A New Perspective on DGP Gravity

TL;DR

This paper critically analyzes DGP gravity, revealing fundamental instabilities and pathologies such as singularities, unbounded energy, and problematic wormhole configurations, challenging its viability as a consistent theory.

Contribution

The paper identifies and discusses previously unnoticed pathologies in DGP gravity, including singularities and instabilities, especially when extending to Schwarzschild geometries.

Findings

Brane hits pressure singularity at finite radius in Schwarzschild bulk.

Self-accelerating branch has unbounded negative energy, indicating instability.

Quantum effects may destabilize classical self-accelerating solutions.

Abstract

We examine brane induced gravity on codimension-1 branes, a.k.a DGP gravity, as a theory of five-dimensional gravity containing a certain class four-dimensional branes. From this perspective, the model suffers from a number of pathologies which went unnoticed before. By generalizing the 5D geometry from Minkowski to Schwarzschild, we find that when the bulk mass is large enough, the brane hits a pressure singularity at finite radius. Further, on the self-accelerating branch, the five-dimensional energy is unbounded from below, implying that the self-accelerating backgrounds are unstable. Even in an empty Minkowski bulk, standard Euclidean techniques suggest that the spontaneous nucleation of self-accelerating branes is unsuppressed. If so, quantum effects will strongly modify any classical intuition about the theory. We also note that unless considered as Z_2-orbifold boundaries, self-accelerating branes correspond to `wormhole' configurations, which introduces the usual problematic issues associated with wormholes. Altogether these pathologies present a serious challenge that any proposed UV completion of the DGP model must overcome.