Light axions can potentially leave a cosmic background, just like neutrinos. We complete the study of thermal axion production across the electroweak scale by providing a smooth and continuous treatment through the two phases. Focusing on both flavor conserving and violating couplings to third generation quarks, we compute the amount of axions produced via scatterings and decays of thermal bath particles. We perform a model independent analysis in terms of axion effective couplings, and we also make predictions for specific microscopic QCD axion scenarios. This observable effect, parameterized as it is conventional by an effective number of additional neutrinos, is above the 1 sigma sensitivity of future CMB-S4 surveys. Moreover, if one assumes no large hierarchies among dimensionless axion couplings to standard model particles, future axion helioscopes will provide a complementary probe for the parameter region we study.
The recent electron recoil excess observed by XENON1T has a possible interpretation in terms of solar axions coupled to electrons. If such axions are still relativistic at recombination they would also leave a cosmic imprint in the form of an additional radiation component, parameterized by an effective neutrino number Δ Neff. We explore minimal scenarios with a detectable signal in future CMB surveys: axions coupled democratically to all fermions, axion-electron coupling generated radiatively, the DFSZ framework for the QCD axion. The predicted Δ Neff is larger than 0.03-0.04 for all cases, close to the 2σ forecasted sensitivity of CMB-S4 experiments. This opens the possibility of testing with cosmological observations the solar axion interpretation of the XENON1T excess.
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, zeta, 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 zeta. Thus, rapid-turn inflation with negatively curved field spaces remains a viable and interesting class of candidate theories of the early universe.
We consider a quintessence field which transitions from a matter-like to a cosmological constant behaviour between recombination and the present time. We aim at easing the tension in the measurement of the present Hubble rate, and we assess the ΛCDM model properly enlarged to include our quintessence field against cosmological observations. The model does not address the scope we proposed. This result allows us to exclude a class of quintessential models as a solution to the tension in the Hubble constant measurements.
Models of inflation where the entropic directions have large and negative masses ms H can be well described by a single-field EFT with an imaginary sound speed cs. Among other features, they predict an exponential enhancement of the spectrum of scalar perturbations which however is not inherited by non-Gaussianities. In this work, I complete the calculation of the trispectrum in this EFT by considering the contributions from the contact interaction and the exchange diagram. While for most shapes the trispectrum is approximately constant, I find that for certain configurations where all the momenta collapse to a line the trispectrum is proportional to (ms/H)5 for the contact interaction and to (ms/H)6 for the exchange diagram, as anticipated by previous work. I also discuss the UV sensitivity of the results and argue why the EFT provides a good order of magnitude estimate. In the end, I confront the different predictions for the scalar spectrum against observations. In models where the entropic mass is proportional to a positive power of the slow-roll parameter, like in hyperinflation, the spectrum grows on small scales and becomes constrained by the overproduction of primordial black holes. Imposing such constraint jointly with the correct amplitude and spectral tilt at CMB scales excludes a large set of potentials. Only those where the spectral tilt is controlled by ms2/H ~ (-0.01), where 2=/( H) is the second slow-roll parameter, are likely observationally viable. Finally, the constraints on the bispectrum generically impose |cs ms|/H 10-02 while those on the trispectrum give a weaker bound when using the constraints on gNL4 as a proxy. For hyperinflation the bispectrum bound translates into ?? 11 where ?? is the turning rate in field-space.
We discuss modifications to the Hawking spectrum that arise when the asymptotic states are supertranslated or superrotated. For supertranslations we find nontrivial off-diagonal phases in the two-point correlator although the emission spectrum is eventually left unchanged, as previously pointed out in the literature. In contrast, superrotations give rise to modifications which manifest themselves in the emission spectrum and depend nontrivially on the associated conformal factor at future null infinity. We study Lorentz boosts and a class of superrotations whose conformal factors do not depend on the azimuthal angle on the celestial sphere and whose singularities at the north and south poles have been associated to the presence of a cosmic string. In spite of such singularities, superrotations still lead to finite spectral emission rates of particles and energy which display a distinctive power-law behavior at high frequencies for each angular momentum state. The integrated particle emission rate and emitted power, on the contrary, while finite for boosts, do exhibit ultraviolet divergences for superrotations, between logarithmic and quadratic. Such divergences can be ascribed to modes with support along the cosmic string. In the logarithimic case, corresponding to a superrotation which covers the sphere twice, the total power emitted still presents the Stefan-Boltzmann form but with an effective area which diverges logarithmically in the ultraviolet.
We perform for the first time a dedicated analysis of cosmological constraints on Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) QCD axion models. Such constructions are especially interesting in light of the recent XENON rrexcess and of hints from stellar cooling. In DFSZ models, for m(a) greater than or similar to 0.1 eV, scatterings of pions and muons can produce a sizable cosmic background of thermal axions, that behave similarly to massive neutrinos. However, the pion coupling depends on the alignment between the vacuum expectation value (vevs) of two Higgs doublets, and can be significantly suppressed or enhanced with respect to the Kim-Shifman-Vainshtein-Zakharov scenario (KSVZ). Using the latest Planck and BAO data, we find m(a) <= 0.2 eV at 95% C.L., when the axion coupling to pions c(a pi) is maximal. Constraints on m(a), instead, can be significantly relaxed when c(a pi) is small. In particular, we point out that in the so-called DFSZ-II model, where the axion coupling to leptons does not vanish simultaneously with c(a pi), production via muons gives m(a) < 0.6 eV at 95% C.L., whereas in the DFSZ-I model bounds on m a can be fully lifted. We then combine cosmological data with recent hints of a DFSZ axion coupled to electrons from the XENON1T experiment, finding in this case that the axion mass is constrained to be in the window 0.07 eV less than or similar to m(a) less than or similar to 1.8(0.3) eV for the DFSZ-I (DFSZ-II) model. A similar analysis with stellar cooling hints gives 3 meV less than or similar to m(a) less than or similar to 0.2 eV for DFSZ-II, while no constraint arises in the DFSZ-I case. Forthcoming cosmic microwave background stage 4 experiments will be able to further test such scenarios; for instance the XENON1T window should be fully probed at 2 sigma for a DFSZ-I axion.