Pulsar timing array constraints on spin-2 ULDM

Abstract: Ultra-light Dark Matter (ULDM) models are suitable candidates for the cosmological Dark Matter that may leave characteristic imprints in many observables. Among other probes, signatures of ULDM can be searched for in pulsar timing data. In this work we describe the effects of spin-2 ULDM on pulsar timing arrays, extending previous results on lower spins. Spin-2 ULDM is universally coupled to standard matter with dimensionless strength $\alpha$. We estimate that current data could constrain this coupling in the mass range $m\lesssim4\times10^{-22}$ eV at the $10^{-5}$ to $10^{-6}$ level, which is the most competitive constraint in this mass range. A crucial feature of the spin-2 ULDM effect on pulsar timing is its anisotropic, quadrupolar shape. This feature can be instrumental in differentiating the effects sourced by spin-2 ULDM from, for instance, scalar ULDM, and the systematics of a PTA experiment.