From Earth, the night sky looks fairly static. Sure, the stars rotate from evening to evening, and the planets move among them. But from a terrestrial perspective, the celestial sphere appears essentially unchanging.
Perception, though, is not reality: our eyeballs don’t hint that beyond nearby planets, stars and galaxies, everything is moving away from us. The universe is constantly expanding—at an ever faster rate.
“When we say that the universe is expanding, we mean something pretty literal,” says Dan Scolnic, an associate professor of physics at Duke University, who studies this cosmic growth. “I think it’s a little different than how people think of it. But we mean that the distance that objects are away from us—particularly other galaxies—is increasing.”
Scientists don’t currently know whether that expansion will continue indefinitely or, if so, whether it will keep accelerating ad infinitum. The universe’s ultimate end state—whether it will expand so rapidly that it will tear itself apart, continue to calmly enlarge and cool off or eventually reverse and contract in on itself—will be determined by the balance of dark matter, dark energy, and regular matter and energy in space. The two unknown, or dark, parts of that equation make up 95 percent of the universe, and their nature continues to elude scientists, who don’t know how the contributions of those components to the universe’s life story might change over time.
For about 100 years, scientists have known about cosmic expansion and that it was a consequence of the big bang—when all the matter and energy in the cosmos exploded (although this is an imperfect metaphor) from a single, dense, hot point and spread outward, expanding space itself as they went. Scientists expected this expansion would slow as the universe aged; the gravitational attraction between bits of matter would act as a brake. And that was true—for a while. But it wasn’t the end of the story.
Humans—from the dawn of our existence to today—seem to live right around the cosmic era that this slowdown in expansion turned into a speedup. Astronomers have detected this flip, although it took them a while. “By the end of the last century [specifically, 1998], we start realizing that the universe isn’t just expanding,” Scolnic says. “It’s accelerating its expansion.”
The explanation for that press on the gas pedal is somewhat unsatisfying. It’s caused by the presence of “dark energy,” a term that describes what is happening but not why. “It’s something that we still don’t understand at a fundamental physics level,” says Wendy Freedman, a professor of astronomy and astrophysics at the University of Chicago. “What is causing that acceleration?”
Dark energy permeates the empty part of the universe—the vacuum. Whatever “it” is, it exerts a repulsive force that pushes everything apart and tugs against gravity. “The more space there is, the more things get pushed away from each other,” Scolnic says, “which means that the universe will expand faster and faster, and things will get pushed away from each other faster and faster.”
To understand what cosmic expansion looks like right now, scientists can observe astronomical signals to measure the so-called Hubble constant. This number represents the current ballooning rate of the universe.
Scientists have several methods of finding this number. The approaches include looking at supernovae and variable stars in distant galaxies and measuring how fast they’re receding, as well as how far away they are. Freedman led a collaboration in the 1990s called the Hubble Space Telescope Key Project, which calculated the Hubble constant more precisely than anyone had before.
But in recent years, astronomers have found that that calculation—and the results of different teams—including Scolnic’s group Supernova H0 for the Equation of State (SH0ES)—don’t match the Hubble constant that other scientists have calculated based on data from the universe’s early years, long before supernovae and variable stars were ever born.
This mismatch is called the “Hubble tension.” It could point to a problem with the way researchers have taken or interpreted the data—or it could be the universe screaming at scientists that they don’t understand its evolution, which would mean they can’t predict its fate either. “If it’s a true disagreement, it’s really important,” Freedman says. “Because it’s suggesting there’s physics that we don’t know about.”
For a long time, Scolnic says, scientists had a story of the universe’s expansion—and its related array of dark matter, dark energy, and light matter and energy—that made sense. But now, he says, the narrative doesn’t quite add up. “That story of what got us to this place will then determine our ultimate destiny—how the universe keeps growing, keeps changing,” he continues. To learn the true end of the story, scientists may need to give the middle chapters a revision.
If physicists’ basic picture of how the universe works is correct, we’re in for a big freeze: the cosmos will keep expanding moderately faster, matter will spread out, stars will die, no new stars will be able to form, and space will go dark and cold with a whimper.
If, on the other hand, dark energy works differently than we assume, we might be heading toward another ending. If, for instance, its strength changes over time and gets stronger as the universe progresses, we’re in for a big rip: the universe will expand fast enough to tear itself apart. “Either way, the answer to the original question is, yes, you’re expanding forever,” Scolnic says. “It’s just kind of a question of violence.”
But not everyone agrees. Paul Steinhardt, a theoretical physicist at Princeton University, says theorized forms of dark energy could be time-dependent in a different way. “It goes from causing the universe to accelerate its expansion to eventually slowing its expansion to eventually slowing it to a halt and then to begin to contract,” he says. That’s called a big crunch. In some cosmological models that Steinhardt is investigating, the contraction might turn back into expansion and produce a kind of cyclical universe.
The gist, as all these possibilities suggest, is that no one knows for sure. “We’ve lost that predictability,” Freedman says.
She’s hopeful, though, that the new James Webb Space Telescope might provide some answers with its ability to see farther and better than past instruments. “I think science proceeds in this way,” Freedman says. “We don’t yet understand what’s going on. And sometimes it takes a very long time.”