Still dark but maybe somewhat more real ... [arXiv:1105.2862, 1105:2948]

Between two competing potential mechanisms behind the observed acceleration of the expansion of the Universe, this is the mysterious, negative-pressure component, which seems to be in favor these days. And this is thanks to a work by Australian researchers from the Wiggle-z team.


Over the last 5 years the  team has mapped the galaxy distribution in a critical for dark energy studies range of intermediate redshifts, 0.3 ≤ z ≤ 1. The survey covered a solid angle of 1000deg² corresponding to the volume two orders of magnitude larger at z ∼ 0.5 than any previous effort. The galactic surveys can be used to constrain the evolution of the Universe in many ways and are therefore a gold-mine for those trying to understand what the Universe is made of and thus in particular, and no surprise, mainly at this time, what the nature and properties of dark energy are. The Wiggle-z team looked at three of those to date: Baryonic Acoustic Oscillation, Paczyński-Alcock test, and a structure growth rate. The two first ones are in their essence two "modern" versions of one of the standard cosmological test: having a standard ruler observed today at some opening angle and placed at known redshift from us one can constrain the matter content of the Universe, and given the constraints coming from the other quarters (mainly CMB observations) directly put a finger on the properties of the unknown, i.e., dark energy. The authors obtain here perfect consistency with other data sets. The geometrical tests, as these two ones are, have one downside - they can not distinguish between two different potential reasons behind the acceleration of the present-day Universe: dark energy and modified gravity. Indeed, dark energy usually, and for shortness, is used to mean both these things, when in the more narrow, and correct, sense the dark energy refers to some rather weird, but physical, component in the Universe, which drives the acceleration of the universal expansion, as opposed to the second option, in which the content of the Universe is left boringly as usual, and it is the gravity itself, which drives the acceleration and which to do so need to be different than the Einstein's one. Both these options are rather capacious in any case, leaving enough freedom, so that within each of them, with some minor tweaking, any geometric evidence can be accounted for. However, not so with the third test applied by the Wiggle-z Team.
The growth rate of the structure formation can in principle tell these two fundamentally different possibilities apart, and in the hands of the Wiggle-z teams indeed does so speaking in favor of the physical component. Lo and behold an evidence is then provided that the dark energy is indeed real !
Measuring the growth rate from the observations is a challenge on its own, analyzing and interpreting the results, another one. No doubt controversy will continue for some time to come before any side throws in a towel for real. And who ultimately is going to do so, does not seem to be engraved in a stone yet. For at least time being, however, the proponents of dark energy component can sleep somewhat more profoundly than its detractors.