Momentum transfer in the dark sector and lensing convergence in upcoming galaxy surveys

We investigated a cosmological model that allows a momentum transfer between dark matter and dark energy. The interaction in the dark sector mainly affects the behaviour of perturbations on small scales while the background evolution matches the $w$CDM solution. As a result of the momentum transfer, these kinds of models help alleviating the $\sigma_8$ discrepancy in the standard model, but do not resolve the so-called $H_0$ tension. We confirm that this is indeed the case by computing cosmological constraints. While our analysis tends to favour $\sigma_8$ values lower than in $\Lambda$CDM, we do not find evidence for a non-vanishing momentum transfer in the dark sector. Since upcoming galaxy surveys will deliver information on scales and red-shift relevant for testing models allowing momentum transfer in the dark sector, we also carried out forecasts using different survey configurations. We assessed the relevance of neglecting lensing convergence $\kappa$ when modelling the angular power spectrum of number counts fluctuations $C_\ell^{\rm ij}(z,z')$. We found that not including $\kappa$ in analyses leads to biased constraints ($\approx 1-5\,\sigma$) of cosmological parameters even when including information from other experiments. Incorrectly modelling $C_\ell^{\rm ij}(z,z')$ might lead to spurious detection of neutrino masses and exacerbate discrepancies in $H_0$ and $\sigma_8$.Comment: 26 pages, 8 figures, 8 tables. Comments welcome Updated version. Matches published versio