Ghost Cosmology: Exact Solutions, Transitions Between Standard Cosmologies and Ghost Dark Energy/Matter Evolution

TL;DR

This paper derives exact solutions in ghost cosmology, showing smooth transitions from early radiation-dominated phases to late-time ghost dark matter or energy dominance, with implications for the universe's evolution and stability.

Contribution

It provides the first exact cosmological solutions for ghost scalar fields, detailing transitions between different cosmological epochs and analyzing stability features.

Findings

Ghost cosmologies enable smooth transitions between radiation and dark energy/matter phases.

The universe's current state is near the non-linear transition phase.

Jeans instability occurs in ghost dark energy but not in ghost dark matter scenarios.

Abstract

The recently proposed infrared modification of gravity through the introduction of a ghost scalar field results in a number of interesting cosmological and phenomenological implications. In this paper, we derive the exact cosmological solutions for a number of scenarios where at late stages, the ghost behaves like dark matter, or dark energy. The full solutions give valuable information about the non-linear regime beyond the asymptotic first order analysis presented in the literature. The generic feature is that these ghost cosmologies give rise to smooth transitions between radiation dominated phases (or more general power-law expansions) at early epochs and ghost dark matter resp. ghost dark energy dominated late epochs. The current age of our universe places us right at the non-linear transition phase. By studying the evolution backwards in time, we find that the dominance of the ghost over ordinary baryonic matter and radiative contributions persists back to the earliest times such that the Friedmann-Robertson-Walker geometry is dictated to a good approximation by the ghost alone. We also find that the Jeans instability occurs in the ghost dark energy scenario at late times, while it is absent in the ghost dark matter scenario.