Thermal relics in cosmology with bulk viscosity

Abstract

In this paper we discuss some consequences of cosmological models in which the primordial cosmic matter is described by a relativistic imperfect fluid. The latter takes into account the dissipative effects (bulk viscosity) arising from different cooling rates of the fluid components in the expanding Universe. We discuss, in particular, the effects of the bulk viscosity on Big Bang Nucleosynthesis and on the thermal relic abundance of particles, looking at recent results of PAMELA experiment. The latter has determined an anomalous excess of positron events, which cannot be explained by conventional cosmology and particle physics.

Cosmology from Strong Interactions

“…[84,286], the evolution of the early universe must include a number of dissipative processes in order to explain the current large value of the entropy per baryon. Some of them, such as the decoupling of neutrinos during the radiation era [120] or different cooling rates of the fluid components in the expanding universe [287], can result from the conventional physics; others, involving more exotic mechanisms, assume entropy production via string creation [288] or the GUT phase transitions [289]. The hydrodynamical description of dissipative effects is summarized in Appendix B.…”

Cosmology from Strong Interactions

“…[82,267], the evolution of the early Universe must include a number of dissipative processes, in order to explain the current large value of the entropy per baryon. Some of them, like the decoupling of neutrinos during the radiation era [116] or different cooling rates of the fluid components in the expanding Universe [268] can result from the conventional physics, other, involving more exotic mechanisms, assume entropy production via string creation [269] or the GUT phase transitions [270]. The hydrodynamical description of dissipative effects is summarized in Appendix B.…”

PeV IceCube signals and Dark Matter relic abundance in modified cosmologies

“…However, any contribution to the energy density (in matter and geometrical sector) modifies the Hubble expansion rate, hence the relic density. In modified cosmologies (MC), the expansion rate of the Universe can be rewritten in the form [63][64][65][66][67][68][69][70] …”

“…2 Investigations along these lines have been performed in different cosmological scenarios [63][64][65][66][67][68][69][70][71][72], where The parameter ν labels cosmological models: ν = 2 in Randall-Sundrum type II brane cosmology [73], ν = 1 in kination models [74][75][76][77], ν = 0 in cosmologies with an overall boost of the Hubble expansion rate [63], ν = − 0.8 in scalar-tensor cosmology [63,78], ν = 2/n − 2 in f (R) cosmology, with f (R) = R + α R n [79,80]. In terms of the modified expansion rate (3.2), it then follows that the inverse decay processes (2.6) takes the form…”