Trading kinetic energy: how late kinetic decoupling of dark matter changes Neff

James A.D. Diacoumis, Yvonne Y.Y. Wong
2019 Journal of Cosmology and Astroparticle Physics  
Elastic scattering between dark matter particles and a relativistic species such as photons or neutrinos leads to a transfer of energy from the latter due to their intrinsically different temperature scaling relations. In this work, we point out that this siphoning of energy from the radiation bath manifests as a change in the effective number of neutrinos N_ eff, and compute the expected shift Δ N_ eff for dark matter-photon and dark matter-neutrino elastic scattering as a function of the dark
more » ... matter mass m_ψ and scattering cross section σ_ψ-X. For (m_ψ,σ_ψ-X)-parameter regions already explored by nonlinear probes such as the Lyman-α forest through collisional and/or free-streaming damping, we find shifts of |Δ N_ eff| ≃ O(10^-2), which may be within the reach of the proposed CMB-S4 experiment. For most of the as-yet-unexplored parameter space, however, we expect |Δ N_ eff| ≲ O(10^-3). An ideal 21 cm tomography survey of the dark ages limited only by cosmic variance is potentially sensitive to |Δ N_ eff| ≃ O(10^-6), in which case dark matter masses up to m_ψ∼ 10 MeV may be probed via their effect on N_ eff.
doi:10.1088/1475-7516/2019/01/001 fatcat:4i6lbqcyofff7pzqmz7mu3axvy