Debate over Hubble constant

Debate intensifies over speed of expanding universe

By Joshua Sokol |

This week, leading experts at clocking one of the most contested numbers in the cosmos—the Hubble constant, the rate at which the universe expands—gathered in hopes that new measurements could point the way out of a brewing storm in cosmology.

No luck so far. A hotly anticipated new cosmic yardstick, reliant on red giants, has served only to muddle the debate about the actual value of the constant, and other measurements brought no resolution. “It was the craziest conference I’ve been to,” said Daniel Scolnic, an astrophysicist at Duke University in Durham, North Carolina. “Everyone felt like they were on this rollercoaster.”

The meeting, at the Kavli Institute for Theoretical Physics in Santa Barbara, California, was the latest episode in a saga stretching back to the 1920s, when Edwin Hubble established that the farther one looks into space, the faster galaxies are speeding away from Earth. Since then, scientists have devoted entire careers to refining the rate of that flow, Hubble’s eponymous constant, or H0. But recently, the problem has hardened into a transdisciplinary dispute.

On one side are cosmologists who gather data from the greatest distances, such as a map of the big bang’s afterglow recorded by the European satellite Planck. They compare the apparent size of features in that afterglow with their actual size, as predicted by theory, to calculate an H0 of about 67. That means distant galaxies should be flying away from the Milky Way 67 kilometers per second faster for every additional megaparsec astronomers gaze out into space.

But when astronomers look at actual galaxies, using delicate chains of inferences to make up for the universe’s frustrating lack of tick marks, they get a different number. Over the past few years, a team led by Nobel laureate Adam Riess from Johns Hopkins University in Baltimore, Maryland, has cataloged standard candles: astrophysical objects with a known brightness, whose distance can be calculated based on how bright they appear from Earth. The team uses the supernovae explosions of white dwarf stars as standard beacons to measure distances far out into the swelling universe; they calibrate the brightness of nearby supernovae by monitoring variable stars, called cepheids, in the same galaxies. The stars’ light waxes and wanes at a rate that signals their intrinsic brightness. Earlier this year, this team, dubbed SH0ES, reported an H0 of about 74, a standard-bearing measurement for the astronomers’ side.

If the discrepancy between the cosmologists and the astronomers can’t be chalked up to a subtle, hidden methodological flaw, modern physics itself could be due for a revision. Theorists, salivating at the possibility, have begun to dream up hidden ingredients in the early universe—new particles or interactions—that could patch over the gulf. But they haven’t found a fix that doesn’t cause new problems. With stakes that high, astronomers put their heads together in Santa Barbara to double and triple check the SH0ES result against other ways to measure the constant.

A team called H0LiCOW relied on gravitational lenses, freak cosmic alignments where the light from a very distant, flickering beacon called a quasar is bent into multiple images on the sky by the gravity of another, intervening galaxy. Each image is formed by light traveling along a different path across expanding space. Because of that, though, the flickers don’t all arrive at Earth at the same time. Based on the time delays and not-so-simple geometry, the team calculated the H0 from six different such systems and came up with a value of roughly 73—“very close” to the SH0ES results, says Geoff Chih-Fan Chen, a team member at the University of California, Davis. The team didn’t check its final number—published just before the meeting on the preprint server arXiv—until the very end of its analysis to avoid bias, Chen says. “Some people will unconsciously want to get the right answer.”

One point for possible new physics. But the meeting brought a twist. On the first evening, the Carnegie-Chicago Hubble Program team, led by Wendy Freedman, a veteran H0 measurer at the University of Chicago in Illinois, uploaded its own long-anticipated paper—already accepted to The Astrophysical Journal—to arXiv. Freedman’s team sought to develop a new type of standard candle. “If we put all our eggs in the cepheid basket,” Freedman says, “we will never uncover our unknown unknowns.”

Instead, her team looked toward old, swollen stars called red giants. These stars have already exhausted the hydrogen fuel at their hearts, converting it to a core of helium that sits, inert, as a hydrogen shell around the core continues to burn. The star, seen from afar, grows brighter and brighter. But at a certain, predictable limit the temperature and pressure in the core grow high enough to burn helium, too, generating an explosive flash of energy that rearranges the interior of the star, ultimately causing it to begin to dim. By finding the very brightest red giants in a distant galaxy—the ones that toe this theoretical limit—the team could use them as standard candles to calculate distances and its own H0.

One day after the paper appeared, Freedman presented the result to the meeting: a surprisingly low H0 of about 70. “It definitely felt like an album drop,” says Scolnic, a SH0ES team member. The value was stuck between the competing sides—and slightly favored the cosmologists. “It has caused at least some people to pause for a second, and say, ‘Well, maybe it’s not as clear cut,’” Freedman says.

The SH0ES team had huddled together as soon as Freedman’s paper came out, and members were ready to question some of her team’s underlying premises after her talk. They also pointed to a trio of other, if less-precise, Hubble results debuted in Santa Barbara that rely on independent astrophysical concepts—clouds of water circling the centers of faraway galaxies, other kinds of variable stars, and the rate at which the luminosities of galaxies fall off from their center to their edge.

A combined measurement that averaged all these astronomical results together still gave a value of 73. Unless hidden biases still lurk in the data, the gulf between that value and the cosmologists’ lower number remains near or above the 5σ statistical standard physicists use to divide possible flukes from the real deal.

In Riess’s mind, at least, astronomers are nearing a consensus that the Hubble gulf highlights a true difference between the ancient and more recent universe. “You’re left with a problem, discrepancy, crisis,” Riess says. “The biggest argument at the meeting, I thought, was about what word to use.”