Defying Gravity

By Marc Airhart. Illustration by Jenna Luecke. 

In November 1915, Albert Einstein unveiled before the Prussian Academy of Sciences a set of elegant equations that would forever change the way we see the universe. The Theory of General Relativity, Einstein’s description of gravity, explained all motion in the cosmos. 

So far, general relativity has held up well to various tests scientists have thrown at it. But there are signs that it’s incomplete. Physicists say that in areas of extreme mass or energy density, the mathematical equations that explain so much of the universe blow up. And in some cases, relativity clashes with the other great pillar of 20th-century physics: quantum mechanics, which describes subatomic particles and how they interact.

Testing Einstein

Einstein’s century-old description of gravity presented physicists with some pretty bizarre predictions. To test them, scientists from the University of Texas at Austin have traveled to the Sahara Desert to observe a rare eclipse. They launched into orbit around Earth an object that’s the densest known thing floating in our solar system. And they used computers to model ripples in space and time unleashed by the mergers of black holes.

Read more about these tests of general relativity in our companion article.

 Scientists now are drawn to a question: what happens in the places in the universe where theories collide?

 “In 100 years, general relativity has never been shown to be wrong,” says Willy Fischler, a professor of physics. “Neither has quantum mechanics. But when we put the two together, we have problems in certain contexts such as black holes. This clash means there’s something missing in our understanding, and we have the chance to make a big leap in understanding, to change paradigms.”

Credit: Richard Matzner

 As scientists at The University of Texas at Austin work to answer some of the biggest questions in astrophysics—about black holes, the beginning of the universe and dark energy—some say finding the answers might require overhauling general relativity.

Inside Black Holes

When a very large star runs out of nuclear fuel and breathes its dying breath, it collapses into a black hole, an object so dense that not even light can escape it. A star ten times as massive as our sun would squeeze down into a ball about the width of Austin. 

In 1962, UT Austin mathematician Roy Kerr provided an exact solution to Einstein’s equations of general relativity, a breakthrough that also described the physics of rotating black holes. At that point, it became clear how beautifully general relativity describes black holes—at least up to a point. Read more about these tests of general relativity in our companion article.

At the very heart of a black hole, things get trickier. According to general relativity, mass and energy become infinitely dense, and space and time become infinitely curved. At such a point, called a singularity, the steps involved in solving Einstein’s equations become infinite. 

That isn’t the only problem. According to general relativity, if you fell into a black hole, you would first cross something called the event horizon, the outer edge of the black hole. Then as you continued falling, the black hole would shred you apart, atom from atom, like some cosmic garbage disposal apparently deleting information about you forever.

Yet according to quantum mechanics, information can never be truly lost. This is called the information paradox.

“Something has to give here,” Fischler says. “What is it in the description of either one or both of these theories that has to be amended?”

Fischler thinks part of the answer might be that, when an object crosses the event horizon, it leaves behind a two-dimensional imprint on the surface of the horizon with all the information about its state, similar to a hologram. So from the point of view of quantum mechanics, the object could fall in, but information about it would not be lost. 

Ultimately, resolving the information paradox and making sense of what happens in a singularity might require a different theory that reconciles general relativity and quantum mechanics. Steven Weinberg, a Nobel laureate and professor at UT Austin, and others have long sought a theory of everything that would do just that.

String theory is probably the most promising way to unify the two theories,” says Weinberg.

How Did our Universe Start?

String theory is the conjecture that all matter and its interactions can be described as vibrations of unimaginably tiny strings of energy. It might explain extreme cases like black holes, as well as another singularity—the birth of our universe as a hot, dense plasma in the Big Bang.

The evolution of the Universe, starting with the Big Bang. The arrow marks the flow of time. Credit: Dana Berry (Skyworks Digital)/NASA

Raphael Flauger, a new assistant professor of physics, is trying to piece together a sort of home movie for our universe from back in its fast-growing infancy. The earliest frame we have in this film comes from about 380,000 years after the Big Bang. Taken by satellites, it shows the oldest light in the universe, called the cosmic microwave background. This ancient glow has tiny variations in brightness, or clumps of energy, that would eventually become the seeds of stars and galaxies. 

Flauger uses string theory to build models of how the early universe might have evolved.

“You try one version of the film, then see if it fits with the snapshot of the cosmic microwave background,” says Flauger. “If it doesn’t agree, you throw it out and make another version of the film and see how well it fits.”

Flauger is part of a team proposing a new satellite mission that will look for a distinct signature in the cosmic microwave background that could help rule out one or more of the competing models.

What is Dark Energy?

In the 1990s, astronomers observed that the expansion of the universe is speeding up, as if some mysterious force is pushing everything apart faster and faster. Nearly 20 years later, one of the biggest unanswered questions in science is: what is this dark energy? Not only was dark energy not predicted by general relativity, but its mere existence might mean that the theory needs to be tweaked or even replaced.

Locked in a cosmic tug of war, dark energy (purple) pushes the universe apart, while gravity (green) pulls it together. At the moment, dark energy is winning.Credit: NASA Ames/JPL-Caltech/T. Pyle

 “Dark energy is the name we apply to our misunderstanding of how the universe is expanding,” says Karl Gebhardt, a professor of astronomy who holds the Herman and Joan Suit Professorship in Astrophysics.

 According to Gebhardt, among astronomers there are two leading ideas for what dark energy might be.

 One is that empty space itself pushes matter apart, sort of like anti-gravity. In this view, space is filled with something called vacuum energy. To account for it, scientists would have to add an extra term to the equations of gravity called the cosmological constant.

Another possibility is that at great distances, gravity becomes weaker than the equations of general relativity say it should. So the accelerating rate of expansion of the universe wouldn’t be due to a mysterious new force, but instead would be due to gravity’s loosening grip on a universe already expanding from the Big Bang.

Not only was dark energy not predicted by general relativity, but its mere existence might mean that the theory needs to be tweaked or even replaced.

Gebhardt is part of a team that could be close to discovering which, if either, of these two explanations for dark energy—each with big implications for the future of general relativity—is correct. The project, based at the University’s McDonald Observatory, is a key piece of the newly upgraded Hobby-Eberly Telescope, now the world’s third-largest telescope. 

The Hobby-Eberly Telescope Dark Energy Experiment will help measure the rate of expansion of the universe at different times in its history, which will help theorists constrain their models of dark energy. Researchers plan to start collecting data in 2016. 

Although the immediate quest is to explain dark energy, their findings might end up taking scientists one step closer to a “theory of everything” that would write the next chapter for general relativity.

Listen to UT scientists describe the search for dark energy on the CNS podcast, Point of Discovery.