UNITY: Multi-wavelength simulations of relativistic cosmology

UNITY: Multi-wavelength simulations of relativistic cosmology

Gravitation in our Universe is best-described by Einstein’s theory of General Relativity. Cosmological simulations typically use a Newtonian approximation for the gravitational field however, which is far simpler to solve, and which proves to be an excellent approximation over a wide range of distance scales. Forthcoming surveys of the large-scale structure of the Universe (e.g. galaxy surveys such as Euclid and LSST, and intensity mapping surveys with MeerKAT and SKA) are now becoming large enough to capture the very largest distance scales in the observable Universe however, which inherently require a relativistic treatment.

To help us precisely understand the effects of the relativistic corrections on observations that will be made by these surveys, we have produced the largest ensemble of fully-relativistic N-body simulations to date, using the cutting-edge gevolution code. This includes a rigorous treatment of relativistic degrees of freedom, such as massive neutrinos, as well as solving for photon paths using the full set of relativistic fields that exist on large scales in the Universe. The result is a particularly accurate representation of a range of different cosmological large-scale structure observables that fully accounts for subtle GR effects.

Figure 1: A simulation of the redshift-space distortion fluctuation field that includes a full set of general relativistic corrections, taken from the UNITY simulations. This figure shows a slice through the lightcone at a redshift of z=0.47, produced using the latest version of the gevolution relativistic N-body simulation code on DiRAC. Another 50 simulations of this kind were produced as part of the project.

As part of the UNITY project, we ran an ensemble of 51 gevolution simulations on the DiRAC Data Intensive system in Leicester, with corresponding lightcones and tomographic maps of various observables taken from each and stored for further processing to add survey-specific observational features, such as different populations of galaxies. The simulations were divided into two sets: an ensemble containing different random realisations of the initial conditions, which can be used to study the statistical properties of observables and methods used to recover them, and a set with fixed initial conditions but systematically different values of cosmological parameters, allowing us to perform a precision study of the cosmology-dependence of the various relativistic correction terms.

Two key works are based on these simulations are currently being prepared for publication: a technical description of the simulations and study of the relativistic dipole in the galaxy distribution (L. Coates et al, in prep.), and a detailed study of GR-induced multipoles in the power spectra and two-point functions of multiple galaxy populations (C. Guandalin et al, in prep.).