Our universe might have originated from a black hole that lies within another universe.
The idea centers on how matter and energy falling into a black hole could in theory come out a "white hole" in another universe. In such a situation, both the black hole and the white hole are mouths of an Einstein-Rosen bridge, popularly known as a wormhole.
With that in mind, theoretical physicist Nikodem Poplawski at Indiana University conjectured that when a black hole forms upon the collapse of a dying star, a universe is born at the same time from the white hole on the other side of the wormhole.
"Our universe could have itself formed from inside a black hole existing inside another universe," he said.
This idea has been suggested before, and now Poplawski has expanded on the thinking.
Although a black hole forming from a star the size of our sun would only be about 2 miles wide, it does not mean that a universe which might originate from a black hole would stay that small.
"Our universe was small a long time ago and expanded," Poplawski said. "From the other side, one would not see our expansion." Essentially, a black hole could seem bigger on the "inside" than how it looked to someone outside.
If anyone survived a trip into a black hole and emerged in another universe, "it would be a one-way trip," Poplawski noted. The event horizon of a black hole is ?boundary at which nothing inside can escape.
In theory, black holes do lose mass, however, as Hawking radiation — particles that emerge from the vacuum right next to their event horizons. Black holes that lose more mass than they gain are expected to shrink and ultimately vanish. This does not mean any universe at the other end of the black hole would cease to exist, Poplawski explained. "We would just be disconnected from the other universe," he said.
Evidence for a black hole birth?
If our universe was born from a black hole in another universe, it would be impossible to cross its event boundary and see the other side, meaning one cannot prove or disprove this idea that way.
So how might one test this conjecture? One implication of Poplawski's concept is that our universe is "closed."
"In cosmology, there are three models for our universe," Poplawski explained. "The first is that it's closed, the second is that it's 'open,' and the third is that it's 'flat.'"
If it is closed, and one tries to venture to the edge of our universe, one would eventually loop back to where one started. "It would be like walking on the surface of the Earth — if you walk to the east, at some point you come back across the west," Poplawski said.
If our universe is flat, and one tries to venture to its edge, one would never reach it, as it continued on infinitely. The same would hold true if the universe is open, "only it would be 'curved,'" Poplawski said. This means that if two beams of light were shot into space parallel to each other, in a flat universe they would stay parallel, while in an open universe they would actually get farther from each other.
If our universe lies "within" another universe, "the prediction would be that our universe is closed," Poplawski said. "That could maybe tested at some point. If the universe is closed, if one watches the stars, their light would be visible on the opposite side of the sky. If the universe is really big, the light from those stars might not have had time to do that yet, but maybe at some point we could see this."
Also, black holes rotate, evidence suggests, and if our universe's parent black hole was spinning, "then one direction might be preferred in our universe, evidence we could see at large scales," Poplawski said.
If our universe was born from a black hole, it could help solve the mystery of what came before the Big Bang and whether our universe was closed, flat or open.
It could also help scientists avoid problems associated with singularities, or infinitely small points with infinite density. In theory, the universe was born from a singularity, and every black hole encloses a singularity. However, singularities are big headaches for theoretical physicists, since the laws of physics break down around them.
"In my model, when the universe is born, there is no problem of having a singularity initially — instead of being infinitely small, it had the radius of its parent black hole," Poplawski said. "Also, instead of black holes ending up with singularities, this model suggests they would end up with universes."
An idea full of holes?
One hole in this idea is one that turns up whenever wormholes are discussed — exotic matter.
In principle, all wormholes are unstable, closing the instant they open. The only way to keep them open is with an exotic form of matter with so-called "negative energy density." Such exotic matter has bizarre properties, including moving in the opposite direction of normal matter when pushed. No one knows if such matter actually exists.
"It is possible that when black holes form, matter can undergo a phase transition to become exotic matter, to allow this initial expansion of a universe inside the black hole," Poplawski suggested.
Also, while this might explain how our universe was born from a black hole, it does not explain how the universe that black hole is from came to be. "So was that universe born from a black hole in another universe, and so on?" Poplawski noted. "It's a problem of an infinite number of universes. That's a very reasonable criticism."
Poplawski will detail his research April 12 in the journal edition of Physics Letters B.