Scalar-induced gravitational wave interpretation of PTA data: the role of scalar fluctuation propagation speed

Understand

Pulsar timing arrays gathered evidence of the presence of a gravitational wave background around nHz frequencies.

• If the gravitational wave background was induced by large and Gaussian primordial fluctuations, they would then produce too many sub-solar mass primordial black holes.
• We show that if at the time of gravitational wave generation the universe was dominated by a canonical scalar field, with the same equation of state as standard radiation but a higher propagation speed of fluctuations, one can explain the gravitational wave background with a primordial black hole counterpart consistent with observations.
• Lastly, we discuss possible ways to test this model with future gravitational wave detectors.