Scientists know so little about the most dominant force in the cosmos that they refer to it with a catch-all placeholder name: dark energy.
This is the stubbornly mysterious something that’s causing the universe’s expansion to accelerate over time. Now, nearly three decades after dark energy’s discovery, astronomers are letting themselves hope that a breakthrough is almost at hand.
That hope comes courtesy of NASA’s Nancy Grace Roman Space Telescope, which the agency will launch as soon as August 30. When Roman becomes fully operational early next year, it will round out a trio of massive survey projects tackling the enigma of dark energy—which is just in time, as recent work has shown that dark energy may be more complicated than scientists have thought.
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“We’re starting to see tensions between measurements of the early universe and measurements of the late universe,” says Jason Rhodes, a cosmologist at NASA’s Jet Propulsion Laboratory (JPL) in California, who works on the Roman team. “Some of us think these tensions may be indicating that we’re ripe for another revolution in cosmology.”
The Trouble with Dark Energy
Dark energy’s mark on the cosmos was discovered in 1998, when astronomers charting the motions and distances of faraway galaxies clocked that the universe wasn’t just expanding but expanding faster and faster over time. This was a revolutionary finding, in no small part because no one had expected anything of the sort—yet, time after time, tests that could’ve dismissed dark energy as a mirage instead reinforced its reality. So cosmologists dutifully factored in dark energy in addition to cold dark matter—which they had already surmised outweighs the ordinary matter we can touch and see—in their appraisals of what constitutes the vast majority of the cosmos.
For years, in cursory follow-up studies of large-scale cosmic structures, dark energy looked to be equivalent to Lambda, the notoriously messy “cosmological constant” hastily enlisted from Albert Einstein’s early work on his general theory of relativity. Thus was born the Lambda cold dark matter model (Lambda-CDM), the so-called standard model of cosmology, which attempts to boil the universe down to a fundamental recipe that faithfully replicates most everything astronomers can see.
That said, not everyone was fully satisfied. “Lambda is, like, the most boring dark energy model,” says Kevork Abazajian, a cosmologist at the University of California, Irvine. “Nothing changes.”
And things never quite added up perfectly for Lambda-CDM. The most persistent problem has been the infamous Hubble tension, by which the rate of cosmic expansion appears inexplicably different when calculated for the near universe versus the distant universe. In recent years additional discrepancies have arisen for theorists to swat at like a game of cosmic whack-a-mole. There’s also a host of small hiccups in the universe’s timeline, as reconstructed by James Webb Space Telescope observations, with supermassive black holes and complex organic molecules forming earlier in the universe’s history than scientists thought possible. And then there’s the sigma-eight tension, which notes that matter appears to be more evenly distributed on cosmic scales than the standard model predicts.
The newest snag comes from a pair of massive ground-based initiatives, the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI), each designed to map huge numbers of galaxies and allow scientists to better see the effects of dark energy on the universe. DES and DESI are largely independent of each other, but in recent years both have uncovered similar tiny-but-tantalizing hints that dark energy may be changing over time. “That’s gotten everyone in a bit of a tizzy,” says Tamara Davis, an astrophysicist at the University of Queensland in Australia, who is a member of both collaborations.
All in all, cosmologists are increasingly questioning the validity of their vaunted standard model, although no one can decide exactly what comes next. “It’s not surprising that it’s starting to break,” says Alexie Leauthaud, a cosmologist at the University of California, Santa Cruz, and a member of the DESI collaboration. “And that’s good news because we actually have some kind of shadow to chase.”
But theorists have struggled to pull together a new approach that matches the extraordinary success of Lambda-CDM while also eliminating the discrepancies. “The type of cracks we’re seeing don’t look to me so much as fundamental shifts in Lambda-CDM as much [as] kind of random fractures showing up in weird places,” says Adrienne Erickcek, a theoretical cosmologist at the University of North Carolina at Chapel Hill.
The most likely conclusion may be simply that the “dark sector” is more complicated than expected. Perhaps there are other “dark” enigmas waiting to be detected, or perhaps dark matter and dark energy indulge in secret shenanigans, either on their own or via interactions with each other. “The way we describe the dark things we know is very vanilla,” says Adam Riess, a cosmologist at Johns Hopkins University, who shared the 2011 Nobel Prize in Physics for dark energy’s discovery. “They have no sprinkles; they have no chocolate chips; they have no other textures or things going on; they’re just really simple—and maybe that is not the case.”
Roman to the Rescue?
Although Roman has only been in active development for about a decade, the core concept for the mission dates back to just a few years after dark energy’s detection. And while it will produce a flood of data for practically every type of astrophysics, dark energy remains a key pillar of Roman’s work.
Roman will chase down three different ways of detecting dark energy, seeking to more precisely define what its effects on the universe are and whether they’re changing over time. The first one involves surveying the skies for a special variety of supernova that’s so predictable that scientists can use it as a beacon for calculating what a host galaxy’s distance is and how fast it’s moving away from us—the same technique that first betrayed dark energy back in the 1990s. The second entails mapping large numbers of galaxies to discern huge, faint bubbles in the cosmos that represent the fossilized signatures of sound waves that froze out of the universe’s primordial plasma as it expanded and cooled. The sizes of those bubbles scale with the size and growth of the very early universe, offering a powerful probe for studying cosmic change over time. The third tracer uses a technique called microlensing, which reveals clumps of dark matter via their warping of background starlight, to fill in scientists’ still incomplete reckoning of matter’s cosmic distribution.
Roman will join two other major projects in that work: the European Space Agency’s Euclid space telescope, which launched in 2023, and the U.S. National Science Foundation’s Vera C. Rubin Observatory in Chile, which began operations last year. The trio will survey the skies, and their torrents of data will feed massive analyses looking for effects of dark energy that the recent DES and DESI findings could scarcely make out.
“They’re doing exactly what we would want after these DESI results,” says Rhodes, who works on Euclid as well as Roman, of the trio of projects. “They’re coming online to do just what we need at just the right time, so that’s either great foresight or luck, or both.”
The three programs are complementary, Rhodes says. Roman won’t scan the sky as broadly as its counterparts, but it will be able to peer deeper into space than Euclid and produce sharper images than Rubin. The overlap between these surveys will be invaluable for distinguishing dark energy’s true impacts from various possible forms of observational bias, scientists say.
“If Roman and Euclid, for example, get the same answer, that will be a really powerful indication that we’re on the right track,” says Katarina Markovic, a cosmologist at JPL, who works on Roman.
There’s no guarantee that even this powerful trio will be able to fully unveil dark energy, Riess says, but he thinks they should be able to answer the crucial question of whether the phenomenon is changing over cosmic time. That alone should guide theorists struggling to make sense of the standard model’s cracks, Rhodes says—proving what dark energy isn’t, even if no one can yet prove what it is.