Scattering and decay processes of thermal bath particles involving heavy leptons can dump hot axions in the primordial plasma around the QCD phase transition. We compute their relic density, parameterized by an effective number ∆N eff of additional neutrinos. For couplings allowed by current bounds, production via scattering yields ∆N eff 0.6 and ∆N eff 0.2 for the cases of muon and tau, respectively. Flavor violating tau decays to a lighter lepton plus an axion give ∆N eff 0.3. Such values of ∆N eff can alleviate the tension between the direct local measurement of the Hubble constant H 0 and the inferred value from observations of the Cosmic Microwave Background, assuming ΛCDM. We analyze present cosmological data from the Planck collaboration and baryon acoustic oscillations with priors given in terms of the axion-lepton couplings. For axions coupled to muons, the tension can be alleviated below the 3σ level. Future experiments will measure ∆N eff with higher precision, providing an axion discovery channel and probing the role of hot axions in the H 0 tension.

Abstract:We consider the presence of an axion like particle, σ, with a generic CP violating axial coupling of the form (α σ/f )FF , where F µν is the gauge field strength of a generic abelian U(1) gauge group, not necessarily associated with the standard electromagnetism, and f is the decay constant of the axion. It has previously been demonstrated that if the axion is identified with the inflaton, such an interaction can lead to measurable cosmological signatures (non-Gaussian modifications of the curvature perturbation spectrum) depending on the parameter ξ = ασ/(f H). In the present paper we will show that the generation of curvature perturbation at horizon crossing due to the axial coupling has a universal form and remains unmodified in terms of the ξ parameter even if the axion, σ, is not identified with the inflaton. As a consequence, it does not appear to be possible to generate CMB tensor perturbations through this mechanism, larger than the vacuum ones, without violating the observational constraints unless we combine this mechanism with a curvaton or if the σ field becomes heavy and decays during inflation. Even in this last case there are non-trivial constraints coming from the slow-roll evolution of the curvature perturbation on super horizon scales which should be taken into account. We also comment on implications for inflationary models where axions play an important role as, for example, models of natural inflation where more than one axion are included and models where the curvaton is an axion.

We study the dynamics of a homogeneous and isotropic Friedmann-Robertson-Walker universe in the context of the Eddington-inspired Born-Infeld theory of gravity. We generalize earlier results, obtained in the context of a radiation dominated universe, to account for the evolution of a universe permeated by a perfect fluid with an arbitrary equation of state parameter w. We show that a bounce may occur for κ > 0, if w is time-dependent, and we demonstrate that it is free from tensor singularities. We argue that Eddington-inspired Born-Infeld cosmologies may be a viable alternative to the inflationary paradigm as a solution to fundamental problems of the standard cosmological model.

Inflation can be supported in very steep potentials if it is generated by rapidly turning fields, which can be natural in negatively curved field spaces. The curvature perturbation, ζ, of these models undergoes an exponential, transient amplification around the time of horizon crossing, but can still be compatible with observations at the level of the power spectrum. However, a recent analysis (based on a proposed single-field effective theory with an imaginary speed of sound) found that the trispectrum and other higher-order, non-Gaussian correlators also undergo similar exponential enhancements. This arguably leads to 'hyper-large' non-Gaussianities in stark conflict with observations, and even to the loss of perturbative control of the calculations. In this paper, we provide the first analytic solution of the growth of the perturbations in two-field rapid-turn models, and find it in good agreement with previous numerical and single-field EFT estimates. We also show that the nested structure of commutators of the in-in formalism has subtle and crucial consequences: accounting for these commutators, we show analytically that the naively leading-order piece (which indeed is exponentially large) cancels exactly in all relevant correlators. The remaining non-Gaussianities of these models are modest, and there is no problem with perturbative control from the exponential enhancement of ζ. Thus, rapidturn inflation with negatively curved field spaces remains a viable and interesting class of candidate theories of the early universe.

We show that when the QCD axion is directly coupled to quarks with c_{i}/f∂_{μ}aq[over ¯]_{i}γ^{μ}γ^{5}q_{i}, such as in Dine-Fischler-Srednicki-Zhitnitsky models, the dominant production mechanism in the early Universe at temperatures 1  GeV≲T≲100  GeV is obtained via q_{i}q[over ¯]_{i}↔ga and q_{i}g↔q_{i}a, where g are gluons. The production of axions through such processes is maximal around T≈m_{i}, where m_{i} are the different heavy quark masses. This leads to a relic axion background that decouples at such temperatures, leaving a contribution to the effective number of relativistic degrees of freedom, which can be larger than the case of decoupling happens the electroweak phase transition, ΔN_{eff}≲0.027. Our prediction for the t quark is 0.027≤ΔN_{eff}≤0.036 for 10^{6}  GeV≲f/c_{t}≲4×10^{8}  GeV and for the b quark is 0.027≤ΔN_{eff}≤0.047 for 10^{7}  GeV≲f/c_{b}≲3×10^{8}  GeV. For the c quark the window can only be roughly estimated as 0.027<ΔN_{eff}≲O(0.1), for f/c_{c}≲(2-3)×10^{8}  GeV, since axions can still be partially produced in a regime of strong coupling, when α_{s}≳1. These contributions are comparable to the sensitivity of future CMB S4 experiments, thus opening an alternative window to detect the axion and to test the early Universe at such temperatures.

We analyze the dynamics of inflationary models with a coupling of the inflaton φ to gauge fields of the form φFF /f , as in the case of axions. It is known that this leads to an instability, with exponential amplification of gauge fields, controlled by the parameter ξ =φ/(2f H), which can strongly affect the generation of cosmological perturbations and even the background. We show that scattering rates involving gauge fields can become larger than the expansion rate H, due to the very large occupation numbers, and create a thermal bath of particles of temperature T during inflation. In the thermal regime, energy is transferred to smaller scales, radically modifying the predictions of this scenario.We thus argue that previous constraints on ξ are alleviated. If the gauge fields have Standard Model interactions, which naturally provides reheating, they thermalize already at ξ 2.9, before perturbativity constraints and also before backreaction takes place. In absence of SM interactions (i.e. for a dark photon), we find that gauge fields and inflaton perturbations thermalize if ξ 3.4; however, observations require ξ 6, which is above the perturbativity and backreaction bounds and so a dedicated study is required. After thermalization, though, the system should evolve non-trivially due to the competition between the instability and the gauge field thermal mass. If the thermal mass and the instabilities equilibrate, we expect an equilibrium temperature of Teq ξH/ḡ whereḡ is the effective gauge coupling. Finally, we estimate the spectrum of perturbations if φ is thermal and find that the tensor to scalar ratio is suppressed by H/(2T ), if tensors do not thermalize.Such a possibility has also been considered by [1,2] under the name of warm inflation, by invoking a dissipation term due to a coupling to a thermal bath of particles 1 . An obvious difficulty is the need of an exponential production of radiation in order to overcome the exponential dilution without spoiling the slow-roll stage. To achieve this goal we propose a thermalized axion inflation model in which we simply couple gauge-fields A µ to an axion-like field φ (which we will think of as the inflaton), through an axial coupling. The phenomenology of such a coupling during inflation has been frequently studied in the literature (see [5][6][7][8][9][10][11] for an incomplete list of references).The interest about this coupling lies in the instability it triggers on the gauge fields equation of motion in presence of a constant field velocityφ, leading to strong particle production, that starts at wavelengths of O((ξH) −1 ), slightly smaller the horizon size. This instability is present already at linear order iṅ φ; the deep reason behind this fact is that the Lagrangian couples φ to a CP-odd (and thus T-odd)term. Another interesting feature of such a coupling is that the gauge field production can become so large that it backreacts on the background and dynamically generates slow-roll even in absence of a flat potential [5,11]. This happens when the parameter th...

Natural inflation is an attractive model for primordial inflation, since the potential for the inflaton is of the pseudo Nambu-Goldstone form, V (φ) = Λ 4 [1 + cos(φ/f )], and so is protected against radiative corrections. Successful inflation can be achieved if f > ∼ few MP and Λ ∼ mGUT where Λ can be seen as the strong coupling scale of a given non-abelian gauge group. However, the latest observational constraints put natural inflation in some tension with data. We show here that a non-minimal coupling to gravity γ 2 (φ)R, that respects the symmetry φ → φ + 2πf and has a simple form, proportional to the potential, can improve the agreement with cosmological data. Moreover, in certain cases, satisfactory agreement with the Planck 2018 TT, TE, EE and low P data can be achieved even for a periodicity scale of approximately Mp.

Soft gravitons produced by the expansion of de Sitter can be viewed as the NambuGoldstone bosons of spontaneously broken asymptotic symmetries of the de Sitter spacetime. We explicitly construct the associated charges, and show that acting with the charges on the vacuum creates a new state equivalent to a change in the local coordinates induced by the soft graviton. While the effect remains unobservable within the domain of a single observer where the symmetry is unbroken, this change is physical when comparing different asymptotic observers, or between a transformed and un-transformed initial state, consistent with the scale-dependent statistical anisotropies previously derived using semiclassical relations. We then compute the overlap, 0|0 , between the unperturbed de Sitter vacuum |0 , and the state |0 obtained by acting N times with the charge. We show that when N → M 2 p /H 2 this overlap receives order one corrections and 0|0 → 0, which corresponds to an infrared perturbative breakdown after a time t dS ∼ M 2 p /H 3 has elapsed, consistent with earlier arguments in the literature arguing for a perturbative breakdown on this timescale. We also discuss the generalization to inflation, and rederive the 3-point and one-loop consistency relations.