Reheating and dark radiation after fibre inflation

Abstract

We study perturbative reheating at the end of fibre inflation where the inflaton is a closed string modulus with a Starobinsky-like potential. We first derive the spectral index n s and the tensor-to-scalar ratio r as a function of the number of efoldings and the parameter R which controls slow-roll breaking corrections. We then compute the inflaton couplings and decay rates into ultra-light bulk axions and visible sector fields on D7-branes wrapping the inflaton divisor. This leads to a reheating temperature of order 10 10 GeV which requires 52 efoldings. Ultra-light axions contribute to dark radiation even if ∆N eff is almost negligible in the generic case where the visible sector D7-stack supports a non-zero gauge flux. If the parameter R is chosen to be small enough, n s 0.965 is then in perfect agreement with current observations while r turns out to be of order r 0.007. If instead the flux on the inflaton divisor is turned off, ∆N eff 0.6 which, when used as a prior for Planck data, requires n s 0.99. After R is fixed to obtain such a value of n s , primordial gravity waves are larger since r 0.01.

Moduli stabilisation and the statistics of axion physics in the landscape

“…where h(F ) ≥ 0 is a non-negative function of the intersection numbers and the gauge flux F on the SM D7-brane stack, this implies that any variation of γ (by varying g s ) should be compensated by a suitable change of h(F ) by considering a different choice of F (if this is allowed by the discreteness of the gauge flux quanta and by tadpole cancellation). Notice that h(F ) vanishes for F = 0, implying from (3.35) γ = 1 and ∆N eff fixed at ∆N eff 0.6 [54]. However h(F ) > 0 for F = 0, and so in this case ∆N eff features a distribution in the flux landscape due to its dependence on g s .…”

“…The inflaton τ 1 is the lightest Kähler modulus and its perturbative decay after the end of inflation produces SM particles together with the QCD axion θ 1 and the ultra-light ALP θ 2 which are both relativistic and yield a g s -dependent contribution to the effective number of relativistic species N eff [54]. One can thus exploit the known distribution of g s to derive the distribution of extra dark radiation in the flux landscape of Fibre Inflation models.…”

“…One can thus exploit the known distribution of g s to derive the distribution of extra dark radiation in the flux landscape of Fibre Inflation models. The amount of extra dark radiation is parameterised by ∆N eff which is determined by the ratio of the inflaton branching ratio into hidden and visible degrees of freedom [54]:…”

“…Using the moduli masses we can study the distribution of the reheating temperature coming from moduli decay [54,62]. The reheating temperature due to the perturbative decay of the i-th Kähler modulus is given by [54]:…”

Geometrical destabilisation of ultra-light axions in string inflation

“…This expression cannot be inverted exactly but we can still express the inflaton at leading order as [22]:φ…”