The universe is expanding too fast. Or is it doing so too slow? Or maybe its expansion is just a steady, ominous trundle toward a long-distant nothingness. Cosmologists have grappled with these distinctly different possibilities for generations. Now, at last, the truth may be in sight.
We know the universe is expanding and, thanks to work that won 2011’s Nobel Prize in Physics, a mysterious “dark energy” seems to be accelerating this expansion. “We needed a force that would be responsible for this acceleration,” says Mathilde Jauzac, an astrophysicist at Durham University in the England. Both dark energy and hypothesized but unseen dark matter would account for some 95 percent of the universe’s total mass and energy. But this “dark sector” of the universe remains deeply mysterious, with no direct evidence for its existence. Now a new telescope is set to probe this murky nothingness like never before in hopes of offering the clearest insight yet into the very conditions that formed the cosmos as we know it and glimpsing which fate awaits us all.
On July 1, around 11 A.M. EDT, the European Space Agency’s (ESA’s) Euclid telescope is set to launch from Florida on a SpaceX Falcon 9 rocket. The $1.5-billion mission has had a tough time of late. Euclid was supposed to lift off last year on a Russian Soyuz rocket, but following Russia’s invasion of Ukraine, ESA pulled the plug on the launch and ended its collaborations with Russia. “We were really stranded,” says Giuseppe Racca, Euclid’s project manager at ESA. With a new European rocket, the Ariane 6, facing delays, ESA instead turned to SpaceX and its Falcon 9 . And while the telescope is the same one that was ready to fly on Soyuz in 2022, additional work was needed to support the switch to a new launch vehicle. “We were not immediately compatible with the Falcon 9,” Racca says. Eventually, the hurdle was surmounted, and Euclid was “go” for launch.
That’s music to the ears of cosmologists. “We are so excited,” Jauzac says. Following the launch, Euclid will take a month to travel to Earth’s second Lagrange point, or L2, a point of gravitational stability 1.5 million kilometers from our planet, well beyond the moon’s orbit. (L2 is also where the James Webb Space Telescope resides.) Here it will begin a six-year mission to study one third of the sky, during which it will image some 10 billion galaxies, gazing as far back as 10 billion years into the universe’s 13.8-billion-year history. Thanks to Euclid’s wide field of view, within just two days of beginning its science operations, it will have observed more of the universe than the Hubble Space Telescope since the latter’s launch more than 30 years ago.
Observing in both visible and near-infrared light, Euclid will not only image galaxies but also precisely measure the age of about 30 million of them by picking apart their light in a technique called spectroscopy. The goal of the mission is to produce a map of these galaxies across the universe and probe their apparent shapes, which can be warped by the intervening dark-matter-suffused space their light has travelled through. “We will measure the distortions in the images of these distant galaxies to see what the dark matter distribution is,” says Rene Laureijs, Euclid’s project scientist at ESA.
An extra benefit of this will be another map, an accurate accounting of the three-dimensional distribution of galaxies throughout the universe. That map, Laureijs says, “will tell us how structure has evolved over time from 10 billion years ago until now.” Seeing how this structure has changed over time will give a measure of the expansion of the universe, potentially helping to pin down the nature of dark energy. One leading candidate traces back to a possibility postulated by Albert Einstein in 1917 to as a mathematical “fix” to his general theory of relativity. To prevent the universe collapsing in his equations, he added what he considered a ham-fisted solution: a “cosmological constant” to counteract gravity’s effects and ensure a static cosmos. “Einstein felt unpleasant about it,” says Ofer Lahav, an astrophysicist at University College London. Einstein himself famously called the cosmological constant his “greatest blunder.”
A century later, that blunder looks like an eerily prescient prediction of dark energy. Its exact value, known as w, remains an open question. The most simplistic model says w is –1, meaning the universe will continue expanding at a steadily accelerating rate forever. But if the value deviates slightly, it could point to a universe that will accelerate exponentially, eventually tearing itself apart, or one that will eventually decelerate and collapse in on itself. “If w equals –1, that’s basically boring dark energy that’s just constant,” says Cora Uhlemann, a cosmologist at Newcastle University in England. Euclid, however, may find otherwise and could conceivably measure the value of w as fluctuating over time since the early universe. “It’s so fundamental for our understanding of physics,” Lahav says. “We really have to pin it down.”
That has already been hinted at by the Hubble constant, a measure of how fast the universe seems to be expanding. In the so-called local universe that surrounds us today, this is calculated at around 73 kilometers per second per megaparsec (that is, per every 3.26 million light-years). But in the distant universe, that expansion rate seems to drop to a value of about 67. This “Hubble tension” is one of the most active and contentious domains in all of cosmology, and missions like Euclid could go a long way toward settling it. “I hope we can resolve this tension,” Laureijs says. Other wide-field surveys of the sky that are set to begin soon will help Euclid in its task to understand both dark energy and dark matter—and the universe’s very nature and ultimate fate. On Earth, the Vera C. Rubin Observatory in Chile will begin its own Legacy Survey of Space and Time (LSST) soon, while NASA’s upcoming Nancy Grace Roman Space Telescope is slated to launch as early as 2026 on a strikingly similar mission to Euclid’s. Roman, however, will be able to peer farther into the universe (albeit across a smaller patch of sky). And as a side project, it will also demonstrate technologies for directly imaging exoplanets.
Euclid will have some secondary science it can conduct, too. Ranga-Ram Chary of the California Institute of Technology is leading one of three U.S.-based investigations using the telescope,. The project will employ Euclid to scrutinize gas in some of the early galaxies it shall see. “By studying [a galaxy’s gas], we want to understand how many stars are forming, how enriched is the galaxy and what the physical conditions are,” he says. Another possibility might be to use the telescope to assist Roman in its exoplanet science, providing complimentary observations of stars in our galaxy to look for the gravitational tug of planets—or even moons orbiting some of these planets, known as exomoons. “Fingers crossed we’ll get that data,” says Eamonn Kerins, an exoplanet scientist at the University of Manchester in England and lead of the Euclid Exoplanet Science Working Group, who predicts it could improve some of Roman’s exoplanet data by a factor of five.
The primary goal is, of course, to enlighten us about the dark universe. For decades astronomers have wondered how the cosmos grew and took shape and what its ultimate fate will be. Thanks to Euclid, we should get closer than ever before to answering those questions. “Euclid is absolutely unique,” Jauzac says. “It will completely transform our view of cosmology.”
