Dark energy, D-branes and pulsar timing arrays

Abstract

Several pulsar timing array (PTA) collaborations recently announced the first detection of a stochasticgravitational wave (GW) background, leaving open the question of its source.We explore the possibility that it originates from cosmic inflation, a guaranteedsource 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 explainsthe 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.