Is Cosmology Broken? This Map May Be a Crucial Puzzle Piece

For centuries, cartographers have sought to map Earth’s land masses and seas to better understand the world and their place in it. Now, astrophysicists have taken a major step toward doing the same with the cosmos itself. They have just completed the largest high-detail map yet of the universe’s early and middle years.

The map sheds new light on a pair of cosmological crises: the debate over the universe’s expansion rate, and a second one about how evenly matter is spread throughout the universe. By showing how light dating back to the Big Bang has been distorted, it provides the clearest picture so far of how fast our universe has been expanding and how quickly gravity has brought together massive structures, like clusters of galaxies and invisible webs of dark matter. Together, these seem to confirm the standard cosmology model of the universe’s growth as well as Einstein’s relativity theory, which describes how cosmic structures grow and how their gravity bends the light from distant objects. At least, the map upholds the model for the universe’s first 8 billion years. After that, strange things seem to happen.

“There’s a lot of excitement about this result. We made a high-resolution dark-matter map of a quarter of the sky,” says Mathew Madhavacheril, a University of Pennsylvania scientist who presented the vast map at a conference in Kyoto, Japan, in April. He is a member of the National Science Foundation-funded Atacama Cosmology Telescope collaboration, an international group of more than 160 members who developed the map. Madhavacheril is the lead author of the team’s new study, which is in peer review at the Astrophysical Journal. They’ll release the map when they complete that process.

The team has been peering through the heavens with a 39-foot-tall millimeter-wave telescope perched on the side of Cerro Toco, a stratovolcano in the Atacama Desert in northern Chile. That’s one of the driest places in the world, and it’s not the easiest spot for researchers to reach, but its unique location makes it easier to discern light from cosmic microwave background radiation, also known as the CMB.

About 380,000 years after the Big Bang, after the universe’s ultrafast expansion known as inflation, it cooled enough to release this embedded radiation. Those photons permeated the universe and are visible today at very long wavelengths. As a result, the CMB provides the earliest snapshot of the structure of the cosmos—a view of the baby universe. 

But the gravitational pull of galaxy clusters and dark matter—the metropolises of the universe—tweak, twist, and wiggle that relic radiation. This phenomenon is called gravitational lensing, and for anyone looking through a telescope, it creates a distorted picture of the cosmos. Yet it presents a boon for astrophysicists, because those distortions are actually clues about how the universe developed after its infant years.

Astrophysicists have been keen to test the standard cosmological model, which uses as its starting point slight temperature fluctuations in the CMB. The model describes the universe’s evolution from there, calculating how the universe has ballooned since its infancy and how clumps of dark matter and galaxies have become more massive over time. It assumes the consensus view on the behavior of dark energy, which permeates the cosmos and somehow accelerates the universe’s expansion, as well as the properties of dark matter, the mysteriously abundant and invisible particles that cluster together, forming the cosmic scaffolding in which galaxies assemble. 

But glaring tensions between model predictions and telescope observations have turned into a full-blown crisis, leading some scientists to fear that that standard model is somehow broken. At first, these discrepancies were large enough that no one was too concerned about them—the uncertainties were so big that they seemed to indicate flawed mismeasurements, not flawed theory. But over the past few years, measurements have became more precise and a clearer discrepancy has emerged. These recent measurements are based on observations from the Hubble Space Telescope, plus others, of the highly predictable locations of certain kinds of stars and supernovas. They show that the universe’s expansion rate in the local universe—the area within a couple billion light-years of Earth—is faster than it should be based on predictions using the CMB. If these measurements are right, could the model be wrong? Astrophysicists refer to this discrepancy as the Hubble constant tension.

And that’s actually only one of two cosmic disputes. The other involves calculations of how fast massive cosmic structures have been growing. The young universe was pretty smooth, like the surface of a snow globe. But then mountain ranges of matter—and canyons that lacked it—grew throughout it. In a sort of cosmic capitalism, the densest spots, with lots of galaxies and dark matter, became even more dense, while their counterparts with less matter became nearly devoid of it. 

Measurements characterizing how these mountain peaks arose in the increasingly lumpy universe don’t agree with each other either. And again, the disagreement pits studies based on the CMB against those based on telescope observations of the nearby universe. But this has drawn less attention than the expansion rate crisis, which was more striking statistically: The Hubble tension had about a one-in-a-million chance of arising from a statistical fluke, versus one in a thousand for the second discrepancy.

Because the ACT map allows scientists to measure both the rate of the universe’s expansion and how fast those structures grew, it serves as the latest test of the prevailing model—and it shows that it actually fares pretty well for most of the history of the universe. “This has told us that the cosmological model isn’t broken. We’ve measured how much cosmic structures have grown, and it’s exactly what we’d predict,” says Jo Dunkley, a Princeton astrophysicist and analysis leader for the ACT team.

But the word “most” is important. The ACT team’s findings agree with studies of the CMB made with instruments like the European Space Agency’s Planck telescope, which together cover the first 8 billion years of the universe’s life. But there are still significant discrepancies between these findings about the young universe and observations made by tracking what’s happened over the past few billion years. (Cosmologically speaking, that’s the recent past.) 

The ACT findings suggest that something might have changed over the past 5 billion years or so, which made the universe’s expansion appear to speed up slightly and made the distribution of matter seem to get lumpier. This recasts physicists’ views of the cosmological crises, because it means that a CMB-based model still works much of the time—but not for the universe’s whole history.

“The exciting prospect is that there might be some new physics that’s going on here,” Madhavacheril says. For example, the standard model assumes that about 32 percent of the universe is made of dark matter—specifically, a particular flavor called “cold dark matter particles,” which move relatively slowly. But he thinks it’s worth exploring the existence of other possible options, like hypothetical particles called axions, which would be extremely light and could form structures differently than cold dark matter.

Another idea, he says, is that perhaps gravity has slightly different effects over vast spatial scales. In that case, gravity’s effects would have gradually changed how the universe took shape, and Einstein’s theory of gravity might need to be modified. 

But to justify such radical solutions, scientists have to be really, really sure about their measurements. That’s where Wendy Freedman, an astronomer at the University of Chicago, comes in. She’s an expert on using pulsating cepheid stars as “standard candles.” These stars have well-known distances and brightnesses that can be used to calibrate measurements of the universe’s expansion. She and her colleagues are making a new Hubble constant assessment with the powerful James Webb Space Telescope, which has 10 times the sensitivity and four times the resolution of Hubble. Her team will compare their results to ACT’s Hubble constant measurements, as well as previous ones from Planck and the South Pole Telescope.

Until then, she argues that caution is warranted when it comes to saying if the model is broken or not. “It’s important to get it right. Planck has set the bar very high. In order to confirm that this is a real discrepancy, you need measurements of the local distance scale that are of comparable accuracy. We’re getting there, but we’re not there yet,” Freedman says.

That said, Freedman thinks it is promising that ACT’s measurements line up with Planck’s, even though they are very different projects. “Here’s another experiment, and they’ve got different detectors, it’s ground-based, they have different frequencies, they have different groups analyzing the data. It’s a completely independent measurement and they’re agreeing extraordinarily well,” she says. 

Other astrophysicists, like Priyamvada Natarajan at Yale who specializes in cosmology, are also impressed by the ACT map. “This is a beautiful piece of work,” she says. 

The ACT collaboration is dramatically improving the precision of cosmological observations, and now theorists need to up their modeling game, she argues. For example, the new findings run contrary to one of the ideas proposed as a resolution for the Hubble tension: “early dark energy.” This theory suggests that the young universe might have contained more of—or a different kind of—dark energy than envisioned in the standard model, and it would have propelled a stronger, earlier expansion. But that theory won’t work if, as the ACT map suggests, the standard model holds up for the first 8 billion years.

Natarajan says this is not the only place where researchers are looking for cracks in the standard model. For example, some physicists using JWST data have argued that big galaxies are forming a bit earlier and structures are assembling faster than expected, implying a cosmic timing problem. Statistical studies have also revealed an apparent timing mismatch between the formation of early galaxies and the formation of the black holes at their centers, possibly another cosmic clocking issue. “There are many other places where tensions are emerging. That is really intriguing. It really does call the model into question, and it behooves us to scrutinize and stress-test it,” Natarajan says.

Freedman has her own kind of independent stress test in the works. In addition to using JWST to make measurements based on cepheid stars, which pulse in a predictable rhythm, she’s also using a different kind of star, called “tip of the red giant branch” stars. These bright objects populate the outer, sparser regions of the Milky Way, making them easier to study than their counterparts in more crowded areas. So far, measurements from these relatively nearby stars suggest an expansion rate closer to what researchers using ACT and Planck have found—which would dissolve the Hubble tension.

It will probably take Freedman and her colleagues a year to complete their observations using the JWST. If they’re out of kilter with the CMB-based projections, they could hint at the “new physics” Madhavacheril is hoping to see. But if they uphold the old model, it may turn out that there’s no cosmological crisis after all.