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Comments: 17 pages, 13 figures, invited contribution submitted to Galaxies, Special Issue "The Role of Halo Substructure in Gamma-Ray Dark Matter Searches"

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Abstract:

Local measurements of the Hubble parameter from type 1a supernovae disagree at almost 4sigma with the value inferred from a ΛCDM fit to the cosmic microwave background (CMB), with local measurements suggesting a higher value. This discrepancy is not easily explained by any obvious systematic effect in either measurement, and so increasing attention is given to the possibility that this “Hubble tension” may be indicating new physics beyond ΛCDM. In this talk, I would like to review promising solutions to this tension. I will first argue that this discrepancy should be interpreted as a tension between our understanding of the early and late universe cosmology, rather than a tension between two datasets. Secondly, I will show that this observation might indicate that our Universe has undergone anomalous expansion due to the presence of an early dark energy (EDE) at redshift z ~ 5000. I will give two examples of a physical model that lead to the desired dynamics. Moreover, I will explain why this solution is favored over modifications of the dark energy dynamics (the so-called “phantom” dark energy) as well as favored over the presence of an additional relativistic degree of freedom.

Abstract:

Local measurements of the Hubble parameter from type 1a supernovae disagree at almost 4sigma with the value inferred from a ΛCDM fit to the cosmic microwave background (CMB), with local measurements suggesting a higher value. This discrepancy is not easily explained by any obvious systematic effect in either measurement, and so increasing attention is given to the possibility that this “Hubble tension” may be indicating new physics beyond ΛCDM. In this talk, I would like to review promising solutions to this tension. I will first argue that this discrepancy should be interpreted as a tension between our understanding of the early and late universe cosmology, rather than a tension between two datasets. Secondly, I will show that this observation might indicate that our Universe has undergone anomalous expansion due to the presence of an early dark energy (EDE) at redshift z ~ 5000. I will give two examples of a physical model that lead to the desired dynamics. Moreover, I will explain why this solution is favored over modifications of the dark energy dynamics (the so-called “phantom” dark energy) as well as favored over the presence of an additional relativistic degree of freedom.