Unimodular-Mimetic Cosmology

TL;DR

This paper introduces a unified unimodular-mimetic gravity framework to address the cosmological constant and dark matter problems, demonstrating its ability to reproduce various cosmological evolutions without matter sources.

Contribution

It formulates a novel combined gravity theory and explores its capacity to realize known and exotic cosmologies, including viable models consistent with observational data.

Findings

Vacuum unimodular mimetic gravity can describe de Sitter and perfect fluid cosmologies without matter.

The framework can produce cosmologies compatible with current observations.

Potential mechanisms for graceful exit from inflation are discussed.

Abstract

We combine the unimodular gravity and mimetic gravity theories into a unified theoretical framework, which is proposed to provide a suggestive proposal for a framework that may assist in the discussion and solution search of the cosmological constant problem and of the dark matter issue. After providing the formulation of the unimodular mimetic gravity and investigating all the new features that the vacuum unimodular gravity implies, by using the underlying reconstruction method, we realize some well known cosmological evolutions, with some of these being exotic for the ordinary Einstein-Hilbert gravity. Specifically we provide the vacuum unimodular mimetic gravity description of the de Sitter cosmology and of the perfect fluid with constant equation of state cosmology. As we demonstrate, these cosmologies can be realized by vacuum mimetic unimodular gravity, without the existence of any matter fluid source. Moreover, we investigate how cosmologically viable cosmologies, which are compatible with the recent observational data, can be realized by the vacuum unimodular mimetic gravity. Since in some cases, the graceful exit from inflation problem might exist, we provide a qualitative description of the mechanism that can potentially generate the graceful exit from inflation in these theories, by searching for the unstable de Sitter solutions in the context of unimodular mimetic theories of gravity.