The Hubble Tension is Still Tense

Title: The Hubble Tension in our own Backyard: DESI and the Nearness of the Coma Cluster

Authors: Scolnic, D., Riess, A. G., Murakami, Y. S., Peterson, E. R., Brout, D., Acevado, M., Carreres, B., Jones, D. O., Said, K., Howlett, C., and Anand, G. S.

First Author’s Institution: Department of Physics, Duke University, Durham, NC 27708, USA

Status: ePrint [open access]

In 1929, Edwin Hubble found that more distant galaxies are moving away from us faster than more nearby galaxies, hence proving that the universe is expanding in all directions. The linear relationship between distance and velocity of galaxies we observe is now known as the Hubble-Lemaître Law, and the slope of that line is known as the Hubble constant.

Astronomers have now measured the velocity of and distance to much more distant galaxies using standard candles such as cepheid variable stars and Type 1a supernovae. A standard candle is an object with a known luminosity, so by comparing the brightness we observe here on Earth to the intrinsic luminosity, astronomers can infer the distance to the object. These techniques have resulted in the Hubble constant being measured around 73 km/s/Mpc.

But there’s a problem. The Hubble constant can also be measured from the cosmic microwave background (CMB) and is found to be around 67.5 km/s/Mpc. Currently, astronomers don’t know why these two results are different, and this issue is known as the Hubble tension. You can read more about the Hubble tension here, here, and here.

Previous work had used galaxies in the Coma cluster (see Figure 1) to estimate the Hubble constant, but they used a known correlation between an elliptical galaxy’s size, luminosity, and the velocity of stars within the galaxy to estimate the distance to the cluster. In order to pin down an exact value of the Hubble constant, you still need an independent measurement of the distance to these galaxies. Today’s authors use Type 1a supernovae to measure the distance to the Coma cluster more precisely than ever before, and therefore infer the value of the Hubble constant.

Some of the stars in these galaxies are part of binary star systems (pairs of stars which orbit each other) containing a dense white dwarf, the remnant of a dead star,  which is orbiting close to its companion star and is pulling mass off of it The white dwarf accretes the matter it pulls off its companion and grows until it reaches a mass 1.4 times the mass of our Sun. At that point, gravity becomes too strong, overcoming the outward electron degeneracy pressure, and triggers a massive explosion known as a Type 1a supernova. Since all Type 1as originate from white dwarfs of the same mass, they all have the same energy and the same luminosity, making them excellent standard candles.

Today’s authors combed through decades of data from multiple different telescopes to identify Type 1a supernovae that went off in the Coma cluster. Supernovae are generally rare: a galaxy experiences one every couple hundred years. But since the Coma cluster contains so many galaxies, the authors could expect to find tens of supernovae within the cluster over the observed decades. And indeed, after making cuts to ensure high-quality data, the authors put together a sample of 12 Type 1a supernovae (green circles in Figure 1).

Past measurements of the distance to Coma all cluster between 95 and 100 megaparsecs (1 Mpc = 3.26 million light years), and the authors measured a distance of 98.5 Mpc (blue point in Figure 2). This implies a Hubble constant of 76.5 km/s/Mpc – still significantly far off from the value implied by the CMB. In order to replicate the CMB Hubble constant, the Coma cluster would have to be 111.8 Mpc (grey shaded region in Figure 2) away from us. Not only is this very far from the value reported in today’s paper, but it’s also very far from multiple other independent measurements (grey, purple, and red points in Figure 2) of the distance to the Coma cluster. 

It seems that improved precision is not the answer to solving the Hubble tension. While astronomers still aren’t sure, the results of this paper suggest that the Hubble tension might not be due to inaccurate measurements, but might actually reflect some intrinsic difference between the two measurement methods. The good news is that there’s lots more to come on the subject: the Dark Energy Spectroscopic Instrument (DESI) is set to observe 100,000 elliptical galaxies in the Coma cluster, as well as other nearby clusters such as Fornax, Virgo, and Leo I, which will provide a substantial improvement in measurement precision! The James Webb Space Telescope is also going to be pointed towards the Coma Cluster, providing even more data points. Stay tuned to see if and how the Hubble tension will be resolved!

Astrobite edited by Storm Colloms

Featured image credit: Adapted from Figure 1 of Today’s Paper