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 scales. The direct evidence we have stretches up to, I don't know, 100AU or so. We're extrapolating that not just to a parsec but to a kiloparsec for galactic scales, a megaparsec for cluster scales, and a gigaparsec for cosmological scales. That has to be justified. But far more importantly, *in GR it is impossible to define an averaged field*. The word "on average" and "on large scales" is peppered through these assumptions. Try and define an average in GR and you'll burn three or four years of your life and end up with something brutally non-covariant and which, worse, only applies on globally hyperbolic spacetimes. Which more practically means it cannot apply whenever there is geodesic crossing -- whenever light rays can cross one another. But that's *precisely* what we have in GR. We have gravitational lenses out there. A single gravitational lens fucks up an averaging procedure, and the universe is riddled with them. For this sole reason, cosmology as an absolute, provable science is currently totally fucked.

Now what this does not mean is that cosmology is worthless or meaningless. The success of Lambda CDM + inflation demands that it be taken extremely seriously. Indeed, up to a redshift of call it 2 or 1 then I wouldn't even think of using an alternative. But at lower redshifts it gets increasingly shaky. We can recreate some aspects of Lambda CDM in pure dust spacetimes simply by swapping from the Robertson-Walker metric. We can prove that the idea of employing the RW metric at all is unjustifiable. That much we can do. What we can't currently do is suggest an alternative, so we're stuck for now with LCDM - because there *is nothing better* that can be founded as securely on reasonable physical bases. (And I even include dark energy in "reasonable physics", with a bit of a twinge.)

Sure, I question the assumptions constantly. So do plenty of others. GR is replaced constantly -- check for f(R) theories and Horava-Lifshitz gravity for two recent explorations in totally different ways to look at gravity, and which produce very different cosmologies that mimic LCDM closely. People consider inhomogeneous models -- non-RW metrics -- or sit there and try and average things more securely. People hack at the underpinnings constantly. But in terms of a standard model we can all look at, like it or not (and many do not), we can't beat LCDM.

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.