Physics Nobel Explainer: Why Is Expanding Universe Accelerating?

New Nobel laureates Saul Perlmutter and Adam Riess of the U.S. and Brian Schmidt of Australia contributed to the discovery that the universe is not only expanding but also speeding up.

The finding led to the now widely accepted theory of dark energy, a mysterious force that repels gravity. Measurements show that dark energy accounts for about 74 percent of the substance of the universe.

But more than a decade after the Nobel-worthy find, scientists are still trying to pin down exactly what dark energy is and and thus solve what some experts call "the most profound problem" in modern physics.

(Also see "New Galaxy Maps to Help Find Dark Energy Proof?")

Does Gravity Work Differently?

Until dark energy, physicists were convinced that gravity should be causing the expansion rate of the universe to slow.

"When I throw my keys up in the air, the gravity of the Earth makes them slow down and return to me," said Mario Livio, a theoretical physicist at the Space Telescope Science Institute (STScI) in Maryland, said during the Decade of Dark Energy Symposium, held in 2008.

But by studying the light from distant supernovae, astronomers saw that the supernovae's host galaxies are flying away from each other at increasing speed.

The observation that the universe's expansion rate is actually speeding up, Livio said, is as if "the keys suddenly went straight up toward the ceiling."

So far, one of the biggest challenges for dark energy researchers is marrying observations to theory.

"We have two known, totally unsatisfactory explanations," said Michael Turner, a cosmologist at the University of Chicago.

One possibility is there is no dark energy, and gravity works differently than scientists think.

(See "Dark Energy's Demise? New Theory Doesn't Use the Force.")

But "physicists are conservative. We don't want to throw away our theory of gravity when we might be able to patch it up," Nobel co-winner Riess, an STScI cosmologist, told National Geographic News.

"Basically it all comes down to the fact that there's one relatively simple equation we work with to describe the universe," Riess said.

"Because we see this extra effect, we can either blame it on the left-hand side of the equation and say we don't understand gravity, or we can blame it on the right-hand side and say there's this extra stuff."

Dark Energy a Product of Quantum Vacuum?

The extra stuff—and a leading contender for explaining dark energy—is quantum vacuum energy.

The idea is tied to quantum mechanics, which predicts that even in the vacuum of space, particles are constantly winking in and out of existence, generating energy.

(Related: "Dark Matter Is an Illusion, New Antigravity Theory Says.")

The trick is that no one has been able to unify the math used in quantum mechanics, which describes the physics of the very small, with the equations in general relativity, which deal with large-scale interactions.

"The two theories use two different sets of rule books, [and] we've always known that these two books are incompatible," Riess said.

Unfortunately, "dark energy is one of the few cases in nature that really requires us to [somehow] use both sets of rules."