U. astrophysicists to join Euclid telescope project

Although dark matter and dark energy comprise about 95 percent of the universe, their nature remains unknown. But three Princeton astrophysicists and a graduate alumnus will seek to advance human knowledge of our universe by participating in the European Space Agency’s Euclid space telescope project.

Senior research scientist Robert Lupton, astrophysics professor and chair David Spergel and astrophysics professor Michael Strauss have been nominated to the 43-person team based out of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. The team will be led by JPL researcher Jason Rhodes GS ’99.

The team is one of three American teams working in the Euclid Consortium, an international collaboration with the ESA. The Euclid space telescope’s six-year study, expected to launch in 2020, will allow scientists to characterize the distribution of dark matter and dark energy in the observable universe.

According to Rhodes, the United States will provide 16 infrared cameras to the ESA. NASA will also assist by developing software in preparation for the 2020 launch.

Measuring dark matter and dark energy

“We’ve known about dark matter for about 80 years, but it’s still very mysterious,” Rhodes said.

Current astrophysics theories hold that dark matter has mass like ordinary matter but is invisible because it does not interact with light, Institute for Advanced Study professor Matias Zaldarriaga explained. Its presence, however, can be inferred through its gravitational effects. Just as the orbit of planets around a central point necessitates the presence of a massive body, like the sun, observations of the motion of clusters of galaxies have indicated a missing mass, or dark matter, he said.

Once Euclid is launched, however, scientists will be able to gain a clearer understanding of dark matter’s characteristics. The telescope has a wide field of view that will allow it to take very detailed pictures of more than a third of the observable universe, unlike previous space telescopes like the Hubble. It also has advantages over earth-based telescopes because it avoids distortions caused by earth’s atmosphere.

“One thing that we can imagine doing [with the Euclid study] is measuring the basic properties of dark matter and dark energy in the context of our cosmological model,” Strauss said, referring to the current theoretical understanding of the universe.

Lupton and Spergel could not be reached for comment because they were traveling.

The study will also attempt to characterize dark energy, the force driving the increasing rate of expansion of the universe. This cosmic acceleration was discovered in 1998, but scientists are not yet sure if this observation is consistent with a constant proposed by Einstein or with competing theoretical models.

Euclid will use two experimental techniques, gravitational lensing and baryon acoustic oscillation, to precisely measure dark matter and dark energy levels in the universe.

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“Weak gravitational lensing is a very powerful probe of the way that dark matter is distributed and what its properties might be,” Colin Hill GS, who studies astrophysics and is a student of Spergel’s, said. Foreground galaxies with enough observable mass and dark matter can distort the light emitted from background galaxies, so the distribution of dark matter throughout the universe can be measured by averaging the distortions over a large sample of galaxies.

Through what is known as baryon acoustic oscillation, high-precision cameras will measure the clustering of distant galaxies. Because of the wide viewing range of the Euclid telescope, the distance between millions of galaxies can be more precisely determined. This standard measurement can be used to track the expansion of the universe over time, from which a characterization of dark energy driving this expansion can be inferred.

A theorist-experimentalist collaboration

The University has a long history of involvement with large astronomical surveys such as Euclid, Strauss said. He added that he is currently collaborating with the National Astronomical Observatory of Japan on the Subaru telescope based in Mauna Kea, Hawaii.

“This Euclid project speaks to our strengths and to our scientific interests with the questions of dark matter and dark energy,” Strauss explained. “It is a natural match to things that people at Princeton are interested in.”

Though it is an observational survey, Euclid will require collaboration between experimentalists and theorists.

“The line between theorists and observers is breaking down,” Strauss said. “These big surveys require people who know about all of these aspects.”

Euclid will provide the data necessary to narrow down the number and types of competing dark energy theories, according to Rhodes. Zaldarriaga said that theorists like him are also enthusiastic about the prospects of the Euclid data set.

While Euclid aims to characterize dark matter and dark energy, it allows potential applications to other areas of research, such as the evolution of galaxies and supernova.

“This is going to be a very powerful survey,” Hill said. “There are probably applications of the data set that people haven’t even thought about yet.”