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 of 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.
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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