Gauge Field Production in Axion Inflation: Consequences for Monodromy, non-Gaussianity in the CMB, and Gravitational Waves at Interferometers (1110.3327v1)

Abstract: Models of inflation based on axions, which owe their popularity to the robustness against UV corrections, have also a very distinct class of signatures. The relevant interactions of the axion are a non-perturbative oscillating contribution to the potential and a shift-symmetric coupling to gauge fields. We review how these couplings affect the cosmological perturbations via a unified study based on the in-in formalism. We then note that, when the inflaton coupling to gauge fields is high enough to lead to interesting observational results, the backreaction of the produced gauge quanta on the inflaton dynamics becomes relevant during the final stage of inflation, and prolongs its duration by about 10 e-foldings. We extend existing results on gravity wave production in these models to account for this late inflationary phase. The strong backreaction phase results in an enhancement of the gravity wave signal at the interferometer scales. As a consequence, the signal is potentially observable at Advanced LIGO/VIRGO for the most natural duration of inflation in such models. Finally, we explicitly compute the axion couplings to gauge fields in string theory construction of axion monodromy inflation and identify cases where they can trigger interesting phenomenological effects.

• The paper demonstrates that gauge field production during axion inflation significantly backreacts on the inflaton dynamics, extending the inflationary phase.
• It quantifies non-Gaussian features in the CMB, predicting observable equilateral non-Gaussianity when the axion-gauge coupling is strong.
• The study reveals that the enhanced gravitational wave signals from axion inflation can be within the detection range of advanced interferometers.

Analysis of Gauge Field Production and Implications for Axion Inflation

The objective of the investigated paper is to explore the interaction between axion inflation models and their consequences on cosmological observables, notably focusing on gauge field production, non-Gaussianity in the Cosmic Microwave Background (CMB), and potential gravitational wave signals observable by interferometers.

The researchers begin by exploring the motivations for employing axion models in inflationary cosmology. Such models are favored for their robustness against ultraviolet (UV) corrections thanks to the axion's shift symmetry. The axionic framework allows for large-field inflationary models where the inflaton traverses a super-Planckian field range. These models are typically generalized to include a non-perturbative oscillating component to the inflaton potential alongside a shift-symmetric coupling to gauge fields.

The authors employ the in-in formalism to paper how these couplings affect cosmological perturbations, emphasizing the effect on non-Gaussian features in the CMB and evaluating the potential for observable gravitational wave signatures. This formalism is pivotal for calculating correlation functions in quantum field theories in curved spacetime, making it particularly suitable for assessing cosmological perturbations produced during inflation.

Key Findings and Numerical Predictions

1. Gauge Field Production: The paper discusses the mechanism by which gauge fields are produced during axion inflation. This is facilitated through a coupling term in the Lagrangian, typically represented as $$\mathcal{L}_{\text{int}} = -\frac{\alpha}{4 f} \varphi F_{\mu\nu}\tilde{F}^{\mu\nu}$$. When the interaction is strong enough, it results in significant backreaction affecting the inflaton dynamics, leading to extended inflation and processes that can amplify gravitational waves affecting observational signatures.
2. Non-Gaussianity in the CMB: The interaction of axions with gauge fields introduces non-Gaussian features in the CMB spectrum, with potential signatures in observable non-Gaussian statistics like the $f_{NL}^{\text{equil}}$ parameter. The paper predicts that if the production of gauge fields is sufficiently strong, it can generate large equilateral non-Gaussianity, provided the axion decay constant and coupling parameters fit within a specified range.
3. Gravitational Waves and Interferometer Scales: An extended result of this exploration is the enhanced production of gravitational waves at frequency ranges detectable by advanced interferometers such as LIGO/VIRGO. White noise from gauge field production, coupled with axionic dynamics, could produce signals strong enough to be measurable, contingent on the duration and character of the inflation epoch.
4. String Theory Realizations: Finally, the authors provide insights into how these axion inflatons might be realized in string theory, via axion monodromy and couplings to gauge fields on NS5 and D5-branes. The paper highlights that for particular string theory moduli configurations, these interactions could lead to fascinating phenomenological effects, provided certain geometric and field configuration are achieved.

Implications and Future Directions

The implications from this paper are manifold. The studied scenarios represent a bridge between high-energy theoretical models, such as those inspired by string theory, and tangible observational cosmology results. Given the potential for observable gravitational waves at scales accessible by current and upcoming experimental facilities, this line of research could substantiate or refute characteristic features expected from particular high-energy physics theories.

The authors delineate several future directions, including possible observational bounds on resonant non-Gaussianity, enhancement of the theoretical frameworks to smaller scales than currently observed, and further precision in determining the feasible string theory setups for these axionic models.

This thorough investigation provides fruitful ground for experimental cosmologists, string theorists, and physicists focused on deciphering inflation's fundamental nature.