Dark Energy (noun, “Dark EN-er-jee”)
Dark energy is a mysterious force causing the universe to expand faster and faster. No one knows exactly what it is. But if it keeps stretching out space, it may someday tear the cosmos to shreds.
The universe has been expanding ever since the Big Bang, about 14 billion years ago. But scientists long thought that gravity would rein in this expansion. Perhaps the universe would keep swelling, but more slowly or someday gravity might cause the universe to collapse back in on itself. That doomsday scenario is called the “Big Crunch.”
In 1998, though, those predictions were upended. Astronomers had been peering at supernovae — the explosions of distant stars. Measuring the distances to those explosions let scientists calculate how fast the universe was expanding. And the results shocked them. The universe appeared to be flying apart faster than ever before. Even now, scientists can’t explain why. But they have dubbed the phantom force pushing the cosmos apart “dark energy.”
Dark energy can’t be measured directly. But scientists can estimate how much there is based on how fast the universe is expanding. Dark energy makes up about 70 percent of all the contents of the universe. (Those contents include both matter and energy.) Another 25 percent of the total stuff in the universe is an invisible substance called dark matter. The rest — a measly 5 percent — is normal matter. That’s the stuff that makes up all visible objects in the universe.
The nature of dark energy is one of science’s greatest mysteries. Maybe it is a property of space. Maybe it’s some kind of energy fluid or field that fills space. Some theorists have dubbed that cosmic broth “quintessence.” Others think the expanding universe could be explained by a new theory of gravity.
Since we don’t know what dark energy is, it’s hard to predict how it will behave. Far in the future, perhaps dark energy will overcome the forces holding the universe together. The universe would then tear itself apart. Such runaway expansion is called the “Big Rip.” So dark energy is key not just for understanding the cosmos today. It’s also key to understanding the ultimate fate of the universe.
Twice a day, seven days a week, from February to November for the past four years, two researchers have layered themselves with thermal underwear and outerwear, with fleece, flannel, double gloves, double socks, padded overalls, and puffy red parkas, mummifying themselves until they look like twin Michelin Men. Then they step outside, trading the warmth and modern conveniences of a science station (foosball, fitness center, 24-hour cafeteria) for a minus-100-degree Fahrenheit featureless landscape, flatter than Kansas and one of the coldest places on the planet.
They trudge in darkness nearly a mile, across a plateau of snow and ice, until they discern, against the backdrop of more stars than any hands-in-pocket backyard observer has ever seen, the silhouette of the giant disk of the South Pole Telescope, where they join a global effort to solve possibly the greatest riddle in the universe: what most of it is made of.
For thousands of years, our species has studied the night sky and wondered if anything else is out there. Last year we celebrated the 400th anniversary of Galileo’s answer: Yes. Galileo trained a new instrument, the telescope, on the heavens and saw objects that no other person had ever seen: hundreds of stars, mountains on the Moon, satellites of Jupiter. Since then we have found more than 400 planets around other stars, 100 billion stars in our galaxy, hundreds of billions of galaxies beyond our own, even the faint radiation that is the echo of the Big Bang.
Now scientists think that even this extravagant census of the universe might be as out-of-date as the five-planet cosmos that Galileo inherited from the ancients. Astronomers have compiled evidence that what we’ve always thought of as the actual universe—me, you, this magazine, planets, stars, galaxies, all the matter in space—represents a mere 4 percent of what’s out there. The rest they call, for want of a better word, dark: 23 percent is something they call dark matter, and 73 percent is something even more mysterious, which they call dark energy.
“We have a complete inventory of the universe,” Sean Carroll, a California Institute of Technology cosmologist, has said, “and it makes no sense.”
Scientists have some ideas about what dark matter might be—exotic and still hypothetical particles—but they have hardly a clue about dark energy. In 2003, the National Research Council listed “What Is the Nature of Dark Energy?” as one of the most pressing scientific problems of the coming decades. The head of the committee that wrote the report, University of Chicago cosmologist Michael S. Turner, goes further and ranks dark energy as “the most profound mystery in all of science.”
The effort to solve it has mobilized a generation of astronomers in a rethinking of physics and cosmology to rival and perhaps surpass the revolution Galileo inaugurated on an autumn evening in Padua. They are coming to terms with a deep irony: it is sight itself that has blinded us to nearly the entire universe. And the recognition of this blindness, in turn, has inspired us to ask, as if for the first time: What is this cosmos we call home?
Scientists reached a consensus in the 1970s that there was more to the universe than meets the eye. In computer simulations of our galaxy, the Milky Way, theorists found that the center would not hold—based on what we can see of it, our galaxy doesn’t have enough mass to keep everything in place. As it rotates, it should disintegrate, shedding stars and gas in every direction. Either a spiral galaxy such as the Milky Way violates the laws of gravity, or the light emanating from it—from the vast glowing clouds of gas and the myriad stars—is an inaccurate indication of the galaxy’s mass.
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