The $\Lambda$CDM model provides a good fit to a large span of cosmological data but harbors areas of phenomenology. With the improvement of the number and the accuracy of observations, discrepancies among key cosmological parameters of the model have emerged. The most statistically significant tension is the $4-6\sigma$ disagreement between predictions of the Hubble constant $H_0$ by early time probes with $\Lambda$CDM model, and a number of late time, model-independent determinations of $H_0$ from local measurements of distances and redshifts. The high precision and consistency of the data at both ends present strong challenges to the possible solution space and demand a hypothesis with enough rigor to explain multiple observations--whether these invoke new physics, unexpected large-scale structures or multiple, unrelated errors. We present a thorough review of the problem, including a discussion of recent Hubble constant estimates and a summary of the proposed theoretical solutions. Some of the models presented are formally successful, improving the fit to the data in light of their additional degrees of freedom, restoring agreement within $1-2\sigma$ between {\it Planck} 2018, using CMB power spectra data, BAO, Pantheon SN data, and R20, the latest SH0ES Team measurement of the Hubble constant ($H_0 = 73.2 \pm 1.3{\rm\,km\,s^{-1}\,Mpc^{-1}}$ at 68\% confidence level). Reduced tension might not simply come from a change in $H_0$ but also from an increase in its uncertainty due to degeneracy with additional physics, pointing to the need for additional probes. While no specific proposal makes a strong case for being highly likely or far better than all others, solutions involving early or dynamical dark energy, neutrino interactions, interacting cosmologies, primordial magnetic fields, and modified gravity provide the best options until a better alternative comes along.[Abridged]

https://iopscience.iop.org/article/10.1088/1361-6382/ac086d/pdf

In the realm of the Hubble tension - a review of solutions

We show how several properties of the QCD axion can be extracted at high precision using only first principle QCD computations. By combining NLO results obtained in chiral perturbation theory with recent Lattice QCD results the full axion potential, its mass and the coupling to photons can be reconstructed with percent precision. Axion couplings to nucleons can also be derived reliably, with uncertainties smaller than ten percent. The approach presented here allows the precision to be further improved as uncertainties on the light quark masses and the effective theory couplings are reduced. We also compute the finite temperature dependence of the axion potential and its mass up to the crossover region. For higher temperature we point out the unreliability of the conventional instanton approach and study its impact on the computation of the axion relic abundance.

/pdf/the-qcd-axion-precisely-1656yn1app.pdf

The QCD axion, precisely

We propose a new strategy to search for dark matter axions in the mass range of $40--400\text{ }\ensuremath{\mu}\mathrm{eV}$ by introducing dielectric haloscopes, which consist of dielectric disks placed in a magnetic field. The changing dielectric media cause discontinuities in the axion-induced electric field, leading to the generation of propagating electromagnetic waves to satisfy the continuity requirements at the interfaces. Large-area disks with adjustable distances boost the microwave signal (10--100 GHz) to an observable level and allow one to scan over a broad axion mass range. A sensitivity to QCD axion models is conceivable with 80 disks of $1\text{ }\text{ }{\mathrm{m}}^{2}$ area contained in a 10 T field.

/pdf/dielectric-haloscopes-a-new-way-to-detect-axion-dark-matter-31yj7ckwju.pdf

Dielectric Haloscopes: A New Way to Detect Axion Dark Matter

Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

Snowmass2021 - Letter of interest cosmology intertwined II: The hubble constant tension

The authors study the cold dark matter axion production in the decay of string-domain wall systems for a scenario where the Peccei-Quinn symmetry remains broken after inflation. Using the most advanced simulations to date, severe constraints for the model parameters (axion mass, decay constant, etc.) are determined, which potentially can be probed in the future experiments.

Axion dark matter from topological defects

We consider decaying dark matter (DDM) as a resolution to the possible tension between cosmic microwave background (CMB) and weak lensing (WL) based determinations of the amplitude of matter fluctuations, σ8. We perform N-body simulations in a model where dark matter decays into dark radiation and develop an accurate fitting formula for the non-linear matter power spectrum, which enables us to test the DDM model by the combined measurements of CMB, WL and the baryon acoustic oscillation (BAO). We employ a Markov chain Monte Carlo analysis to examine the overlap of posterior distributions of the cosmological parameters, comparing CMB alone with WL+BAO. We find an overlap that is significantly larger in the DDM model than in the standard CDM model. This may be hinting at DDM, although current data is not constraining enough to unambiguously favour a non-zero dark matter decay rate Γ. From the combined CMB+WL data, we obtain a lower bound Γ−1≥ 97 Gyr at 95 % C.L, which is less tight than the constraint from CMB alone.

Decaying dark matter and the tension in σ8

We show that the ${H}_{0}$ tension can be resolved by making recombination occur earlier, keeping the fit to cosmic microwave background (CMB) data almost intact. We provide a suite of general necessary conditions to give a good fit to CMB data while realizing a high value of ${H}_{0}$ suggested by local measurements. As a concrete example for a successful scenario with early recombination, we demonstrate that a model with a time-varying ${m}_{e}$ can indeed satisfy all of the conditions. We further show that such a model can also be well fitted to low-$z$ distance measurements of baryon acoustic oscillations (BAO) and type Ia supernovae (SNeIa) with a simple extension of the model. A time-varying ${m}_{e}$ in the framework of ${\mathrm{\ensuremath{\Omega}}}_{k}\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ is found to be a sufficient and excellent example of a solution to the ${H}_{0}$ tension, yielding ${H}_{0}={72.3}_{\ensuremath{-}2.8}^{+2.7}\text{ }\mathrm{km}/\mathrm{sec}/\mathrm{Mpc}$ from the combination of CMB, BAO, and SNeIa data even without incorporating any direct local ${H}_{0}$ measurements. Employing the Bayesian posterior predictive distribution, we find that this model can reduce the ${H}_{0}$ tension in the reference $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model from $4.8\ensuremath{\sigma}$ down to $2.2\ensuremath{\sigma}$. Apart from the ${H}_{0}$ tension, this model is also favored from the viewpoint of the CMB lensing anomaly.

Early recombination as a solution to the H 0 tension

We discuss mixed inflaton and spectator field models where both the fields are responsible for the observed density fluctuations. We combine the angular power spectrum of the CMB temperature anisotropy from the Planck 2013 result and other ground-based CMB observations in order to constrain both the general mixed model as well as some specific representative scenarios. Based on the Markov Chain Monte Carlo method, in addition to constraints on model parameters, we obtain the predictive posterior distributions of the CMB spectral μ distortion for those models. We demonstrate that the standard single-field inflaton model typically predicts μ ~ 10−8 with a relatively narrow distribution, whereas for the mixed models, the distribution turns out to be much broader, and μ could be larger by almost an order of magnitude. Hence future experiments of μ distortion could provide a tool for the critical testing of the mixed source models of the primordial perturbation.

Mixed inflaton and spectator field models: CMB constraints and μ distortion

Although cosmic microwave background (CMB) is a critical component of cosmological probes of neutrino masses, it has trouble with local direct measurements of ${H}_{0}$, and this is called the ${H}_{0}$ tension. Since neutrino masses are correlated with ${H}_{0}$ in CMB, one can expect the cosmological bound on neutrino masses would be much affected by the ${H}_{0}$ tension. We investigate what impact this tension brings to the cosmological bound on neutrino masses by assuming a model with early recombination in the framework allowing a nonflat Universe which has been shown to resolve the tension. We argue that constraints on neutrino masses become significantly weaker in models where the ${H}_{0}$ tension can be resolved.

Toyokazu Sekiguchi

Papers

Axion dark matter from topological defects

Decaying dark matter and the tension in σ8

Early recombination as a solution to the H 0 tension

Mixed inflaton and spectator field models: CMB constraints and μ distortion

Cosmological bound on neutrino masses in the light of H 0 tension