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This study examines how axion inflation, a theoretical model of the early universe's rapid expansion, can generate detectable high-frequency gravitational waves through interactions between the inflaton field and gauge fields. The researchers focus on the weak backreaction regime and include analysis of the reheating phase after inflation ends, finding that the oscillatory behavior of the inflaton during reheating excites both helical modes of the gauge field. Their calculations show that axion inflation can produce some of the strongest predicted primordial gravitational wave signals at high frequencies, which could be detected through measurements of the effective number of neutrino species and future high-frequency gravitational wave detectors.
Why it matters
This work identifies a potentially detectable signature of cosmic inflation that could be measured with upcoming precision cosmological observations and motivates development of new high-frequency gravitational wave detection technologies. If confirmed, these signals would provide direct evidence about the physics of the early universe and help validate or constrain inflationary models.
Understand the Science
arXiv:2607.23152v1 Announce Type: cross
Abstract: Axion inflation, characterized by a Chern-Simons interaction between the inflaton and a gauge field, provides a powerful mechanism for generating primordial gravitational waves (GWs) through tachyonic enhancement of the gauge field. While recent literature has predominantly focused on the Strong Backreaction (SB) regime to maximize GW signals for future interferometers, this regime suffers from computational complexities as well as the risk of overproducing scalar perturbations. In this work, we investigate gauge field amplification and GW production strictly within the theoretically safer Weak Backreaction (WB) regime, with a particular focus on the largely unexplored non-instantaneous reheating phase. Because the tachyonic enhancement during slow-roll typically increases as inflation approaches its end, it is crucial to investigate how the production of GWs behaves at the very end of inflation and thereafter. By continuously tracking the evolution from slow-roll through reheating to radiation domination, we present a complete picture of inflationary and post-inflationary GW production in this framework. A particularly interesting feature of the post-inflationary phase is that the oscillatory behavior of the inflaton during reheating leads to frequent sign-flips of the instability parameter $xi$, exciting both helical modes of the gauge field. Our analysis reveals that axion inflation can naturally generate one of the strongest known primordial GW signals at high-frequency bands. The yield is relevant for future precision measurements of the effective number of neutrino species, $N_{{rm eff}}$, and also strongly motivates the development of novel high-frequency GW detectors.
Source: High-frequency gravitational waves from axion inflation in the weak-backreaction regime