Eur. Phys. J. C (2016) 76: 18
https://doi.org/10.1140/epjc/s10052-015-3861-3

Regular Article - Theoretical Physics

Inflation in a viscous fluid model

¹ Division of Human Support System, Faculty of Symbiotic Systems Science, Fukushima University, Fukushima, 960-1296, Japan
² Institut de Ciencies de lEspai (IEEC-CSIC), Campus UAB, Carrer de Can Magrans, Cerdanyola del Valles, 08193, Barcelona, Spain
³ Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys, 23 08010, Barcelona, Spain

^* e-mail: bamba@sss.fukushima-u.ac.jp

Received: 14 October 2015
Accepted: 20 December 2015
Published online: 18 January 2016

Abstract

We explore a fluid description of the inflationary universe. In particular, we investigate a fluid model in which the equation of state (EoS) for a fluid includes bulk viscosity. We find that the three observables of inflationary cosmology, i.e., the spectral index of the curvature perturbations, the tensor-to-scalar ratio of the density perturbations, and the running of the spectral index, can be consistent with the recent Planck results. We also reconstruct the explicit EoS for a fluid from the spectral index of the curvature perturbations compatible with the Planck analysis. In the reconstructed models of a fluid, the tensor-to-scalar ratio of the density perturbations can satisfy the constraints obtained from the Planck satellite. The running of the spectral index can explain the Planck data. In addition, it is demonstrated that in the reconstructed models of a fluid, the graceful exit from inflation can be realized. Moreover, we show that the singular inflation can occur in a fluid model. Furthermore, we show that a fluid description of inflation can be equivalent to the description of inflation in terms of scalar field theories.