Cosmological constraints on the velocity-dependent baryon-dark matter coupling

Abstract

We present the cosmological constraints on the cross section of baryon-dark matter interactions for the dark matter mass below the MeV scale from the Planck CMB (cosmic microwave background) and SDSS (Sloan Digital Sky Survey) Lyman-alpha forest data. To explore the dark matter mass m(chi) less than or similar to 1 MeV for which the dark matter's free-streaming effect can suppress the observable small scale density fluctuations, in addition to the acoustic oscillation damping in existence of the baryon-dark matter coupling, we apply the approximated treatment of dark matter free-streaming analogous to that of the conventional warm dark matter. We demonstrate the mass dependence of the baryon-dark matter cross section bounds (for the dark matter mass down to m(chi) similar to 5 keV), in contrast to the dark matter mass independence of the cross section constraints for the light dark matter below the MeV scale claimed in the previous literature. We also investigate the degeneracy between the baryondark matter coupling and the running spectral index in the scalar primordial spectrum. We find that this degeneracy is weak and the constraint on the baryon-dark matter coupling is not affected by the inclusion of the running spectral index in our MCMC analysis. c 2019 IOP Publishing Ltd and Sissa Medialab