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

Dark energy, D-branes, and Pulsar Timing Arrays