Sure but you know what I'm meaning. In physics we have a very distinct hierarchy of descriptions. At the base are the "fundamental" theories, which are assumed to hold for point particles and fundamental forces (ie forces that don't reduce to different ways of viewing another force). These are the likes of the electroweak theory, the strong theory, and any random speculation about quantum gravity that you choose to believe (or not - I don't believe any of the theories of quantum gravity at the minute, though the approaches of loop quantum gravity, dynamical triangulations, or non-commutative geometries are to my mind a lot more promising than the approach of superstring theories; less ambitious and following more closely the ethos that lead to QED in the first place). Above that is the emergent theory of quantum mechanics, which in principle should arise out of QED but which in practice is a staggeringly successful and ultimately phenomenological theory. The two are related, however, and both involve similar quantisations of classical concepts - a Lagrangian density in the case of the "fundamental" theories which are therefore quantum field theories describing the fundamental forces; and a Hamiltonian or Hamiltonian density in the case of quantum mechanics which is therefore a direct quantum theory of particles.
Then above that, quite a long way, sit the likes of thermodynamics and fluid mechanics. Thermodynamics arises by describing a system of interacting particles -- atoms, molecules, what have you -- through a Hamiltonian and then applying a statistical average of this. What emerges is a simple description of the behaviour of vast numbers of particles which is coincidentally the phenomenological theory developed in the 19th century, except that entropy is now a determined quantity rather than defined only to within an additive constant. Fluid mechanics can be recovered by setting up a Boltzmann equation, which is itself a statistical quantity, and then integrating out the lower moments, which become density, momentum, momentum flux etc. Similarly chemistry in principle can be recovered by modelling atoms with Schroedinger equations, and that's certainly done but it's computationally expensive.
Then above *that* (which holds on our scales) lies cosmology. Cosmology is the extrapolation to gigaparsec scales, of all things, of a force known to operate on scales between a millimetre and, with mounting inaccuracy in the measurements, solar system scales. It seems likely to hold on at least parsec scales but the extrapolation to kiloparsec scales is questionable, and the application in galaxies is actually ill-defined (meaning that it is impossible with current knowledge to calculate the mean-field of a galaxy due to the numerous gravitational lenses a galaxy is filled with, and the fact that such lenses break any spatial average we've currently been able to define in metric-based theories; we're also still working on straight statistical averages for gravitating systems). The application up to first mega- and then gigaparsec scales is subject to similar caveats.
So yes, in principle I totally agree wtih you that ultimately all numerical science is phenomenology, but the way the word is typically used in physics is slightly different -- a phenomenology is a theory that is not assumed to be related in any direct way to the theories that are assumed to hold on laboratory or, particularly, atomic scales and below.
I absolutely agree with your first paragraph. (Actually with pretty much your entire post.)
For the record, my feeling on dark energy is that it's a mirage caused by analysing recent (highly inhomogeneous) cosmology through a model that at its base assumes homogeneity and isotropy, which a hunch would suggest is liable to cause odd effects, and which closer study through inhomogeneous models imply that at the least the effects of dark energy can be generated in pure dust (ie baryon + CDM) universes. My own research focusses on another, related issue -- cosmology is a theory built on averages, but that average *is never defined anywhere*. An average over a tensor field is ill-defined, and any average that we've been able to write that more or less makes sense relies on a globally-hyperbolic spacetime, meaning no geodesic crossings. Alas, the universe is full of gravitational lenses which are nothing but massive regions where null geodesics cross, so we have an inbuilt issue with the idea of mapping up from small-scale (well, galaxy scale) objects to an entire cosmology -- it's currently quite impossible.
Dark matter is a slightly different topic. You can argue that the effects of dark matter might also arise from averaging effects (ie constructing a model which is after all nothing more than a homogeneous and isotropic solution fitted to the inhomogeneous reality), and some have attempted to do so, with a bit more success than with dark energy. (Practically every attempt to find a dark component from averaging has found dust, except the most refined and plausible which have found curvature.) I suspect that's a part. You can also argue that GR simply does not operate on such large scales, and that even GR should be modified on large scales before we apply an average, and I suspect that's a part, too. Then we have particles that can act as a dark matter -- neutrinos are massive, and as you point out definitely contribute, but they certainly can't be the whole answer because they imply a washed-out structure because they're relativistic for too much of the universe's evolution, meaning that they tend to propagate out of gravity wells rather than falling into them. There are other, speculative, particles that could contribute; if SUSY has any basis in fact (and I hope not but even now it may still do) then there's a lightest supersymmetric particle, which would be stable and massive and act as a dark matter. We *also* have that gravity may not be described by GR, and may not map up to cosmological scales with a simple average, but also that since it's geometric, attempting to describe smaller scales on which evidence much of the basis for dark matter rests is a bit suspect: we have to check what the dynamics of stars are in the geometry produced by a galaxy. That's basically impossible (and is linked to the averaging issue, since we'd be wanting a mean-field, along with the backreaction of the star itself on the mean-field) but it's also got to be done before we believe particle physicists when they blithely declare that they've found "the" dark matter.
My suspicion is that dark energy doesn't exist and is a mirage, while dark matter is a combination of all the above: modifications to GR, effects of averaging GR, the related effects of stellar motion in realistic geometries, massive neutrinos, perhaps even sterile neutrinos, some currently unknown particle or particles that arise in high energy particle physics... and given the number of parameters that are introduced from all of that, also probably impossible to constrain. Hey ho.
Time, yes. Not sure what you're referring to with thermodynamics - it's just a statistical theory that emerges when you deal with vast numbers of particles. (And if I did want to treat thermodynamics as inviolate, which it basically is for large enough systems of particles, there is no issue with conservation of energy with the loss of energy due to cosmological expansion. I'm not totally sure why you'd think there is: energy conservation is inherent in the system. There's nothing controversial about the idea that if you work in an expanding spacetime then photons that are not being pumped by an external source of energy will be stretched. Similarly if you work in a collapsing spacetime then photons not being drained with be blue-shifted. The Friedmann equation can ultimately be interpreted as an energy conservation equation if one is so minded, it just comes from the Hamiltonian constraint.)
"If there is more dark matter in the universe than ordinary matter (by a factor of 4:1 they say), wouldn't you expect it to somehow figure in the "calculations" going back to the big bang?"
Yes. And yes, it does, it "figures" right from the start.
"I saw no mention of it in the article."
Who died and made this article God?
"In fact, come to think of it, you seldom hear much about that big elephant dark matter in the room in the first minutes after the bing bag."
That's chiefly becase in the first few minutes after the big bang the universe was radiation dominated, meaning that the density of photons (and neutrinos) was vastly greater than that of dark and normal matter. The transition between radiation and matter domination is governed by the density of dark matter just as much as baryons. Where on Earth are you getting this idea that dark matter is an "elephant in the room"? Here's an interesting fact for you - you know there are waves imprinted both on the CMB and on the large-scale structure of galaxies, right? If you "love reading about cosmology" you must, right? Those waves are the result of oscillations while the universe was radiation-dominated, caused by baryons tending to cluster together under gravity, and a restoring force introduced by radiation pressure, which set up ringing oscillations across the universe. Without dark matter to provide extra clustering under gravity those waves are at totally the wrong wavelength. From the CMB *alone* you can find how much dark matter there has to be relative to normal matter. How's that for an "elephant in the room"?
"I think readers should be warned this is a very speculative field of study."
As is all theory. However, I think readers should be warned that the fundamentals of cosmology are very far from speculative - even if the results might in principle be phenomenological, they will not change. Cosmology, particularly in the early universe but after the first microsecond, say, is based on well-understood science and is anything *but* speculative, and questions about whether dark matter or dark energy are physical quantities or are emergent in one way or another are not unique to cosmology but also arise on astrophysical scales. (And in some ways are irrelevant, since whatever dark matter and dark energy actually are, they have to work as cosmology describes them anyway. Small changes to the Lambda CDM model cause large disagreements with the data.)
"I'm reminded of my physics professor of many years ago who claimed "Cosmology is as mature as botany was before Darwin." "
Err, yeah. How many years ago? If he held the same opinion now I'd be surprised. If he held that opinion after the late 90s then he was ignorant of the field. That's OK, my Masters supervisor, in the early 2000s, is a brilliant physicist and held a similar opinion (although not stated quite so... badly, with a lousy analogy that could never work), and he was wrong too, increasingly so as the datasets grow ever huger and the tools with which to analyse them evermore sophisticated. This kind of view is untenable, and I say that as a man who has gone on record repeatedly with statements such as "cosmology is wrong. It is demonstrably wrong, it is wrong in its fundamentals and it is wrong in its principles" - because that's a statement I also surround with caveats. Cosmology is "wrong" in the same way that thermodynamics is "wrong", or that much of chemistry is "wrong", or much of biology is "wrong", in that it's at heart a descriptive, phenomenological theory. (Before chemists or biologists come to me to scream, those fields are typically phenomenological, although both contain subfields that are avowedly not so; but ultimately if you're not mapping up from the behaviour of the individual atoms you're dealing with phenomenology, and I'm well aware of how brutally difficult it is to do chemistry directly from the Schroedinger equation, which is what that implies. 'Phenomenology' is not a criticism, unless it's taken as so by people who mistakenly think they're dealing with the underlying science directly.)
Sorry, it was a throwaway remark - I was referring to Poul Anderson's "The Stars are Also Fire" which won the Prometheus Award back in the 90s. I spent seven years as a professional cosmologists so I'd be extremely worried if I didn't know that stars run on fusion:)
That isn't, strictly speaking, true as much as it's one postulated result. If the expansion of the universe continues to (appear to) accelerate then eventually, yes, everything that isn't virially bound to us is going to be swept over the horizon, which would leave us with a shattered fragment within view, a pathetic remnant of everything that's actually out there which will still be monumentally vast and monumentally impressive; it will effectively be the Virgo supercluster. In extreme cases where the acceleration is caused by a phantom field then *maybe* -- and that's a very big maybe, because the physics used to argue this is grossly misapplied -- that supercluster itself might start getting swept across the horizon and we'd end up with the local cluster or even the local group. And in the hard extreme we end up with everything ripped to shreds.
The reason I don't think that that latter will happen, even if it does turn out that dark energy is a physical phenomenon (as opposed to an apparent phenomenon; the distinction does not necessarily affect the fact that distant galaxies will retreat over a horizon), is that the expansion of spacetime is a facet of *Robertson-Walker* geometry, which does not apply to bound structures. You get different behaviour in other geometries, and if RW has any physical validity at all (which, strictly speaking, it doesn't save as a loose approximation whose errors we have literally no handle on -- we just slap it in and tune its parameters) then it is only on scales at which that validity holds that "dark energy" acts the way we tend to assume. Not to say that a strongly phantom field wouldn't also cause havoc in Schwarzschild geometries, of course, but we certainly don't have a strongly phantom field because we'd see it in the orbits of planets in the solar system.
Anyway, a long and slightly off-topic digression there, but just to say I don't think the far future would be anything like as gloomy. The most likely end point of the current acceleration (whether it's physical or apparent) will be the local supercluster sitting glimmering in the darkness. And the local supercluster is *vast*. It still means that we're around in the period at which we can see there's more to the universe than just the local supercluster. A civilisation in the far future -- assuming that the reasoning that lead *here* even holds -- would have no way to know this bar theory, and since their universe would not betray large-scale homogeneity it's possible that they wouldn't end up with a cosmology anything like ours at all.
To be fair, the statement "Dark matter is just some stable electrically neutral particle" is pretty strong. "Dark matter could just be some stable electrically neutral particle" would be better, particularly if you went on to some well-motivated examples such as the neutralino or axino, but there are always alternatives, particularly in this kind of case with plenty of alternatives and plenty of doubt that we even understand how gravity works on galactic scales, in clusters, superclusters, cosmologically, etc. A particulate contribution to dark matter certainly seems plausible and perhaps even likely, but it is far from certain and even less far from certain that it is even the dominant, let alone sole, contribution.
"* Lambda CDM is wrong. It is dead wrong. It is wrong in principle. It is questionable from a particle physics perspective, particularly where it comes to dark energy, but far more importantly, it cannot be justified with general relativity."
That does not mean that it is inaccurate -- there are now at least two questions from here. How do we get a theory of cosmology that can be properly derived from physical underpinnings, and to what extent is Lambda CDM (or any other Robertson-Walker based model) inaccurate? No-one can really answer either of those questions, although there's certainly a lot of interest in both.
"Is there anyone who thinks that \LCDM isn't built on phenomenology?"
Sure. Everyone who without question says "the universe is homogenous and isotropic on average", derives the (Friedman-Lemaitre)-Robertson-Walker metric on those assumptions, perturbs it, and then slaps it straight into the Einstein equations without any caveats. You'd perhaps be surprised at how many people in current cosmology haven't really realised that this is invalid. And then those that *have* realised it's invalid, due not least to people like myself (and to people more well-known and influential in the field than I -- I'm hardly trying to attribute any undue credit on myself here; journeyman all the way) giving seminars and trying to drum up awareness of this, don't really seem to put much further thought into it. Which is quite frustrating. Everyone is interested, and no-one wants to do anything with it, or thinks "it has to be insignificant". Which can certainly be argued, even perhaps persuasively, but *cannot be demonstrated* and always relies ultimately on Newtonian reasoning.
I'm not saying people all think that Lambda CDM is reality. Everyone in the field is looking at alternatives, and the vast bulk accept that dark energy may well not be a cosmological constant, although you'd also be surprised at how few will actually question that dark matter may not (or may not entirely) be particulate and may instead be due to modifications of gravity or even due to a ham-fisted application of gravity. There are assumptions that are deep-rooted, and there are even quite a few cosmologists who seem to find them unremarkable and the attempt to chisel at them somehow unconstructive. Which does irritate me a bit, but I don't mean to put things too strongly as a result -- I'd say the majority of researchers could ultimately be persuaded of total alternatives and theorists are *almost* all aware that ultimately we deal in theories and marrying yourself to a theory is silly.
My experience is the more senior the researcher, the less likely they are to throw their ego into a construction, which probably makes sense.
"if I'm allowed to make stuff up whenever I want to make my theory fit the model, I can do at least as well as the Lambda CDM"
Go ahead - you're more than welcome to. Empty assertions don't show much but new cosmological models are welcomed. *I* welcome them, anyway; I've never liked Lambda CDM much and it's obviously a phenomenological model. But they have to be predictive, and founded on firm principles.
I didn't actually want to suggest you're an idiot because I think it's apparent you're not, but this type of post at the same time implies that *cosmologists* are idiots and brainwashed into a model that doesn't really make much sense. And in some cases that's actually true -- there are more and more cosmologists trained into cosmology rather than general relativity and it's a bit dangerous -- but on the whole I don't think many people *like* LCDM. There are too many unanswered questions in it, and everyone is looking to answer those. Just some people work more tightly within its framework than others.
"is there a point where you would ever consider reexamining the questions of the assumptions? Why haven't we reached that point yet?"
Oh, don't misunderstand me -- I *constantly* question and re-examine the assumptions. At some point, if you're genuinely interested, flip back through my posts on Slashdot; I've made my position I think fairly clearly. Boiling it down and putting it in bullet form it goes something like this:
* The "big bang theory", and Lambda CDM in particular, is an astonishingly successful theory, particularly when attached to an inflationary period in the early universe or something that mimics its observational results closely * The successes of Lambda CDM -- such as the predicted abundances from Big Bang nucleosynthesis, the *prediction* of the angular power spectra of the CMB (temperature auto-correlation, temperature/E mode cross correlation, E mode polarisation auto-correlation and now the B mode polarisation auto-correlation) from a simple early primordial power spectrum, the direct mapping between the wavelength of the sound horizon at last scattering as seen on the CMB and that same wavelength imprinted on large-scale structure and *observed* as the baryon acoustic oscillations, and their ilk -- are far too numerous and significant to be ignored. * Any alternative absolutely has to preserve these, and they're all extremely sensitive * Lambda CDM is wrong. It is dead wrong. It is wrong in principle. It is questionable from a particle physics perspective, particularly where it comes to dark energy, but far more importantly, it cannot be justified with general relativity.
Lambda CDM rests on a few main assumptions:
* The universe is on average isotropic around the Earth. OK, fine, we can't argue that; the CMB is proof enough.
* Since the Earth is nowhere special, the universe is on average isotropic around every point: homogeneous. Well, this is debatable since the Earth *is* in a particular position, but on the whole this is probably at least approximately true.
* Gravity is best described on large scales (ie > mm) by a metric theory. This is currently practically unquestionable; metric-based theories of gravity are vastly more succesful than any alternative.
* Gravity is described by general relativity. OK, now we're entering questionable territory but GR remains our best example of a metric-based theory and is yet to be seriously challenged (though there are many, myself among them, who point out that the appearance of dark matter on galactic scales, and the addition of dark energy on cosmological scales, may very well imply that actually we cannot apply gravity on such scales or else that it simply doesn't act this way on large scales)
* GR can be applied directly on large scales. This is extraordinarily shaky. Actually, it's unjustifiable. We've got two main objections here: firstly, there is no reason to assume that gravity actually obeys GR on large s
One of the greatest living cosmologists is George Ellis, now an emeritus professor at the University of Cape Town. Ellis is one of very few people to pioneer an approach at perturbations in cosmology -- effectively describing how structure can form out of a smooth background -- laying foundations in the 60s and 70s with the likes of Hawking and then applying it fully to cosmology in the late 80s with a new generation of students such as Bruni, Hwang and Dunsby. He's amongst the most respected gravitational physicists of the 20th and 21st centuries, has been scientific advisor to the South African government, and was active in the 70s and 80s against apartheid. He's also won the Templeton Prize (for Progress Toward Research or Discoveries about Spiritual Realities) and so far as I know is firmly Quaker. Religion does not have to stand in opposition to science, even within the same person. Personally I can't do as Prof. Ellis has and I lost what faith I had quite early in studying theoretical physics and then cosmology, but I've no lack of respect for people who can.
"A viable alternate theory is that light gives up some energy while traveling extremely long distances, which shows up as red-shift. Where does the energy go? It could be the source of energy for the CMBR. It could go somewhere else. In any case, as a theory, it explains the red-shift just as well as expansion."
Excellent! Now repeat the rest of the predictions of the Lambda CDM model. Ah, no, you'll have trouble with that one.
"Another viable alternate theory is that the absorption/emission spectra of atoms differs with space/time. Perhaps atoms farther away or longer ago created and absorbed light at lower frequencies, this making older light appear red-shifted by current frequency comparisons. This theory is even harder to test, but just as good at explaining the observations. "
Even better! Now repeat the rest of the predictions of the Lambda CDM model. I think you'll have problems with that one, too.
Actually, I'll give you a bye -- all I want to see is the position of the first peak on the CMB *and* the wavelength of the oscillations in the large-scale structure, with one predicted consistently from the other. Once you've done that, if you can further get out supernovae 1a redshift/distance plots I'll give you extra credit, but since the progentiors aren't fully understood I'll give you a bye on that one, too.
See, the word "viable" has certain caveats. It has to satisfy the observations it's been built to explain *at least* as well as the theory it's replacing. Second, it has to -- self-consistently -- predict further observations that fit *at least* as well as the theory it's replacing. I'm no fan of Lambda CDM but its successes should convince anyone who's actually looked seriously at them that there's something close to reality there, even if ultimately it's a phenomenology close to reality (which it is; I can prove it's phenomenology -- rigorously -- but I can't demonstrate how wrong it actually is, and neither can anyone else at present, but I can at least assert that up until very recent times it's so close as to be indistinguishable and no that I fit all observations, and even very recently it's exceedingly good).
You're a decade early. What you say is *true* but the relevant time would be the late 1910s when Einstein was applying general relativity (1915) to cosmology and introduced the cosmological constant to engineer a static universe.
Who do you think predicted the gravitational radiation in the first place? There's a reason we've been wanting to find them since the 1990s and it's not that observational cosmologists were arguing with theoreticians that they were definitely there.
You evidently have a different definition of "elegant" to me. My definition of "elegant" does not include "theories containing hand-crafted and unjustified potentials of a strikingly bizarre form inserted purely phenomenologically into a theory that is a phenomenological attempt to see what might happen if some facets of M theory are put onto large scales". That's not to defend inflation too much but the potentials of inflatons are typically quadratic or quartic, which is a lot neater than the potential in ekpyrosis, and the setup with two infinite, flat branes is at least as contrived as the inflaton.
All of that said, let's wait until the dust settles before we rule models out based on gravitational radiation alone. Their position is certainly looking somewhat precarious, but there may be ways out yet.
Radio waves still have polarisation, just the same as optical and gamma radiation does. You need some pretty refined equipment to study it in detail but you can build it. Check out bolometers and radiometers.
"First of all, how is it that all stars moving apart from each other rapidly is not "first direct evidence" of the universe's expansion?"
Because it isn't. The first direct evidence of the universe's expansion is typically accredited to Hubble in the 1920s and was very firmly established a good couple of generations back. Don't believe all you read in/. summaries...
"And secondly, how could the expansion of the universe amplify gravitational waves? Space stretching would thin out the waves because they would be expressed over a wider area."
Yes, it does indeed do so.
What this summary missed is that the universe was both extraordinarily small *and* undergoing inflation at the time. That's significant, because inflation was driven (in the theory) by the inflaton, a quantum field, and the size of the universe implied that the matter content was governed by (semi-classical) quantum theory -- ie quantum field theory on a curved but classical spacetime. Even earlier, it would be described by quantum gravity and we don't know what would happen.
Anyway, if you start looking at quantum fluctuations of a field such as the inflaton acting in something close to "slow-roll" (necessary for simple models to actually get out an inflation -- you need a field close to frozen in its potential so that it mimics a cosmological constant) then you get out an impressive array of density and metric perturbations. The density (and "scalar" metric) perturbations are what lead to the entire observable structure in the universe. Far smaller -- but, according to the results today, not *that* much smaller with an amplitude as large as 20% of the scalar -- are gravitational waves coming out. Entertainingly, even though you get out density and gravitational waves, you get basically negligible vorticity.
The results today are the first direct signal of inflation -- competing theories can produce the density perturbations and, just as importantly, their power spectrum, but frequently predict unobservably small gravitational radiation.
I would advise caution on these results since they rely on a remarkable "tilt" to the scalar power spectrum, which Bicep2 introduced to resolve a strong tension with Planck but which may (or may not) itself be in tension with Planck. That's going to be the first thing attacked -- in an investigative sense of the word, not an aggressive one -- by the community.
Mod points anyone still reading this thread please - this is very interesting stuff.
I must admit to my shame that using supernovae type II for distance measurements hasn't really been on my radar at all, although I must have been in quite a few talks discussing it. Anything that can add redundant checks to the distance ladder absolutely has to be pursued.
Perlmutter et al.: http://uk.arxiv.org/abs/astro-... "...All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation..."
The reason is that SN1a can be standardised -- although that's an empirical (i.e. phenomenological) relationship rather than a theoretical one, it seems to be basically robust, as this paper has demonstrated -- and therefore used as standard candles. Other types of supernovae can not be used in the same way; one cannot necessarily correlate a (corrected) brightness against a (corrected) redshift.
This doesn't say that samples aren't contaminated by supernovae that aren't actually Type 1a (and a few years back an explanation for tension between the so-called "Gold sample" and other datasets was that it may have been more contaminated), but the intention is to only look at Type 1as.
I'd also argue that they weren't particularly high redshift, but then for me a redshift of 3 or 4 is very much low redshift. Come to that, redshifts of 300 are low redshift.
Hell, I still sometimes play Elite 2 and Grand Prix 4 more than anything other than Perfect Dark (N64 or XBox 360 remake, I'm not fussy) and Time Splitters: Future Perfect. More recent gaming has kind of passed me by.
Slashdot Mirror
Sure but you know what I'm meaning. In physics we have a very distinct hierarchy of descriptions. At the base are the "fundamental" theories, which are assumed to hold for point particles and fundamental forces (ie forces that don't reduce to different ways of viewing another force). These are the likes of the electroweak theory, the strong theory, and any random speculation about quantum gravity that you choose to believe (or not - I don't believe any of the theories of quantum gravity at the minute, though the approaches of loop quantum gravity, dynamical triangulations, or non-commutative geometries are to my mind a lot more promising than the approach of superstring theories; less ambitious and following more closely the ethos that lead to QED in the first place). Above that is the emergent theory of quantum mechanics, which in principle should arise out of QED but which in practice is a staggeringly successful and ultimately phenomenological theory. The two are related, however, and both involve similar quantisations of classical concepts - a Lagrangian density in the case of the "fundamental" theories which are therefore quantum field theories describing the fundamental forces; and a Hamiltonian or Hamiltonian density in the case of quantum mechanics which is therefore a direct quantum theory of particles.
Then above that, quite a long way, sit the likes of thermodynamics and fluid mechanics. Thermodynamics arises by describing a system of interacting particles -- atoms, molecules, what have you -- through a Hamiltonian and then applying a statistical average of this. What emerges is a simple description of the behaviour of vast numbers of particles which is coincidentally the phenomenological theory developed in the 19th century, except that entropy is now a determined quantity rather than defined only to within an additive constant. Fluid mechanics can be recovered by setting up a Boltzmann equation, which is itself a statistical quantity, and then integrating out the lower moments, which become density, momentum, momentum flux etc. Similarly chemistry in principle can be recovered by modelling atoms with Schroedinger equations, and that's certainly done but it's computationally expensive.
Then above *that* (which holds on our scales) lies cosmology. Cosmology is the extrapolation to gigaparsec scales, of all things, of a force known to operate on scales between a millimetre and, with mounting inaccuracy in the measurements, solar system scales. It seems likely to hold on at least parsec scales but the extrapolation to kiloparsec scales is questionable, and the application in galaxies is actually ill-defined (meaning that it is impossible with current knowledge to calculate the mean-field of a galaxy due to the numerous gravitational lenses a galaxy is filled with, and the fact that such lenses break any spatial average we've currently been able to define in metric-based theories; we're also still working on straight statistical averages for gravitating systems). The application up to first mega- and then gigaparsec scales is subject to similar caveats.
So yes, in principle I totally agree wtih you that ultimately all numerical science is phenomenology, but the way the word is typically used in physics is slightly different -- a phenomenology is a theory that is not assumed to be related in any direct way to the theories that are assumed to hold on laboratory or, particularly, atomic scales and below.
I absolutely agree with your first paragraph. (Actually with pretty much your entire post.)
For the record, my feeling on dark energy is that it's a mirage caused by analysing recent (highly inhomogeneous) cosmology through a model that at its base assumes homogeneity and isotropy, which a hunch would suggest is liable to cause odd effects, and which closer study through inhomogeneous models imply that at the least the effects of dark energy can be generated in pure dust (ie baryon + CDM) universes. My own research focusses on another, related issue -- cosmology is a theory built on averages, but that average *is never defined anywhere*. An average over a tensor field is ill-defined, and any average that we've been able to write that more or less makes sense relies on a globally-hyperbolic spacetime, meaning no geodesic crossings. Alas, the universe is full of gravitational lenses which are nothing but massive regions where null geodesics cross, so we have an inbuilt issue with the idea of mapping up from small-scale (well, galaxy scale) objects to an entire cosmology -- it's currently quite impossible.
Dark matter is a slightly different topic. You can argue that the effects of dark matter might also arise from averaging effects (ie constructing a model which is after all nothing more than a homogeneous and isotropic solution fitted to the inhomogeneous reality), and some have attempted to do so, with a bit more success than with dark energy. (Practically every attempt to find a dark component from averaging has found dust, except the most refined and plausible which have found curvature.) I suspect that's a part. You can also argue that GR simply does not operate on such large scales, and that even GR should be modified on large scales before we apply an average, and I suspect that's a part, too. Then we have particles that can act as a dark matter -- neutrinos are massive, and as you point out definitely contribute, but they certainly can't be the whole answer because they imply a washed-out structure because they're relativistic for too much of the universe's evolution, meaning that they tend to propagate out of gravity wells rather than falling into them. There are other, speculative, particles that could contribute; if SUSY has any basis in fact (and I hope not but even now it may still do) then there's a lightest supersymmetric particle, which would be stable and massive and act as a dark matter. We *also* have that gravity may not be described by GR, and may not map up to cosmological scales with a simple average, but also that since it's geometric, attempting to describe smaller scales on which evidence much of the basis for dark matter rests is a bit suspect: we have to check what the dynamics of stars are in the geometry produced by a galaxy. That's basically impossible (and is linked to the averaging issue, since we'd be wanting a mean-field, along with the backreaction of the star itself on the mean-field) but it's also got to be done before we believe particle physicists when they blithely declare that they've found "the" dark matter.
My suspicion is that dark energy doesn't exist and is a mirage, while dark matter is a combination of all the above: modifications to GR, effects of averaging GR, the related effects of stellar motion in realistic geometries, massive neutrinos, perhaps even sterile neutrinos, some currently unknown particle or particles that arise in high energy particle physics... and given the number of parameters that are introduced from all of that, also probably impossible to constrain. Hey ho.
There doesn't seem to be any actual news here - just a link to someone's post about the hydrogen content of the universe.
Time, yes. Not sure what you're referring to with thermodynamics - it's just a statistical theory that emerges when you deal with vast numbers of particles. (And if I did want to treat thermodynamics as inviolate, which it basically is for large enough systems of particles, there is no issue with conservation of energy with the loss of energy due to cosmological expansion. I'm not totally sure why you'd think there is: energy conservation is inherent in the system. There's nothing controversial about the idea that if you work in an expanding spacetime then photons that are not being pumped by an external source of energy will be stretched. Similarly if you work in a collapsing spacetime then photons not being drained with be blue-shifted. The Friedmann equation can ultimately be interpreted as an energy conservation equation if one is so minded, it just comes from the Hamiltonian constraint.)
Eh?
"If there is more dark matter in the universe than ordinary matter (by a factor of 4:1 they say), wouldn't you expect it to somehow figure in the "calculations" going back to the big bang?"
Yes. And yes, it does, it "figures" right from the start.
"I saw no mention of it in the article."
Who died and made this article God?
"In fact, come to think of it, you seldom hear much about that big elephant dark matter in the room in the first minutes after the bing bag."
That's chiefly becase in the first few minutes after the big bang the universe was radiation dominated, meaning that the density of photons (and neutrinos) was vastly greater than that of dark and normal matter. The transition between radiation and matter domination is governed by the density of dark matter just as much as baryons. Where on Earth are you getting this idea that dark matter is an "elephant in the room"? Here's an interesting fact for you - you know there are waves imprinted both on the CMB and on the large-scale structure of galaxies, right? If you "love reading about cosmology" you must, right? Those waves are the result of oscillations while the universe was radiation-dominated, caused by baryons tending to cluster together under gravity, and a restoring force introduced by radiation pressure, which set up ringing oscillations across the universe. Without dark matter to provide extra clustering under gravity those waves are at totally the wrong wavelength. From the CMB *alone* you can find how much dark matter there has to be relative to normal matter. How's that for an "elephant in the room"?
"I think readers should be warned this is a very speculative field of study."
As is all theory. However, I think readers should be warned that the fundamentals of cosmology are very far from speculative - even if the results might in principle be phenomenological, they will not change. Cosmology, particularly in the early universe but after the first microsecond, say, is based on well-understood science and is anything *but* speculative, and questions about whether dark matter or dark energy are physical quantities or are emergent in one way or another are not unique to cosmology but also arise on astrophysical scales. (And in some ways are irrelevant, since whatever dark matter and dark energy actually are, they have to work as cosmology describes them anyway. Small changes to the Lambda CDM model cause large disagreements with the data.)
"I'm reminded of my physics professor of many years ago who claimed "Cosmology is as mature as botany was before Darwin." "
Err, yeah. How many years ago? If he held the same opinion now I'd be surprised. If he held that opinion after the late 90s then he was ignorant of the field. That's OK, my Masters supervisor, in the early 2000s, is a brilliant physicist and held a similar opinion (although not stated quite so... badly, with a lousy analogy that could never work), and he was wrong too, increasingly so as the datasets grow ever huger and the tools with which to analyse them evermore sophisticated. This kind of view is untenable, and I say that as a man who has gone on record repeatedly with statements such as "cosmology is wrong. It is demonstrably wrong, it is wrong in its fundamentals and it is wrong in its principles" - because that's a statement I also surround with caveats. Cosmology is "wrong" in the same way that thermodynamics is "wrong", or that much of chemistry is "wrong", or much of biology is "wrong", in that it's at heart a descriptive, phenomenological theory. (Before chemists or biologists come to me to scream, those fields are typically phenomenological, although both contain subfields that are avowedly not so; but ultimately if you're not mapping up from the behaviour of the individual atoms you're dealing with phenomenology, and I'm well aware of how brutally difficult it is to do chemistry directly from the Schroedinger equation, which is what that implies. 'Phenomenology' is not a criticism, unless it's taken as so by people who mistakenly think they're dealing with the underlying science directly.)
Sorry, it was a throwaway remark - I was referring to Poul Anderson's "The Stars are Also Fire" which won the Prometheus Award back in the 90s. I spent seven years as a professional cosmologists so I'd be extremely worried if I didn't know that stars run on fusion :)
That isn't, strictly speaking, true as much as it's one postulated result. If the expansion of the universe continues to (appear to) accelerate then eventually, yes, everything that isn't virially bound to us is going to be swept over the horizon, which would leave us with a shattered fragment within view, a pathetic remnant of everything that's actually out there which will still be monumentally vast and monumentally impressive; it will effectively be the Virgo supercluster. In extreme cases where the acceleration is caused by a phantom field then *maybe* -- and that's a very big maybe, because the physics used to argue this is grossly misapplied -- that supercluster itself might start getting swept across the horizon and we'd end up with the local cluster or even the local group. And in the hard extreme we end up with everything ripped to shreds.
The reason I don't think that that latter will happen, even if it does turn out that dark energy is a physical phenomenon (as opposed to an apparent phenomenon; the distinction does not necessarily affect the fact that distant galaxies will retreat over a horizon), is that the expansion of spacetime is a facet of *Robertson-Walker* geometry, which does not apply to bound structures. You get different behaviour in other geometries, and if RW has any physical validity at all (which, strictly speaking, it doesn't save as a loose approximation whose errors we have literally no handle on -- we just slap it in and tune its parameters) then it is only on scales at which that validity holds that "dark energy" acts the way we tend to assume. Not to say that a strongly phantom field wouldn't also cause havoc in Schwarzschild geometries, of course, but we certainly don't have a strongly phantom field because we'd see it in the orbits of planets in the solar system.
Anyway, a long and slightly off-topic digression there, but just to say I don't think the far future would be anything like as gloomy. The most likely end point of the current acceleration (whether it's physical or apparent) will be the local supercluster sitting glimmering in the darkness. And the local supercluster is *vast*. It still means that we're around in the period at which we can see there's more to the universe than just the local supercluster. A civilisation in the far future -- assuming that the reasoning that lead *here* even holds -- would have no way to know this bar theory, and since their universe would not betray large-scale homogeneity it's possible that they wouldn't end up with a cosmology anything like ours at all.
The stars are also fire.
There is a Gnome 2 foundation -- it's called MATE. Knock yourself out: http://mate-desktop.org/
To be fair, the statement "Dark matter is just some stable electrically neutral particle" is pretty strong. "Dark matter could just be some stable electrically neutral particle" would be better, particularly if you went on to some well-motivated examples such as the neutralino or axino, but there are always alternatives, particularly in this kind of case with plenty of alternatives and plenty of doubt that we even understand how gravity works on galactic scales, in clusters, superclusters, cosmologically, etc. A particulate contribution to dark matter certainly seems plausible and perhaps even likely, but it is far from certain and even less far from certain that it is even the dominant, let alone sole, contribution.
From another post I made in this thread:
"* Lambda CDM is wrong. It is dead wrong. It is wrong in principle. It is questionable from a particle physics perspective, particularly where it comes to dark energy, but far more importantly, it cannot be justified with general relativity."
That does not mean that it is inaccurate -- there are now at least two questions from here. How do we get a theory of cosmology that can be properly derived from physical underpinnings, and to what extent is Lambda CDM (or any other Robertson-Walker based model) inaccurate? No-one can really answer either of those questions, although there's certainly a lot of interest in both.
"Is there anyone who thinks that \LCDM isn't built on phenomenology?"
Sure. Everyone who without question says "the universe is homogenous and isotropic on average", derives the (Friedman-Lemaitre)-Robertson-Walker metric on those assumptions, perturbs it, and then slaps it straight into the Einstein equations without any caveats. You'd perhaps be surprised at how many people in current cosmology haven't really realised that this is invalid. And then those that *have* realised it's invalid, due not least to people like myself (and to people more well-known and influential in the field than I -- I'm hardly trying to attribute any undue credit on myself here; journeyman all the way) giving seminars and trying to drum up awareness of this, don't really seem to put much further thought into it. Which is quite frustrating. Everyone is interested, and no-one wants to do anything with it, or thinks "it has to be insignificant". Which can certainly be argued, even perhaps persuasively, but *cannot be demonstrated* and always relies ultimately on Newtonian reasoning.
I'm not saying people all think that Lambda CDM is reality. Everyone in the field is looking at alternatives, and the vast bulk accept that dark energy may well not be a cosmological constant, although you'd also be surprised at how few will actually question that dark matter may not (or may not entirely) be particulate and may instead be due to modifications of gravity or even due to a ham-fisted application of gravity. There are assumptions that are deep-rooted, and there are even quite a few cosmologists who seem to find them unremarkable and the attempt to chisel at them somehow unconstructive. Which does irritate me a bit, but I don't mean to put things too strongly as a result -- I'd say the majority of researchers could ultimately be persuaded of total alternatives and theorists are *almost* all aware that ultimately we deal in theories and marrying yourself to a theory is silly.
My experience is the more senior the researcher, the less likely they are to throw their ego into a construction, which probably makes sense.
Anyway, I'm rambling, sorry.
"if I'm allowed to make stuff up whenever I want to make my theory fit the model, I can do at least as well as the Lambda CDM"
Go ahead - you're more than welcome to. Empty assertions don't show much but new cosmological models are welcomed. *I* welcome them, anyway; I've never liked Lambda CDM much and it's obviously a phenomenological model. But they have to be predictive, and founded on firm principles.
I didn't actually want to suggest you're an idiot because I think it's apparent you're not, but this type of post at the same time implies that *cosmologists* are idiots and brainwashed into a model that doesn't really make much sense. And in some cases that's actually true -- there are more and more cosmologists trained into cosmology rather than general relativity and it's a bit dangerous -- but on the whole I don't think many people *like* LCDM. There are too many unanswered questions in it, and everyone is looking to answer those. Just some people work more tightly within its framework than others.
"is there a point where you would ever consider reexamining the questions of the assumptions? Why haven't we reached that point yet?"
Oh, don't misunderstand me -- I *constantly* question and re-examine the assumptions. At some point, if you're genuinely interested, flip back through my posts on Slashdot; I've made my position I think fairly clearly. Boiling it down and putting it in bullet form it goes something like this:
* The "big bang theory", and Lambda CDM in particular, is an astonishingly successful theory, particularly when attached to an inflationary period in the early universe or something that mimics its observational results closely
* The successes of Lambda CDM -- such as the predicted abundances from Big Bang nucleosynthesis, the *prediction* of the angular power spectra of the CMB (temperature auto-correlation, temperature/E mode cross correlation, E mode polarisation auto-correlation and now the B mode polarisation auto-correlation) from a simple early primordial power spectrum, the direct mapping between the wavelength of the sound horizon at last scattering as seen on the CMB and that same wavelength imprinted on large-scale structure and *observed* as the baryon acoustic oscillations, and their ilk -- are far too numerous and significant to be ignored.
* Any alternative absolutely has to preserve these, and they're all extremely sensitive
* Lambda CDM is wrong. It is dead wrong. It is wrong in principle. It is questionable from a particle physics perspective, particularly where it comes to dark energy, but far more importantly, it cannot be justified with general relativity.
Lambda CDM rests on a few main assumptions:
* The universe is on average isotropic around the Earth. OK, fine, we can't argue that; the CMB is proof enough.
* Since the Earth is nowhere special, the universe is on average isotropic around every point: homogeneous. Well, this is debatable since the Earth *is* in a particular position, but on the whole this is probably at least approximately true.
* Gravity is best described on large scales (ie > mm) by a metric theory. This is currently practically unquestionable; metric-based theories of gravity are vastly more succesful than any alternative.
* Gravity is described by general relativity. OK, now we're entering questionable territory but GR remains our best example of a metric-based theory and is yet to be seriously challenged (though there are many, myself among them, who point out that the appearance of dark matter on galactic scales, and the addition of dark energy on cosmological scales, may very well imply that actually we cannot apply gravity on such scales or else that it simply doesn't act this way on large scales)
* GR can be applied directly on large scales. This is extraordinarily shaky. Actually, it's unjustifiable. We've got two main objections here: firstly, there is no reason to assume that gravity actually obeys GR on large s
One of the greatest living cosmologists is George Ellis, now an emeritus professor at the University of Cape Town. Ellis is one of very few people to pioneer an approach at perturbations in cosmology -- effectively describing how structure can form out of a smooth background -- laying foundations in the 60s and 70s with the likes of Hawking and then applying it fully to cosmology in the late 80s with a new generation of students such as Bruni, Hwang and Dunsby. He's amongst the most respected gravitational physicists of the 20th and 21st centuries, has been scientific advisor to the South African government, and was active in the 70s and 80s against apartheid. He's also won the Templeton Prize (for Progress Toward Research or Discoveries about Spiritual Realities) and so far as I know is firmly Quaker. Religion does not have to stand in opposition to science, even within the same person. Personally I can't do as Prof. Ellis has and I lost what faith I had quite early in studying theoretical physics and then cosmology, but I've no lack of respect for people who can.
Could you let me know which bits are meaningless and betray ignorance? Genuine question.
"A viable alternate theory is that light gives up some energy while traveling extremely long distances, which shows up as red-shift. Where does the energy go? It could be the source of energy for the CMBR. It could go somewhere else. In any case, as a theory, it explains the red-shift just as well as expansion."
Excellent! Now repeat the rest of the predictions of the Lambda CDM model. Ah, no, you'll have trouble with that one.
"Another viable alternate theory is that the absorption/emission spectra of atoms differs with space/time. Perhaps atoms farther away or longer ago created and absorbed light at lower frequencies, this making older light appear red-shifted by current frequency comparisons. This theory is even harder to test, but just as good at explaining the observations. "
Even better! Now repeat the rest of the predictions of the Lambda CDM model. I think you'll have problems with that one, too.
Actually, I'll give you a bye -- all I want to see is the position of the first peak on the CMB *and* the wavelength of the oscillations in the large-scale structure, with one predicted consistently from the other. Once you've done that, if you can further get out supernovae 1a redshift/distance plots I'll give you extra credit, but since the progentiors aren't fully understood I'll give you a bye on that one, too.
See, the word "viable" has certain caveats. It has to satisfy the observations it's been built to explain *at least* as well as the theory it's replacing. Second, it has to -- self-consistently -- predict further observations that fit *at least* as well as the theory it's replacing. I'm no fan of Lambda CDM but its successes should convince anyone who's actually looked seriously at them that there's something close to reality there, even if ultimately it's a phenomenology close to reality (which it is; I can prove it's phenomenology -- rigorously -- but I can't demonstrate how wrong it actually is, and neither can anyone else at present, but I can at least assert that up until very recent times it's so close as to be indistinguishable and no that I fit all observations, and even very recently it's exceedingly good).
You're a decade early. What you say is *true* but the relevant time would be the late 1910s when Einstein was applying general relativity (1915) to cosmology and introduced the cosmological constant to engineer a static universe.
Who do you think predicted the gravitational radiation in the first place? There's a reason we've been wanting to find them since the 1990s and it's not that observational cosmologists were arguing with theoreticians that they were definitely there.
Tit.
"And it was such a damn elegant model, too."
You evidently have a different definition of "elegant" to me. My definition of "elegant" does not include "theories containing hand-crafted and unjustified potentials of a strikingly bizarre form inserted purely phenomenologically into a theory that is a phenomenological attempt to see what might happen if some facets of M theory are put onto large scales". That's not to defend inflation too much but the potentials of inflatons are typically quadratic or quartic, which is a lot neater than the potential in ekpyrosis, and the setup with two infinite, flat branes is at least as contrived as the inflaton.
All of that said, let's wait until the dust settles before we rule models out based on gravitational radiation alone. Their position is certainly looking somewhat precarious, but there may be ways out yet.
Radio waves still have polarisation, just the same as optical and gamma radiation does. You need some pretty refined equipment to study it in detail but you can build it. Check out bolometers and radiometers.
"First of all, how is it that all stars moving apart from each other rapidly is not "first direct evidence" of the universe's expansion?"
Because it isn't. The first direct evidence of the universe's expansion is typically accredited to Hubble in the 1920s and was very firmly established a good couple of generations back. Don't believe all you read in /. summaries...
"And secondly, how could the expansion of the universe amplify gravitational waves? Space stretching would thin out the waves because they would be expressed over a wider area."
Yes, it does indeed do so.
What this summary missed is that the universe was both extraordinarily small *and* undergoing inflation at the time. That's significant, because inflation was driven (in the theory) by the inflaton, a quantum field, and the size of the universe implied that the matter content was governed by (semi-classical) quantum theory -- ie quantum field theory on a curved but classical spacetime. Even earlier, it would be described by quantum gravity and we don't know what would happen.
Anyway, if you start looking at quantum fluctuations of a field such as the inflaton acting in something close to "slow-roll" (necessary for simple models to actually get out an inflation -- you need a field close to frozen in its potential so that it mimics a cosmological constant) then you get out an impressive array of density and metric perturbations. The density (and "scalar" metric) perturbations are what lead to the entire observable structure in the universe. Far smaller -- but, according to the results today, not *that* much smaller with an amplitude as large as 20% of the scalar -- are gravitational waves coming out. Entertainingly, even though you get out density and gravitational waves, you get basically negligible vorticity.
The results today are the first direct signal of inflation -- competing theories can produce the density perturbations and, just as importantly, their power spectrum, but frequently predict unobservably small gravitational radiation.
I would advise caution on these results since they rely on a remarkable "tilt" to the scalar power spectrum, which Bicep2 introduced to resolve a strong tension with Planck but which may (or may not) itself be in tension with Planck. That's going to be the first thing attacked -- in an investigative sense of the word, not an aggressive one -- by the community.
Mod points anyone still reading this thread please - this is very interesting stuff.
I must admit to my shame that using supernovae type II for distance measurements hasn't really been on my radar at all, although I must have been in quite a few talks discussing it. Anything that can add redundant checks to the distance ladder absolutely has to be pursued.
At redshifts of 0.03-0.08 you can hardly see the Hubble flow, man!
" In fact, I'm not quite sure whether the dark energy research that got the Nobel was strictly limited to type Ia supernovae..."
No, they were definitely intended to be SN1a.
Riess et al.: http://arxiv.org/abs/astro-ph/...
"We present observations of 10 type Ia supernovae (SNe Ia)..."
Perlmutter et al.: http://uk.arxiv.org/abs/astro-...
"...All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation..."
The reason is that SN1a can be standardised -- although that's an empirical (i.e. phenomenological) relationship rather than a theoretical one, it seems to be basically robust, as this paper has demonstrated -- and therefore used as standard candles. Other types of supernovae can not be used in the same way; one cannot necessarily correlate a (corrected) brightness against a (corrected) redshift.
This doesn't say that samples aren't contaminated by supernovae that aren't actually Type 1a (and a few years back an explanation for tension between the so-called "Gold sample" and other datasets was that it may have been more contaminated), but the intention is to only look at Type 1as.
I'd also argue that they weren't particularly high redshift, but then for me a redshift of 3 or 4 is very much low redshift. Come to that, redshifts of 300 are low redshift.
Hell, I still sometimes play Elite 2 and Grand Prix 4 more than anything other than Perfect Dark (N64 or XBox 360 remake, I'm not fussy) and Time Splitters: Future Perfect. More recent gaming has kind of passed me by.
"The big bang theory is a pretty stupid idea, as it's creationism under a thin veneer of speculative pseudoscience."
Would you care to back up that astonishing claim, ideally with some concrete science that references a respectable theory of gravity?