Ring-like patterns in the sky could be ghostly imprints of a universe that existed before the big bang, according to a controversial new study.
If the theory is correct, the cosmic rings may be the first real-world evidence that our current universe is just the latest in an endless string of recycled universes, the study authors say. (Related: "Universe Reborn Endlessly in New Model of the Cosmos.")
Astronomers found the circular patterns in a new analysis of the cosmic microwave background (CMB), which is the radiation left over from the big bang that now permeates the universe.
Within the newly described rings, many of which are nested like ripples in a pond, the temperature is more uniform than elsewhere in the CMB sky. (See pictures of other odd rings and spirals in space.)
One possible explanation for these rings is that they were created when black holes collided in a previous universe, according to Roger Penrose of the University of Oxford in the U.K. and Vahe Gurzadyan of the Yerevan Physics Institute in Armenia.
Scientists think that when two black holes crash together, they emit ripples of energy known as gravitational waves. The more massive the colliding black holes, the more numerous and powerful the waves.
Gravitational waves distort the fabric of space-time and—according to Penrose and Gurzadyan—the waves can leave imprints of their passage in the form of ring-like patterns.
If our universe is one in a series of reborn universes, this would mean that the rings survived our big bang and now "allow us to 'see through' the big bang into the previous aeon," Penrose and Gurzadyan write in their study. Neither study author could be reached for comment.
(Get the ten biggest cosmic mindblowers of 2010 as chosen by National Geographic News editors.)
If the concept of patterns surviving a big bang event sounds far-fetched, don't worry: Even other astrophysicists find it hard to believe.
"That part confuses me as well," said James Zibin, of Canada's University of British Columbia. "It just seems odd to me that the effects of these ripples could survive at all during these transitions. ... That's one of the many details that hasn't been fleshed out."
Universe Gets Recycled?
The new paper, published earlier this month on the Cornell University website arXiv.org, is the latest piece in a model of a cyclical universe that Penrose has developed over several years.
According to his theory, the big bang that created our universe is not unique. At least one earlier big bang occurred, giving rise to a universe that existed before ours. Countless more universes could have existed before that one, he thinks.
Penrose calls each cycle of the universe an aeon, and each aeon lasts for an unimaginably long time—much longer than the age of the current universe, which is already 13.7 billion years old.
An aeon starts with a big bang, and over time the newborn universe evolves from a sea of dilute and homogenous particles into ever more complex structures such as galaxies, stars, planets, and life.
All the while, the universe is expanding at an accelerated rate, presumably stretched apart by the same mysterious dark energy that is driving the expansion of space-time in our current universe. (Related: "New Proof Unknown 'Structures' Tug at Our Universe.")
Inevitably, however, all the matter in the universe is ingested by the supermassive black holes that lurk in the hearts of large galaxies, such as the one in our own Milky Way, according to Penrose's theory.
These black holes grow as they feed, colliding and combining into even larger black holes. The gravitational waves created by these black hole collisions are what drive the CMB rings, the theory goes.
Eventually the monster black holes consume matter until nothing is left. Physicist Stephen Hawking proposed in the 1970s that, when black holes stop taking in matter, they lose mass via radiation, with smaller black holes losing mass faster than larger ones.
Based on that theory, over billions more years the remaining black holes would "evaporate," converting their ingested matter into radiation. At this point in the aeon, the universe—now old and large beyond imagining—is once again a sea of uniform particles.
Penrose theorizes that the universe then undergoes a transformation that essentially compacts it back into an infinitesimally small point, setting the stage for the next big bang.
According to this theory, "everything that is big becomes small, and everything cold becomes hot, so that the [ancient] universe, which is big and cold, becomes dense and hot," said astrophysicist Amir Hajian of the Canadian Institute for Theoretical Astrophysics (CITA) in Toronto.
Penrose and Gurzadyan don't explain what drives this transformation, though. As with other aspects of the model, this part needs more explanation, said Hajian, who is a co-author of one of three independent papers, all recently posted on arXiv.org, that challenge the new study's claims.
"In principle, it works on paper, but it's still missing details and quantitative predictions," Hajian said.
Space Circles Are "Perfectly Normal"
None of the challengers doubt the existence of the CMB rings: "We confirmed that they were there," said the University of British Columbia's Zibin, who is a co-author of another one of the rebuttal papers.
"We could see exactly the signals that they described. So that part isn't in question. ... Where we differ is in our interpretation of the significance the signals."
Instead, all three papers note that Penrose's cyclic model doesn't include a process called inflation.
The inflationary theory says that the early universe went through a period of hyperexpansion that helped it reach its current size and shape. Inflation helps solve a number of problems with the big bang theory, some of which were revealed by studies of the CMB.
For example, one potentially puzzling aspect of our universe is that it appears to be homogeneous—that is, when viewed on the large scale of galaxy clusters, different parts of the universe look essentially the same.
Inflation explains the universe's uniformity, because any clumping of matter in the early universe was smoothed out by hyperexpansion.
However, according to Penrose, if an earlier universe existed before our own, and that universe expanded at an accelerated rate the same way ours is doing now, inflation is not needed.
The idea is that "the accelerated expansion in the previous aeon smoothed the universe out, and that takes the place of inflation in the standard picture," Zibin said.
Each of the rebuttal papers describes the results of computer models that simulate hundreds of CMB skies resembling the actual radiation pattern found in our universe. Each simulation used inflationary theory as its foundation.
If the ripples in the CMB are evidence of the cyclic model of the universe, then the ripples should not be present in the simulated CMB skies because, according to Penrose, they should not appear in an inflationary universe.
But the rings do show up in the simulations.
"We looked for these patterns, and we could see exactly the same kinds of patterns. We could see similar numbers of them, with similar shapes and sizes," Zibin said. "If [Penrose] wants to argue that this cyclic model is the correct one, he has to find some signature that would distinguish between his model and inflation. These rings are not that signature."
CITA's Hajian agreed: "The circles are not anomalous in any way. They are perfectly normal, and [our models] show that they can happen in a universe that is consistent with inflationary theory."
Instead of evidence for previous universes, the rings could be a kind of optical illusion created by natural variations in the CMB, Zibin added.
"If you look hard enough, you can find all sorts of things" in the CMB, he said. "That's one of the lessons of this, I think."