Unlocking the Shadowy Cosmos: How the Nancy Grace Roman Space Telescope Will Revolutionize Astrophysics
For decades, astronomers have known an invisible substance fills every corner of the cosmos. Only detectable through its gravitational pull on visible matter, dark matter makes up roughly 85% of all mass in the universe, wrapping entire galaxies in massive, branching, interconnected spherical halos that form the backbone of our universe. Yet the exact details of this grand cosmic architecture remain deeply mysterious. What is more, the universe is constantly expanding at an accelerating rate, driven by an enigmatic force researchers call dark energy.
A new NASA space telescope, scheduled to launch no earlier than August 30 from the agency’s Kennedy Space Center in Florida, aims to untangle the fundamental nature of these dark intergalactic puzzles. Boasting the same sharp resolution as the Hubble Space Telescope paired with a field of view roughly 100 times larger, the Nancy Grace Roman Space Telescope will open an entirely new window on the universe that could help solve some of astrophysics’ biggest open questions.
“An observatory like Roman is inherently a discovery machine,” says Julie McEnery, NASA’s senior project scientist for the mission. “We’re going to find rare objects, unusual phenomena, entirely new discoveries, and results that will surprise us.”
Over its five-year primary mission, Roman will map enormous swathes of sky to build a holistic, comprehensive portrait of galaxies and dark matter, helping scientists trace how the universe’s largest structures have evolved over time. Named for NASA’s first chief astronomer, the telescope will map cosmic structures by measuring subtle gravitational distortions of light, allowing researchers to study exactly how dark energy drives cosmic change. “We aren’t just measuring a single property of something in the universe,” McEnery explains of the work. “We’re working to understand how the universe we inhabit fundamentally works.”
Roman’s sharp, wide field of view is also projected to detect up to 200,000 new exoplanets—an extraordinary leap from the roughly 6,300 confirmed exoplanets documented to date. Most of these new worlds will be spotted when they block a small fraction of their host star’s light during a transit, but the telescope’s extreme sensitivity will let it uncover an additional 1,000 or so planets by measuring the gravitational effect these worlds have on starlight. Given the telescope’s powerful resolution and massive survey area, no one can predict exactly what other discoveries it will make. “The most exciting science from Roman,” McEnery says, “may well be something we cannot even imagine right now.”
Whole New Worlds
In many ways, Roman will act as a complementary partner to the James Webb Space Telescope. Both primarily observe infrared light, and like Webb, Roman will orbit at Lagrange Point 2, a spot nearly 1 million miles from Earth where telescopes can easily block out interfering light from the Sun, Earth, and Moon. But where Webb is designed to peer deep into small patches of the distant cosmos, Roman is built to capture sweeping, broad views of the sky.
“If we look at what the James Webb Space Telescope does, it’s optimized to study the very distant universe … but just a tiny patch of sky,” says Rachel Mandelbaum, an astrophysicist at Carnegie Mellon University. “In contrast, Roman can observe a huge area of the sky all at once.”
This broad view lets Roman scan hundreds of millions of stars in search of new planets. Like previous exoplanet missions, the observatory will find most of its targets by looking for planets that transit across the face of their host stars. This method is highly effective for finding large planets with short orbital periods, but Roman will use a second technique called microlensing to detect entirely different types of worlds.
When two stars align perfectly with one in front of the other from Earth’s perspective, the gravity of the foreground star warps and magnifies light from the more distant background star. If the foreground star hosts a planet, Roman can detect how the planet’s gravity adds an extra boost to that magnified light. “This method can detect planets that are farther from their stars than other methods can reach,” says Matthew Penny, an exoplanet researcher at Louisiana State University. “You don’t need to wait for the planet to complete an entire orbit before you can spot it.”
Roman is so sensitive to these microlensing signals that it will be able to detect planets with less mass than Mercury. It will also be capable of spotting free-floating “rogue planets” that have been ejected from their home star systems or formed on their own in interstellar space. By studying these new populations of planets, scientists can learn more about the conditions that create systems like our own, and by extension, life-bearing planets like Earth.
“Our solar system is potentially quite unique, and we don’t really know that yet,” Penny says. “There are a lot of seemingly lucky coincidences in our solar system that make life possible.” Probing the outer regions of planetary systems will fill key gaps in this puzzle. For example, Penny notes, the formation of large planets far from their host star can let smaller rocky planets form closer in, within the habitable zone where liquid water can exist. “Without a complete census of the whole range of possible planets, we don’t really know how common planets like Earth are, or planetary systems like our own.”
The Dark Universe
In addition to thousands of new worlds, Roman’s gaze will capture more than a billion galaxies. It will specifically focus on galaxies that formed between 2 billion and 6 billion years after the Big Bang, a critical era in cosmic evolution.
“One of the things we try to study is the history and growth of cosmic structure,” Mandelbaum says. “In the very early universe, matter was almost uniformly distributed. But there were tiny perturbations, tiny regions that were just a fraction more dense or less dense than surrounding space.”
Over time, those slightly denser regions attracted more and more material, eventually forming immense clumps of dark matter scientists call halos. Gas then sank to the center of these dark matter halos, where it ignited to form stars that grouped together to create galaxies.
Roman will build an extraordinarily detailed map of visible galaxies, and will map the distribution of dark matter with greater precision than ever before using a technique called weak gravitational lensing. “Anytime a light ray from a distant galaxy travels through the universe, past all kinds of structures, to reach us, the path of that light ray gets slightly perturbed,” Mandelbaum says. By comparing these perturbed light signals from thousands of galaxies, scientists can trace the cosmic web of dark matter that surrounds and connects galaxy clusters.
Comparing the maps Roman generates to observations of the infant universe will let scientists study exactly how cosmic structures have changed over time. “We’re trying to answer fundamental questions about the universe, and it turns out to be incredibly challenging to do that,” Mandelbaum says. “We need really powerful observatories like Roman.”
Addressing Cosmic Mysteries
In recent years, three notable discrepancies have emerged in cosmological research that challenge our current standard model of the universe. First, the universe appears to be expanding faster today than models built from early Big Bang conditions predict. Second, matter across the universe is distributed more evenly than researchers expected. Third, recent data from the Dark Energy Spectroscopic Instrument (DESI) in Arizona suggests dark energy—the force driving cosmic expansion—may be unexpectedly weakening over time.
These three “tensions,” as scientists call them, each point to gaps in the standard cosmological model that may be hiding key details. “It’s kind of tantalizing, because there’s hints that something is very wrong with our current understanding,” McEnery says.
Roman will deliver valuable new data to unpack each of these unsolved mysteries. For studies of the universe’s expansion rate, the telescope is expected to capture tens of thousands of supernova explosions, several times more than have been observed to date. Specific types of supernovae act as standard cosmic markers to measure the current expansion rate, meaning Roman will let researchers calculate this value with far greater accuracy.
The observatory’s maps of large-scale cosmic structures will also give researchers a clearer picture of how clumped matter is across the universe. And its measurements of dark matter’s gravitational effects will create a detailed record of how these structures grew over time, letting scientists test whether dark energy’s strength really has changed as DESI’s data suggests. If it has, “we’re pretty much guaranteed to be able to confirm what they’re seeing, because it’s exactly the thing we were designed to do,” McEnery says.
Roman’s vast view of the universe will reveal countless new details, and as scientists piece together this enormous cosmic puzzle, they may revolutionize our understanding of cosmology. “We had a major breakthrough in the last century on the very small with the development of quantum mechanics and the discovery of elementary particles,” McEnery says. “Maybe this is the century of understanding the very big.”
