Measuring Unified Dark Matter with 3D Cosmic Shear

TL;DR

This paper forecasts how future 3D cosmic shear surveys can precisely estimate parameters of Unified Dark Matter models, especially the sound speed parameter c_inf, and demonstrates that these surveys could strongly favor UDM over LCDM.

Contribution

It provides the first detailed forecast showing that 3D cosmic shear surveys can tightly constrain UDM model parameters and distinguish them from LCDM.

Findings

3D weak lensing can estimate c_inf with errors as low as 0.00003.

Survey data would favor UDM models over LCDM for c_inf > 0.001.

High-precision constraints on the sound speed parameter c_inf are achievable.

Abstract

We present parameter estimation forecasts for future 3D cosmic shear surveys for a class of Unified Dark Matter (UDM) models, where a single scalar field mimics both Dark Matter (DM) and Dark Energy (DE). These models have the advantage that they can describe the dynamics of the Universe with a single matter component providing an explanation for structure formation and cosmic acceleration. A crucial feature of the class of UDM models we use in this work is characterised by a parameter, c_inf (in units of the speed of light c=1), that is the value of the sound speed at late times, and on which structure formation depends. We demonstrate that the properties of the DM-like behaviour of the scalar field can be estimated with very high precision with large-scale, fully 3D weak lensing surveys. We found that 3D weak lensing significantly constrains c_inf, and we find minimal errors 0.00003, for the fiducial value c_inf=0.001, and 0.000026, for c_inf=0.012. Moreover, we compute the Bayesian evidence for UDM models over the LCDM model as a function of c_inf. For this purpose, we can consider the LCDM model as a UDM model with c_inf=0. We find that the expected evidence clearly shows that the survey data would unquestionably favour UDM models over the LCDM model, for the values c_inf>0.001.