It’s an exhilarating and sobering thought: All the planets, galaxies, starlight, and other objects that we can see and measure in the universe make up just 5 percent of existence. The other 95 percent are eaten up by two enigmas, dark matter and dark energy, known to scientists by their apparent gravitational effects on the surrounding universe, but not directly detectible.
On July 1, however, a new European Space Agency mission could help scientists get a little closer to solving the twin mysteries of dark matter and dark energy. The Euclid space telescope will take flight from Cape Canaveral Space Force Station no earlier than 11:11 a.m. EDT atop a SpaceX Falcon 9 rocket. NASA will live stream the launch beginning at 10:30 a.m.
Following blastoff, Euclid will take about 30 days to reach its operational orbit around Lagrange Point 2 (L2), an area a million miles toward the outer solar system where Euclid can maintain a constant position relative to Earth. The James Webb Space Telescope also orbits L2.
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Once on location and operational, Euclid will begin what is expected to be a six-year mission where it will survey around a third of the sky, carefully measuring the shapes of billions of galaxies up to 10 billion light-years away to catch a glimpse at the ways dark matter and dark energy shape our cosmos. To do that, the roughly 4,600-pound space telescope will use its four-foot-wide primary mirror to collect and focus visible and near-infrared wavelengths of light on two instruments: the VISible instrument camera and Near-Infrared Spectrometer and Photometer, which helps determine the distance to far off galaxies.
“The awesomeness of how many galaxies Euclid will be able to measure and at what amazing precision—it’s just an amazing feat of human engineering,” says Lindley Winslow, a professor of physics at MIT who designs experiments to detect dark matter, but is not directly involved with this mission. “The fact that we can do precision cosmology is awesome.”
Cosmologists, who study the formation, evolution, and structure of the universe, have a model called Lambda-CDM that might explain why everything is the way it is. Lambda is the cosmological constant, the force that appears to be causing the universe to expand at an accelerating rate and which scientists believe is related to or manifests in mysterious dark energy. CDM stands for “cold dark matter,” which interacts with normal matter gravitationally.
”Those are the two ingredients that have sculpted the universe that we know,” Winslow says. Dark energy drives universal expansion, while “in the early universe, it was this cold dark matter that pulled visible matter that we see now into potential wells, that then allowed it to contract and form galaxies and stars.”
Lambda-CDM helps us construe a lot of the large-scale universe, according to Winslow, but it doesn’t tell us how it fits together with the theory that explains how the small scale universe works: the Standard Model of particle physics. Euclid is one of several attempts to learn more about how the universe expands and revise Lambda-CDM.
“What we’re really interested in is, can we get more data? Winslow says. “And can we find something that Lambda-CDM doesn’t explain?”
To hunt for that evidence, Euclid will use a technique known as weak gravitational lensing. This is similar to the strong gravitational lensing technique employed by JWST, where the mass of a foreground object, such as a galaxy cluster, is used to magnify a more distant background object. With weak gravitational lensing, scientists are more interested in the way the mass of the foreground objects, including dark matter, creates subtle distortions in the shape of background galaxies.
“We’re using the background galaxies to learn about the matter distribution in the foreground,” says Rachel Mandelbaum, an astrophysicist at Carnegie Mellon University who is a member of the US portion of the Euclid Consortium, a group of thousands of scientists and engineers. “We’re trying to measure the effects of all of the matter between the distinct galaxy shape and us.”
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This method will also help them measure the effects of dark energy, Mandelbaum adds. Since dark matter helps all other forms of matter clump together, and dark energy counteracts the gravitational effects of dark matter, by measuring how clumpy matter is over a range of distance from Earth, “we can measure how cosmic structure is growing and use that to infer the effects of dark energy on the matter distribution.”
Euclid will not be the first large sky survey using weak gravitational lensing to search for signs of dark matter and dark energy, but it will be the first survey of its kind in orbit. Previous studies, such as the Dark Energy Survey, have all been conducted by ground-based telescopes, according to Mandelbaum. Being up in space offers a different advantage.
“Ground-based telescopes see blurrier images than space-based telescopes because of the effects of the Earth’s atmosphere on the light of distant stars and galaxies,” Mandelbaum says. Euclid’s view from L2 will be helpful when “we’re trying to measure these very subtle distortions in the shapes of galaxies.”
But dark matter and dark energy are tough enigmas to crack, and scientists can use all the data they can collect, from as many angles as possible. The Vera Rubin Observatory, currently under construction in Chile and scheduled to open in 2025, will host the ground-based Legacy Survey of Space and Time and scan the entire southern sky for similar phenomena. Efforts like these will help ensure the reproducibility of findings by Euclid, and vice versa, according to Mandelbaum.
”Euclid is a really exciting experiment within a broader landscape of surveys that are trying to get at the same science, but with very different datasets that have different assumptions,” she says. “They’re going to be doing somewhat different things that give us a different approach to answering these really fundamental questions about the universe.”
