Is the Hubble constant not…Constant?

Cosmologists have been struggling to understand an apparent tension in their measurements of the present-day expansion rate of the universe, known as the Hubble constant. Observations of the early cosmos – mostly the cosmic microwave background – point to a significantly lower Hubble constant than the value obtained through observations of the late universe, primarily from supernovae. A team of astronomers have dug into the data to find that one possible way to relieve this tension is to allow for the Hubble constant to paradoxically evolve with time. This result could point to either new physics…or just a misunderstanding of the data.

“The point is that there seems to be a tension between the larger values for late universe observations and lower values for early universe observation,” said Enrico Rinaldi, a research fellow in the University of Michigan Department of Physics and coauthor on the study. “The question we asked in this paper is: What if the Hubble constant is not constant? What if it actually changes?”

Something’s rotten in the state of the Universe

Cosmologists employ a variety of probes and observations to determine the fundamental properties of our universe. They try to measure its age, its contents, its expansion rate, and more. After almost a century of intense scrutiny, those cosmologists have developed a coherent, consistent model of the universe. In short, our cosmos is about 13.77 billion years old, is constantly expanding, and is made of mostly dark energy and dark matter – with normal matter like stars and planets and clouds of gas making up a brightly-lit minority of the ingredients of the universe.

Leaving aside the gigantic mysteries of the true nature of dark energy and dark matter, in recent years cosmologists have run into another frustrating puzzle: different probes disagree about the present-day expansion rate, known as the Hubble constant.

Measurements taken of the young universe, like the cosmic microwave background (the afterglow light pattern that was released when the universe cooled from a plasma state when it was 380,000 years old), tell us that the Hubble constant is somewhere around 68 km/s/Mpc (which means that for every million parsecs away from our vantage point, the expansion rate of the universe increases by 68 kilometers per second).

But more local, late-universe measurements, like observations of supernovae, lean towards a different answer: a Hubble constant of more like 74 km/s/Mpc.

Bin it

A team of astronomers led by Maria Dainotti, an assistant professor at the National Astronomical Observatory of Japan and the Graduate University for Advanced Studies, SOKENDAI in Japan and an affiliated scientist at the U.S. Space Science Institute dug into this discrepancy more. The work was published in May in The Astrophysical Journal.

The team focused their work on Type-1a supernovae, which are a particular kind of explosion that happens when white dwarf stars accumulate too much mass from a companion star, which triggers a runaway nuclear fusion event. This fusion event has roughly the same brightness every time it happens, so astronomers can use these supernovae as “standard candles” – since they know how bright the supernovae should be, they can compare that to how bright they appear to be and calculate a distance. By combining many such measurements over a wide range of distances, astronomers can calculate the expansion history of the universe.

The team used a catalog of over 1,000 supernovae observations and separated them into different bins of distance ranges, with each bin representing the same number of supernovae. They then used each bin to measure the Hubble constant. In the standard cosmological picture, the expansion rate of the universe is constantly changing as the cosmos evolves, but the Hubble constant is a fixed number – it’s the expansion rate of the universe right now.

Each bin of supernovae should yield the same Hubble constant, but in their analysis the researchers allowed the Hubble constant to not be so constant – they allowed for the possibility that it could change with time. By using different bins, they could test to see if the Hubble constant stayed fixed across the different bins, or if it did indeed vary.

“If it’s a constant, then it should not be different when we extract it from bins of different distances. But our main result is that it actually changes with distance,” Rinaldi said. “The tension of the Hubble constant can be explained by some intrinsic dependence of this constant on the distance of the objects that you use.”

Not very constant

Ultimately, the astronomers found in the study that by adding a little bit of flexibility to the standard cosmological models – by allowing the Hubble constant to change with time – they could relieve almost all of the tension between the supernovae and cosmic microwave background measurements. The researchers were able to extrapolate their evolving Hubble constant back to the time of the cosmic microwave background and match it up with those results.

“The extracted parameters are still compatible with the standard cosmological understanding that we have,” he said. “But this time they just shift a little bit as we change the distance, and this small shift is enough to explain why we have this tension.”

The new results are not altogether surprising. It’s always possible to make differing observations agree by adding more complexity to models. In this case, the researchers added a new variable – how quickly the Hubble constant changes with time – and they were able to find a way to connect the early- and late-time measurements of the Hubble constant. Also, the work did not find a statistically significant measurement of this varying Hubble constant. Although they were able to relieve the tension in cosmological observations, they were not able to conclusively say that the Hubble constant is changing with time.

These results, if they hold up, could give theorists a pathway to introducing new physics into the universe to explain the Hubble constant tension. Or it might also mean that supernovae aren’t as “standard” as we think they are, and that perhaps some bias is creeping into the observations to spoil those measurements of the Hubble constant.