Gauge field production in axion inflation: Consequences for monodromy, non-Gaussianity in the CMB, and gravitational waves at interferometers

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 scales of interferometers. 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.

Inflation and String Theory

“…63 Besides the model-independent gravitational wave signal, a host of additional modeldependent signatures have been explored, including oscillations in the power spectrum [36], deviations from scale-invariance [759], non-Gaussianity [37,760], chiral gravitational waves [260,759], and primordial black holes [761]. Since all of these effects are tied to the underlying axion shift symmetry, one has the hope of finding correlated signatures across different observational channels.…”

“…65 We will assume α ≤ 1. The case α 1 is discussed in [767], while estimates for α in type IIB string theory appear in [759]. Both bottom-up and top-down naturalness of the regime α 1 remain to be established.…”

Planck2015 results

“…2.4) naturally allows (from an effective field theory point of view) for a coupling between a pseudoscalar axion inflaton field and a gauge field of the type L ⊃ −(α/4 f )φF µνF µν , where the parameter α is dimensionless and f is the axion decay constant (F µν = µνγβ F γβ /2). This scenario has a rich and interesting phenomenology both for scalar and tensor primordial fluctuations (see, e.g., Sorbo 2011;Barnaby et al , 2012cLinde et al 2013;Meerburg & Pajer 2013;Ferreira & Sloth 2014). Gauge field quanta are produced by the background motion of the inflaton field, and these in turn source curvature perturbations through an inverse decay process of the gauge field.…”

Creation of matter in the universe and groups of type E7

“…In non-supersymmetric theories the coupling aFF of vectors with axion field a(x) was studied intensely in QFT and in applications to cosmology, see for example [47] and references therein. The couplings of vectors to moduli fields e φ F F were also studied in various situations in QFT and cosmology.…”