Speaker
Description
A rolling axion inflaton can amplify one polarization of a gauge field through its pseudoscalar coupling. The amplified fields source scalar fluctuations and gravitational waves, and their backreaction can substantially change the inflaton’s motion. In this talk, I will present our numerical study of this coupled evolution and discuss when the homogeneous-backreaction approximation remains reliable. We assess this by comparing the inflaton gradient energy with the kinetic energy of the homogeneous field.
We find examples in which oscillations in the inflaton velocity produce potentially detectable peaks in the gravitational-wave spectrum while gradients remain subdominant. In this regime, enhanced scalar fluctuations can also form primordial black holes (PBHs) that make up all dark matter in the asteroid-mass window. Because the statistics of the rare fluctuations responsible for PBH formation remain uncertain, we consider both Gaussian and chi-squared distributions. For both benchmarks, the gravitational-wave background contains contributions sourced directly by the gauge fields and induced by scalar fluctuations, with a total signal within LISA’s projected sensitivity. For the same PBH abundance, the Gaussian case requires larger scalar fluctuations and produces a stronger gravitational-wave signal.