Expansion history with decaying vacuum: a complete cosmological scenario

Abstract

We propose a novel cosmological scenario with the space-time emerging from a pure initial de Sitter stage and subsequently evolving into the radiation, matter and dark energy dominated epochs, thereby avoiding the initial singularity and providing a complete description of the expansion history and a natural solution to the horizon problem. The model is based on a dynamical vacuum energy density which evolves as a power series of the Hubble rate. The transit from the inflation into the standard radiation epoch is universal, giving a clue for a successful description of the graceful exit. Since the resulting late time cosmic history is very close to the concordance ΛCDM model, the new unified framework embodies a more complete past cosmic evolution than the standard cosmology.

Primordial gravitational waves in running vacuum cosmologies

“…[25,26], the very late time behavior of the model analized here is exactly ΛCDM. The GW equation is derived and its time-dependent part numerically integrated since the primordial de Sitter stage.…”

“…Finally, it should be reinforced that the model discussed here is a starting point for the investigation of more complex and rich decaying vacuum cosmologies, like the general class proposed in Ref. [25,26]. Such models with Λ(H) ∝ H n , n > 2 deserves a closer scrutiny since they furnish a complete cosmological history.…”

“…The dynamical Λ(H)-term was phenomenologically modeled as Λ(H) = Λ b + H 3 /H I . This kind of model undergone a smooth transition from an early inflation to the standard radiation phase which can analytically be described [3,4,25,26]. Interestingly, the model explain the present day entropy content of the Universe stored in the relic radiation [27,28] without necessity of a highly non-adiabaticreheating stage.…”

“…As remarked before, it will be assumed here that the Λ(H)-term is given by a particular case of the general class studied by Perico et al [25,26] (for closely related works see also [3,4])…”

“…In this connection, a large class of nonsingular models, where the vacuum energy density evolves phenomenologically as a truncated power-series in the Hubble parameter, has been proposed a couple of years ago [25,26] (its dominant term ρ vac (H) ∝ H n , n > 2). This class of models has some interesting features, among them: a nonsingular origin for the expanding universe (no horizon problem) with a deflationary process also without "exit problem", that is, the model evolves smoothly from the primeval nonsingular de Sitter state to the radiation phase; its late-time cosmic expansion history is very close to the concordance model and it also furnishes a smooth link between the initial and final de Sitter stages through the radiation and matter dominated phases.…”