Constraints on small-field axion inflation

Abstract

We study general class of small-field axion inflations which are the mixture of polynomial and sinusoidal functions suggested by the natural and axion monodromy inflations. The axion decay constants leading to the successful axion inflations are severely constrained in order not to spoil the Big-Bang nucleosynthesis and overproduce the isocurvature perturbation originating from the QCD axion. We in turn find that the cosmologically favorable axion decay constants are typically of order the grand unification scale or the string scale which is consistent with the prediction of closed string

Polyinstanton axion inflation

“…Its value is negative and rather large, α s ∼ −10 −3 . A similar value has been obtained in other axion inflation models [9,14]. This behavior might be a generic feature in a certain class of the axion inflation models.…”

“…The running is negative and rather large. A similar value has been obtained in other axion inflation models [9,14]. Finally, let us estimate the reheating temperature.…”

“…The simple natural inflation may lead to disfavored predictions on observables, but modulation terms would significantly change predictions on the observables [7][8][9]. Correction terms in monodromy inflation are also important and they change significantly predictions on the observables [9,10].…”

“…For example, we can analyze explicitly the reheating processes after the axion inflation. The axion couplings to the gauge bosons, quarks, and leptons may be rather weak, and the reheating temperature may be rather low [9,13,15]. The axions would also appear in phases of couplings among the quarks, leptons, and the Higgs fields.…”

Extranatural inflation redux

“…It turns out that the effect of adding such extra fermions in the bulk is the addition of more sinusoidal functions to the potential of natural inflation. Though scenarios with such potentials have been studied for a long time in the context of both natural inflation [23][24][25][26][27][28][29][30] and extranatural inflation [31][32][33][34][35], the possibility of obtaining CMB predictions similar to those of Starobinsky model has not been explored (see however [36,37]). This paper is organized as follows: in §II, we begin by explaining the issue of UV sensitivity of inflation and then review the relevant details of extranatu-ral inflation.…”