Astronomers have known for decades that an invisible substance permeates the cosmos. Detected only by its gravitational influence on visible objects, dark matter accounts for some 85 percent of all matter, enveloping entire galaxies in great spheroids that branch and interweave to form the scaffolding of the universe.
But the specifications of this grand architecture remain shrouded in mystery. Driven by a ghostly force that scientists call dark energy, the universe is ever-changing, constantly expanding at an accelerating rate.
A new space telescope, launching as soon as August 30 from NASA’s Kennedy Space Center in Florida, will attempt to untangle the nature of these dark intergalactic phenomena. With the resolution of Hubble and a field of view about 100 times as big, the Nancy Grace Roman Space Telescope will provide a new way to look at the universe that could help solve some of the biggest questions in astrophysics.
“An observatory Roman is intrinsically a discovery machine,” says Julie McEnery, NASA’s senior project scientist for the space telescope. “We’re going to find rare things, unusual things, new things, surprising things.”
During its five-year primary mission, Roman will map expansive swaths of space to develop a comprehensive picture of galaxies and dark matter, helping scientists understand how the universe’s largest formations change over time. The telescope, named for the first chief astronomer of NASA, will trace astrophysical structures by measuring subtle distortions of light caused by gravity, allowing scientists to study how dark energy is driving cosmic evolution.
“We’re not measuring the property of something in the universe,” McEnery says of this work. “We’re understanding how the universe we’re in fundamentally works.”
Roman’s sharp, wide view is also expected to capture as many as 200,000 new planets, an extraordinary leap from the roughly 6,300 confirmed exoplanets. The telescope will spot most of these worlds as they block some of the light from their host stars, but its high sensitivity will allow it to detect an additional 1,000 or so by measuring the planets’ gravitational effects on starlight.
Given the space telescope’s powerful resolution and wide area of study, there’s no telling exactly what else it may find.
“The most exciting science from Roman,” McEnery says, “may well be something that we can’t even imagine now.”
Whole New Worlds
In many ways, Roman will be a partner to the James Webb Space Telescope. Both primarily observe infrared light, and Webb, Roman will orbit a point in space nearly a million miles away from Earth called Lagrange Point 2, where the telescopes can effectively block out light from the sun, Earth, and moon.
But where Webb is designed to peer deep into the cosmos, Roman will take a wide view.
Photograph: NASA/Sydney Rohde
“If we consider what the James Webb Space Telescope is doing, it’s optimized towards looking at the very distant universe … but just a tiny patch of the sky,” says astrophysicist Rachel Mandelbaum of Carnegie Mellon University. “In contrast, Roman is able to look at a large area of the sky at once.”
That broad view will allow Roman to scour hundreds of millions of stars in search of planets. Similar to previous missions, the space observatory will find most of its targets by looking for planets that transit in front of their host stars. This is a highly effective technique to find large planets with short orbits, but to look for different kinds of worlds, Roman will use another trick called microlensing.
When two stars line up one in front of the other, the light from the background star is warped and magnified by the gravity of the star in the foreground. If that foreground star happens to host a planet, Roman will be able to detect how the planet’s gravity further magnifies the starlight coming from behind.
“This method can detect planets that are farther away from their stars than other methods,” says Matthew Penny, an exoplanet researcher at Louisiana State University. “You don’t need the planet to complete an entire orbit before you see it.”
Roman is so sensitive to these microlensing effects that it should be able to detect planets with less mass than Mercury. It will also have the ability to spot free-floating “rogue” planets that have been cast away from their stars or that formed alone in the void.
By studying these new populations of planets, scientists can learn more about the conditions that may produce a system our own and, by extension, a life-bearing planet Earth.
“Our solar system is potentially quite unique, although we don’t really know at this point,” Penny says. “There’s a lot of potentially lucky coincidences in the solar system that make life possible.”
Probing the outer regions of planetary systems will provide key missing pieces to this puzzle. For example, the formation of large planets farther from a star may allow smaller planets to form closer in, Penny says, near the habitable zone where liquid water could form.
“Without a complete census of the whole range of possible planets, we don’t really know how common planets Earth are,” Penny says, “or planetary systems our own.”
The Dark Universe
In addition to many new worlds, Roman’s gaze will fall upon more than a billion galaxies. In particular, it will focus on a population of galaxies between about 2 billion and 6 billion years after the Big Bang.
“One of the things that we try to study is the history and growth of cosmic structure,” Mandelbaum says. “In the very early universe, the matter was almost uniformly distributed. But there were tiny perturbations or tiny regions that were just a fraction more dense or less dense.”
Those slightly denser regions attracted more and more material over time, eventually forming immense clumps of dark matter that scientists call halos. Gas was then driven to the center of these dark matter haloes, where it ignited stars that swirled together to form galaxies.
Roman will create an extraordinarily detailed map of visible galaxies, and it will map the distribution of dark matter with more precision than ever before using a technique called weak gravitational lensing.
“Anytime we have a light ray from one galaxy that has to pass through the universe, including all kinds of structures, to us, the path of that light ray is getting slightly perturbed,” Mandelbaum says.
By comparing those perturbed light rays from many galaxies, scientists can trace the cosmic web of dark matter that surrounds and connects clusters of galaxies. Comparing the maps generated by Roman to observations of the infant universe will allow scientists to study how things 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 Roman.”
Addressing Cosmic Mysteries
In recent years, three notable aberrations have cropped up in the study of cosmology. The first is that the universe appears to be expanding faster today than it should be, according to models based on the conditions shortly after the Big Bang. The second is that the universe’s matter is distributed more evenly than expected.
And the third is a surprising recent find from the Dark Energy Spectroscopic Instrument (DESI) in Arizona that suggests dark energy, the force driving the expansion of the universe, may be unexpectedly weakening over time.
NASA / Sydney Rohde (Rocz)
These three tensions, as scientists call them, each point to an area where the standard model of cosmology may be missing key details.
“It’s kind of tantalizing, because there’s hints that something is very wrong,” McEnery says.
For each of these unsolved mysteries, Roman will provide valuable data. Regarding the expansion rate of the universe, the space telescope is expected to capture tens of thousands of supernova explosions, several times as many as have been observed so far. Certain supernovae can be used to measure the expansion rate of the universe today, meaning Roman will allow researchers to calculate this value more accurately.
The observatory’s maps of large-scale structures will also give researchers a better sense of the universe’s matter distribution, or clumpiness. And its measurements of dark matter’s gravitational effects will provide a detailed picture of how these structures have grown over time, allowing scientists to test whether the strength of dark energy truly has been changing as DESI 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 put the immense puzzle together, 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.”
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