Dark energy, D-branes, and Pulsar Timing Arrays

TL;DR

This paper proposes a string theory-inspired cosmological model where early universe dynamics enhance primordial gravitational waves, potentially explaining recent PTA detections and linking dark energy phenomena with gravitational wave observations.

Contribution

It introduces a novel inflationary scenario driven by Dirac-Born-Infeld scalar dynamics that explains PTA GW signals and addresses the H0 tension through early and late dark energy phases.

Findings

GW energy density matches PTA observations

Model links dark energy with primordial GW background

Provides a testable connection between string theory and cosmology

Abstract

Several pulsar timing array (PTA) collaborations recently announced the first detection of a stochastic gravitational wave (GW) background, leaving open the question of its source. We explore the possibility that it originates from cosmic inflation, a guaranteed source of primordial GW. The inflationary GW background amplitude is enhanced at PTA scales by a non-standard early cosmological evolution, driven by Dirac-Born-Infeld (DBI) scalar dynamics motivated by string theory. The resulting GW energy density has a broken power-law frequency profile, entering the PTA band with a peak amplitude consistent with the recent GW detection. After this initial DBI kination epoch, the dynamics starts a new phase mainly controlled by the scalar potential. It provides a realization of an early dark energy scenario aimed at relaxing the $H_0$ tension, and a late dark energy model which explains the current cosmological acceleration with no need of a cosmological constant. Hence our mechanism - besides providing a possible explanation for the recent PTA results - connects them with testable properties of the physics of the dark universe.