Cosmic Acceleration and the Helicity-0 Graviton

TL;DR

This paper investigates a scalar-tensor model derived from a non-linear covariant extension of massive gravity, demonstrating self-acceleration, stability of fluctuations, and potential to screen the cosmological constant, but with phenomenological limitations.

Contribution

It introduces a unique scalar-tensor theory from massive gravity that achieves self-acceleration and vacuum energy screening while maintaining stability and avoiding ghosts.

Findings

The model admits a self-accelerated solution with the Hubble parameter set by graviton mass.

Fluctuations of the helicity-0 mode decouple from sources, ensuring stability.

The theory can screen large cosmological constants but faces constraints from fifth force experiments.

Abstract

We explore cosmology in the decoupling limit of a non-linear covariant extension of Fierz-Pauli massive gravity obtained recently in arXiv:1007.0443. In this limit the theory is a scalar-tensor model of a unique form defined by symmetries. We find that it admits a self-accelerated solution, with the Hubble parameter set by the graviton mass. The negative pressure causing the acceleration is due to a condensate of the helicity-0 component of the massive graviton, and the background evolution, in the approximation used, is indistinguishable from the \Lambda CDM model. Fluctuations about the self-accelerated background are stable for a certain range of parameters involved. Most surprisingly, the fluctuation of the helicity-0 field above its background decouples from an arbitrary source in the linearized theory. We also show how massive gravity can remarkably screen an arbitrarily large cosmological constant in the decoupling limit, while evading issues with ghosts. The obtained static solution is stable against small perturbations, suggesting that the degravitation of the vacuum energy is possible in the full theory. Interestingly, however, this mechanism postpones the Vainshtein effect to shorter distance scales. Hence, fifth force measurements severely constrain the value of the cosmological constant that can be neutralized, making this scheme phenomenologically not viable for solving the old cosmological constant problem. We briefly speculate on a possible way out of this issue.