C. Carreau / ESA An artist's impression of Euclid.

Scientists are in the business of solving mysteries—or trying to, anyway. That’s true across all disciplines, but astronomers and physicists are the only ones who get to think about questions that are literally cosmic.

And even in that rarefied category, in which the subject matter ranges from black holes to neutron stars to the search for Earth-like planets across interstellar space, it doesn’t get any more esoteric than the ongoing quest to uncover the secrets of dark matter and dark energy. Together, they make up a whopping 96 percent of the cosmos—but to this day, nobody can say with any confidence what either one of them actually is.

The European Space Agency (ESA) is hoping to change all that: by 2020, if all goes according to plan, the Euclid space mission will go into orbit, trying to sniff out the nature of these similarly named but (presumably) unrelated phenomena. And now, NASA is on board as well: a few weeks ago, the space agency formally joined the Euclid project, funding 43 U.S.-based scientists to work with their international counterparts. “Once Europe commits to a mission, they do it,” says Charles Bennett, of Johns Hopkins University, part of the NASA contingent, who has served on “more committees than I care to think about” trying to get a similar U.S. undertaking off the ground. While America dithered, Europe moved, and NASA’s best option to participate in the cutting-edge research meant accepting an unfamiliar supporting position on the mission.

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The matter of whose flag goes on the telescope pales, however, compared with the grandeur of the questions Euclid could help answer. The mystery of dark matter goes all the way back to the 1930’s, when Caltech astronomer Fritz Zwicky noted that some galaxies seemed to be orbiting each other so fast that they should be slowly separating—each galaxy remaining discrete and intact, but the distances among them opening wider and wider. In the 1960’s, the Carnegie Institution’s Vera Rubin and others realized that something similar ought to be true within individual galaxies—that they were whirling so fast they should rip themselves apart. And by the 1980’s, astronomers were forced to accept the idea that the gravity from some mysterious, invisible form of matter had to be holding them all together.

Today, the consensus is that dark matter consists of vast clouds of some still-undiscovered subatomic particle that surround galaxies and galactic clusters. The shapes and sizes of those clouds could provide a valuable clue to the particles’ properties, and while Euclid can’t see the clouds directly (“dark” here is a synonym for “utterly invisible”), it can deduce their shapes and sizes by looking at the galaxies that lie beyond them.

“We’ll use a technique called ‘weak lensing,’” says Jason Rhodes, of the Jet Propulsion Laboratory, the research leader of the NASA contingent. “It’s my particular area of interest, and it’s what got me interested in Euclid in the first place.” The idea, based on Einstein’s General Theory of Relativity, is that a massive foreground object warps spacetime, distorting the images of objects in the background. To map out the dark matter in a nearby cluster of galaxies, therefore, you look at the distortions of thousands of other galaxies behind it; the pattern of distortions tells you the size and shape of the dark-matter cloud that must have caused it.

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Dark energy is something entirely different—indeed, in some ways it’s the exact opposite: it’s a still-unknown force, discovered in the 1990’s, that makes the universe expand faster and faster all the time. (Einstein originally came up with this idea, but eventually abandoned it). You can think of dark energy as a type of antigravity, but exactly what type — whether it fluctuates in strength over time, for example — is yet to be determined.

Euclid will tackle this problem as well, by looking at the distances among tens of millions of galaxies at many different stages of cosmic history—with objects more distant from Earth representing images that come to us from earlier in time. Using measurements of the primordial light left over from the Big Bang, theorists can predict how those distances should change as the universe evolves, both with and without dark energy in its various possible forms. By comparing the theories with what Euclid actually sees, they’ll be able to get a handle on which theory matches what’s happening in the cosmos.

(MORE: Dark Energy: Astronomers Inch Toward Solving Space Riddle)

Euclid probably won’t solve the mysteries of dark matter or dark energy by itself. “These are big questions,” says Bennett, “and it’s not possible to do everything with one satellite.” At the same time the Euclid team is gearing up, physicists are thus searching for individual dark-matter particles here on Earth, for example, since they should pervade, not just surround, the Milky Way. But without complementary measurements from Euclid and other cosmic dark-matter searches, says Rhodes, “we wouldn’t know how dark matter behaves in bulk.”

As for dark energy, telescopes on the ground are already trying to do the same sort of measurements Euclid will do from space, and while the Europeans’ orbiting telescope will be free from atmospheric distortion, its relatively small mirror means it can’t easily detect light from the faintest galaxies. In coming years, the U.S.-built Large Synoptic Survey Telescope could significantly expand this ground-based campaign. And a new Hubble-like space telescope donated to NASA free of charge by the National Reconnaissance Office could become Euclid’s partner in orbit.

That growing mix of high-tech eyes looking steadily up and out is exactly what’s needed for puzzles as big as dark mater and dark energy, says Bennett. “My advisor at MIT, Bernie Burke, used to tell me ‘you can’t discover anything unless you’re looking at the sky,’” he recalls. Obvious? Maybe, but when you’re trying to answer these most cosmic of questions, it’s not a bad thing to keep in mind.

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