# Phantom solution in a non-linear Israel-Stewart theory

## TL;DR

This paper demonstrates a phantom solution with a big rip singularity within a non-linear Israel-Stewart framework, enabling accelerated cosmic expansion without exotic matter.

## Contribution

It extends the Israel-Stewart formalism to describe accelerated expansion with a constant, phantom-like EoS parameter, crossing the phantom divide without exotic fluids.

## Key findings

- Achieves a phantom solution with a big rip singularity.
- Allows crossing the phantom divide in a viscous fluid model.
- Effective EoS parameter remains below -1 and constant.

## Abstract

In this paper we present a phantom solution with a big rip singularity in a non-linear regime of the Israel-Stewart formalism. In this framework it is possible to extend this causal formalism in order to describe accelerated expansion, where assumption of near equilibrium is no longer valid. We assume a flat universe filled with a single viscous fluid ruled by a barotropic EoS, $p=\omega \rho$, which can represent a late time accelerated phase of the cosmic evolution. The solution allows to cross the phantom divide without evoking an exotic matter fluid and the effective EoS parameter is always lesser than $-1$ and constant in time.