Old Physics, New Tricks, and the Theory of Atomic Dark Matter
February 06, 2012
Abstract: Cold dark matter (CDM) is a central pillar of the current cosmological paradigm. While CDM predictions are in good agreement with observations of the cosmic microwave background, large-scale structures, and the dynamics of clusters and galaxies, the physics of dark matter is unknown. Recent observations probing small astrophysical scales might indicate deviations from the standard CDM scenario. We introduce a model of dark-matter physics in which the dark sector, due to a new dark force, is made of atom-like bound states. This model predicts different dark-matter properties on small scales but retains the success of CDM on cosmological scales. In the early Universe, the dark sector forms a tightly-coupled plasma sustaining acoustic wave that can leave observable signatures in the matter power spectrum. The kinetic decoupling of dark matter in this model is significantly delayed compared to a typical WIMP model leading to a significant damping of fluctuations on small scales. We revisit the atomic physics necessary to capture the the thermal history of the dark sector and show significant improvements over the standard hydrogen calculation are required to make accurate predictions. We discuss constraints on Atomic Dark Matter from galactic and cluster dynamics, cosmic microwave background, the matter power spectrum, and big-bang nucleosynthesis.