by Ira Polans

The first meeting of the season will be held on September 10 at 7:30 PM in the auditorium of Peyton Hall on the Princeton University campus. The talk is on the “Engineering the Measurement of the Hubble Constant” by Rachael Beaton, Research Fellow, Princeton University

The local expansion rate of the Universe, the Hubble constant, is one of the fundamental parameters in our current concordance cosmology and one that anchors the expansion history of the Universe. The resolution of the historical factor-of-two controversy in the Hubble constant nearly two decades ago (e.g., the Hubble Space Telescope Key Project) has evolved into a > 3.8-sigma tension between the traditional Cepheid-distance ladder measurements and that determined from modelling anisotropies in the cosmic microwave background (CMB; Planck Collaboration). At the heart of the tension, is not only a difference in method, but also a fundamental difference in the state of the observed Universe: the distance ladder measures the local rate in the nearby universe (e.g., z~0), whereas the CMB anisotropy measurements use the very young Universe (z ~1100).

Resolution of the tension requires (i) a full-scale evaluation of the systematic effects in either technique or (ii) “new physics” added to the standard cosmological model. The trigonometric parallaxes provided by Gaia in the near term permit an unprecedented opportunity to use alternative standard candles and construct a full end-to-end distance ladder without Cepheids. The Carnegie-Chicago Hubble Program is doing just that; we are in the middle of building a new distance ladder that relies on the tip of the red giant branch (TRGB). As I will demonstrate, this not only provides a direct cross-check on the Cepheid path, but there are numerous advantages to using a distance indicator that, as a standard candle from old stellar populations, is nearly ubiquitously present low-crowding and low-extinction components of galaxies. More specifically, by being able to calibrate every ‘local’ SNe Ia and easily probing ever-larger volumes with JWST and WFIRST, the TRGB-based distance ladder paves a clear path to a measurement that is engineered for the highest precision and accuracy.

Two changes are being made for the 10 minute talks this season. First, the talk will be given after the intermission. Second, we are instituting a 10 minute limit. The speaker will be given a 90 second warning to wrap up the talk. If you’re interested in giving a 10 minute talk please contact either Rex or me.

There will be a meet the speaker dinner at 6 PM an Wiberries in Palmer Square. Please contact program@princetonastronomy.org by noon on September 10 if you are interested in attending the dinner.