Sustaining quasi de-Sitter inflation with bulk viscosity

TL;DR

This paper explores how bulk viscosity within a causal relativistic hydrodynamics framework can naturally drive a quasi de-Sitter inflationary phase in the early universe, matching observational data without extra scalar fields.

Contribution

It introduces a novel bulk-viscosity driven inflation model based on causal hydrodynamics, demonstrating its equivalence to scalar field inflation and analyzing its observational viability.

Findings

Bulk viscosity can sustain quasi de-Sitter inflation without scalar fields.

The model's predictions for spectral index and tensor-to-scalar ratio align with observations.

The transition from inflation to standard expansion depends on bulk viscosity magnitude.

Abstract

We here investigate bulk-viscosity driven quasi de-Sitter inflation, that is, the period of accelerated expansion in the early universe during which $-\dot{H}\ll H^2$, with $H(t)$ being the Hubble expansion rate. We do so in the framework of a causal theory of relativistic hydrodynamics that takes into account non-equilibrium effects associated to bulk viscosity that may be present as the early universe undergoes an accelerated expansion. In this framework, the existence of a quasi de-Sitter universe emerges as a natural consequence of the presence of bulk viscosity, without requiring to introduce additional scalar fields. As a result, the equation of state, determined by numerically solving the generalized momentum-conservation equation involving bulk-viscosity pressure turns out to be time-dependent. The transition timescale characterising its departure from an exact de-Sitter phase is intricately related to the magnitude of the bulk viscosity. We examine the properties of the new equation of state, as well as the transition timescale in presence of bulk-viscosity pressure. In addition, we construct a fluid description of inflation and demonstrated that, in the context of the causal formalism, it is equivalent to the scalar field theory of inflation. Our analysis also shows that the slow-roll conditions are realised in the bulk-viscosity supported model of inflation. Finally, we examine the viability of our model by computing the inflationary observables, namely, the spectral index and the tensor-to-scalar ratio of the curvature perturbations, and compare them with a number of different observations finding good agreement in most cases.