When the universe expands too fast: relentless dark matter

Abstract

We consider a modification to the standard cosmological history consisting of introducing a new species φ whose energy density red-shifts with the scale factor a like ρ φ ∝ a −(4+n) . For n > 0, such a red-shift is faster than radiation, hence the new species dominates the energy budget of the universe at early times while it is completely negligible at late times. If equality with the radiation energy density is achieved at low enough temperatures, dark matter can be produced as a thermal relic during the new cosmological phase. Dark matter freeze-out then occurs at higher temperatures compared to the standard case, implying that reproducing the observed abundance requires significantly larger annihilation rates. Here, we point out a completely new phenomenon, which we refer to as relentless dark matter: for large enough n, unlike the standard case where annihilation ends shortly after the departure from thermal equilibrium, dark matter particles keep annihilating long after leaving chemical equilibrium, with a significant depletion of the final relic abundance. Relentless annihilation occurs for n ≥ 2 and n ≥ 4 for s-wave and p-wave annihilation, respectively, and it thus occurs in well motivated scenarios such as a quintessence with a kination phase. We discuss a few microscopic realizations for the new cosmological component and highlight the phenomenological consequences of our calculations for dark matter searches.

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“…The DM relic abundance is usually expressed in terms of the parameter Ω DM h 2 where h ∼ 0.7 is the value Hubble expansion rate at present times in units of 100 (km/s)/Mpc 1 See Ref. [22] for an exception ("relentless" DM) for modified expansion histories. 2 We make also use of: while Ω DM represents the ratio between the DM energy density ρ DM and the so called critical energy density ρ cr , namely:…”

“…(21) and the second Lagrangian of Eq. (22) are substantially equivalent for self-conjugate (i.e., real scalar or Majorana fermion) and non self-conjugate (i.e., complex scalar and/or Dirac fermion) DM with ξ = 1/2 and 1, respectively. For this reason we are explicitly reporting Lagrangians only for the case of a real scalar and a Dirac fermionic DM.…”

“…The first Lagrangian of Eq. (22) can only be written for a complex scalar DM. In the case of a spin-1 DM with spin-1 mediator, two sensitively different Lorentz structures are allowed for non-self conjugate (third Lagrangian of Eq.…”

“…SM portals: These represent a special case of s-channel portals, i.e., Eqs. (21) and (22), in which S = h and Z = Z with h and Z being the SM Higgs and Z bosons, respectively. 10 These are the most predictive models since the couplings of the mediators with the SM states are fixed.…”

“…BSM s-channel portals: These are the models fully described by Eqs. (21) and (22). Here the DM is coupled with the SM fermions either by a new spin-0 (real) or by a spin-1 electrically neutral state.…”