Cool dark sector, concordance, and a low σ_8

We investigate a cosmological model in which a fraction of the dark matter is atomic dark matter (ADM). This ADM consists of dark versions of the electron and of the proton, interacting with each other and with dark photons just as their light sector versions do, but interacting with everything else only gravitationally. We find constraints given current cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) data, with and without an H_0 prior, and with and without enforcing a big bang nucleosynthesis consistent helium abundance. We find that, at low dark photon temperature, one can have consistency with BAO and CMB data, with a fraction of dark matter that is ADM (f_ adm) as large as ∼ 0.1. Such a large f_ adm leads to a suppression of density fluctuations today on scales below about 60 Mpc that may be of relevance to the σ_8 tension. Our work motivates calculation of nonlinear corrections to matter power spectrum predictions in the ADM model. We forecast parameter constraints to come from future ground-based CMB surveys, and find that if ADM is indeed the cause of the σ_8 tension, the influence of the ADM, primarily on CMB lensing, will likely be detectable at high significance.