The Hubble constant H0 is one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. The distance ladder relying on supernovae yields values of H0 higher than those inferred from the inverse distance ladder, which is instead based on early-time physics and relies on observations typically involving the cosmic microwave background, in combination with galaxy surveys. Such discrepancy has come to be known as the 'Hubble tension'. In this Focus Meeting Adam Riess and Licia Verde will illustrate the different methods of measurement of H0, and analyze possible resolutions of the Hubble tension.
The Hubble tension has motivated the exploration of extensions to the standard cosmological model in which higher values of H0 can be obtained from CMB measurements and galaxy surveys. The trouble, however, goes beyond H0; such modifications affect other quantities too, such as cosmic times and the matter density. Any Hubble trouble has implications well beyond H0 itself. Licia Verde will analyze the tension in both a model-dependent and a model-independent way, and will propose a new representation of parameter constraints that, hopefully, will help us find a resolution.
A streamlined distance ladder constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provides < 2% precision and offer the means to do much better. By steadily improving the precision and accuracy of the Hubble constant, we now see evidence for significant deviations from the standard model, referred to as LambdaCDM, and thus the exciting chance, if true, of discovering new fundamental physics such as exotic dark energy, a new relativistic particle, or a small curvature to name a few possibilities. Adam Riess will review recent and expected progress, most recently based on measurements from Gaia EDR3 released in December, 2020.