The nature of dark energy and dark matter is still mysterious. Ongoing and upcoming galaxy surveys (eBOSS, DES, KIDS, DESI, Euclid, LSST, SKA) will measure the position and apparent shape of billions of galaxies with the aim of constraining the nature of the dark sector of the Universe. To do this, however, it is necessary to understand the link between the universe as it is (’real’ position and shape of the sources) and the universe as seen through telescopes (’apparent’ position and shape of the pictures). Between the two, the path of photons governed by general relativity is unfortunately not straight, but is on the contrary deflected by the curvature of the metric created by the massive large-scale structures of the Universe (galaxies, groups, clusters) : we mean gravitational lensing effects. In the same way the energy of the photons is modified in particular according to the velocity (Doppler effect) and the gravitational potential of the sources. In cosmology, distances are precisely derived from the redshift of atomic spectra. Also the energy changes will lead to fluctuations in the apparent distance of the sources : these are redshift-space distortions. All these interferences related to the dynamics of the photons which seem a priori to blurr out the original signal are in reality a rich source of information on the universe because they depend directly on its energy content and its gravitational metric.
In order to study these effects in detail, an international team of researchers has carried out new cosmological simulations of the formation of large-scale structures called RayGal for two different dark energy models (the standard model and an alternative model). Combining large volume and high resolution, these simulations make it possible to properly resolve the curvature of the metric created by galaxies, groups and clusters of galaxies which are the main light deflectors. For the first time, billions of photons have been launched within cosmological simulations and their trajectory has been finely calculated thanks to the equations of general relativity. In addition, a new technique has made it possible to identify the rays emitted by the sources and which pass exactly through the observer. Thus the researchers were able to precisely reconstruct the Universe as it would be seen by a virtual observer located within the simulation and taking into account all the relativistic effects : gravitational lensing effects and redshift space distortions. Unique data from the RayGal simulation suite is made available to the community : the scope at the intersection of cosmology, general relativity and galaxies is very broad. They should allow, in combination with the surveys of galaxies to come, to see a little more clearly the nature of the dark sector of the universe...
References
The RayGalGroupSims cosmological simulation suite for the study of relativistic effects : an application to lensing-matter clustering statistics, Rasera, Y., Breton, M-A., Corasaniti, P-S. et ak. : 2022, A&A in press
https://arxiv.org/abs/2111.08745
Imprints of relativistic effects on the asymmetry of the halo cross-correlation function : from linear to non-linear scales, Breton, M-A., Rasera, Y., Taruya, A., et al. : 2019, MNRAS, 483, 2671
https://arxiv.org/abs/1803.04294