Axion-like fields coupled to the Nieh-Yan term generate a chiral GW background during radiation domination, with parameter space explored for detectability in PTA and space-based observatories.
Gauss-Bonnet Chern-Simons gravitational wave leptogenesis
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abstract
The gravitational Chern-Simons term coupled to an evolving axion is known to generate lepton number through the gravitational anomaly. We examine this leptogenesis scenario in the presence of the Gauss-Bonnet term over and above the gravitational Chern-Simons term. We find that the lepton production can be exponentially enhanced. The Gauss-Bonnet term creates CP-violating instability of gravitational waves that may appear transiently after inflation, and during the period of instability elliptically polarized gravitational waves are exponentially amplified at sub-horizon scales. This instability does not affect the spectrum of the cosmic microwave background as it occurs at much shorter length scales. In a typical scenario based on natural inflation, the observed baryon asymmetry of the Universe corresponds to the UV cutoff scale at $10^{14-16}$ GeV.
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Fuzzy dark matter induces frequency-dependent amplitude birefringence in gravitational waves with periodic time modulation set by the scalar mass, but no velocity birefringence.
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Chiral Gravitational Wave Background from Audible Axion via Nieh-Yan Term
Axion-like fields coupled to the Nieh-Yan term generate a chiral GW background during radiation domination, with parameter space explored for detectability in PTA and space-based observatories.
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Gravitational Wave Birefringence from Fuzzy Dark Matter
Fuzzy dark matter induces frequency-dependent amplitude birefringence in gravitational waves with periodic time modulation set by the scalar mass, but no velocity birefringence.
- Constraining Quintessential Inflation with ACT: A Gauss-Bonnet Gateway