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Astronomy Colloquium

Wednesday, November 3, 2021
4:00pm to 5:00pm
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Online and In-Person Event
Increasing Accuracy in Measuements of the Hubble Constant: Is There Evidence for New Physics?
Wendy Freedman, John and Mary Sullivan University Professor of Astronomy and Astrophysics, Department of Astronomy and Astrophysics, University of Chicago,

An important and unresolved question in cosmology today is whether there is new physics that is missing from our current standard Lambda Cold Dark Matter (LCDM) model. Recent measurements of the Hubble constant, Ho -- based on Cepheids and Type Ia supernovae (SNe) -- are discrepant at the 4-5-sigma level with values of Ho inferred from measurements of fluctuations in the cosmic microwave background (CMB). The latter assumes LCDM, and the former assumes that systematics have been fully accounted for. If real, the current discrepancy could be signaling a new physical property of the universe. I will present new results based on an independent calibration of SNe Ho based on measurements of the Tip of the Red Giant Branch (TRGB). The TRGB marks the luminosity at which the core helium flash in low-mass stars occurs and provides an excellent standard candle. Moreover, the TRGB method is less susceptible to extinction by dust, to metallicity effects, and to crowding/blending effects than Cepheid variable stars. I will address the current uncertainties in both the TRGB and Cepheid distance scales, as well as discuss the current tension in Ho and whether there is need for additional physics beyond the standard LCDM model.

In-person attendance is limited to Caltech community members only, with a 74 person capacity limit in Hameerman Auditorium. Masks must be worn at all times.

To view this talk via YouTube, please visit :

For more information, please contact Jim Fuller by email at or visit