Observing Invisible Axions with Gravitational Waves

Abstract

If the Peccei-Quinn symmetry associated to an axion has ever been restored after inflation, axion strings inevitably produce a contribution to the stochastic gravitational wave background. Combining effective field theory analysis with numerical simulations, we show that the resulting gravitational wave spectrum has logarithmic deviations from a scale invariant form with an amplitude that is significantly enhanced at low frequencies. As a result, a single ultralight axion-like particle with a decay constant larger than 10 14 GeV and any mass between 10 −18 eV and 10 −28 eV leads to an observable gravitational wave spectrum and is compatible with constraints on the post-inflationary scenario from dark matter overproduction, isocurvature and dark radiation. Since the spectrum extends over a wide range of frequencies, the resulting signal could be detected by multiple experiments. We describe straightforward ways in which the Peccei-Quinn symmetry can be restored after inflation for such decay constants. We also comment on the recent possible NANOgrav signal in light of our results.

The scalar, vector, and tensor modes in gravitational wave turbulence simulations

“…Recently, the NANOGrav collaboration has reported the detection of a signal, which could be compatible with a stochastic GW background [19]. If this detection is confirmed in the future, it could be a manifestation of the stochastic GW background originated from the epoch around the QCD energy scale [20][21][22][23][24][25][26]. Interestingly, the source of these GWs might be the presence of primordial magnetic fields at the QCD scale [21] .…”

Approach to scaling in axion string networks

“…The evolution of axion strings and axion radiation is governed by the classical field equations of the underlying scalar field theory, and due to their non-linearities, lattice simulations are required to go beyond order-of-magnitude estimates. In recent years, several groups have studied the evolution of axion strings and the production of axions using lattice simulations [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42].…”

Gravitational Waves from Global Cosmic Strings and Cosmic Archaeology

“…We included the contribution from these high modes in this updated study, which leads to substantial modifications to the GW spectrum at low f for standard thermal history as well as at high f with the presence of an early matter domination epoch. We also discuss the consequence for the prediction of SGWB if the non-scaling behavior found in some simulation results for early evolution sustains in the late-time evolution of a global string network, compare with the results found in [46,65], and suggest potential modifications to the VOS model to accommodate such a feature. We will dive into the time-frequency correspondence for global strings, which is the guiding principle for testing standard cosmology.…”

“…With a semianalytical approach based on the Velocity-dependent One-Scale (VOS) model, our earlier work [24] demonstrated that the GW signal from global strings, albeit notably smaller than that from its NG string counterpart, can be within reach of future GW experiments such as LISA [6,7], AEDGE [47], DECIGO and BBO [48]. Such a positive prospect of detection has been confirmed by simulation-based work [45,46], although details differ which will be addressed in this work.…”

“…In contrast, a global string network as a potential source of GWs has been largely ignored since by naive estimate GW radiation would be overwhelmed by Goldstone emission which occurs with a much larger rate. Very recently, inspired by its intimate connections to axion dark matter physics, significant progress has been made in simulating global topological defects and on the GW signals originated from it [42][43][44][45][46]. With a semianalytical approach based on the Velocity-dependent One-Scale (VOS) model, our earlier work [24] demonstrated that the GW signal from global strings, albeit notably smaller than that from its NG string counterpart, can be within reach of future GW experiments such as LISA [6,7], AEDGE [47], DECIGO and BBO [48].…”