Large Non-Gaussianity in Axion Inflation

Abstract

The inflationary paradigm has enjoyed phenomenological success, however, a compelling particle physics realization is still lacking. The key obstruction is that the requirement of a suitably flat scalar potential is sensitive to Ultra-Violet (UV) physics. Axions are among the best-motivated inflaton candidates, since the flatness of their potential is naturally protected by a shift symmetry. We re-consider the cosmological perturbations in axion inflation, consistently accounting for the coupling to gauge fields φFF , which is generically present in these models. This coupling leads to production of gauge quanta, which provide a new source of inflaton fluctuations, δφ. For an axion decay constant < ∼ 10 −2 Mp, this effect typically dominates over the standard fluctuations from the vacuum, and saturates the current observational bounds on nongaussianity of the CMB anisotropies. Since sub-Planckian values of the decay constant are typical for concrete realizations that admit a UV completion (such as N-flation and axion monodromy), we conclude that large nongaussianity is easily obtained in very minimal and natural realizations of inflation.

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.…”

Production of high stellar-mass primordial black holes in trapped inflation

“…Instead of pursuing this computationally heavy task, we estimate the tensor perturbation by using the results based on analytic photon mode functions found in refs. [40][41][42], where the present energy density per logarithmic k interval of the gravitational waves relative to the critical density is given by…”

“…This may solve the long standing problem in the slow-roll inflation that a flat potential is required. Moreover, the curvature perturbation induced by the particle production has distinctive non-Gaussianities [40,41,47]. In the case of a quadratic inflation, the measurements of CMB power spectra and bispectrum has constrained the inflaton-photon coupling α < 32 [40,41,47].…”

“…Moreover, the curvature perturbation induced by the particle production has distinctive non-Gaussianities [40,41,47]. In the case of a quadratic inflation, the measurements of CMB power spectra and bispectrum has constrained the inflaton-photon coupling α < 32 [40,41,47]. Furthermore, using α < 23, the formation of PBHs with mass M BH < 10 8 g due to the photon production at the end of inflation has been discussed [14][15][16][17][18].…”

Generation of large-scale magnetic fields, non-Gaussianity, and primordial gravitational waves in inflationary cosmology

“…In Refs. [32,33], it has been indicated that a coupling of the pseudoscalar inflaton field to the electromagnetic fields can generate non-Gaussianity [34,35] of power spectrum of the curvature perturbations coming from the quantum fluctuations of the inflaton field.…”