Tension mounts over the Hubble tension: It's getting really . . . . not dull

Left to right: Big bang, primordial soup period,

opaque universe period, modern universe we can see

(the JWST is on the right looking back in time)

As we all know, the Hubble tension is the discrepancy between the rate of the expansion of the universe that the standard model of the universe predicts compared to expansion rate that the Hubble space telescope revealed. The standard model predicts that the universe should be expanding at a rate of 67.4 km/s/Megaparsec (1 Mpc = 3.26 million light years). Measurements from Hubble and now also the JWST (James Webb Space Telescope) indicate a faster rate of expansion, but with different rates depending on the kind of star used for the calculation.

As you can see, the mess keeps getting messier. Things are tense. Quanta Magazine comments:

Two rival teams have led the effort to measure the cosmic expansion rate, which is known as the Hubble constant, or H₀. One of these teams, led by Adam Riess of Johns Hopkins University, has consistently measured H₀ to be about 8% higher than the theoretical prediction for how fast space should be expanding, based on the cosmos’s known ingredients and governing equations. This discrepancy, known as the Hubble tension, suggests that the theoretical model of the cosmos might be missing something — some extra ingredient or effect that speeds up cosmic expansion. Such an ingredient could be a clue to a more complete understanding of the universe.

Riess and his team released their latest measurement of H₀ based on Webb data this spring, getting a value that agrees with their earlier estimates.

But for years a rival team led by Wendy Freedman of the University of Chicago has urged caution, arguing that cleaner measurements were needed. Her team’s own measurements of H₀ have invariably landed closer than Riess’ to the theoretical prediction, implying that the Hubble tension may not be real.

That the three methods disagree “is not telling us about fundamental physics,” Freedman said. “That’s telling us there’s some systematic [error] in one or more of the distance methods.”

Freedman’s results have been submitted to The Astrophysical Journal but have not yet undergone formal peer review, where outside researchers anonymously check the data and analysis. Saul Perlmutter, a Nobel Prize-winning cosmologist at the University of California, Berkeley, who was shown the team’s preprint prior to its release, told Quanta that the results suggest “we may have a Hubble tension just within the [star-based] measurements. That’s the tension that we really have to be trying to figure out more than trying to invent new [cosmological] models.”

Riess, after studying the preprint, told Quanta that he takes issue with the small set of supernovas that Freedman’s team used in one step of the analysis, which he says could bias the results. “The new measurements are lovely and in fact are in excellent agreement with the same measurements obtained … several years ago by our group, so the distance measurements seem under control,” he said. “However, I fear this study of such a small supernova sample gives a somewhat misleading impression of the value of the Hubble constant.”

The results come after months of behind-the-scenes drama, as Freedman initially thought her analysis had killed the Hubble tension, only to see it come roaring back to life. “It’s been really … not dull, I’ll put it that way,” she said.

Hubble, his pipe and

a telescope

(peek-a-boo!)

Measurements of H₀ improved as astronomers got better at calibrating the relationship between Cepheids’ pulsation frequency and their luminosity. Still, the whole approach was limited because Cepheids are only so bright. To measure the distance to galaxies across the vastness of the universe, scientists would need a new approach.

In the 1970s, researchers started using Cepheids to calibrate the distances to bright supernovas, enabling more accurate measurements of H₀. Then as now, two research teams led the way, using supernovas anchored to Cepheids and arriving at disagreeing values of 50 km/s/Mpc and 100 km/s/Mpc. “There was no meeting of minds ever; they were just completely polarized,” said George Efstathiou, an astrophysicist at the University of Cambridge.

The 1990 launch of the Hubble Space Telescope gave astronomers a new, crisp view of the universe. Freedman led a multiyear observing campaign using Hubble, and in 2001, she and her colleagues announced an expansion rate of 72 km/s/Mpc, estimating that this was at most 10% off.

The JWST -- it's a big little dude

Anyway, we can see that there is tension about the Hubble tension. Maybe in a couple of years this will resolve. Maybe the standard model will be vindicated and the Hubble tension gets dissipated into a dust bunny as a matter of systematic error in the measurements. Or, maybe there is something new and fundamental that we need to figure out.

By Germaine