The analysis of the Sloan Digital and DEEP2 sky surveys data has uncovered a redshift-dependent anisotropy in the distribution of apparent (i.e, redshift-space) relative angles of galaxy pairs with respect to the line of sight. Such a dependence could have been predicted and can be considered as an example of the Alcock-Paczynski effect, where the redshift-space anisotropy of an otherwise spherically symmetric distribution is due to the cosmological expansion. Therefore at least in principle such observations could be used to set constraints on the energy content of the Universe in general and on the dark energy in particular.
However usually the signature of the expansion is overwhelmed by the peculiar velocities of the studied objects, making the inference difficult and prone to systematic errors. This is also the case for the galaxy pairs studied in this work. However, a simple physical context of this case permits a reliable analytical modeling of their contributions, which then can be strictly taken into account in the analysis. This opens up a path to setting cosmological constraints with such a probe.
Indeed the analysis performed in this study confirms the overall geometrical flatness of the Universe, constraining ΩK = 0.1 +0.2 -0.35 (1σ errors). Moreover if the flatness is assumed, and extra constraints from the SDSS data based on Baryonic Acoustic Oscillations analysis are added, the density of the dark energy is constrained to be between 0.6 and 0.8 of the critical density (1 σ limits) and it s equation of state exponent to be between -0.85 and -1.12 (1σ again).
Some caveats remain, e.g., possibility of some intrinsic anisotropy of the galaxy pair distributions, and will be certainly dwelt upon by some skeptics. These will not change however the fact that it is a nice application of the "classic" geometrical test and an interesting, complementary constrain on dark energy.
More details can be found in this Nature paper and the related editorial.
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