Ghosts in the self-accelerating universe

TL;DR

This paper reviews the self-accelerating universe, focusing on the DGP model, and discusses the ghost instability that challenges its viability as an alternative to dark energy.

Contribution

It analyzes the origin and implications of the ghost in the DGP model's self-accelerating universe, highlighting stability issues.

Findings

The ghost in the DGP model carries negative energy densities.

The ghost leads to spacetime instability.

The paper clarifies the physical implications of the ghost presence.

Abstract

The self-accelerating universe realizes the accelerated expansion of the universe at late times by large-distance modification of general relativity without a cosmological constant. The Dvali-Gabadadze-Porrati (DGP) braneworld model provides an explicit example of the self-accelerating universe. Recently, the DGP model becomes very popular to study the observational consequences of the modified gravity models as an alternative to dark energy models in GR. However, it has been shown that the self-accelerating universe in the DGP model contains a ghost at the linearized level. The ghost carries negative energy densities and it leads to the instability of the spacetime. In this article, we review the origin of the ghost in the self-accelerating universe and explore the physical implication of the existence of the ghost.