You know that dream where you’re about to take a final exam, only to realize that you have neglected to study and, moreover, to put on pants? I imagine that’s what astronomers must have felt like in 1998, when they found out that most of the cosmos had somehow escaped their notice.
Astronomers knew that the universe had been expanding since the Big Bang, but they assumed that the gravitational pull of all the stuff inside it was gradually slowing that expansion. So they were caught off guard when supernova observations showed that in fact the expansion was speeding up, thanks to a mysterious phenomenon they dubbed “dark energy.”
Nearly 16 years and a trio of Nobel Prizes later, the initial shock has worn off, but the sense of chagrin lingers. “About 70% of the universe is dark energy, so it’s embarrassing not to know what it is,” says Valeria Pettorino, a physicist and cosmologist at the University of Geneva in Switzerland and the University of Heidelberg in Germany.
That uncertainty means physicists can’t predict whether the acceleration will continue at the same pace, speed up, or slow down, leading to no small amount of existential angst over the fate of the cosmos. “We don’t know what’s going to happen,” says Daniel Eisenstein of Harvard University. “The expansion of the universe is really being controlled by this new phenomenon that we don’t [understand].”
So when will we unmask this cosmic meddler? It could take physicists a decade or more to narrow down its hundreds of possible identities, many of which are devilishly hard to distinguish from one another. But researchers are attacking the problem from a number of angles and are already collecting some intriguing hints as to the true nature of dark energy—or at least what it’s not.
The astrophysical observation campaign “is really hitting its stride,” says Eisenstein. The initial finding in 1998 was based on measurements of fewer than 100 supernovae that appeared to be dimmer, and thus farther away, than expected. Since then, researchers have studied about 1,000 supernovae and developed a number of other techniques to gauge how the universe’s expansion rate has changed over time.
So far, all the observations can be accounted for with the simplest explanation: that any cubic meter of space froths with a set amount of repulsive energy, so that as space expands over time, the amount of repulsive energy grows as well.
But this simple solution, known as the cosmological constant, suffers from what may be a fatal flaw. Quantum physics suggests that the vacuum of space does contain energy, thanks to “virtual” particles that constantly pop into and out of existence, but when researchers calculate that energy based on known physics, they come up with a value that is some 10120 times larger than dark energy’s observed strength. “That’s a really embarrassing mismatch,” says cosmologist Michael Mortonson of the University of California, Berkeley, and Lawrence Berkeley National Lab. It’s easier to imagine how to cancel out the vacuum energy completely in the equations than have it retain a small, but non-zero, value, he says.
That has led researchers to ponder alternative explanations, such as the possibility that dark energy is caused by a quantum field that changes strength over time. One such field is the leading candidate to explain inflation, the period of super-fast expansion thought to have taken place just after the Big Bang. Another possibility is that our current understanding of gravity, Einstein’s theory of general relativity, is incomplete. “General relativity is very well tested at the scale of our solar system,” says Pettorino. “What we are trying to understand is if somehow it could be different at scales that are much larger.”
Some of these alternatives already appear to be on shaky ground, including the exotic notion that gravity could be leaking into an extra spatial dimension, making the universe expand faster than it otherwise should, says Mortonson.
But many others are still in the running, and new observations have bolstered the case for certain models. For example, last year researchers analyzing data from Europe’s Planck satellite calculated a value for the expansion rate of the universe today, known as the Hubble constant, that differs from Hubble Space Telescope observations by as much as 10%. The conflicting results can be reconciled by a model called “phantom energy,” a hypothetical dark energy that grows stronger over time. The catch: phantom energy could cause the universe to end in a “big rip,” tearing apart stars, planets, atoms, and their constituents. But don’t panic just yet. Pettorino says the discrepancy can also be explained if there is an extra force in the universe that causes dark energy to interact with another unknown component of the universe: the dark matter, whose presence has only been detected through its gravitational pull on visible matter.
Or the discrepancy, which is small to begin with, may be a flaw in the data analysis. “I think [that's] the more likely explanation,” says Mortonson. “Every team is checking their analysis,” says Pettorino, who is a member of the Planck collaboration.
Future observations should help narrow down the possibilities. Dark energy’s behavior over time, which is measured by the ratio of its pressure and energy density (called its equation of state, or w) is measured to a precision of about 5% today. But in the next five years, new observations, including those with a special camera fitted onto a telescope in Chile called the Dark Energy Survey, will increase the precision to 2% or 3%, says Mortonson. Future ground- and space-based missions, including a planned European space mission called Euclid and a possible US probe called WFIRST, could make even finer measurements.
These missions will not only probe the expansion history of the universe but also chronicle how the distribution of matter has changed over time. One way to do this is with a method called weak gravitational lensing, which looks for distortions in the light from distant galaxies due to any mass that the light passes on its way to a telescope. If dark energy changes over time or if gravity behaves unexpectedly at large scales, we might see evidence of it in the changing “clumpiness” of matter over space and time.
But Eisenstein points out that there are always going to be exotic dark energy models that behave just like the cosmological constant. “The worry is that if we do all these very accurate measurements, and it still looks like a cosmological constant, then we haven’t actually ruled out a lot of the models,” he says. “I think we have a major challenge on the theory side to try to understand what else we can look for.”
“[To understand] dark energy, we will probably require more time, and I would say not less than 10 years,” says Pettorino.
But it’s worth trying to get to the bottom of the mystery, say the researchers. “We thought we had four forces of nature: gravity, electromagnetism, and the weak and strong nuclear forces. Dark energy is either some new force, or some substantial modification to gravity,” says Eisenstein. “It’s a major actor in cosmology and in the history of the universe.”
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The New York Times: The Universe, Dark Energy and Us
In this Op-Ed, Harvard astrophysicist Robert Kirshner reacts to the 2011 physics Nobel award and reflects on the future of dark energy research.
physicsworld: Dark energy: how the paradigm shifted
Explore the history of dark energy and the cosmological constant.
Talks at Google: Dark Energy and the Runaway Universe
In this video, astrophysicist Alex Filippenko, a member of the teams that discovered the accelerating expansion of the universe, discusses the history and implications of dark energy.