Fading Gravity and Self-Inflation

TL;DR

This paper explores a modified gravity theory where gravity weakens at short distances, leading to novel self-inflationary solutions with distinctive scalar and tensor perturbation spectra, differing from standard inflation models.

Contribution

It introduces a ghost-free, perturbatively local modified gravity model that naturally produces self-inflation without scalar fields, and analyzes its unique perturbation spectra.

Findings

Self-inflationary solutions arise from vacuum equations without scalar fields.

Scalar spectrum is slightly red, similar to standard inflation.

Tensor spectrum can be blue-tilted, unlike typical inflation models.

Abstract

We study the cosmology of a toy modified theory of gravity in which gravity shuts off at short distances, as in the fat graviton scenario of Sundrum. In the weak-field limit, the theory is perturbatively local, ghost-free and unitary, although likely suffers from non-perturbative instabilities. We derive novel self-inflationary solutions from the vacuum equations of the theory, without invoking scalar fields or other forms of stress energy. The modified perturbation equation expressed in terms of the Newtonian potential closely resembles its counterpart for inflaton fluctuations. The resulting scalar spectrum is therefore slightly red, akin to the simplest scalar-driven inflationary models. A key difference, however, is that the gravitational wave spectrum is generically not scale invariant. In particular the tensor spectrum can have a blue tilt, a distinguishing feature from standard inflation.