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The scalar and tensor fluctuations produced during inflation can be correlated, if arising from the same underlying mechanism. In this paper we investigate such correlation in the model of axion inflation, where the rolling inflaton produces quanta of a U(1) gauge field which, in turn, source scalar and tensor fluctuations. We compute the primordial correlator of the curvature perturbation, ζ, with the amplitude of the gravitational waves squared, hijhij, at frequencies probed by gravitational wave detectors. This two-point function receives two contributions: one arising from the correlation of gravitational waves with the scalar perturbations generated by the standard mechanism of amplification of vacuum fluctuations, and the other coming from the correlation of gravitational waves with the scalar perturbations sourced by the gauge field. Our analysis shows that the latter effect is generally dominant. The correlator, normalized by the amplitude of ζ and of hijhij, turns out to be of the order of 10−2×(fequilNL)1/3, where fequilNL measures the scalar bispectrum sourced by the gauge modes. 

The coupling between a pseudo-scalar inflaton and a gauge field leads to an amount of additional density perturbations and gravitational waves (GWs) that is strongly sensitive to the inflaton speed. This naturally results in enhanced GWs at (relatively) small scales that exited the horizon well after the CMB ones, and that can be probed by a variety of GW observatories (from pulsar timing arrays, to astrometry, to space-borne and ground-based interferometers). This production occurs in a regime in which the gauge field significantly backreacts on the inflaton motion. Contrary to earlier assumptions, it was later shown that this regime is characterized by an oscillatory behavior of the inflaton speed, with a period of O ( 5 ) e-folds. Bursts of GWs are produced at the maxima of the speed, imprinting nearly periodic bumps in the frequency-dependent spectrum of GWs produced during inflation. This can potentially generate correlated peaks appearing in the same or in different GWs experiments. While recent lattice studies show that the inclusion of inflaton gradients can modify significantly the dynamics of this system in the strong backreaction regime, this is not the case for the first oscillation or two of the inflaton speed, so that we expect our results to be robust for modes that were excited during that epoch. 

Axion inflation coupled to Abelian gauge fields via a Chern-Simons-like term of the form$$ \phi F\overset{\sim }{F} $$ $\mathrm{\varphi F}\tilde{F}$represents an attractive inflationary model with a rich phenomenology, including the production of magnetic fields, black holes, gravitational waves, and the matter-antimatter asymmetry. In this work, we focus on a particular regime of axion inflation, the so-called Anber-Sorbo (AS) solution, in which the energy loss in the gauge-field production provides the dominant source of friction for the inflaton motion. We revisit the AS solution and confirm that it is unstable. Contrary to earlier numerical works that attempted to reach the AS solution starting from a regime of weak backreaction, we perform, for the first time, a numerical evolution starting directly from the regime of strong backreaction. Our analysis strongly suggests that, at least as long as one neglects spatial inhomogeneities in the inflaton field, the AS solution has no basin of attraction, not even a very small one that might have been missed in previous numerical studies. Our analysis employs an arsenal of analytical and numerical techniques, some established and some newly introduced, including (1) linear perturbation theory along the lines of ref. [1], (2) the gradient expansion formalism (GEF) developed in ref. [2], (3) a new linearized version of the GEF, and (4) the standard mode-by-mode approach in momentum space in combination with input from the GEF. All these methods yield consistent results confirming the instability of the AS solution, which renders the dynamics of axion inflation in the strong-backreaction regime even more interesting than previously believed. 

Abstract Adiabatic subtraction is a popular method of renormalization of observables in quantum field theories on a curved spacetime. When applied to the computation of the power spectra of light ( m ≪ H ) fields on de Sitter space with flat Friedmann-Lemaître-Robertson-Walker slices, the standard prescriptions of adiabatic subtraction, traceable back to [1,2], lead to results that are significantly different from the standard expectations not only in the ultraviolet ( k ≫ aH ) but also at intermediate ( m ≪ k / a ≲ H ) wavelengths. In this paper we review those results and we contrast them with the power spectra obtained using an alternative prescription for adiabatic subtraction applied to quantum field theoretical systems by Dabrowski and Dunne [3,4]. This prescription eliminates the intermediate-wavelength effects of renormalization that are found when using the standard one. 

Abstract We perform an analytical study of the stability of the background solution [1] of the model in which an inflaton, through an axionic coupling to a U(1) gauge field, causes an amplification of the gauge field modes that strongly backreact on its dynamics. To this goal, we study the evolution of the gauge field modes coupled to the inflaton zero mode, treating perturbatively the deviation of the inflaton velocity from its mean-field value. As long as the system is in the strong backreaction regime we find that the inflaton velocity performs oscillations of increasing amplitude about the value it would have in the approximation of constant velocity, confirming an instability that has been observed in numerical studies. 

Abstract We investigate the sensitivity of the Laser Interferometer Space Antenna (LISA) to the anisotropies of the Stochastic Gravitational Wave Background (SGWB). We first discussthe main astrophysical and cosmological sources of SGWB which are characterized by anisotropies in the GW energy density, and we build a Signal-to-Noise estimator to quantify the sensitivity of LISA to different multipoles. We then perform a Fisher matrix analysis of the prospects of detectability of anisotropic features with LISA for individual multipoles, focusing on a SGWB with a power-law frequency profile. We compute the noise angular spectrum taking into account the specific scan strategy of the LISA detector. We analyze the case of the kinematic dipole and quadrupole generated by Doppler boosting an isotropic SGWB. We find that β Ω GW ∼ 2 × 10 -11 is required to observe a dipolar signal with LISA. The detector response to the quadrupole has a factor ∼ 10 3 β relative to that of the dipole. The characterization of the anisotropies, both from a theoretical perspective and from a map-making point of view, allows us to extract information that can be used to understand the origin of the SGWB, and to discriminate among distinct superimposed SGWB sources.