Calendar of Events

Speaker: Jo Dunkley, Princeton University & Wendy Freedman, University of Chicago

Host: Chair Jack Hughes

Introduction: The Hubble constant, one of the most important cosmological quantities, is the expansion rate of the universe at the current time.  Its symbol is H_0 (read as "H naught") and is given in units of km/s/Mpc (Mpc = megaparsec, a unit of distance in astronomy). For about 100 years now astronomers have measured this quantity by determining the distances and velocities of special classes of objects (variable stars, evolved stars,supernovae) in the nearby universe. This yields a direct measure of H_0, the slope of the distance vs velocity relation.  More recently improved observations of the cosmic microwave background (CMB) by the Planck
satellite has allowed H_0 to be determined indirectly from the power spectrum of the CMB in the context of a cosmological model.  The standard cosmological model, Lambda-Cold Dark Matter, very successfully explains most of the observations we have of the universe, with only six parameters.  However the value of H_0 determined this way differs from the local, direct measurements by about 5 standard deviations.  This is the so-called Hubble tension.  Today we have an expert in each of the two methodologies for determining H_0 to take us through the details and tell us about possible solutions to resolving the tension.

Here is a Scientific American article with more information:https://www.scientificamerican.com/article/hubble-tension-headache-clashing-measurements-make-the-universes-expansion-a-lingering-mystery/
 

Jo Dunkley: I will describe current estimates of the Hubble constant derived from the cosmic microwave background (CMB) and other large scale structure probes. These estimates naturally depend on the cosmological model; I will describe how changing the model could change the inferred Hubble constant. I will discuss near-term prospects for using new ground-based CMB polarization data to improve estimates, and to distinguish between alternative models.

Wendy Freedman: The measurement of extragalactic distances has improved enormously in recent decades. The discrepancy in measurements of the Hubble constant has dropped from a factor of two to less than 10%. I will present results for the local determination of the Hubble constant based on the Tip of the Red Giant Branch applied to Type Ia supernovae in the Hubble flow, discuss the difference in the calibration using Cepheids, present early new and exciting results from the James Webb Space Telescope, and discuss remaining challenges and future prospects.