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Pulsar timing arrays gathered evidence of the presence of a gravitationalwave background around nHz frequencies. If the gravitational wave backgroundwas induced by large and Gaussian primordial fluctuations, they would thenproduce too many sub-solar mass primordial black holes. We show that if at thetime of gravitational wave generation the universe was dominated by a canonicalscalar field, with the same equation of state as standard radiation but ahigher propagation speed of fluctuations, one can explain the gravitationalwave background with a primordial black hole counterpart consistent withobservations. Lastly, we discuss possible ways to test this model with futuregravitational wave detectors.

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