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Adam Riess was 27 years old when he began the work that earned him the Nobel Prize in Physics, and just 41 when he received it. Earlier this year, Riess, who is now in his early 50s, pulled a graph-paper notebook off a bookshelf in his office at Johns Hopkins University so that I could see the yellowing page on which he’d made his famous calculations. He told me how these pen scratches led to a new theory of the universe. And then he told me why he now thinks that theory might be wrong.
For nearly a century, astronomers have known that the universe is expanding, because the galaxies that we can see around us through telescopes are all rushing away. Riess studied how they moved. He very carefully measured the distance of each one from Earth, and when all the data came together, in 1998, the results surprised him. They were “shocking even,” he told his colleagues in a flustered email that he sent on the eve of his honeymoon. A striking relationship had emerged: The farther away that galaxies were, the faster they were receding. This “immediately suggested a profound conclusion,” he said in his Nobel Prize lecture. Something is causing the expansion of the universe to accelerate.
Riess’s genius lies in making precise observations, but the task of explaining the accelerating expansion that he discovered fell to theorists. They proposed the existence of dark energy: a faint, repulsive force that pervades all of empty space. The amount of dark energy that fits inside your bedroom, say, isn’t very strong. It won’t blow the walls out. But when dark energy’s power sums across truly cosmic volumes of space, it can drive galaxy clusters apart. And as this process puts more space between those galaxies, the repulsive force only strengthens, speeding up the expansion of the universe. Telescopes can see hundreds of billions of galaxies today, but trillions upon trillions of years from now, dark energy will have driven them all out of sight. Eventually, it will dilute every last bit of matter and energy into a cold equilibrium, a thin gruel of nothingness.
By doing the work that led to the discovery of dark energy, Riess had helped add the final piece to what has since come to be called the “standard model of cosmology.” Indeed, few people played a larger role in establishing the standard model as the field’s dominant theory of how the universe began, how it organized itself into galaxies, and how it will end. But in recent years, cosmologists, the people who study the universe on the largest scales of space and time, have begun to worry that this story, and particularly its final act, might be wrong. Some talk of revolution. A growing number now say that the standard model should be replaced.
Adam Riess is among them.
Whenever a big theory of the universe is teetering, the old guard tends to close ranks; hence, the classic joke about science progressing one funeral at a time. Riess easily could have joined the old guard. He could have been its commanding officer. When he returned from Stockholm with his prize in 2011, he found that his academic life had changed. People around him started to behave oddly, he told me. Some clammed up. Others argued with him about trivial things, he said, perhaps so they could boast of having dunked on a Nobel laureate. Riess was besieged with invitations to sit on panels, give talks, and judge science fairs. He was asked to comment on political issues that he knew nothing about. He told me he was even recruited to run major scientific institutions.
Riess wondered about that path—being the big boss of a NASA mission or gliding around a leafy university as its chancellor. He could see the appeal, but he hated fundraising, and unlike other, older Nobel laureates, he said, Riess still felt that he had scientific contributions to make, not as an administrator, but as a frontline investigator of capital-n Nature. “Scientists sometimes tell themselves this myth: I’ll go lead this thing, and then I’ll come back and do research,” he told me. But then, by the time they’ve finished up with their administrative roles, they’ve lost touch with the data. They become clumsy with the latest software languages. “The science passes them by,” Riess said.
Riess decided to stick with research. There was plenty to do. The standard model had not solved cosmology. Even in 2011, people knew that the theory was lacking some important details. For one, 96 percent of the standard model’s universe is made up of dark energy and dark matter—and yet no scientist had ever detected either one directly. Cosmologists had good reasons to believe that both exist in some form, but any intuitions about how one might find either in the actual universe had not proved out. Something major seemed to be missing from the picture.
To get a better handle on these mysteries, theorists needed some new data. They badly wanted to know the rate at which the universe expands at different times, and for that they had to know the distances to galaxies from Earth with greater precision. This was Riess’s specialty: He would wait until he saw a certain kind of star explode in a far-off galaxy, and then he’d photograph its unfolding detonation in real time. He knew these supernovas always reached a certain luminosity, which meant he could figure out how far away they were by measuring their brightness in his telescope. The dimmer they were, the farther away.
I’m making this sound a lot easier than it is. Taking a snapshot of an exploding star from tens of millions of light-years away involves many subtleties. You have to subtract out light from the bright stars that surround it, in its own galaxy. The glow of the Milky Way will also sneak into your images, and so will the sun’s; you have to get rid of those too. At the same time, interstellar dust clouds near the star will block some of its light, as will dust in the Milky Way. These dimming effects must be accounted for. The circuits and other parts of your telescope will add noise to your image. The hundreds of thousands of pixels in your camera aren’t all the same, and their differences will need to be sussed out ahead of every observation.
Riess had never stopped trying to master these delicate additions and subtractions of light. Within the field, his measurements have long been regarded as the most precise, according to Colin Hill, a cosmologist at Columbia who does not work with Riess. But in 2011, Riess and his team developed an even better technique for measuring cosmic distances with the Hubble Space Telescope. (The idea came to him in the swimming pool, he said.)
As these new and better data piled up, a problem soon emerged. With each measured distance to another galaxy, Riess would update his calculation of the current expansion rate of the universe. To his alarm, the answers he was getting differed from those produced another way. Some cosmologists don’t bother with the distances to galaxies and look, instead, at the afterglow of the Big Bang. They can then take the expansion rate that they see in that snapshot of the early universe and extrapolate it forward on the basis of assumptions from the standard model. In other words, the latter approach takes it as a given that the standard model is correct.
Riess expected that this discrepancy between the two expansion rates would fade with further observations. But it was stubborn. The more he looked at distant galaxies, the more pronounced the difference became. Indeed, the mere fact of its existence presented the cosmologists with a serious problem. They became so vexed that they had to give it a name: the Hubble tension.
Riess wondered if the observations of the early universe that fed into the other measurement’s equations might be wrong. But neither he nor anyone else could find fault with them. To Riess, this suggested that the Hubble tension could be a product of a broken theory. “It smelled like something might be wrong with the standard model,” he told me.
If the standard model were to topple, the field of cosmology would be upended, and so would an important part of the grand story that we’ve been telling ourselves about the end of the universe. And so, naturally, with weighty matters of career, ego, and the very nature of existence at stake, the Hubble tension has led to a bit of tension among cosmologists.
Some of the field’s most prominent scientists told me that they still expect the problem to disappear with more data, and that Riess may be getting ahead of himself. Wendy Freedman, a professor at the University of Chicago, has made her own measurements of the local universe, using different exploding stars, and the Hubble tension shows up in her data too. But it’s smaller. She told me it’s too soon to tell what the problem is: her measurements, the standard model, or something else. She would want to know the distances to many more galaxies before deciding on the culprit. She would also want to see multiple methods of measurement converging. At a minimum, hers and Riess’s should match up. Hill, the cosmologist from Columbia, expressed a similar view.
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David Spergel, the president of the Simons Foundation, who has for decades held a lot of sway in the field, agrees that it’s premature to start dancing on the standard model’s grave. “Adam speaks very loudly,” Spergel said. “He argues vociferously with whoever disagrees with him.”
Riess does indeed prosecute his case with vigor. Still, no one has been able to find an error in his measurements, and not for lack of trying. His numbers have been cross-checked with observations from both the Hubble and James Webb Space Telescopes. Sean Carroll, a cosmologist and philosopher at Johns Hopkins who is not on Riess’s team, told me that Riess has done a “heroic job” of knocking systematic errors out of his measurements. But Carroll said that it is still too early to tell if the Hubble tension will hold up, and definitely too early to throw out the standard model. “If the implications weren’t so huge, people wouldn’t be so skeptical,” Carroll said.
Riess grew visibly exasperated when we discussed these objections. He blamed them on the “sociology” of the field. He said that a clique of cosmologists—Spergel and “other graybeards”—who work on the early universe have tended to dismiss conflicting data. (For the record, Riess’s own goatee is observably gray.) Even so, at least one of them had come around to his view, he said. Riess had sent data to George Efstathiou, a well-respected early universe cosmologist who’d been a vocal skeptic of the Hubble tension. On his desktop computer, Riess showed me Efstathiou’s reply: “Very convincing!”
I didn’t want to make too much of what might have been politeness, so I followed up with Efstathiou myself. In the email that he wrote to me, he was more circumspect than he had been with Riess: “I don’t have much to say on the Hubble tension.” So far as he could tell, Riess’s measurements didn’t contain any errors, but he couldn’t rule out the possibility that something in them was wrong.
Riess believes that in time he will be vindicated. He believes that the Hubble tension will likely grow more pronounced and that more cosmologists will start to question the standard model. For someone who helped stand up that theory, he comes off as gleeful about this possibility. Maybe this is just his scientific mindset: always deferential to the data. Or perhaps he simply craves the thrill of being right, again, about the fundamental nature of the universe.
When I visited Riess, back in January, he mentioned he was looking forward to a data release from the Dark Energy Spectroscopic Instrument, a new observatory on Kitt Peak, in Arizona’s portion of the Sonoran Desert. DESI has 5,000 robotically controlled optic fibers. Every 20 minutes, each of them locks onto a different galaxy in the deep sky. This process is scheduled to continue for a total of five years, until millions of galaxies have been observed, enough to map cosmic expansion across time. The observatory was preparing to release its second batch of data. Riess thought the information might produce another challenge to the standard model.
In the simplest version of the theory, the strength of dark energy—the faint, repulsive force that’s everywhere in the universe, pushing it apart—is fixed for all eternity. But DESI’s first release, last year, gave some preliminary hints that dark energy was stronger in the early universe, and that its power then began to fade ever so slightly. On March 19, the team followed up with the larger set of data that Riess was awaiting. It was based on three years of observations, and the signal that it gave was stronger: Dark energy appeared to lose its kick several billion years ago.
This finding is not settled science, not even close. But if it holds up, a “wholesale revision” of the standard model would be required, Hill told me. “The textbooks that I use in my class would need to be rewritten.” And not only the textbooks—the idea that our universe will end in heat death has escaped the dull, technical world of academic textbooks. It has become one of our dominant secular eschatologies, and perhaps the best-known end-times story for the cosmos. And yet it could be badly wrong. If dark energy weakens all the way to zero, the universe may, at some point, stop expanding. It could come to rest in some static configuration of galaxies. Life, especially intelligent life, could go on for a much longer time than previously expected.
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If dark energy continues to fade, as the DESI results suggest is happening, it may indeed go all the way to zero, and then turn negative. Instead of repelling galaxies, a negative dark energy would bring them together into a hot, dense singularity, much like the one that existed during the Big Bang. This could perhaps be part of some larger eternal cycle of creation and re-creation. Or maybe not. The point is that the deep future of the universe is wide open.
I called Riess after the DESI results came out, to see how he was feeling. He told me that he had an advance look at them. When he’d opened the data file in his office, a smile spread across his face. He’d been delighted to see another tough result for the standard model. He compared the theory to an egg that is breaking. “It’s not going to cleave neatly in one place,” he said. “You would expect to see multiple cracks opening up.”
Whether the cracks—if they really are cracks—will widen remains to be seen. Many new observations will come, not just from DESI, but also from the new Vera Rubin Observatory in the Atacama Desert, and other new telescopes in space. On data-release days for years to come, the standard model’s champions and detractors will be feverishly refreshing their inboxes. For the moment, though, Riess believes that the theorists have become complacent. When he reaches out to them for help in making sense of his empirical results, their responses disappoint him. “They’re like, Yeah, that’s a really hard problem,” he said. “Sometimes, I feel like I am providing clues and killing time while we wait for the next Einstein to come along.”
When I talked to Riess for the last time, he was at a cosmology conference in Switzerland. He sounded something close to giddy. “When there’s no big problems and everything’s just kind of fitting, it’s boring,” he said. Now among his colleagues, he could feel a new buzz. The daggers are out. A fight is brewing. “The field is hot again,” he told me. A new universe suddenly seems possible.
