The Most Recent Chapter on the Hubble Constant

Astronomers agree that the universe is expanding in all directions, a notion now called the "Hubble expansion" to refer discoverer Mr. Edwin Hubble.

A useful analogy to understand the Hubble expansion is to draw dots onto a balloon to represent galaxies in the universe. As you blow up the balloon the dots expand away from each other. While there seems to be no way around a universal Hubble expansion, now there is controversy brewing regarding the exact value for this rate of expansion. What is at stake may be a tiny misunderstanding in how we make the measurements, or may be a signal of new physics.

Oh, we all agree now on the approximate answer, that the space between galaxies grows such that for every 3.3 million light years a galaxy moves in distance away from us, the velocity of that distant galaxy becomes 70 km/s faster. Equivalently, in astronomer’s jargon we say that the rate of expansion (H0) equals 70 km/s/Mpc, with the symbol H0 used again to recognize the work of Mr. Hubble.

Although this law was first established in 1929, astronomers are still working on getting its exact value nearly 100 years later. To recap the 21st century part of the story, in 2015 one research group measured H0 by studying distant exploding stars, or supernovae. This group, led by Nobel Laureate Dr. Adam Reiss, reported a value for H0 from their high quality data set in the range of 71.2 - 74.8 km/s/Mpc.

Somewhat surprisingly, a different group working with also with high quality data this time from the space satellite “Planck” compute a value for H0 of 67 km/s/Mpc with quoted error bars that make it incompatible with the supernova-based measurement.

This mismatch of the value for H0 between the two groups may sound a bit like scientists being nitpicky. Afterall, we could very well find out later that one or more of the measurements that went into the computations mentioned above have larger uncertainties than originally projected. Then again, if the numbers do hold, then this could be a gentle but persistent beacon that queues us into new physics.

Dr. Brenda Frye

About Dr. Brenda Frye

Brenda L. Frye is an observational cosmologist at the Department of Astronomy/Steward Observatory, University of Arizona. She earned her Ph. D. in Astrophysics from the University of California at Berkeley, assisted by a National Science Foundation Graduate Research Fellowship.

Her thesis work involved measuring the concentration of the total mass of visible plus dark matter in the fields of massive galaxy clusters, a program requiring the use of some of the largest telescopes in the world.

Moving a mile from her Ph. D. institution, she assumed a postdoctoral position with the Supernova Cosmology Project at Lawrence Berkeley National Laboratory under the direction of Professor Saul Permutter.

She then treked across the country to take a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship and a Princeton Council on Sciences and Technology Fellowship both at Princeton University.

Moving further east, she became a Lecturer in Physics at Dublin City University in Dublin, Ireland, where a number of European collaborations were formed.

From there she crossed back across the pond to the west coast of the U. S. to become a tenure-track Assistant Professor of Physics at the University of San Francisco.

Her travels have now landed her at her Alma Mater in Tucson, where she teaches and does research. The aims of her research continue to be to use gravitational telescopes in space as 'lenses' to study the properties of dark matter and those of distant galaxies back to when the universe was <900 million years old.

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The Most Recent Chapter on the Hubble Constant — 2 Comments

  1. The Planck spacecraft H0 of 66.9 – 68.7 km/s/Mpc computation was based on measurement of Cosmic Microwave Background Radiation(CMBR) that is limited to the GigaHz spectral range.
    Perhaps Planck missed something expressed in Dr. Adam Reiss’ more direct H0 measurement of 71.2 – 74.8 km/s/Mpc.

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