Hot axions and the H₀ tension

Abstract

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.

Late time transitions in the quintessence field and the H0 tension

“…We find that there is hardly any effect on the discrepancy between the different measurements of the Hubble constant. Our results suggest, in line with earlier work on the subject [14][15][16][17][18][19], that the resolution to the problem needs to be found in the early Universe, possibly through a modified sound horizon at recombination.…”

“…The pressure and the energy density in the quintessence field are respectively the numerator and the denominator in Eq. (15). The sum of all continuity equations can be rephrased to express the conservation law for the total energy density ρ ≡…”

Constraining the Self-Interacting Neutrino Interpretation of the Hubble Tension

“…The standard cosmological model, ΛCDM, may need to be augmented if this "H 0 tension" is not resolved by observational systematics. Intriguingly, this tension cannot be addressed by modifying ΛCDM at low redshift [6][7][8][9], but adding new physics before recombination seems more promising [10][11][12][13][14][15][16]. Furthermore, low redshift measurements of the matter density fluctuation amplitude on 8 Mpc scales, σ 8 , also appear to be lower than predicted by ΛCDM from the CMB.…”

“…Furthermore, low redshift measurements of the matter density fluctuation amplitude on 8 Mpc scales, σ 8 , also appear to be lower than predicted by ΛCDM from the CMB. This milder σ 8 tension is not ameliorated in the models of [11][12][13][14][15][16].…”

Fractional Dark Matter decay: cosmological imprints and observational constraints

“…Many attempts have been made so far to remedy the tensions on H 0 and σ 8 by taking account of some new physics beyond the ΛCDM model, for instance, a holographic dark energy plus sterile neutrino model [12], non-zero coupling in the dark sector components [13], a minimally coupled and slowly-or-moderately rolling quintessence field [14], multiple Dark Energy (DE) models [15] and the early dark energy [16]. Alternatively, one can take into account modifications of the standard cold DM scenario, including a cannibal dark matter [17], partially acoustic dark matter models [18], dissipative dark matter models [19], hot axions [20], charged DM with chiral photons [21] and decaying DM scenarios [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36], etc., which provide other possible solutions to resolve the tensions. Moreover, DM self-interactions [37] and DM-DE interacting models [38][39][40][41][42][43][44][45][46][47][48][49] as well as the modified gravity [50,51] are also possible solutions to fix such the tensions.…”