power and zero use as a physical model.
In return, I am *not* pretending to say anything about the nature of humanity. Why would I? I deal with numbers, physical laws, and how the universe seems to behave. I draw conclusions from that, postulate a model, and test it against other bits of the universe. Metaphysics, by its very nature, is a bit outside of my domain of expertise. Likewise, physical cosmology is totally outside the domain of expertise of metaphysicists, philosophers, theologists, and random internet nerds with a hard-on for anything from the ancient East.
Hmm, I'll have a think. Nothing jumps immediately to mind (except Sean Carroll's online cosmology primer (http://preposterousuniverse.com/writings/cosmologyprimer/) but that's not quite what you're asking for. I think Rachel Bean wrote a book quite recently which was probably pretty good. Unless she was merely talking about it because I can't find it anywhere, but I know she's keen on what we call "outreach". I'll have a think.
Hmmm, big question. I'll try and give the quick answer
The first ripples are seen on the cosmic microwave background radiation. This is a bath of microwave radiation that surrounds us, at almost exactly 2.71K and the most perfect blackbody ever observed. It is virtually impossible to explain the existence of this without having something very similar to modern cosmology. People tried when they were trying to keep steady-state cosmologies in the 60s but ultimately they failed; it's seriously difficult to explain something with a blackbody spectrum and isotropic to one part in a thousand (to one part in ten thousand if you subtract off a dipole which is almost certainly a Doppler shift caused by our motion with respect to the CMB) unless the universe started from a compact, very nearly uniform state.
Basically, if you link the isotropy of the CMB with the idea that the Earth isn't at the centre of the universe, you're lead almost inevitably to modern cosmology: the universe is isotropic around the Earth, but the Earth isn't at the centre, so the universe must be isotropic around *every place in the universe*. That means it's both homogeneous and isotropic.
The next assumption is that gravity is metric-based -- that is, that on scales larger than a few micometres, that the effects of gravity are due to distortions in space-time. This is an extremely safe assumption on solar system scales but it's only that - an assumption - on larger scales. Still, we've got no sensible alternatives so let's stick with it. (It's very hard to build a working model of gravity that isn't metric-based.)
The next assumption, and this is much weaker but is the best we can currently do, is that Einstein's general relativty applies on very large scales. GR is a particular form of a metric-based theory and is the simplest, most intuitively clean of them. So let's stick with it. But it's quite weak.
Doing that, we're lead to only one possible model for the universe -- the Friedman-LeMaitre-Robertson-Walker model. Basically that says that if you've got 3D spatial surfaces that have to be homogeneous and isotropic, you can chose to make them flat, saddle-shaped or spherical, and then pile them together to fill the whole of spacetime. It then tells you the behaviour of these surfaces given the matter you put into it.
An immediate consequence of saying "The universe contains photons and baryons" (which is obvious; as cosmologists use the word, *everything* is baryonic except for neutrinos and photons, and no-one would deny that we exist, or that photons exist, or that neutrinos exist, so you put them all in there) is the CMB. It exists, and we can calculate when it formed. The CMB is formed when the temperature of the universe becomes low enough that photons don't continuously reionise hydrogen. Basically a small universe is a hotter universe, so at some point in the distant past (which turns out to be before the universe was about 370,000 years old) the universe was hot enough that if an electron combined with a proton to form hydrogen, a photon immediately came along and smacked the electron back out again. This tied protons, electrons and photons together. The universe was opaque and it was all a massive chaotic game of pool. Without any pockets.
When the universe became cold enough that that no longer happened, the electrons all condensed into the protons, the universe suddenly went neutral, and the photons could stream free. Those photons are the CMB. Originally they were very hot but as the universe has expanded they've been redshifted until they reached teh current temperature barely above absolute zero.
Now, when the photons and protons were bound together it wasn't all *entirely* smooth. There are waves go through any plasma. (Without them the Sun would be a very boring place.) These waves are those ripples in the CMB I mentioned. At the formation of the CMB the photons suddenly broke free and the waves stopped, err, waving. This left an imprint of the ripples in the *baryons* on the photons. Ba
"into" is the problem there but if you change that to "in" instead you've got a good picture of chaotic inflation (and, indeed, pretty much all current models of early cosmology). the "multiverse" is a conglomeration of regions of spacetime with different conditions and we live in one where an inflaton appeared with such and such a potential and drove an expansion that then lead to the current universe. "outside" our bubble are bubbles with different conditions, some that only survived for milliseconds before collapsing and others undergoing big rips and so forth.
Yeah, these "Eastern systems" have been *superb* at predicting the anisotropies on the CMB. Why, I even once saw an ancient Hindu text that gave a beautiful B-mode polarisation map with the foregrounds cleaned out, proving both the existence of primordial magnetic fields from a coupling of the inflaton with the electromagnetic field, *and* confirming the nature of the inflaton itself! Some of the concrete, testable predictions of these religions are well beyond the best our supercomputers can come out with! I'm currently working on a proposal for the Euclid satellite and I'm basing a lot of my statistical predictions on old Buddhist texts. Those ancient dudes sure knew how to model the baryon acoustic peaks in different cosmologies and how to observe them without having to build in assumptions from a particular cosmology!
To be fair, there are a lot of fudges in ekpyrosis. They just might be more acceptable than are in inflationary cosmology, depending on your point of view. (My view is that both are just phenomenology.) In inflation you have to postulate the existence of one or more inflationary fields (typically scalar fields, as yet unobserved in nature which is a problem since the Higg's is a scalar field) and a specific form of potential. In ekpyrosis you have to postulate two *perfectly parallel* 5D branes, and a very specific form of potential governing their interaction. That potential was pulled out of a hat to give the dynamics they want, just as inflationary potentials are pulled out of a hat; and that setup is pretty contrived and unlikely, just as the existence of, say, one single scalar field in addition to the Higg's is pretty contrived and unlikely.
Horses for courses, really. If we see a nice signature of gravitational waves on the CMB with Planck then ekpyrosis is dead until Neil can find a way of arguing that actually there *should* be a signature. If we don't, then neither inflation nor ekpyrosis is dead because we can tune inflationary models to make the signal vanishingly small anyway.
Gravity getting "weaker" is probably referring to a scale-dependent Newton constant (or however else they've phrased the strength of gravity). Something like Horava-Lifshitz gravity is perhaps worth a look if you're interested in things like that and can find a popular-level thing on it (I think I read a survey of Horava-Lifshitz gravity on the New Scientist website a month or two back which was OK if a bit... undercited, shall we say). That's basically a theory of gravity that modifies the Newton constant on very small scales to make it easier to link with the other three forces. Failing that you could hunt out things on "Brans-Dicke" gravity, or "Scalar/Tensor" gravity, which are effectively theories with a *space*-dependent (rather than scale-dependent) Newton constant.
Easy reading on FLRW cosmology? Hmmm. For whatever flaws he might have, Sean Carroll's a very good communicator. http://preposterousuniverse.com/writings/cosmologyprimer/index.html is worth a look. (He might not use the words "FLRW" anywhere in it, which if so proves he's smarter than me when he's talking with non-specialists...)
I'd try and defend my profession but I won't because you're quite right. We can happily build models for pre-big bang theories but until we've got a good reason to believe in a way to go with high-energy physics, it's all just phenomenology -- a mathematical way of waving your hands, basically. No-one's actually denying this; if you read the papers on this kind of model they'll tend to wave their hands madly and talk about modifications arising from M theory and low-energy effective field theories. All that is just gloss, motivations for your own model which you'll never seriously pretend is fundamental.
What I would say though is that putting the bounds on your effective theory at least gives you a handle on your inaccuracies. Not many religions do that...
"the theorie of Black Holes gave the idea to the BigBang theory - they just applied time-reversal!."
No 'they' didn't. Black holes are based on inhomogeneous solutions to Einstein's equations -- the first being the Schwarzschild solution describing a spherical, uncharged body embedded in flat spacetime, with Reisser-Nordstrom, Kerr and Kerr-Newman adding in electromagnetic fields, rotation and then both respectively.
Cosmology is based on Friedman-LeMaitre-Robertson-Walker solutions, which impose maximal symmetry on spatial surfaces of constant time. You might be interested to note that no black hole solution can be maximally-symmetric since only three surfaces are -- normal flat space, a (hyper)sphere and a (hyper)saddle.
There really isn't much connection. "Reversing" time on a black hole solution (which happen when you take, for example, a Schwarzschild solution and allow it to exist all the way to the centre of the system instead of cutting it off with a stellar surface partway down, which is what happens in the solar system) gives you a white hole.
Basically. You're talking about the critical density of the universe. This is about 1, meaning that the universe is "flat" -- so it's infinite in extent and basically composed of a load of flat sheets rather than saddle shapes or spheres. So far as we can tell it's exactly 1. (It's pretty easy to tell, actually. We can look at the ripples in the universe back from when it was 370,000 years old, and then look at *those same ripples* from when the universe was about 10 billion years old. Those ripples have a particular wavelength, so from that we can tell how much the universe had to expand. It pins things down really quite nicely.)
Our problem comes from counting how much actual normal ("baryonic" though it includes more than just baryons) matter there is, by looking at everything that glows (and also by looking at the amount of hydrogen and helium, which was produced in the first few seconds of the universe's life; the ratio between the two is extremely sensitive to how much baryonic matter there was). It gives us a density parameter of about 0.05. Shit. So we then look at how much clumping matter we need, which would include "dark matter", whatever form that takes. We find that we need about a density parameter of 0.3 -- so 25% of the universe is dark matter.
Shit. We *still* only have 30% of the universe even accounting for dark matter.
So we're forced to add about 70% of the universe in something else. That can't clump and for other reasons it has to act as an "anti-gravity". That's called "dark energy".
Rather surprisingly, this model fits all the available data...
That's not really so much "consensus" as "the result of the model we're currently using". No-one *likes* the standard model of cosmology; it's obviously just phenomenology, but it happens to fit all the data at least as well as any alternative. The standard model of cosmology is "lambda CDM", the lambda being a cosmological constant which drives an accelerating expansion in the current universe, and the CDM being cold dark matter which was responsible for the clustering of matter and the formation of galaxies and so forth.
The problem is that if it *is* just a cosmological constant then it will grow to dominate the universe and things will, indeed, expand forever.
But it's probably not a cosmological constant. The "best" prediction from quantum field theory -- and it's not really a prediction so much as the only way of estimating the size of the constant -- comes from evaluating the vacuum energy. Doing this suggests that it should be about 10^120 times bigger than we see in reality. That's a pretty big difference. Weinberg described it as the most embarrassing mismatch between theory and observation in the history of science, and he's got a point. The conclusion is that there's probably some mechanism (coming from trans-Planckian physics, maybe, or something else) that cancels the cosmological constant. If that's true, then it's almost certain to cancel it perfectly because the fine-tuning necessary to produce the *observed* constant is horrific, whereas a symmetry principle could wipe the whole thing much more easily.
That leaves you open to more general dark energy models to explain the accelerating expansion and that's where you have more fun. There are plenty of ways to get an observed acceleration. Some of them lead to big rips, which is where eventually the universal expansion will tear galaxies, then solar systems, then stars, planets and eventually even atoms and nuclei apart. Others lead to the decay of whatever field is responsible for acceleration -- like if you couple a scalar field into dark matter you can tune it such that the scalar field grows to dominate and then starts transferring its energy into dark matter, which would cause the universe to reclump again (and then probably the dark matter would dump energy back into the scalar field causing more acceleration). Those models are horribly contrived and unrealistic, but at least they're alternatives.
And the cosmological constant is pretty contrived and unrealistic in the first place...
Anyway. Before I got side-tracked my point was that there isn't really a consensus so much as a model that fits observations and predicts eternal expansion, but that it's not the only model and it's not even the best motivated model, merely the simplest. Other models can still lead to crunches while fitting pretty much all the data as well. And others can lead to cyclic universes, which is ultimately what Penrose is talking about in one form or another.
Disclaimer: I am a cosmologist but I've not actually read the article. This is Slashdot, after all.
Slashdot Mirror
Hear fucking hear. I'm no supporter of string theory in the slightest (relativist turned cosmologist here) but this summary bears no resemblance to the actual story. All I take from this is "that uppermost tip of braneworld theories that people used because it wasn't yet ruled out has now been ruled out to the surprise of absolutely nobody since nobody seriously treated any of these as anything but toy models".
I wish you'd not posted AC because maybe a few more people would have read your comment and it would now be +5 Informative and have spawned a lot of discussion. I'm impressed by /. that it's at +4 Interesting even though it's AC.
The CMB dipole isn't a "universal velocity reference", it's a dipole in the CMB. If one wishes one can attribute it to our velocity with respect to the CMB (although it could be other things; for example, we could live near the centre of a large void in which case the dipole would be related to our displacement from the void centre, or it could be a sign of an intrinsic anisotropy in the universe from a magnetic field with a coherence length greater than the horizon scale, or a super-horizon cosmic string, or even just an imprint from anisotropic inflation), but that doesn't say that the dipole is a "universal velocity reference". It says that the *CMB* is a "universal velocity reference" in that the existence of the CMB provides us with a fundamental observer with which to define cosmology. (In that we can do a spacetime split parallel to the CMB velocity, which acts as a "time", and onto its rest-frame, perpendicular to the velocity.)
But as someone else also pointed out, that doesn't even begin to go against one of the "tenets" of relativity (either special or general). It just says that the CMB -- the monopole, not the dipole -- is a convenient reference frame. If we really wanted, and we often do, we can pick a reference frame defined with respect to cold dark matter instead. The benefit of the CMB is we actually observe it, while CDM is a parameter in the equations. The benefit of the CDM is that it makes all the actual calculations easier.
The joy of relativity isn't that the CMB destroys a central tenet, it's that we can describe the same physics from a reference frame attached to the CMB, a reference frame attached to (possibly but probably not fictional dark matter), a reference frame attached to hydrogen atoms or a reference frame attached to people who post on Slashdot.
I think one of the main issues you have is you criticise relativity for being an ad hoc mathematical model. I won't argue that -- it is. The problem is *so is all of physics*. Physics is a collection of algorithms that are postulated to explain the behaviour of a part of nature and applied to another part of nature to test their validity. We can make good, persuasive arguments (and there are many that very convincingly argue that what we see as "gravity" is, at least on large scales, metric in nature -- that is, that gravity is a fictional force related to the curvature of a four or more dimensional spactime) but ultimately that's all they are. Arguments to describe nature.
Yes, we can instead try and quantise "space and time" (time being the particularly difficult part of that). But what does that *mean*? It means that we're simply applying a different collection of totally ad hoc mathematical models to gravity. There's nothing particularly fundamental about quantum mechanics, especially not in its first quantised form. After all, when Heisenberg first formulated matrix mechanics, which is ultimately the more conceptually pleasing formulation of quantum mechanics given its extremely close relationship with classical mechanics, what he did was collect all the observables he could think of together and fucked around until he got a set of mathematical rules that fitted experiment. Totally arbitrary! It was only afterwards that someone (Jordan, I think) told him that he'd rediscovered matrices and helped him formulate it as matrix mechanics instead of just a collection of equations. Still totally arbitrary.
And what about Schroedinger? He took a classical energy equation, postulated a form for a "wavefunction" (which had no meaning; and people still debate what the "wavefunction" actually means even if the Copenhagen interpretation is taught in universities) which lead to an operator that describes momentum, and lumped them together. What came out was the Schroedinger equation. Totally arbitrary.
Second quantisation? Equally arbitrary. Successful? Yes, absolutely. QED is I think still the best-tested theory we possess and it's astonishingly accurate, to one part in 10^12 or something like that these days. Th
Unfortunately such reasoning is certainly suspect in general -- Aristotle did it by postulating that nature was designed with niches to fit the various animals perfectly because he was unaware of the theory of evolution which has animals evolving into those niches -- but often quite successful in particle physics. The neutrino was first postulated totally arbitrarily to help explain weak decays and wasn't discovered for decades after, but people generally accepted it existed because the theory worked nicely. Much the same happened with things like W and Z bosons which weren't detected for ten or fifteen years after they were postulated to fit the symmetries of an apparently arbitrary theory. The current example is the Higgs' boson which we've *still* not discovered but which will cause ructions across all of high-energy physics if it doesn't exist.
That doesn't mean I *like* it. I get a distinct flavour of epicycles from a lot of this kind of thing too, but I can't deny it works.
(For the record, I hope the Higgs' doesn't exist. We'll have to junk about 50 years worth of theory and I quite like the idea of rebuilding without basing the whole lot on ideas from QED and group theory.)
You're missing the firt-order effects. At the "background level" -- so on average -- it's exactly as you've said. But the universe is by nature quantum-mechanical. If we assume inflation is roughly an accurate theory then we've got an inflaton field, which is a scalar field that comes into operation at extremely high energies, and "on average" it's totally smooth and drives an exponential expansion. But since it's a quantum field it also has random fluctuations all over it. These fluctuations themselves seed fluctuations in matter (and radiation) and in the spacetime curvature. Earlier in inflation these random ripples just get wiped out by the expansion but at the end of inflation they don't. So the ripples that were there at the end of inflation (which are about 4,500,000,000 times smaller than the background field) are allowed to evolve on and eventually became galaxy clusters and galaxies and so on.
Gravity is weak but it's extremely pervasive. If there is the slightest ripple at the start of the universe it will collapse under its own weight. Have a random distribution of these (of the right form) and you get the massive network of globules and filaments and voids that we see around us, or can simulate in things like the Millennium simulation.
It's one of the most impressive results of inflationary theory that it predicts the right kind of initial fluctuations when it wasn't designed to. True, you can also build inflationary models that don't work at all, but in general they work very well.
There are other ways of dealing with the problems inflation was invented to solve (flatness, horizon, defects) but all of them have to have a way of seeding these perturbations and get them to look about right. And they all do, generally quantum mechanical in nature. (Though I remember years back seeing Joao Magueijo giving a talk where he generated the perturbations through thermal effects. Last I knew, which was a couple of years back when I last met him, he was still working on that and it wasn't published.)
I'm actually a cosmologist by trade. I've been steeped in this stuff for about ten years now... :) Kaku's generally very good actually. I'm not a fan of when he's interviewed on TV but I like his books a lot. They're really clearly (and cleanly) written.
Glad to be of service, if it actually makes sense. When you've steeped yourself in something for years it's hard to realise whether it makes sense to normal people or not. Or even to slashdotters...
"Big Bang theory borrowed the idea of calculating the evolution of a system according to GR from black hole theory."
That may even be true - I'm not sure. But what's actually done is very different; at most, the motivation of looking at the evolution of a system with GR would be there. And that I'm seriously unconvinced by but I'm happy to be proven wrong. The thing is that the actual solutions bear no real resemblance to one-another. Schwarzschild is an isotropic but inhomogeneous, spherically-symmetric solution. FLRW is an isotropic and homogeneous solution. That makes an enormous difference. (Totally off-topic but McVittie metrics, amongst others, are exact solutions which are Schwarzschild embedded in FLRW.)
So yeah, the motivation for Friedman, or Lemaitre, or Robertson and Walker, might have been from looking at people modelling the formation of black holes. I've never heard that but it doesn't mean it's not true. Obviously. Even my ego isn't *that* big...
No problem, if it makes any sense...
Yep, not long after the CMB anisotropies were first predicted, too.
It's a bit of a crime that there were no Nobel prizes forthcoming for any of this, except for Penzias and Wilson, who originally thought it might be pigeon shit in their antennae. (To be fair to them they had to rule out all possible sources of noise -- but the people who initially predicted the existence of the thing such as Gamow and his ilk, and then the structure of it, were totally overlooked. It seems seriously unfair. I still feel that Jim Peebles should get a very belated Nobel for his services to statistical cosmology.)
i'm not knowledgeable about philosophy, unfortunately :( but i wouldn't say it implies eternal recurrence as i understand it -- such that everything that happens repeats. the problem is that the things that are repeating are cosmological, so it's only things on the very largest scales. that means that the bulk properties of hte universe would repeat in a cyclic model (although entropy is an issue in that), but it doesn't say that anything on smaller scales will. each time it's extremely likely that the actual distribution of matter will be different since gravitational collapse is an extremely complicated process (and can be chaotic), and that means that even the massive networks of galaxy clusters will look different on each cycle, let alone the suns, planets and sentient beings.
this is because the initial perturbations, in almost every model, are quantum in nature and seeded in the first microseconds (or before) -- being quantum in nature they're also random in nature. so each time through the universe as a whole will be identical, but you'll get different ripples on top of it and a different "micro-structure", if i can use "micro-structure" to describe objects as large as superclusters of galaxies.
not sure if that answers your question though, unfortunately.
"classic", yeah, but not sure about "best". but i last read it 15 years back before i started doing any of this stuff properly so don't quote me on that :)
No offence meant, btw. For some reason I just felt like having a go at people linking religion and cosmology and kind of got off-topic from your post :)
Also, you forgot fireworks.
I've never read her papers, actually. I'll give them a look, thanks.
Hahaha. I'd say the same about the other guy, too.
Thank you. Your permission means a lot to me. From my side, please continue to walk around talking shit about ancient religions having anything pertinent to say on physical cosmology. When you can get out a prediction for the CMB sky please get back to me.
Caveat: don't misunderstand me, I couldn't care the slightest about ancient cosmological models one way or the other; they're absolutely fine by me. People can have any religion they want and that's also fine by me. I know some superb cosmologists, much better than I am, who are devoutly religious. But pretending that you can get a feasible cosmological model out of a religion is sheer delusion. A cosmological model is about predictive power -- basically, it involves numbers. No religion and particularly not ancient religions, are built on that premise. They're not about physics. Pretty obviously, they're about religion. And that's a good thing and quite how it should be. Physics killed my own belief in religion but that's my problem. Basically, physics is about how the world behaves and *nothing more*. It's algorithms. Set up a scenario, run your algorithm, and get out a prediction. That's not at all what religions are set up to do. I can sit there and dig in religious texts and support an argument if I like, but attempting to pin any scientific meaning to it is both missing the point and is, in all reality, grossly offensive to the believers of that religion while at the same time saying nothing of value to science.
You might not like that answer, but science is just about numbers. Religion has nothing to say about that. I'm a cosmologist, meaning ultimately I care about the CMB and the distribution of galaxy clusters. Until your vaunted Buddhist cosmology can give me a concrete prediction about the CMB and galaxy clusters I'm going to (rightly) dismiss it, because it has zero predictive power and zero use as a physical model.
In return, I am *not* pretending to say anything about the nature of humanity. Why would I? I deal with numbers, physical laws, and how the universe seems to behave. I draw conclusions from that, postulate a model, and test it against other bits of the universe. Metaphysics, by its very nature, is a bit outside of my domain of expertise. Likewise, physical cosmology is totally outside the domain of expertise of metaphysicists, philosophers, theologists, and random internet nerds with a hard-on for anything from the ancient East.
Hmm, I'll have a think. Nothing jumps immediately to mind (except Sean Carroll's online cosmology primer (http://preposterousuniverse.com/writings/cosmologyprimer/) but that's not quite what you're asking for. I think Rachel Bean wrote a book quite recently which was probably pretty good. Unless she was merely talking about it because I can't find it anywhere, but I know she's keen on what we call "outreach". I'll have a think.
Hmmm, big question. I'll try and give the quick answer
The first ripples are seen on the cosmic microwave background radiation. This is a bath of microwave radiation that surrounds us, at almost exactly 2.71K and the most perfect blackbody ever observed. It is virtually impossible to explain the existence of this without having something very similar to modern cosmology. People tried when they were trying to keep steady-state cosmologies in the 60s but ultimately they failed; it's seriously difficult to explain something with a blackbody spectrum and isotropic to one part in a thousand (to one part in ten thousand if you subtract off a dipole which is almost certainly a Doppler shift caused by our motion with respect to the CMB) unless the universe started from a compact, very nearly uniform state.
Basically, if you link the isotropy of the CMB with the idea that the Earth isn't at the centre of the universe, you're lead almost inevitably to modern cosmology: the universe is isotropic around the Earth, but the Earth isn't at the centre, so the universe must be isotropic around *every place in the universe*. That means it's both homogeneous and isotropic.
The next assumption is that gravity is metric-based -- that is, that on scales larger than a few micometres, that the effects of gravity are due to distortions in space-time. This is an extremely safe assumption on solar system scales but it's only that - an assumption - on larger scales. Still, we've got no sensible alternatives so let's stick with it. (It's very hard to build a working model of gravity that isn't metric-based.)
The next assumption, and this is much weaker but is the best we can currently do, is that Einstein's general relativty applies on very large scales. GR is a particular form of a metric-based theory and is the simplest, most intuitively clean of them. So let's stick with it. But it's quite weak.
Doing that, we're lead to only one possible model for the universe -- the Friedman-LeMaitre-Robertson-Walker model. Basically that says that if you've got 3D spatial surfaces that have to be homogeneous and isotropic, you can chose to make them flat, saddle-shaped or spherical, and then pile them together to fill the whole of spacetime. It then tells you the behaviour of these surfaces given the matter you put into it.
An immediate consequence of saying "The universe contains photons and baryons" (which is obvious; as cosmologists use the word, *everything* is baryonic except for neutrinos and photons, and no-one would deny that we exist, or that photons exist, or that neutrinos exist, so you put them all in there) is the CMB. It exists, and we can calculate when it formed. The CMB is formed when the temperature of the universe becomes low enough that photons don't continuously reionise hydrogen. Basically a small universe is a hotter universe, so at some point in the distant past (which turns out to be before the universe was about 370,000 years old) the universe was hot enough that if an electron combined with a proton to form hydrogen, a photon immediately came along and smacked the electron back out again. This tied protons, electrons and photons together. The universe was opaque and it was all a massive chaotic game of pool. Without any pockets.
When the universe became cold enough that that no longer happened, the electrons all condensed into the protons, the universe suddenly went neutral, and the photons could stream free. Those photons are the CMB. Originally they were very hot but as the universe has expanded they've been redshifted until they reached teh current temperature barely above absolute zero.
Now, when the photons and protons were bound together it wasn't all *entirely* smooth. There are waves go through any plasma. (Without them the Sun would be a very boring place.) These waves are those ripples in the CMB I mentioned. At the formation of the CMB the photons suddenly broke free and the waves stopped, err, waving. This left an imprint of the ripples in the *baryons* on the photons. Ba
"into" is the problem there but if you change that to "in" instead you've got a good picture of chaotic inflation (and, indeed, pretty much all current models of early cosmology). the "multiverse" is a conglomeration of regions of spacetime with different conditions and we live in one where an inflaton appeared with such and such a potential and drove an expansion that then lead to the current universe. "outside" our bubble are bubbles with different conditions, some that only survived for milliseconds before collapsing and others undergoing big rips and so forth.
Yeah, these "Eastern systems" have been *superb* at predicting the anisotropies on the CMB. Why, I even once saw an ancient Hindu text that gave a beautiful B-mode polarisation map with the foregrounds cleaned out, proving both the existence of primordial magnetic fields from a coupling of the inflaton with the electromagnetic field, *and* confirming the nature of the inflaton itself! Some of the concrete, testable predictions of these religions are well beyond the best our supercomputers can come out with! I'm currently working on a proposal for the Euclid satellite and I'm basing a lot of my statistical predictions on old Buddhist texts. Those ancient dudes sure knew how to model the baryon acoustic peaks in different cosmologies and how to observe them without having to build in assumptions from a particular cosmology!
To be fair, there are a lot of fudges in ekpyrosis. They just might be more acceptable than are in inflationary cosmology, depending on your point of view. (My view is that both are just phenomenology.) In inflation you have to postulate the existence of one or more inflationary fields (typically scalar fields, as yet unobserved in nature which is a problem since the Higg's is a scalar field) and a specific form of potential. In ekpyrosis you have to postulate two *perfectly parallel* 5D branes, and a very specific form of potential governing their interaction. That potential was pulled out of a hat to give the dynamics they want, just as inflationary potentials are pulled out of a hat; and that setup is pretty contrived and unlikely, just as the existence of, say, one single scalar field in addition to the Higg's is pretty contrived and unlikely.
Horses for courses, really. If we see a nice signature of gravitational waves on the CMB with Planck then ekpyrosis is dead until Neil can find a way of arguing that actually there *should* be a signature. If we don't, then neither inflation nor ekpyrosis is dead because we can tune inflationary models to make the signal vanishingly small anyway.
Gravity getting "weaker" is probably referring to a scale-dependent Newton constant (or however else they've phrased the strength of gravity). Something like Horava-Lifshitz gravity is perhaps worth a look if you're interested in things like that and can find a popular-level thing on it (I think I read a survey of Horava-Lifshitz gravity on the New Scientist website a month or two back which was OK if a bit... undercited, shall we say). That's basically a theory of gravity that modifies the Newton constant on very small scales to make it easier to link with the other three forces. Failing that you could hunt out things on "Brans-Dicke" gravity, or "Scalar/Tensor" gravity, which are effectively theories with a *space*-dependent (rather than scale-dependent) Newton constant.
Easy reading on FLRW cosmology? Hmmm. For whatever flaws he might have, Sean Carroll's a very good communicator. http://preposterousuniverse.com/writings/cosmologyprimer/index.html is worth a look. (He might not use the words "FLRW" anywhere in it, which if so proves he's smarter than me when he's talking with non-specialists...)
I'd try and defend my profession but I won't because you're quite right. We can happily build models for pre-big bang theories but until we've got a good reason to believe in a way to go with high-energy physics, it's all just phenomenology -- a mathematical way of waving your hands, basically. No-one's actually denying this; if you read the papers on this kind of model they'll tend to wave their hands madly and talk about modifications arising from M theory and low-energy effective field theories. All that is just gloss, motivations for your own model which you'll never seriously pretend is fundamental.
What I would say though is that putting the bounds on your effective theory at least gives you a handle on your inaccuracies. Not many religions do that...
"the theorie of Black Holes gave the idea to the BigBang theory - they just applied time-reversal!."
No 'they' didn't. Black holes are based on inhomogeneous solutions to Einstein's equations -- the first being the Schwarzschild solution describing a spherical, uncharged body embedded in flat spacetime, with Reisser-Nordstrom, Kerr and Kerr-Newman adding in electromagnetic fields, rotation and then both respectively.
Cosmology is based on Friedman-LeMaitre-Robertson-Walker solutions, which impose maximal symmetry on spatial surfaces of constant time. You might be interested to note that no black hole solution can be maximally-symmetric since only three surfaces are -- normal flat space, a (hyper)sphere and a (hyper)saddle.
There really isn't much connection. "Reversing" time on a black hole solution (which happen when you take, for example, a Schwarzschild solution and allow it to exist all the way to the centre of the system instead of cutting it off with a stellar surface partway down, which is what happens in the solar system) gives you a white hole.
Basically. You're talking about the critical density of the universe. This is about 1, meaning that the universe is "flat" -- so it's infinite in extent and basically composed of a load of flat sheets rather than saddle shapes or spheres. So far as we can tell it's exactly 1. (It's pretty easy to tell, actually. We can look at the ripples in the universe back from when it was 370,000 years old, and then look at *those same ripples* from when the universe was about 10 billion years old. Those ripples have a particular wavelength, so from that we can tell how much the universe had to expand. It pins things down really quite nicely.)
Our problem comes from counting how much actual normal ("baryonic" though it includes more than just baryons) matter there is, by looking at everything that glows (and also by looking at the amount of hydrogen and helium, which was produced in the first few seconds of the universe's life; the ratio between the two is extremely sensitive to how much baryonic matter there was). It gives us a density parameter of about 0.05. Shit. So we then look at how much clumping matter we need, which would include "dark matter", whatever form that takes. We find that we need about a density parameter of 0.3 -- so 25% of the universe is dark matter.
Shit. We *still* only have 30% of the universe even accounting for dark matter.
So we're forced to add about 70% of the universe in something else. That can't clump and for other reasons it has to act as an "anti-gravity". That's called "dark energy".
Rather surprisingly, this model fits all the available data...
That's not really so much "consensus" as "the result of the model we're currently using". No-one *likes* the standard model of cosmology; it's obviously just phenomenology, but it happens to fit all the data at least as well as any alternative. The standard model of cosmology is "lambda CDM", the lambda being a cosmological constant which drives an accelerating expansion in the current universe, and the CDM being cold dark matter which was responsible for the clustering of matter and the formation of galaxies and so forth.
The problem is that if it *is* just a cosmological constant then it will grow to dominate the universe and things will, indeed, expand forever.
But it's probably not a cosmological constant. The "best" prediction from quantum field theory -- and it's not really a prediction so much as the only way of estimating the size of the constant -- comes from evaluating the vacuum energy. Doing this suggests that it should be about 10^120 times bigger than we see in reality. That's a pretty big difference. Weinberg described it as the most embarrassing mismatch between theory and observation in the history of science, and he's got a point. The conclusion is that there's probably some mechanism (coming from trans-Planckian physics, maybe, or something else) that cancels the cosmological constant. If that's true, then it's almost certain to cancel it perfectly because the fine-tuning necessary to produce the *observed* constant is horrific, whereas a symmetry principle could wipe the whole thing much more easily.
That leaves you open to more general dark energy models to explain the accelerating expansion and that's where you have more fun. There are plenty of ways to get an observed acceleration. Some of them lead to big rips, which is where eventually the universal expansion will tear galaxies, then solar systems, then stars, planets and eventually even atoms and nuclei apart. Others lead to the decay of whatever field is responsible for acceleration -- like if you couple a scalar field into dark matter you can tune it such that the scalar field grows to dominate and then starts transferring its energy into dark matter, which would cause the universe to reclump again (and then probably the dark matter would dump energy back into the scalar field causing more acceleration). Those models are horribly contrived and unrealistic, but at least they're alternatives.
And the cosmological constant is pretty contrived and unrealistic in the first place...
Anyway. Before I got side-tracked my point was that there isn't really a consensus so much as a model that fits observations and predicts eternal expansion, but that it's not the only model and it's not even the best motivated model, merely the simplest. Other models can still lead to crunches while fitting pretty much all the data as well. And others can lead to cyclic universes, which is ultimately what Penrose is talking about in one form or another.
Disclaimer: I am a cosmologist but I've not actually read the article. This is Slashdot, after all.