Constraining dark matter late-time energy injection: decays and p-wave
  annihilations
  We use the latest cosmic microwave background (CMB) observations to provide
updated constraints on the dark matter lifetime as well as on p-wave suppressed
annihilation cross sections in the 1 MeV to 1 TeV mass range. In contrast to
scenarios with an s-wave dominated annihilation cross section, which mainly
affect the CMB close to the last scattering surface, signatures associated with
these scenarios essentially appear at low redshifts ($z \lesssim 50$) when
structure began to form, and thus manifest at lower multipoles in the CMB power
spectrum. We use data from Planck, WMAP9, SPT and ACT, as well as
Lyman-$\alpha$ measurements of the matter temperature at $z \sim 4$ to set a 95
% confidence level lower bound on the dark matter lifetime of $\sim 4 \times
10^{25}$ s for $m_\chi = 100$ MeV. This bound becomes lower by an order of
magnitude at $m_\chi = 1$ TeV due to inefficient energy deposition into the
intergalactic medium. We also show that structure formation can enhance the
effect of p--wave suppressed annihilation cross sections by many orders of
magnitude with respect to the background cosmological rate, although even with
this enhancement, CMB constraints are not yet strong enough to reach the
thermal relic value of the cross section.
