$\Lambda$vCF: Extending $\Lambda$CDM into a Unified Model with Particle Creation

TL;DR

This paper introduces a unified cosmological model extending $ ext{Lambda}$CDM by incorporating viscous cosmic fluids with particle creation, providing analytical solutions for all cosmic epochs and insights into inflation and cosmic fluid behavior.

Contribution

It develops a novel analytical framework for a unified cosmic evolution model with viscous fluids and particle creation, linking inflation to late acceleration.

Findings

Determines transition points and boundary conditions for cosmic epochs.

Estimates inflationary Hubble parameter as approximately 10^{13} GeV.

Models inflation as an ultra-slow-roll hilltop scenario with a graceful exit.

Abstract

We present a novel extended version of the $\Lambda$CDM model that provides analytical solution for Hubble parameter uniting all epochs of cosmic evolution starting from inflation to late-acceleration, with intermediate radiation and matter-dominated epochs. This is achieved by relaxing the perfect fluid assumption in the standard model and considering a general viscous cosmic fluid (vCF) with non-zero particle creation rate and evolving adiabatic equation of state. Transition points of the Universe and the finite boundary connecting them is exactly determined. We then propose a novel method to determine the early-time viscous coefficient and inflation energy scale using the Cosmic Mode Index value postulated by Padmanabhan. Considering the data from the Planck 2018 analysis, this yields an inflationary Hubble parameter of $H_{I}\approx10^{13}$\,GeV. An equivalent scalar-field description for the inflationary epoch is then constructed and inferences are made regarding the nature of inflation. Notably, we find that the model describes an ultra-slow-roll hilltop inflation scenario with a graceful exit to radiation-dominated epoch. Subsequently, we show that bulk viscosity in this model can be expressed as Israel-Stewart equation in relativistic dissipative hydrodynamics with an appropriate underlying viscous coefficient and relaxation time that satisfy the causality constraint in its extreme limit. Finally, by comparing the evolution of this causal relation and its Navier-Stokes counterpart, we infer that the evolution from inflation to radiation era signifies a fluid transitioning from viscoelastic to pseudoplastic behavior.