Scalar perturbations from inflation in the presence of gauge fields

Axion-inflation models are a compelling candidate as a mechanism behind the accelerated expansion in the early universe in light of the possibility to embed them in higher dimensional UV complete theories and the exciting prospect of testing them with next-generation cosmological probes. Adding an Abelian gauge sector to axion-inflation models makes for a rich, interesting, phenomenology spanning from primordial black holes to gravitational waves (GWs). Several recent studies employ an approximate analytic (Gaussian) template to characterize the effect of gauge field production on cosmological perturbations. In this work we go beyond such approximation and numerically study particle production and the ensuing scalar and tensor spectra. We find a significant deviation from results based on log-normally distributed vector field excitations. As an important phenomenological application of the improved method, we study the expected chirality and spectral index of the sourced GW background at scales relevant for current and next-generation GW detectors. One striking feature is that of a scale-dependent chirality. We derive a consistency relation between these two observables that can serve as an important tool in identifying key signatures of multi-field dynamics in axion inflation.

Scale-dependent chirality as a smoking gun for Abelian gauge fields during inflation

Özsoy,

Papageorgiou,

Fasiello

2024

CMB constraints on natural inflation with gauge field production

Alam,

Dutta,

Jaman

2024

The natural inflation model with a periodic cosine potential is ruled out by recent Planck 2018 data for the decay constant f ≲ 5.5 M Pl. If the Planck data is combined with the BICEP Keck array and BAO data, the model is excluded (at 2-σ) for all values of f. In this context, we revisit the model when the pseudoscalar inflation ϕ is coupled with a gauge field via a coupling of the form α/fϕ FF̃, where F(F̃) denotes the gauge field (dual) strength tensor, and α is the coupling constant. The back-reactions associated with the gauge field production during the later stages of inflation extend the duration of inflation. We numerically evaluate the dynamics of the fields while neglecting the effects due to the perturbations in the inflaton field. It allows us to determine the scalar and tensor power spectra leading to the calculations of observables at the Cosmic Microwave Background (CMB) scales. We find that the natural inflation model survives the test of the latest data only for a certain range of the coupling constant α. Our analysis shows that the latest constraints coming from the scalar spectral index are more stringent than the ones arising from the non-gaussianities and the running of the scalar spectrum. This leads to lower and upper bounds on ξ *, the parameter that controls the growth of the gauge field.

Magnetogenesis from axion-SU(2) inflation

Brandenburg,

Iarygina,

Sfakianakis

et al. 2024

We describe a novel proposal for inflationary magnetogenesis by identifying the non-Abelian sector of Spectator Chromo Natural Inflation (SCNI) with the SU(2)L sector of the Standard Model. This mechanism relies on the recently discovered attractor of SCNI in the strong backreaction regime, where the gauge fields do not decay on super-horizon scales and their backreaction leads to a stable new trajectory for the rolling axion field. The large super-horizon gauge fields are partly transformed after the electroweak phase transition into electromagnetic fields. The strength and correlation length of the resulting helical magnetic fields depend on the inflationary Hubble scale and the details of the SCNI sector. For suitable parameter choices we show that the strength of the resulting magnetic fields having correlation lengths around 1 Mpc are consistent with the required intergalactic magnetic fields for explaining the spectra of high energy γ rays from distant blazars.