Abstract
An axion rotating in field space can produce dark photons in the early universe via tachyonic instability. This explosive particle production creates a background of stochastic gravitational waves that may be visible at pulsar timing arrays or other gravitational wave detectors. This scenario provides a novel history for dark photon dark matter. The dark photons may be warm at a level detectable in future 21-cm line surveys. For a consistent cosmology, the radial direction of the complex field containing the axion must be thermalized. We explore a concrete thermalization mechanism in detail and also demonstrate how this setup can be responsible for the generation of the observed baryon asymmetry.
Highlights
May instead be initially rotating in field space, with important consequences
We explore the parameter space motivated by the potential gravitational wave (GW) signal reported by NANOGrav [20] and for a smaller signal strength within the reach of Square Kilometer Array (SKA) [29]
Such a rotation is expected when the radial direction of the global symmetry breaking field takes on a large initial field value and the global symmetry is explicitly broken by higher dimensional operators
Summary
We consider a field theoretical axion, a pseudo-Nambu-Goldstone boson associated with the spontaneous breaking of a global U(1)P symmetry. As conventionally assumed, the axion field has zero initial velocity, it begins oscillations when its mass is comparable to the Hubble expansion rate, mφ ∼ H. Where M is a cut-off scale, is enhanced at large field values Such terms are expected when the U(1)P symmetry accidentally arises as a result of another exact symmetry [34,35,36,37]. This explicit breaking can initiate an angular motion for P. We discuss the consequences of axion rotation when the axion couples to a dark gauge field
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