Is Dark Energy Really "Repulsive Gravity"?

Antimatter could be behind mysterious force, new theory says.
A time line shows the universe to be ever expanding and accelerating—traits often ascribed to dark energy.

A powerful repulsion between normal matter and hidden pockets of antimatter could be an alternate explanation for the mysterious force known as dark energy, according to a controversial new theory.

In 1998 scientists discovered that the universe is not only expanding but that its expansion is accelerating.

This totally unexpected behavior has been called the "most profound problem" in physics, because our current understanding of gravity says that attractions between mass in the universe should be causing the expansion to slow down.

The leading theory to explain the accelerating expansion is the existence of a hypothetical repulsive force called dark energy. (Related: "New Galaxy Maps to Help Find Dark Energy Proof?")

But in the new study, Massimo Villata, an astrophysicist at the Observatory of Turin in Italy, suggests the effects attributed to dark energy are actually due to a kind of "antigravity" created when normal matter and antimatter repel one another.

"Usually this repulsion is ascribed to a mysterious dark energy that would uniformly permeate the cosmos, but nobody knows what it is nor why it behaves this way," Villata said in an email.

"We are replacing an unknown force caused by an unknown element with the repulsive gravity of the well-known antimatter."

Antimatter Hiding in "Holes" in the Universe?

According to Villata, the keys to accelerated expansion lie in large-scale voids that are seen scattered throughout the cosmos.

These holes in our map of the universe—which can each be millions of light-years wide—are inexplicably empty of galaxies and galaxy clusters. The nearest hole to us is called the Local Void, bordering the Virgo supercluster of galaxies.

Villata thinks these voids harbor vast quantities of antimatter, which could even be organized into antimatter galaxies, complete with antimatter stars and planets.

All this antimatter doesn't emit radiation that can be detected by current sensors, making it effectively invisible, Villata said.

"There can be various reasons why antimatter in voids should be invisible, but we do not know which of them is the right one," Villata said. "Moreover, antimatter in laboratories could have different behavior, since it is 'immersed' in a world of matter."

While we can't see antimatter superstructures, we can observe their effects on our visible universe, Villata argues, because antimatter must repel the normal matter in galaxies, pushing them farther from one another.

Villata says his theory, which will appear in an upcoming issue of the journal Astrophysics and Space Science, has the potential to solve other cosmic mysteries, such as the universe's "missing antimatter" problem.

According to standard physics, matter and antimatter particles should have been created in equal amounts during the big bang. Yet the visible universe appears to be dominated by structures made up of normal matter.

To determine how much antimatter might be contained in the Local Void, Villata calculated how much would be needed to create a repulsive force strong enough to explain the so-called Local Sheet. This collection of normal matter, which includes our Milky Way and other nearby galaxies, is all moving at the same speed.

"If each void contains a mass of antimatter similar to that calculated for our Local Void ... then our universe would host an amount of antimatter equivalent to that of matter, and [there] would finally be a matter-antimatter symmetric universe," Villata said.

But Do Matter and Antimatter Repel?

While Villata's theory doesn't require mysterious forces created from nothing, it does rely on the untested assumption that matter and antimatter are mutually repulsive.

There is as yet "no [experimental] evidence that antimatter repels matter," said physicist Frank Close of the University of Oxford in the U.K., although, he added, plans are underway at the European Organization for Nuclear Research (CERN) in Switzerland to test the idea.

In fact, Dragan Hajdukovic, a physicist at CERN, recently proposed a separate antigravity theory that also relies on repulsion between matter and antimatter to explain dark energy and dark matter.

Hajdukovic called Villata's theory "an interesting idea," be he added that he disagrees with the hypothesis of a matter-antimatter symmetric universe.

"The major problem is why [such] big quantities of antimatter in the voids are not observed," Hajdukovic said.

In Hajdukovic's theory, antimatter particles spontaneously pop in and out of existence in the quantum vacuum, which is the name physicists give to seemingly empty space.

"I use the reality of the quantum vacuum. For a physicist, it is more natural and plausible," Hajdukovic said.

"In order to explain the invisibility of antimatter, proponents of a matter-antimatter symmetric universe would be forced to invoke an additional hypothesis"—such as the emission by antimatter of so-called advanced photons, which travel backward in time and so wouldn't be detectable to current instruments.

"It is not a good sign for a theory if one hypothesis immediately demands introduction of other hypotheses."

But study author Villata argues that the assumptions in his theory—including matter-antimatter repulsion and advanced photons—are predicted by well-established theories in physics.

In that sense, he said, there is "no introduction of other hypotheses."