University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Colloquium

Thursday, September 28th 2017
3:35 pm:
Speaker: Wendy Freedman, University of Chicago
Subject: A New Calibration of the Hubble Constant
Fall 2017 Misel Lecturer

photo by John Zich Image M100 Galaxy. Credit: Hubble Legacy Archive, NASA, ESA. Processing & Licence: Judy Schmidt
The accuracy with which we can measure the Hubble
constant, Ho has been steadily increasing over the past
decade. The direct and traditional means to measure Ho
is based on measurement of distances and velocities to
galaxies in the local universe; for example, using Cepheid
variables and Type Ia supernovae. A model-dependent
Ho can be inferred from applying a cosmological model
to measurements of anisotropies in the cosmic microwave
background. Recently, these two precise techniques have
yielded values of Ho that disagree at more than 3-sigma.
This disagreement may be signaling errors in one or both
techniques. Alternatively, it could be signaling new physics
not currently included in the standard model of cosmology.
The Chicago-Carnegie Hubble Program is undertaking a
completely independent calibration of the Hubble constant
using red giant stars in the nearby universe. These stars
are proving to be both more precise and more accurate
than the traditional Cepheid variables. Moreover, with
the imminent launch of the James Webb Space Telescope
and new geometric parallaxes measured by Gaia, they will
provide a means of extending the distance scale beyond
the realm of Cepheids, and for measuring Ho to both a
precision and accuracy of 1%.

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