Wendy L. Freedman

University of Chicago

Research field

Observational cosmology, the extragalactic distance scale

Wendy Freedman co-led the Hubble Space Telescope Key Project, which fixed the Hubble constant to 10% in 2001, and has since built a distance ladder that replaces Cepheids with red giant stars at the tip of the red giant branch. Her 2021 paper combines four independent geometric calibrations of that tip and finds a Hubble constant of 69.8 km/s/Mpc, between the Cepheid value and the value inferred from the cosmic microwave background. The unit uses this work to show how a ladder measurement is built and why a few hundredths of a magnitude in a zero point matter.

In the masterclass

Section 6 explains the tip of the red giant branch method and presents her 2021 measurement in the research spotlight.

The research is presented in Unit 13: The Expanding Universe and Relativistic Cosmology, in a research spotlight box that explains the result at the level of the unit and links it to the physics you have just learned.

The paper

Measurements of the Hubble Constant: Tensions in Perspective