Abstract
Abstract
A coherently oscillating ultra-light axion can behave as dark matter. In particular, its
coherently oscillating pressure perturbations can source an oscillating scalar metric
perturbation, with a characteristic oscillation frequency which is twice the axion Compton
frequency. A candidate in the mass range 10(-24,-21) eV can provide a signal in the
frequency range tested by current and future Pulsar Timing Array (PTA) programs. Involving the
pressure perturbations in a highly nonlinear environment, such an analysis demands a relativistic
and nonlinear treatment. Here, we provide a rigorous derivation of the effect assuming
weak-gravity and slow-motion limit of Einstein's gravity in zero-shear gauge and show that dark
matter's velocity potential determines the oscillation phase and frequency change. A monochromatic
PTA signal correlated with the velocity field would confirm the prediction, for example, by
cross-correlating the PTA results with the future local velocity flow measurements.
Subject
Astronomy and Astrophysics