Astronomy Colloquium for 2018-04-04

Series: Astronomy Colloquium
Date: Wednesday 04-Apr-2018
Time: 16:05-17:00 (4:05-5:00 pm)
Location: ATL 2400
Speaker: Dr. Rachael Beaton (Princeton University)
Title: Engineering the Measurement of the Hubble Constant

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; Freedman et al. 2001) has evolved into a 3.4-sigma tension between the traditional Cepheid-distance ladder measurements (Riess et al. 2016, Freedman et al. 2012, Freedman et al. 2001) and that determined from modelling anisotropies in the cosmic microwave background (CMB; Planck Collaboration et al. 2016). 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 uses 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; Beaton et al. 2016). 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 1% measurement within the foreseeable future.

Colloquia are usually preceded by espresso and ice cream at 1:45 pm outside room PSC 1150 and are followed by an informal reception at 5:15 pm in that room. Anyone interested in talking with the speaker, or being added to the colloquium announcement email list, should contact the colloquium organizer. Lunch will normally be reserved for the speaker to talk to graduate students.

Colloquium Organizer: Prof. L. Drake Deming


Special accommodations for individuals with disabilities can be made by calling (301) 405-3001. It would be appreciated if we are notified at least one week in advance.


Directions and information about parking can be found here.

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