When cosmologist Laura Mersini-Houghton first looked at the evidence for a giant void in space, what she saw left her cold. It was a gaping hole nearly a billion light-years across, containing virtually no galaxies or matter and showing up as a cold spot in a heat map put together by NASA’s WMAP satellite. The heat map is a plot of the cosmic microwave background — remnant heat left over from the Big Bang. The void represents a volume of space with temperatures between 20 and 45 percent lower than the average for the rest of the skies.

The void is somewhere in the range of six to ten billion light-years away from Earth, and it’s forty times bigger than the void formerly known as the biggest in space.

Voids in space are not a novelty, but one this big was just what Mersini-Houghton needed. To her, the void is evidence in favor of a theory that borders on the outlandish — the existence of a parallel universe.

In August 2007 Lawrence Rudnick, a physicist at the University of Minnesota in Minneapolis, uncovered the first experimental evidence of the void using the Very Large Array radio telescope in New Mexico. The telescope located the void in the direction of the constellation Eridanus, in the same place where scientists had previously noted the cold spot picked up by WMAP.

It all started with a hunt for the origin of our universe, Mersini-Houghton says. A universe that begins with very high energy, such as our own at the Big Bang, would have very low entropy. Entropy is the measure of a system’s disorder and is inversely related to energy. A universe at very low entropy could not have stood a chance of being born. Entropy explains how our fledgling universe managed to escape its own gravitational attraction by stretching space out and flinging matter far enough apart to prevent the universe from coalescing back into the singularity it was when it began. Cosmologists call this the theory of inflation.

“Cosmologists have shown that the probability that our universe would come to be is basically zero,” Mersini-Houghton says. And yet, it happened. If an event so unlikely as the birth of our universe can happen, Mersini-Houghton says, virtually anything is possible. Including multiple universes.

Cosmologists turned to string theory to find out why such an unlikely windfall came to pass. “String theory continues to be the leading candidate for a solution to the problem of our existence,” Mersini-Houghton says. But string theorists predicted 10⁵⁰⁰ possible universes, whereas many scientists had hoped that string theory would predict a unique universe. “That kind of dashed hopes,” she adds.

Until recently, these results have been considered a major crisis in string theory, Mersini-Houghton says. Rather than solving cosmic problems, applying the theory created more challenging ones. Most of the cosmological community, at least in the United States, responded to the dilemma by taking an anthropic approach. They suggest that we may have ended up with this universe because it supports life.

Mersini-Houghton, who never took anthropic reasoning very seriously, looked for a more straightforward explanation. “Something was missing in the picture,” she says. She decided to solve the problem by upending it.

Mersini-Houghton reasoned that any good theory that is meant to explain the origin of our universe must indeed predict the possibility of multiple universes.

“We’re asking, ‘Why did I start with this universe?’ But the question does not make sense if all you have is one sample,” she says. “That question immediately begs another: Compared to what else?”

That’s when Mersini-Houghton hypothesized multiple universes, each with their own physical properties and constants. Each of those baby universes, she says, would have started as a tiny patch in the fabric of space, distinct from others in the amount of matter and energy it contained. Matter, which tends to clump, vies with energy, which tends to cause matter to expand. Depending on the outcome of this cosmic tug-of-war, some of those tiny patches would have survived to become universes, while others would have collapsed into cosmic obscurity. This theory explains how a universe that started with very high energy might have survived the fatal pull of matter.

Mersini-Houghton began to look for testable signatures of her theory. “That’s where the void comes into the picture. We predicted the void in 2006, and in fact, we were lucky that it was discovered a mere eight months after we predicted it,” she says.

In a multiverse scenario, our neighboring universes exert a gravitational tug on our universe, causing matter to shift toward the attracting universes. This would create a hole in our universe, Mersini-Houghton says. It’s akin to creating a hole in a piece of stretched fabric while trying to pull at it from one point.

Mersini-Houghton’s calculations of this gravitational force predicted a void of the exact scale that Rudnick’s team observed a few months later.

When the WMAP satellite recorded the cold spot in their heat map in 2004, NASA put it down to an instrumental snafu or an experimental artifact, because such a humongous void could not be explained by standard cosmology. But Rudnick’s observations with the radio telescopes confirmed the void’s presence. “It was in the exact place in the sky where WMAP had seen the cold spot. Then we knew for sure,” Mersini-Houghton says. “That was the first real test of our model.”

Leonard Parker, a cosmologist at the University of Wisconsin-Milwaukee, says, “The idea of a very large metauniverse, of which our universe is a relatively small part, goes all the way back to the nineteenth century. What is new in Mersini-Houghton’s work is the possibility that regions of the meta-universe with which we cannot communicate (what you could call parallel universes) may still have influence on our universe because of correlations between our universe and other ‘parallel’ universes.”

But Mersini-Houghton’s dissenters have come up with alternative explanations for the void. Some say that the void may represent a giant knot in space called a topological defect, while others say the void results from textural aberrations in the fabric of space.

“It’s madly courageous to even contemplate the possibility of addressing the beginnings of our universe,” Mersini-Houghton says. She adds that we may never be sure of how it all started, but testing more predictions could make the theory stronger.

She is now working on predictions of more voids. And another of her theory’s predictions will be tested this year by the Large Hadron Collider in Switzerland, where scientists can recreate some of the conditions present in the early universe.

Mersini-Houghton says her approach is based on the Copernican view of nature. “No, we are not at the center,” she says, “and our whole universe is not at the center.”