Is the Evidence for Dark Energy Secure?

[cristo]

This paper came out on the arxiv a few weeks ago. I've not had a chance to read it fully yet, but it looks like an interesting paper. http://arxiv.org/abs/0710.5307

Several kinds of astronomical observations, interpreted in the framework of the standard Friedmann-Robertson-Walker cosmology, have indicated that our universe is dominated by a Cosmological Constant. The dimming of distant Type Ia supernovae suggests that the expansion rate is accelerating, as if driven by vacuum energy, and this has been indirectly substantiated through studies of angular anisotropies in the cosmic microwave background (CMB) and of spatial correlations in the large-scale structure (LSS) of galaxies. However there is no compelling direct evidence yet for (the dynamical effects of) dark energy. The precision CMB data can be equally well fitted without dark energy if the spectrum of primordial density fluctuations is not quite scale-free and if the Hubble constant is lower globally than its locally measured value. The LSS data can also be satisfactorily fitted if there is a small component of hot dark matter, as would be provided by neutrinos of mass 0.5 eV. Although such an Einstein-de Sitter model cannot explain the SNe Ia Hubble diagram or the position of the `baryon acoustic oscillation' peak in the autocorrelation function of galaxies, it may be possible to do so e.g. in an inhomogeneous Lemaitre-Tolman-Bondi cosmology where we are located in a void which is expanding faster than the average. Such alternatives may seem contrived but this must be weighed against our lack of any fundamental understanding of the inferred tiny energy scale of the dark energy. It may well be an artifact of an oversimplified cosmological model, rather than having physical reality.

Here's an article in the news section of Nature on the same topic: Bursting Dark Energy's Bubble.

 

[Wallace]

Based on the abstract ( I haven't read the full paper ) this all sounds reasonable ( and by no means original, people have been pointing all of this out since dark energy first was suggested). SN1A could be spoofing us by evolving in some as yet unknown way, and the rest of the probes may indeed by spoofed in the ways he suggests. However, there are two issues here I think. The first is one of simplicity, either we have one model that concisely describes all of what we observe with needed tweaks here and there to satisfy each observation, or we need a contrived model like the one suggested by Sarkar.

Please nobody starting abusing poor little Ockam and his razor, we could argue till next millennium whether LCDM is 'simpler' because it has only a few parameters, or whether the kind of thing Sarkar suggests is 'simpler' because it doesn't need exotic dark energy. Let's not pretend that our subjective judgments are objective just because we mis-represent a much abused principle!

I'd prefer to talk about something more concrete, for instance predictions. The LCDM model predicted the location of the acoustic peak in the galaxy-galaxy auto-correlation function, based on previous observations of very different things (the CMB and SN1A predominantly). We've seen marginal evidence for this peak in existing galaxy surveys and will hopefully have that confirmed soon by new surveys. Sarkar's suggestion however requires that the mechanism for the peak is totally unrelated to anything else in cosmology, and just happens to occur on the scale predicted by LCDM.

If we want to talk about physical plausibility, it seems quite reasonable to have a 'not quite scale free' inflationary fluctuation spectrum yet we have no idea of how this physically would occur. All physically motivated theories describing the origin of the fluctuations predict that there would be no preferred scales.

At this stage, in the end, we are not going to settle this kind of theoretical argument. With next generation BAO, SN and CMB probes however, we may either confirm LCDM (or more generally wCDM) to ridiculous precision or indeed find that the model starts to break down.

 

[Garth]

As already discussed here.

As I asked in the Critique of Mainstream Cosmology thread, "Mundane checks" anyone?"

Garth

 

[cristo]

As already discussed here.

Ahh, sorry, I don't delve into that thread too much!

 

[andrewj]

black holes

all black holes have the same mass

 

[Garth]

all black holes have the same mass

What on Earth made you say that?

Black Holes that formed from the collapse of stellar cores and through mergers have masses from
~ 3 M_Solar up to 10⁸ M_Solar, mini BH's may have formed in the BB as over dense areas collapsed into themselves, which may have had small masses of a few Kg upwards.

Garth