On dark matter-dark radiation interaction and cosmic reionization

The dark matter sector of our universe can be much richer than the conventional picture of a single weakly interacting cold dark matter particle species. An intriguing possibility is that the dark matter particle interacts with a dark radiation component. If the non-gravitational interactions of the dark matter and dark radiation species with Standard Model particles are highly suppressed, then astrophysics and cosmology could be our only windows into probing the dynamics of such a dark sector. It is well known that such dark sectors would lead to suppression of small scale structure, which would be constrained by measurements of the Lyman-alpha forest. In this work we consider the cosmological signatures of such dark sectors on the reionization history of our universe. Working within the recently proposed "ETHOS" (effective theory of structure formation) framework, we show that if such a dark sector exists in our universe, the suppression of low mass dark matter halos would also reduce the total number of ionizing photons, thus affecting the reionization history of our universe. We place constraints on the interaction strengths within such dark sectors by using the measured value of the optical depth from the Planck satellite, as well as from demanding a successful reionization history. We compare and contrast such scenarios with warm dark matter scenarios which also suppress structure formation on small scales. In a model where dark matter interacts with a sterile neutrino, we find a bound on the ETHOS parameter a(4) less than or similar to 1.2 x 10(6) Mpc(-1). For warm dark matter models, we constrain the mass m(WDM) greater than or similar to 0.7 keV, which is comparable to bounds obtained from Lyman-a measurements. Future 21-cm experiments will measure the global history of reionization and the neutral hydrogen power spectrum, which could either lead to stronger constraints or discovery of secret dark sector interactions.