The radiation instability in modified gravity

TL;DR

This paper investigates the instability of radiation-dominated universes in quadratic gravity theories, showing that solutions have fewer arbitrary functions than general solutions, with implications for polynomial extensions of general relativity.

Contribution

It demonstrates the instability of inhomogeneous radiation universes in quadratic gravity and constructs formal series solutions revealing reduced degrees of freedom.

Findings

Solutions have fewer arbitrary functions than general solutions.

Instability persists in polynomial extensions of general relativity.

Formal series expansions characterize the solutions.

Abstract

We study the problem of the instability of inhomogeneous radiation universes in quadratic lagrangian theories of gravity written as a system of evolution equations with constraints. We construct formal series expansions and show that the resulting solutions have a smaller number of arbitrary functions than that required in a general solution. These results continue to hold for more general polynomial extensions of general relativity.