Measuring unified dark matter with 3D cosmic shear

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 characterized by a parameter, c∞ (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∞, and we find minimal errors Δc∞= 3.0 × 10−5, for the fiducial value c∞= 1.0 × 10−3 and Δc∞= 2.6 × 10−5, for c∞= 1.2 × 10−2. Moreover, we compute the Bayesian evidence for UDM models over the ΛCDM model as a function of c∞. For this purpose, we can consider the ΛCDM model as a UDM model with c∞= 0. We find that the expected evidence clearly shows that the survey data would unquestionably favour UDM models over the ΛCDM model, for the values c∞≳ 10−3.