Interpreting Pulsar Timing Array data of Gravitational Waves with Ekpyrosis-Bouncing Cosmology

Abstract: Recent pulsar timing array (PTA) experiments have reported strong evidence of the stochastic gravitational wave background (SGWB). If interpreted as primordial Gravitational Waves (pGWs), the signal favors a strongly blue-tilted spectrum. On the other hand, the Ekpyrosis-bouncing cosmology with a strongly blue-tilted GW spectrum, i.e., $n_T \simeq 2$, offers a potential explanation for the observed SGWB signal. In this paper, we construct a concrete Ekpyrosis-bouncing model, and show its capacity to intepret the PTA result without pathologies. Both tensor and scalar perturbations are analysed with constraints from the current observations.