Journal of Cosmology and Astroparticle Physics

Preferred axion models are minimal realizations of the Peccei-Quinn solution to the strong CP problem while providing a dark matter candidate. These models invoke new heavy quarks that interact strongly with the Standard Model bringing them into thermal equilibrium in the early Universe. We show that for a number of these models, the heavy quarks will decay after axions have decoupled from the Standard Model thermal bath. As a consequence, any axion products in the decay form a component of dark radiation. This provides the potential to differentiate between preferred axion models through measurements of the number of relativistic degrees of freedom. The most sensitive of which comes from the Planck collaboration's measurements of the Cosmic Microwave Background. We find that existing constraints allow us to rule out regions of parameter space for 40% of the canonical preferred axion models.

Motivated by the hint for time-dependent dynamical dark energy from an analysis of the DESI Baryon Accoustic Oscillation (BAO) data together with information from the Cosmic Microwave Background (CMB) and Supernovae (SN), we relax the assumption of a vanishing initial velocity for a quintessence field. In particular we focus on pseudo-Nambu-Goldstone-Boson (PNGB) quintessence in the form of an axion like particle, that can arise as the phase of a complex scalar and could possess derivative couplings to fermions or topological couplings to abelian gauge fields, without upsetting the necessary flatness of its potential. We discuss mechanisms from the aforementioned interactions for sourcing an initial axion field velocity _i at redshifts 3 ≤ z ≤ 10, that will "kick" it into motion. Driven by this initial velocity the axion will first roll up in its potential, similar to "freezing" dark energy. After it has reached the pinnacle of its trajectory, it will start to roll down, and behave as "thawing" quintessence. As a proof of concept we undertake a combined fit to BAO, SN and CMB data at the background level. We find that a scenario with _i = (1) m_a, where m_a is the axion mass, is slightly preferred over both ΛCDM and the conventional "thawing" quintessence with _i = 0. The best fit points for this case exhibit transplanckian decay constants and very flat potentials, which both are in tension with conjectures from string theory.

Local primordial non-Gaussianity (LPNG) couples long-wavelength cosmological fluctuations to the short-wavelength behavior of galaxies. This coupling is encoded in bias parameters including b_ϕ and b_δϕ at linear and quadratic order in the large-scale biasing framework. We perform the first field-level measurement of b_ϕ and b_δϕ using Lagrangian bias and non-linear displacements from N-body simulations. We compare our field level measurements with universality predictions and separate universe results, finding qualitative consistency, but disagreement in detail. We also quantify the information on f_NL available in the field given various assumptions on knowledge of b_ϕ at fixed initial conditions. We find that it is not possible to precisely constrain f_NL when marginalizing over b_ϕf_NL even at the field level, observing a 2-3X degradation in constraints between a linear and quadratic biasing model on perturbative field-level mocks, suggesting that a b_ϕ prior is necessary to meaningfully constrain f_NL at the field level even in this idealized scenario. For simulated dark matter halos, the pure f_NL constraints from both linear and quadratic field-level models appear biased when marginalizing over bias parameters including b_ϕ and b_δϕ due largely to the f_NLb_ϕ degeneracy. Our results are an important consistency test of the large-scale bias framework for LPNG and highlight the importance of physically motivated priors on LPNG bias parameters for future surveys.

Stochastic inflation, together with the ΔN formalism, provides a powerful tool for estimating the large-scale behaviour of primordial fluctuations. In this work, we develop a numerical code to capture the non-perturbative statistics of these fluctuations and validate it to obtain the exponential non-Gaussian tail of the curvature perturbations. We present a numerical algorithm to compute the non-perturbative curvature power spectrum and apply it to both slow-roll (SR) and ultra-slow-roll (USR) single-field models of inflation. We accurately generate a non-perturbative scale-invariant power spectrum in the SR scenario. In the USR case, we obtain a peak in the power spectrum that, in the time-independent regime, aligns with the structure of its perturbative counterpart. Additionally, We underscore how the evolving nature of the super-Hubble perturbations in the USR model complicates the numerical computation of the non-perturbative spectrum.

In the context of non-standard cosmologies, an early matter-dominated (EMD) era can significantly alter the conventional dark matter (DM) genesis. In this work, we reexamine the impact of an EMD on the weakly- and feebly-interacting massive particle (WIMP and FIMP) paradigms. EMD eras significantly modify the genesis of DM because of the change in the Hubble expansion rate and the injection of entropy. The WIMP paradigm can be realized with couplings much smaller than in the standard cosmological scenario, whereas much larger couplings are required in the FIMP case. Using the singlet-scalar DM model as a case study, we show that these results can lead to a continuous transition between the WIMP and FIMP scenarios, with results that are also applicable to other DM models. This broadens the parameter space consistent with observed DM levels and suggests that even elusive FIMP scenarios may be within the reach of future experimental searches.

Motivated by the hint for time-dependent dynamical dark energy from an analysis of the DESI Baryon Accoustic Oscillation (BAO) data together with information from the Cosmic Microwave Background (CMB) and Supernovae (SN), we relax the assumption of a vanishing initial velocity for a quintessence field. In particular we focus on pseudo-Nambu-Goldstone-Boson (PNGB) quintessence in the form of an axion like particle, that can arise as the phase of a complex scalar and could possess derivative couplings to fermions or topological couplings to abelian gauge fields, without upsetting the necessary flatness of its potential. We discuss mechanisms from the aforementioned interactions for sourcing an initial axion field velocity _i at redshifts 3 ≤ z ≤ 10, that will "kick" it into motion. Driven by this initial velocity the axion will first roll up in its potential, similar to "freezing" dark energy. After it has reached the pinnacle of its trajectory, it will start to roll down, and behave as "thawing" quintessence. As a proof of concept we undertake a combined fit to BAO, SN and CMB data at the background level. We find that a scenario with _i = (1) m_a, where m_a is the axion mass, is slightly preferred over both ΛCDM and the conventional "thawing" quintessence with _i = 0. The best fit points for this case exhibit transplanckian decay constants and very flat potentials, which both are in tension with conjectures from string theory.

In the context of non-standard cosmologies, an early matter-dominated (EMD) era can significantly alter the conventional dark matter (DM) genesis. In this work, we reexamine the impact of an EMD on the weakly- and feebly-interacting massive particle (WIMP and FIMP) paradigms. EMD eras significantly modify the genesis of DM because of the change in the Hubble expansion rate and the injection of entropy. The WIMP paradigm can be realized with couplings much smaller than in the standard cosmological scenario, whereas much larger couplings are required in the FIMP case. Using the singlet-scalar DM model as a case study, we show that these results can lead to a continuous transition between the WIMP and FIMP scenarios, with results that are also applicable to other DM models. This broadens the parameter space consistent with observed DM levels and suggests that even elusive FIMP scenarios may be within the reach of future experimental searches.

We summarize and explain the current status of time variations of the electron mass in cosmology, showing that such variations allow for significant easing of the Hubble tension, from the current ∼ 5σ significance, down to between 3.4σ and 1.0σ significance, depending on the precise model and data. Electron mass variations are preferred by Cosmic Microwave Background (CMB) data in combination with the latest results on baryonic acoustic oscillations (BAO) and type Ia supernovae at a level of significance between 2σ and 3.6σ depending on the model and the data. This preference for a model involving an electron mass variation is neither tightly constrained from light element abundances generated during big bang nucleosynthesis nor from post-recombination observations using quasars and atomic clocks, though future data is expected to give strong evidence in favor of or against this model.

Using the effective field theory of quantum gravity at second order in curvature, we calculate quantum corrections to the metric of gravastars and the closely related dark energy stars. We find that the quantum corrections in the exterior region depend on the equation of state of the gravastar, thus providing an example of quantum gravitational hair. We continue by calculating the induced quantum corrections to the photon sphere and the bending of light rays in the weak field regime. These corrections, albeit Planck scale suppressed, allow in principle to distinguish these objects from black holes observationally.

Inflationary models equipped with Chern-Simons coupling between their axion and gauge sectors exhibit an array of interesting signals including a testable chiral gravitational wave spectrum. The energy injection in the gauge sector triggered by the rolling axion leads to a well-studied enhancement of gauge field fluctuations. These may in turn affect observables such as the scalar and tensor spectra and also account for non-linear corrections to field propagators. In this work, we focus on non-Abelian gauge sectors. We show that gauge field self-interactions and axion-gauge field non-linear couplings significantly renormalize the gauge field mode function. Operating within the regime of validity of the perturbative treatment places strong constraints on the accessible parameter space of this class of models. We calculate corrections to the gauge field propagator that are universally present in these scenarios. Enforcing perturbativity on such propagators leads to bounds that are competitive with those stemming from analytical estimates on the onset of the strong backreaction regime.

In a mean-field approximation within Kinetic Field Theory, it is possible to derive an accurate analytic expression for the power spectrum of present-day non-linear cosmic density fluctuations. It depends on the theory of gravity and the cosmological model via the expansion function of the background space-time, the growth factor derived from it, and the gravitational coupling strength, which may deviate from Newton's constant in a manner depending on time and spatial scale. In earlier work [1], we introduced a functional Taylor expansion around general relativity and the cosmological standard model to derive the effects of a wide class of modified-gravity theories on the non-linear power spectrum, assuming that such effects need to be small given the general success of the standard model. Here, we extend this class towards theories with small-scale screening, modeling screening effects by a suitably flexible interpolating function. We compare the Taylor expansion with full mean-field solutions and find good agreement where expected. We find typical relative enhancements of the non-linear power spectrum between a few and a few ten per cent in a broad range of wave numbers between k ≳ 0.1-10 h Mpc, in good qualitative agreement with results of numerical simulations. Taking nDGP gravity as a quantitative example we compare our results to N-body simulations and find percent-level agreement for wavenumbers k ≲ 2 h Mpc^-1, if the scale where screening sets in, k^*, is adapted appropriately. This extends the application of our analytic approach to non-linear cosmic structure formation to essentially all classes of modified-gravity theories.

V-mode polarization of the cosmic microwave background is expected to be vanishingly small in the ΛCDM model and, hence, usually ignored. Nonetheless, several astrophysical effects, as well as beyond standard model physics could produce it at a detectable level. A realistic half-wave plate — an optical element commonly used in CMB experiments to modulate the polarized signal — can provide sensitivity to V modes without significantly spoiling that to linear polarization. We assess this sensitivity for some new-generation CMB experiments, such as the LiteBIRD satellite, the ground-based Simons Observatory and a CMB-S4-like experiment. We forecast the efficiency of these experiments to constrain the phenomenology of certain classes of BSM models inducing mixing of linear polarization states and generation of V modes in the CMB. We find that new-generation experiments can improve current limits by 1-to-3 orders of magnitude, depending on the data combination. The inclusion of V-mode information dramatically boosts the sensitivity to these BSM models.

This paper calculates the stochastic gravitational wave background from dark binaries with finite-range attractive dark forces, complementing previous works which consider long-range dark forces. The finiteness of the dark force range can dramatically modify both the initial distributions and evolution histories of the binaries. The generated gravitational wave spectrum is enhanced in the intermediate frequency regime and exhibits interesting "knee" and "ankle" features, the most common of which is related to the turn on of the dark force mediator radiation. Other such spectral features are related to changes in the binary merger lifetime and the probability distribution for the initial binary separation. The stochastic gravitational wave background from sub-solar-mass dark binaries is detectable by both space- and ground-based gravitational wave observatories.

While pulsar timing array experiments have recently found evidence for the existence of a stochastic gravitational wave (GW) background, its origin is still unclear. If this background is of astrophysical origin, we expect the distribution of GW sources to follow the one of galaxies. Since galaxies are not perfectly isotropically distributed at large scales, but follow the cosmological large-scale structure, this would lead to an intrinsic anisotropy in the distribution of GW sources. In this work, we develop a formalism to account for this anisotropy, by considering a Gaussian ensemble of sources in each realization of the universe and then taking ensemble averages over all such realizations. We find that large-scale galaxy clustering has no impact on the expectation value of pulsar timing residual correlations, described by the Hellings-Downs curve. However, it introduces a new contribution to the variance of the Hellings-Downs correlation. Hence, the anisotropic distribution of sources contributes to the amount by which the measurements of pulsar timing residual correlations, in our single realization of the universe, may differ from the Hellings-Downs curve.

We investigate loosely bound composite states made of dark matter, where the binding energy for constituent particles is considerably less than the constituent mass. We focus on models of nuclear and molecular dark matter, where constituents are separated by length scales larger than the inverse constituent mass, just like nuclei and atoms in the Standard Model. The cosmology, structure, and interactions at underground experiments are described. We find that loosely bound composites can have a very large cross section for scattering with nuclei that scales with nucleon number like ∼ A⁴. For some couplings, these composites produce extremely soft (≪ keV) individual atomic recoils while depositing a large amount of total recoil energy (≫ keV) in a single passage through a detector, implying an interesting new class of signatures for low threshold direct detection.