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A fundamental disagreement in the measurement of the universe's expansion rate could be explained away, new data suggests.
In a new paper, a major player in this dilemma takes a look at the available information and concludes that the best observations might be pointing to a triumph for our standard picture of how the universe has grown over time.
Scientists know that the universe is expanding but have disagreed for a decade about just how fast this process is happening. Data that uses the cosmic microwave background (CMB), a leftover light from shortly after the Big Bang, has suggested that the value of the Hubble constant, which measures this expansion, should be about 46,200 mph per million light-years, or 67.4 kilometers per second per megaparsec in cosmologists' units. (A megaparsec is equal to 3.26 million light-years.)
Related: Big Bang to present: Snapshots of our universe through time
Yet telescopes trained on stars in the nearby universe have instead come up with a Hubble constant measurement of 50,400 mph per million light-years (73.4 km/s/Mpc). The two numbers aren't all that different, but each is quite precise and they can’t be reconciled with one another.
The tension between these two numbers has been an ongoing headache for researchers, with some invoking the idea that the discrepancy requires them to overturn their favored model of the universe, which explains how giant cosmic structures like galactic clusters have arisen and evolved since the dawn of time. Perhaps, researchers wondered, new physics beyond what we currently know could be used to bridge the gap.
"I think it's a really interesting question: 'Is there new physics beyond the standard cosmological model?'" Wendy Freedman, a cosmologist at the University of Chicago, told Live Science.
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Freedman has spent much of her career observing what are known as Cepheid variable stars. These stars, which pulsate regularly, have a relationship between the period of the fluctuations in their light and their intrinsic brightness, meaning how bright they would be if we were standing right next to them. By knowing this intrinsic brightness and a Cepheid’s luminosity as seen from Earth, astronomers can calculate its distance from us and then measure the speed at which the universe is expanding at that point in space.
Cepheid data is one of the linchpins of the higher value of the Hubble constant, but Freedman and her collaborators have always wondered if perhaps they were making systematic errors in their observations. They have long searched for independent methods to corroborate or contest their results.
A few years ago, she and her colleagues found one method in the light of giant red stars. These objects, which represent a later life stage for stars with a mass similar to our sun, reach a specific peak brightness at a certain point in their evolution. Much like with the Cepheids, astronomers can look at how dim they appear from Earth to get a good estimate of their distance.
In 2019, Freedman and her team provided a number for the Hubble constant that sat just between the two other measurements: 47,300 mph per million light-years (69.8 km/s/Mpc). That result was calibrated using giant red stars in the Large Magellanic Cloud, a dwarf galaxy that orbits the Milky Way whose distance from us is relatively well determined.
Since then, the researchers have added more data points, calibrating the distance to giant red stars in three other galaxies and regions of space, which ups the precision of their Hubble constant measurements. These findings, which found essentially the same middle-ground estimate, appeared in a paper that was published to the preprint database arXiv on June 29, and which has been accepted for publication in the Astrophysical Journal.
"It's landing in the same place, just shy of 70 [km/s/Mpc] with an uncertainty of just over 2%," Freedman said of the new Hubble constant estimate from the red giant stars. "If we compare those results to the CMB, we wouldn't say there's an issue."
These latest red giant measurements point to the possibility of systematic errors in the Cepheid observations, Freedman said. Obscuring dust and background light from the universe are some possible culprits, she added, though it will take time to actually discover if that is the case.
"I'm really impressed with the work and the details," Simon Birrer, a cosmologist at Stanford University in California, who was not involved in the study, told Live Science. The paper really highlights the specific advantages of the red giant star observations, he added.
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But Birrer, who has been part of a team that looked at how massive galaxies warp light to provide another independent Hubble constant measurement, doesn't think the saga is quite over yet. "Is this the beginning of the end of the tension? We’re still working on it," he said.
Astronomers have now provided many different estimates of the universe's expansion, some of which agree and some which don't. Each team is striving for the best accuracy they can provide, Birrer said, and sorting out which might contain the ultimate answer is still unclear.
Freedman agreed, saying that she and her colleagues have recently been approved to use the upcoming James Webb Space Telescope to look at both Cepheids and red giants. Those observations should help clear up some of the remaining systematic uncertainties and hopefully get closer to the true value of the Hubble constant.
Originally published on Live Science.
Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike.