Another Look at the Hubble Constant

The Cosmic Companion
Oct 23 · 4 min read

A new measurement of the expansion rate of the Universe could have far-reaching consequences to our understanding of the Cosmos.

The Hubble constant is one of the critical calculations which astronomers and astrophysicists use to determine the nature of the Universe. One of the great challenges faced by researchers today is determining the exact value of this number. Various means of measuring the Hubble Constant produce different results, suggesting different possible pasts, and futures, for our Universe and everything in it.

Simply put, the Hubble Constant is a measure of how quickly objects in the Universe are flying away from each other. Galaxies and stars located further away from us are receding at greater velocities than those closer to us. This does not mean we are in any special location, but these same conditions would be seen by observers near any star in any galaxy in the Universe.

One of a pair of 10-meter telescopes at the Keck Observatory in Hawaii used in a new study of the Hubble constant from UC Davis. Image credit: Scott Kardel

Astronomers and other researchers use the Hubble constant to determine the age and size of the Universe, as well as determine the distances between distant bodies.

The Hubble constant is typically expressed in terms of kilometers per second per megaparsec (km/s/Mpc). The proposed speed has changed many times since the idea first became known in the opening decades of the 20th Century. Today, typically accepted figures are found around 73 km/s/Mpc.

A megaparsec (MPc) is equal to a little more than 3.25 million light years (or 30 million trillion kilometers). Using the most-commonly expressed value for the Hubble constant, objects located 3.25 million light years (one megaparsec) from us are (on average) flying away from us at around 73 km/sec (163,300 MPH), while objects twice that distant are travelling twice that speed.

Eyes on the Prize

A team of researchers from the University of California, Davis (UC Davis) looked at distant, ancient galaxies located behind closer, large galaxies. Light from the more distant body was bent by the gravitational influence of the intermediary galaxy.

By measuring the amount of time each path of the light takes to bend around the galaxy (and it’s accompanying dark matter), the team was able to calculate their own, independent value for the Hubble constant.

Light from distant galaxies bent by gravity from more nearby galaxies.
Light from distant galaxies bent by gravity from more nearby galaxies.
LIght from three different galaxies is seen bending around more nearby galaxies in these images taken by the W.M. Keck Observatory. Image credit: Chris Fassnacht, UC Davis

Using this method, the UC Davis team found a value for the Hubble constant of 76.8 km/s/Mpc. However, this number is at the high end of several recent studies. An investigation announced in 2017, using the same methods employed by the UC Davis team, calculated a value of 71.9 km/s/Mpc.

Adam Reiss of Johns Hopkins University calculated just over 74 km/s/Mpc by looking at highly-predictable variable stars, known as Cepheids. Examination of supernovae and red giant stars by a team from the University of Chicago suggests the constant sits at 69.8 km/s/Mpc.

When Hubble looked at the Cosmic Microwave Background radiation (the echo of the Big Bang), researchers calculated a value of 67.4 km/s/Mpc (if the standard model of the evolution of the Universe is correct). This figure shows the greatest difference from that suggested by the UC Davis research.

Although those differences may seem small, differing values for the Hubble constant lead to radically different values for the age, size, and ultimate fate of our Universe. If the constant is far from 73 km/s/Mpc, it could call for major changes in the way we understand the Universe.

“More and more scientists believe there’s a real tension here. If we try to come up with a theory, it has to explain everything at once,” Geoff Chen, graduate student at UC Davis stated.

A diagram showing a three-step process to calculate the Hubble constant.
A diagram showing a three-step process to calculate the Hubble constant.
To calculate the Hubble constant, the distances to nearby variable stars is measured, using parallax created as Earth travels around the Sun over the course of a year. Once their true brightness is determined, astronomers know the distance to similar stars at greater distances, including those within other galaxies. These variable stars, along with highly-regular Type 1a supernoave, allow researchers to determine the distance to more distant galaxies. Image credit: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)

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The UC Davis team used the W.M. Keck telescopes in Hawaii to collect their data. Adaptive optics, a technique which removes most of the distortion from the atmosphere typically seen as the twinkling of stars, was used to enhance detail seen during observations.

Analysis of the study was published in the Monthly Notices of the Royal Astronomical Society.

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