Who's Afraid of the Dark? Alternatives to Dark Energy
Last week, we asked whether astronomers could be wrong about dark matter, the invisible stuff that seems to help hold galaxies together. Is it possible that dark matter doesn’t really exist?
This week, we’ll investigate whether there are viable alternatives to the idea of dark energy, the mysterious stuff that astrophysicists believe is pushing our universe apart.
In every direction we look, galaxies are hurtling away from us. That isn’t surprising in itself—after all, the Big Bang sent space and everything in it flying apart. One would expect that the gravitational pull of all the “stuff” in the cosmos would gradually slow down this expansion, bringing it to a dead stop or even collapsing everything back together in a "Big Crunch." Yet instead, astronomers see that the galaxies in our universe are rushing apart faster and faster.
What could be causing this acceleration? Physicists call it dark energy, and it could make up more than 70 percent of the cosmos. But so much remains unknown about dark energy that some scientists are asking whether it exists at all.
What if, instead of a mysterious unseen energy, "there is something wrong with gravity?" asks Sean Carroll, a theoretical physicist at the California Institute of Technology.
Einstein's theory of general relativity represents gravity as the curvature of space and time. Perhaps this idea "is still right, but we're not solving the equations correctly," suggests Carroll. "We're used to thinking of the universe expanding perfectly smoothly, and we know it isn't, and maybe these deviations are important." If we accounted for how the universe is clumpy instead of smooth, it might turn out that the gravitational pull of clusters of galaxies and other large agglomerations of matter alter spacetime more than previously appreciated. Distant objects would thus appear to be farther away than they actually are, leading to the false conclusion that the universe's expansion is accelerating.
The problem with this kind of model, Carroll says, is that while it suggests that these clumped-up astronomical bodies might distort our view of the universe more than suspected, gravity still remains the weakest of the known fundamental forces of nature. Also, these astronomical clumps would evolve in size and gravitational strength over time. In contrast, the mysteries that dark energy was invoked to solve require something with a lot of energy that changes less over time.
Another approach is to modify the laws of gravity to do away with dark energy. This tack suggests that "the laws of gravity as we know them work better on relatively small scales such as our solar system,” says Carroll, but perhaps they need “tweaks” to work on cosmic scales. Carroll and other theorists have developed alternative descriptions of gravity that could explain why the universe evolved as it did. One set of scenarios suggests that the strength of gravity increases over time and has different values depending on the distances involved. But critics argue that, to avoid contradicting well-established features of general relativity, these models are unacceptably contrived.
Another family of alternative gravity models analyzes how gravity behaves if there are extra dimensions of reality, as suggested by string theory. But this approach has problems of its own: It leads to empty space "decaying" into particles in potentially detectable ways, Carroll says.
To avoid modifying gravity, some theorists have suggested that our galaxy and its neighborhood might lie within a giant void, an emptier-than-average region of space roughly 8 billion light years across. With so little matter to slow down its expansion, the void would expand faster than the rest of the universe. If we lived near the heart of this void, our observations of accelerating cosmic expansion would be an illusion.
"The advantage of giant void models is that they don't require any new physics to explain the apparent acceleration of the universe, like the existence of some weird dark energy or a modified theory of gravity," says theoretical cosmologist Phil Bull at the University of Oxford.
Still, "there are lots and lots of problems with void scenarios," says theoretical physicist Malcolm Fairbairn at King's College London. "It's very difficult to get them to fit existing data—for instance, the cosmic microwave background (CMB) radiation usually gets distorted in these models compared to what we actually see." For the void model to match observations of CMB radiation, we would need to be very close to the center of the void, to within one part in 100 million. That "seems like an unacceptable 'fine-tuning' to some people,” says Bull. “Why should we find ourselves so close to the center?"
In addition, astronomers using NASA's Hubble Space Telescope recently found evidence against the existence of such a void. After refining their measurements of the rate at which the universe is expanding, they all but ruled out the possibility that the accelerating expansion is an illusion created by a void. In addition, if we are living inside a void, Bull and his colleagues argue, we should see very strong fluctuations of cosmic microwave background radiation reflected off hot gas in the clusters of galaxies surrounding the void. Yet we do not see any reflections that strong. "This was pretty much the final nail in the coffin for void models," Bull says.
To support the existence of dark energy—or vindicate one of these alternatives—we need giant sky surveys which will clock the speeds of even more galaxies, Fairbairn says. The colorful scenarios that theorists are dreaming up "ultimately show what an interesting and weird universe we live in," Carroll says. "It's one where we must keep an open mind as to what the answers may be."
Editor's picks for further reading
COSMOS: Doubts Over Dark Energy
Reexamining the evidence for dark energy.
New York Times Magazine: Out There
In this article, Richard Panek explores the evidence for dark energy.
NPR’s 13.7: Dark Energy and the Joy of Being Wrong
In this blog post, Adam Frank recounts the history of the discovery of dark energy.