New physics in light of the H_{0} tension: An alternative view
Abstract
The strong discrepancy between local and earlytime (inverse distance ladder) estimates of the Hubble constant H_{0} could be pointing towards new physics beyond the concordance Λ CDM model. Several attempts to address this tension through new physics rely on extended cosmological models, featuring extra free parameters beyond the six Λ CDM parameters. However, marginalizing over additional parameters has the effect of broadening the uncertainties on the inferred parameters (including H_{0}), and it is often the case that within these models the H_{0} tension is addressed due to larger uncertainties rather than a genuine shift in the central value of H_{0}. In this paper I consider an alternative viewpoint: what happens if a physical theory is able to fix the extra parameters to a specific set of nonstandard values? In this case, the degrees of freedom of the model are reduced with respect to the standard case where the extra parameters are free to vary. Focusing on the dark energy equation of state w and the effective number of relativistic species N_{eff}, I find that physical theories able to fix w ≈1.3 or N_{eff}≈3.95 would lead to an estimate of H_{0} from cosmic microwave background, baryon acoustic oscillation, and type Ia supernovae data in perfect agreement with the local distance ladder estimate, without broadening the uncertainty on the former. These two nonstandard models are, from a modelselection perspective, strongly disfavored with respect to the baseline Λ CDM model. However, models that predict N_{eff}≈3.45 would be able to bring the tension down to 1.5 σ while only being weakly disfavored with respect to Λ CDM , whereas models that predict w ≈1.1 would be able to bring the tension down to 2 σ (at the cost of the preference for Λ CDM being definite). Finally, I estimate dimensionless multipliers relating variations in H_{0} to variations in w and N_{eff}, which can be used to swiftly repeat the analysis of this paper in light of future more precise local distance ladder estimates of H_{0}, should the tension persist. As a caveat, these results were obtained from the 2015 Planck data release, but these findings would be qualitatively largely unaffected were I to use more recent data.
 Publication:

Physical Review D
 Pub Date:
 July 2020
 DOI:
 10.1103/PhysRevD.102.023518
 arXiv:
 arXiv:1907.07569
 Bibcode:
 2020PhRvD.102b3518V
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology
 EPrint:
 28 pages, 10 figures. Added references and discussion on physical models able to fix w or Neff to nonstandard values (Section IVD). Clarified the aim of this work more clearly (I am not trying to evade or ignore additional degrees of freedom), many thanks to the referee for help! Abstract abridged. The busy reader should skip to Fig. 1, 2, 4, 5, and 10