Abstract
Understanding the nature of the Dark Matter (DM) is one of the current challenges in modern astrophysics and cosmology. Knowing the properties of the DM particle would shed light on physics beyond the Standard Model and even provide us with details of the early Universe. In fact, the detection of such a relic would bring us information from the pre-Big Bang Nucleosynthesis (BBN) period, an epoch from which we have no direct data, and could even hint at inflation physics. In this work, we assume that the expansion rate of the Universe after inflation is governed by the kinetic energy of a scalar field ϕ , in the so-called “kination” model. Adding to previous work on the subject, we assume that the ϕ field decays into both radiation and DM particles, which we take to be Weakly Interacting Massive Particles (WIMPs). The present abundance of WIMPs is then fixed during the kination period through either a thermal “freeze-out” or “freeze-in” mechanism, or through a non-thermal process governed by the decay of ϕ . We explore the parameter space of this theory with the requirement that the present WIMP abundance provides the correct relic budget. Requiring that BBN occurs during the standard cosmological scenario sets a limit on the temperature at which the kination period ends. Using this limit and assuming the WIMP has a mass m χ = 100 GeV, we obtain that the thermally averaged WIMP annihilation cross section has to satisfy the constraints 4 × 10 − 16 GeV − 2 ≲ ⟨ σ v ⟩ ≲ 2 × 10 − 5 GeV − 2 in order for having at least one of the production mechanism to yield the observed amount of DM. This result shows how the properties of the WIMP particle, if ever measured, can yield information on the pre-BBN content of the Universe.
Highlights
The existence of a Dark Matter (DM) component in the Universe has long been established [1,2], with a Weakly Interacting Massive Particle (WIMP) being among the best motivated particle candidates [3,4]
In the simplest scenario of the early Universe, WIMPs of mass mχ interact with the Standard Model (SM) particles at a sufficiently high rate so that the chemical equilibrium is attained
Owing to the expansion rate of the Universe, when the temperature falls below Tf.o. ≈ mχ /20 WIMPs chemically decouple from the plasma and “freeze-out” of the equilibrium distribution [5,6,7,8,9,10,11,12]
Summary
The existence of a Dark Matter (DM) component in the Universe has long been established [1,2], with a Weakly Interacting Massive Particle (WIMP) being among the best motivated particle candidates [3,4]. In the Low Reheat Temperature Scenario (LRTS) [43,44,45,46], φ is a massive modulus which drives an early matter-dominated epoch, eventually decaying into SM particles and, possibly, WIMPs. In the pre-BBN LRTS, the thermal (both freeze-out and freeze-in) and non-thermal production of WIMPs have both been extensively studied [39,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63]. [79] discusses an intermediate model between LRTS and KS, in which a sub-dominant massive scalar field reheats the Universe during a kination period governed by an additional field.
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