If the early universe is dominated by an energy density which evolves other than radiation-like the normal Hubble-temperature relation H∝ T² is broken and dark matter relic density calculations in this era can be significantly different. We first highlight that with a population of states sourcing an initial expansion rate of the form H∝ T^2+n/2, for n≥−4, during the period of appreciable decays the evolution transitions to H∝ T⁴. The decays of imply a source of entropy production in the thermal bath which alters the Boltzmann equations and impacts the dark matter relic abundance. We show that the form of the initial expansion rate leaves a lasting imprint on relic densities established while H∝ T⁴ since the value of the exponent n changes the temperature evolution of the thermal bath. In particular, a dark matter relic density set via freeze-in or non-thermal production is highly sensitive to the temperature dependance of the initial expansion rate. This work generalises earlier studies which assumed initial expansion rates due to matter or kination domination.

We present a detailed study of the oscillations and optical properties for vacuum, in a model for the dark sector that contains axion-like particles and hidden photons. We provide bounds for the coupl

We analyze the early stage of evolution of a universe with two scale factors proposed in Falomir et al. [ Phys. Rev. D 96, 083534 (2017)]. when matter is present. The scale factors describe two causally disconnected patches of the universe interacting trough a nontrivial Poisson bracket structure in the momentum sector characterized by one parameter $\kappa$. We studied two scenarios in which one of the patches is always filled with relativistic matter while the other contains relativistic matter in one case, and nonrelativistic matter in the second case. By solving numerically the set of equations governing the dynamics, we found that the energy content of one sector drains to the other and from here it is possible to constrain the deformation parameter $\kappa$ by supposing that the decay of the energy density happens, at most, at the Big Bang Nucleosynthesis temperature in order to return to the usual behavior of radiation. The relation with nonstandard cosmologies is also addressed.

In this paper, we explore an extension of the classical non-standard cosmological scenario in which the new field, ϕ, which interacts with the radiation component in the early universe, experiences dissipative processes in the form of a bulk viscosity. Assuming an interaction term given by Γ _ϕ ρ _ϕ , where Γ _ϕ accounts for the decay rate of the field and ρ _ϕ corresponds to its energy density, and a bulk viscosity according to the expression ξ=ξ ₀ ρ _ϕ ϕ^1/2 in the framework of Eckart's theory, we apply this novel non-standard cosmology to study the parameters space for WIMPs Dark Matter candidate production. This parameter space shows deviations from the classical non-standard cosmological scenario, obtaining new regions to search for this candidate. In particular, for certain combinations of the free parameters, we found large regions in which the model can establish the DM and reproduce the current observable relic density.