Softly Massive Gravity

TL;DR

This paper explores a brane-induced gravity model that introduces a soft graviton mass, addressing theoretical issues in large-distance modifications of gravity and their implications for the cosmological constant problem.

Contribution

It demonstrates that a consistent 4D unitarity can be maintained in higher-dimensional brane-induced gravity models, despite challenges with analyticity and causality.

Findings

4D unitarity is preserved in the model.

4D analyticity may be violated at large scales.

The model avoids strong-coupling problems at intermediate scales.

Abstract

Large-distance modification of gravity may be the mechanism for solving the cosmological constant problem. A simple model of the large-distance modification -- four-dimensional (4D) gravity with the hard mass term-- is problematic from the theoretical standpoint. Here we discuss a different model, the brane-induced gravity, that effectively introduces a soft graviton mass. We study the issues of unitarity, analyticity and causality in this model in more than five dimensions. We show that a consistent prescription for the poles of the Green's function can be specified so that 4D unitarity is preserved. However, in certain instances 4D analyticity cannot be maintained when theory becomes higher dimensional. As a result, one has to sacrifice 4D causality at distances of the order of the present-day Hubble scale. This is a welcome feature for solving the cosmological constant problem, as was recently argued in the literature. We also show that, unlike the 4D massive gravity, the model has no strong-coupling problem at intermediate scales.