That July morning in a room with an obstructed view of the Pacific, Riess seemed to have a second Nobel Prize in his sights. Among the 100 experts in the crowd — invited representatives of all the major cosmological projects, along with theorists and other interested specialists — nobody could deny that his chances of success had dramatically improved the Friday before.

Ahead of the conference, a team of cosmologists calling themselves H0LiCOW had published their new measurement of the universe’s expansion rate. By the light of six distant quasars, H0LiCOW pegged H 0 at 73.3 kilometers per second per megaparsec — significantly higher than Planck’s prediction. What mattered was how close H0LiCOW’s 73.3 fell to measurements of H 0 by SH0ES — the team led by Riess. SH0ES measures cosmic expansion using a “cosmic distance ladder,” a stepwise method of gauging cosmological distances. SH0ES’ latest measurement in March pinpointed H 0 at 74.0, well within H0LiCOW’s error margins.

“My heart was aflutter,” Riess told me, of his early look at H0LiCOW’s result two weeks before Santa Barbara.

For six years, the SH0ES team claimed that it had found a discrepancy with predictions based on the early universe. Now, the combined SH0ES and H0LiCOW measurements have crossed a statistical threshold known as “five sigma,” which typically signifies a discovery of new physics. If the Hubble constant is not 67 but actually 73 or 74, then ΛCDM is missing something — some factor that speeds up cosmic expansion. This extra ingredient added to the familiar mix of matter and energy would yield a richer understanding of cosmology than the rather bland ΛCDM theory provides.

During his talk, Riess said of the gulf between 67 and 73, “This difference appears to be robust.”

“I know we’ve been calling this the ‘Hubble constant tension,’” he added, “but are we allowed yet to call this a problem?”

He put the question to fellow Nobel laureate David Gross, a particle physicist and the former director of the Kavli Institute for Theoretical Physics (KITP), where the conference took place.

“We wouldn’t call it a tension or problem, but rather a crisis,” Gross said.

“Then we’re in crisis.”

To those trying to understand the cosmos, a crisis is the chance to discover something big. Lloyd Knox, a member of the Planck team, spoke after Riess. “Maybe the Hubble constant tension is the exciting breakdown of ΛCDM that we’ve all been, or many of us have been, waiting and hoping for,” he said.

The Hubble Constant Surd

When talks ended for the day, many attendees piled into a van bound for the hotel. We drove past palm trees with the ocean on the right and the Santa Ynez Mountains to the distant left. Wendy Freedman, a decorated Hubble constant veteran, perched in the second row. A thin, calm woman of 62, Freedman led the team that made the first measurement of H 0 to within 10% accuracy, arriving at a result of 72 in 2001.

The driver, a young, bearded Californian, heard about the Hubble trouble and the issue of what to call it. Instead of tension, problem or crisis, he suggested “surd,” meaning nonsensical or irrational. The Hubble constant surd.

Freedman, however, seemed less giddy than the average conferencegoer about the apparent discrepancy and wasn’t ready to call it real. “We have more work to do,” she said quietly, almost mouthing the words.

Freedman spent decades improving H 0 measurements using the cosmic distance ladder method. For a long time, she calibrated her ladder’s rungs using cepheid stars — the same pulsating stars of known brightness that SH0ES also uses as “standard candles” in its cosmic distance ladder. But she worries about unknown sources of error. “She knows where all the skeletons are buried,” said Barry Madore, Freedman’s white-whiskered husband and close collaborator, who sat up front next to the driver.

Freedman said that’s why she, Madore and their Carnegie-Chicago Hubble Program (CCHP) set out several years ago to use “tip of the red giant branch” stars (TRGBs) to calibrate a new cosmic distance ladder. TRGBs are what stars like our sun briefly turn into at the end of their lives. Bloated and red, they grow brighter and brighter until they reach a characteristic peak brightness caused by the sudden igniting of helium in their cores. Freedman, Madore and Myung Gyoon Lee first pointed out in 1993 that these peaking red giants can serve as standard candles. Now Freedman had put them to work. As we unloaded from the van, I asked her about her scheduled talk. “It’s the second talk after lunch tomorrow,” she said.

“Be there,” said Madore, with a gleam in his eye, as we parted ways.

When I got to my hotel room and checked Twitter, I found that everything had changed. Freedman, Madore and their CCHP team’s paper had just dropped. Using tip-of-the-red-giant-branch stars, they’d pegged the Hubble constant at 69.8 — notably short of SH0ES’ 74.0 measurement using cepheids and H0LiCOW’s 73.3 from quasars, and more than halfway to Planck’s 67.4 prediction. “The Universe is just messing with us at this point, right?” one astrophysicist tweeted. Things were getting surd.

Dan Scolnic, a bespectacled young member of SH0ES based at Duke University, said that he, Riess and two other team members had gotten together, “trying to figure out what was in the paper. Adam and I then went out to dinner and we were pretty perplexed, because in what we had seen up to this point, the cepheids and TRGBs were in really good agreement.”

They soon homed in on the key change in the paper: a new way of measuring the effects of dust when gauging the intrinsic brightness of TRGBs — the first rung of the cosmic distance ladder. “We had a bunch of questions about this new method,” Scolnic said. Like other participants scattered throughout the Best Western Plus, they eagerly awaited Freedman’s talk the next day. Scolnic tweeted, “Tomorrow is going to be interesting.”

To Build a Distance Ladder

Tension, problem, crisis, surd — there has been a Hubble constant something for 90 years, ever since the American astronomer Edwin Hubble’s plots of the distances and recessional speeds of galaxies showed that space and everything in it is receding from us (Hubble’s own refusal to accept this conclusion notwithstanding). One of the all-time greatest cosmological discoveries, cosmic expansion implies that the universe has a finite age.

The ratio of an object’s recessional speed to its distance gives the Hubble constant. But whereas it’s easy to tell how fast a star or galaxy is receding — just measure the Doppler shift of its frequencies, an effect similar to a siren dropping in pitch as the ambulance drives away — it’s far harder to tell the distance of a pinprick of light in the night sky.

It was Henrietta Leavitt, one of the human “computers” at the Harvard College Observatory, who discovered in 1908 that cepheid stars pulsate with a frequency that’s proportional to their luminosity. Big, bright cepheids pulsate more slowly than small, dim ones (just as a big accordion is harder to compress than a tiny one). And so, from the pulsations of a distant cepheid, you can read off how intrinsically bright it is. Compare that to how faint the star appears, and you can tell its distance — and the distance of the galaxy it’s in.

In the 1920s, Hubble used cepheids and Leavitt’s law to infer that Andromeda and other “spiral nebulae” (as they were known) are separate galaxies, far beyond our Milky Way. This revealed for the first time that the Milky Way isn’t the whole universe — that the universe is, in fact, unimaginably vast. Hubble then used cepheids to deduce the distances to nearby galaxies, which, plotted against their speeds, revealed cosmic expansion.

Hubble overestimated the rate as 500 kilometers per second per megaparsec, but the number dropped as cosmologists used cepheids to calibrate evermore accurate cosmic distance ladders. From the 1970s on, the eminent observational cosmologist and Hubble protégé Allan Sandage argued that H 0 was around 50. His rivals claimed a value around 100, based on different astronomical observations. The vitriolic 50-versus-100 debate was raging in the early ’80s when Freedman, a young Canadian working as a postdoc at the Carnegie Observatories in Pasadena, California, where Sandage also worked, set out to improve cosmic distance ladders.

To build a distance ladder, you start by calibrating the distance to stars of known luminosity, such as cepheids. These standard candles can be used to gauge the distances to fainter cepheids in farther-away galaxies. This gives the distances of “Type 1a supernovas” in the same galaxies — predictable stellar explosions that serve as much brighter, though rarer, standard candles. You then use these supernovas to gauge the distances to hundreds of farther-away supernovas, in galaxies that are freely moving in the current of cosmic expansion, known as the “Hubble flow.” These are the supernovas whose ratio of speed to distance gives H 0 .

But although a standard candle’s faintness is supposed to tell its distance, dust also dims stars, making them look farther away than they are. Crowding by other stars can make them look brighter (and thus closer). Furthermore, even supposed standard-candle stars have inherent variations due to age and metallicity that must be corrected for. Freedman devised new methods to deal with many sources of systematic error. When she started getting H 0 values higher than Sandage’s, he became antagonistic. “To him, I was a young upstart,” she told me in 2017. Nevertheless, in the ’90s she assembled and led the Hubble Space Telescope Key Project, a mission to use the new Hubble telescope to measure distances to cepheids and supernovas with greater accuracy than ever before. The H 0 value of 72 that her team published in 2001 split the difference in the 50-versus-100 debate.

Freedman was named director of Carnegie Observatories two years later, becoming Sandage’s boss. She was gracious and he softened. But “until his dying day,” she said, “he believed that the Hubble constant had a very low value.”

A few years after Freedman’s measurement of 72 to within 10% accuracy, Riess, who is a professor at Johns Hopkins University, got into the cosmic distance ladder game, setting out to nail H 0 within 1% in hopes of better understanding the dark energy he had co-discovered. Since then, his SH0ES team has steadily tightened the ladder’s rungs — especially the first and most important: the calibration step. As Riess put it, “How far away is anything? After that, life gets easier; you’re measuring relative things.” SH0ES currently uses five independent ways of measuring the distances to their cepheid calibrators. “They all agree quite well, and that gives us a lot of confidence,” he said. As they collected data and improved their analysis, the error bars around H 0 reduced to 5% in 2009, then 3.3%, then 2.4%, then 1.9% as of March.

Meanwhile, since 2013, the Planck team’s increasingly precise iterations of its cosmic microwave background map have enabled it to extrapolate the value for H 0 evermore precisely. In its 2018 analysis, Planck found H 0 to be 67.4 with 1% accuracy. With Planck and SH0ES more than “four sigma” apart, a desperate need arose for independent measurements.

Tommaso Treu, one of the founders of H0LiCOW and a professor at the University of California, Los Angeles, had dreamed ever since his student days in Pisa of measuring the Hubble constant using time-delay cosmography — a method that skips the rungs of the cosmic distance ladder altogether. Instead, you directly determine the distance to quasars — the flickering, glowing centers of faraway galaxies — by painstakingly measuring the time delay between different images of a quasar that form as its light bends around intervening matter.