Superluminality in DGP

TL;DR

This paper demonstrates explicit stable five-dimensional solutions in the DGP model that support superluminal excitations, analyzing their causality and stability, and proposes using superluminality constraints to refine the model's parameters.

Contribution

It provides explicit stable solutions in the full DGP theory showing superluminal propagation without relying on scaling limits, and introduces a method to constrain model parameters based on causality.

Findings

Explicit stable 5D solutions with superluminal excitations

Retarded Green's function analysis confirms causality and stability

Superluminal propagation occurs with a cosmological constant and scale hierarchy

Abstract

We reconsider the issue of superluminal propagation in the DGP model of infrared modified gravity. Superluminality was argued to exist in certain otherwise physical backgrounds by using a particular, physically relevant scaling limit of the theory. In this paper, we exhibit explicit five-dimensional solutions of the full theory that are stable against small fluctuations and that indeed support superluminal excitations. The scaling limit is neither needed nor invoked in deriving the solutions or in the analysis of its small fluctuations. To be certain that the superluminality found here is physical, we analyze the retarded Green's function of the scalar excitations, finding that it is causal and stable, but has support on a widened light-cone. We propose to use absence of superluminal propagation as a method to constrain the parameters of the DGP model. As a first application of the method, we find that whenever the 4D energy density is a pure cosmological constant and a hierarchy of scales exists between the 4D and 5D Planck masses, superluminal propagation unavoidably occurs.