Gauge-preheating and the end of axion inflation

Abstract: We study the onset of the reheating epoch at the end of axion-driven inflation where the axion is coupled to an Abelian, $U(1)$, gauge field via a Chern-Simons interaction term. We focus primarily on $m^2\phi^2$ inflation and explore the possibility that preheating can occur for a range of coupling values consistent with recent observations and bounds on the overproduction of primordial black holes. We find that for a wide range of parameters preheating is efficient. In certain cases the inflaton is seen to transfer all its energy to the gauge fields within a few oscillations. In most cases, we find that the gauge fields on sub-horizon scales end preheating in an unpolarized state due to the existence of strong rescattering between the inflaton and gauge-field modes. We also present a preliminary study of an axion monodromy model coupled to $U(1)$ gauge fields, seeing a similarly efficient preheating behavior as well as indications that the coupling strength has an effect on the creation of oscillons.

• The paper demonstrates that tachyonic and parametric resonances efficiently transfer energy, sometimes completing preheating in a single oscillation.
• It shows that strong gauge field couplings yield nearly instantaneous preheating while remaining consistent with primordial black hole constraints.
• The study finds that rescattering between inflaton and gauge fields eliminates initial polarization biases, resulting in an unpolarized sub-horizon state.

Preheating Dynamics in Axion Inflation Coupled to Gauge Fields

The paper "Gauge-preheating and the end of axion inflation" by Adshead et al. provides a comprehensive study of preheating dynamics at the end of axion-driven inflation, focusing particularly on interactions between axions and gauge fields via a Chern-Simons term. The authors explore the efficiency and implications of preheating when axions couple to a $U(1)$ gauge field, offering a numerical exploration alongside analytic insights.

Key Findings

1. Reheating Mechanism: The research investigates how preheating occurs when axions are coupled to gauge fields. It emphasizes both tachyonic and parametric resonance as mechanisms contributing to preheating. The paper identifies that, depending on the coupling strength, tachyonic resonance can lead to rapid energy transfer from the inflaton to the gauge fields, often completing reheating within a single oscillation.
2. Coupling Dependence: The work examines a range of coupling values, constrained by limits on primordial black hole production. For larger couplings, preheating is found to be efficient and nearly instantaneous due to strong tachyonic resonance, whereas smaller couplings rely more on parametric resonance.
3. Back Reaction and Polarization: The study highlights that strong rescattering between the inflaton and gauge-field modes results in an unpolarized final state of gauge fields at sub-horizon scales, despite initial polarization asymmetries induced during inflation. This rescattering efficiently generates the second polarization, nullifying initial biases.
4. Potential Variants: Two potential forms for the axion—$m^2\phi^2$ and axion monodromy—were explored. The research notes that while both exhibit efficient preheating behavior, axion monodromy inflation shows slightly reduced reheating efficiency due to slower initial inflaton velocities.
5. Oscillon Formation: In the monodromy scenario, the emergence of oscillons is noted, indicating a potential matter-dominated phase immediately post-inflation. However, the dominance of the gauge field induced radiation keeps the universe in a radiation-dominated state post-preheating.

Implications and Future Directions

The paper implies substantial theoretical implications for reheating following axion inflation. It suggests that efficient energy transfer via gauge fields is plausible under constrained couplings without causing undesirable black hole overproduction. From a practical perspective, the model offers a viable pathway from inflation to the hot big bang phase, complemented by perturbative decay processes at lower couplings.

For future research directions, this study indicates the need to explore the effects of these preheating dynamics on observable cosmological phenomena, such as non-Gaussianity in the cosmic microwave background or potential gravitational wave signatures. Further inquiry into the role of oscillons in early universe dynamics, particularly in axion models with gauge fields, could also be fruitful.

Conclusion

Adshead et al.'s investigation into axion inflation with gauge field couplings stands as a critical contribution to understanding reheating mechanisms in cosmology. It provides detailed insight into preheating processes, helping to bridge the gap in knowledge between the inflationary epoch and the standard cosmological model, paving the way for further theoretical and observational studies in the field.