According to a new theory, the Big Bang, shown here in an artist's concept, may not have been the beginning of everything. [Credit: Stephen van Vuuren]
** Editor’s Note: The staff of Scienceline is taking a short break to work on future stories. This article originally appeared February 27, 2008.**
For decades, the Big Bang has been taught in high school physics classes as the leading theory for the way the universe began. But despite the overwhelming evidence supporting it, several questions linger for physicists. How could something come from nothing? And why do the laws of physics not hold up at the bang?
Now, some scientists say that the Big Bang was not the beginning, and that there was a universe before ours. The key is a new concept of gravity, which explains how such a universe could exist without violating the laws of physics.
If correct, Martin Bojowald, main architect of the theory and Penn State physicist, will have overcome the inability to explain the early universe. This problem has left scientists, including the likes of Einstein, perplexed for years.
“In my opinion, this is the single most embarrassing problem of physics,” said Max Tegmark, an astrophysicist at Massachusetts Institute of Technology.
In simple terms, Bojowald’s theory, published in August’s Nature Physics, can be described as a Big Crunch followed by a Big Bounce. He suggests there was a universe before ours that was collapsing and getting hotter (the crunch). Then, when it reached a maximum density and temperature, it was driven apart (the bounce), forming our current universe.
The main ingredient of Bojowald’s crunch-bounce theory is loop quantum gravity, a new concept that combines the traditional understanding of gravity with the quantum effect, which says that matter behaves differently at the subatomic level. Many scientists believe it is the absence of the quantum effect in equations that describe the Big Bang that lead to impossible phenomena like infinite temperature and density.
In Bojowald’s model, without the quantum effect, there would be no force to drive it apart; the collapsing universe that preceded ours would simply collapse into oblivion. But with the quantum effect, reaching a certain temperature and density threshold would trigger repulsive forces to drive the universe apart.
“The repulsive forces would stop the complete collapse and also turn it around into an expansion,” Bojowald said, effectively preventing temperature and density from approaching infinity.
While the crunch-bounce solves this problem of infinite temperature and density, the challenge becomes finding evidence to support the existence of the previous universe. To see if there was anything before the bounce, you would have to observe the distribution of particles small enough to have passed through the very dense early universe. Currently, neutrinos, very tiny particles that travel close to the speed of light, fit the bill. But they are very hard to detect.
If the method of detecting neutrinos improves, they could point to a previous universe. However, Bojowald said that in order to do this he first needs to answer the following question: If there was a universe before the bounce, what would the distribution of neutrinos look like today? Then, using equations and computer simulations he could make predictions and compare those with actual observations of neutrinos.
Other scientists find the crunch-bounce theory intriguing, but point out that there are still problems. Sean Carroll, a physicist from the California Institute of Technology, said his main concern is that the model overlooks the idea of entropy.
One way that entropy can be defined is the tendency for particles, energy or heat to disperse rather than remain clumped together. For example, smoke leaving a chimney will spread out, rather than stay in one place, thus increasing in entropy.
The expanding universe also acts in this manner and is continually increasing in entropy. An unusual observation, according to Carroll, is the universe’s low entropy near the Big Bang. Since this is such an unusual phenomenon, he said it was necessary for any new theory to explain why entropy is so low at that point.
“It would be like walking into a room and finding all the air molecules on one side,” Carroll said.
Aside from the crunch-bounce theory, other credible challenges to the Big Bang are emerging, too. Cambridge’s Neil Turok and Princeton’s Paul Steinhardt have a model they call the Cyclic Universe. These physicists believe that the Big Bang was not a unique event, but one in a series of bangs with more bangs yet to come.
Bojowald’s theory does not discount this idea of “multiverses.” And neither theory estimates the number of previous universes or when the next crunch will happen. Turok and Steinhardt’s theory differs from Bojowald’s mainly in the mathematics they use to describe it.
They explain their model using string theory, a branch of physics that views matter as one-dimensional “strings.” Using the equations of string theory, they come up with a different picture of the early universe.
Like Bojowald, they were driven by the belief that the current Big Bang model leaves too many unanswered questions. Steinhardt said that the more scientists understand about the universe, the more the current model does not work. “Maybe we’re on the wrong track,” he said.
So far, though, none of the new models have overturned the Big Bang to emerge as the leading alternative, primarily because proving the existence of a previous universe is so difficult. Steinhardt says much more data and evidence are necessary before a paradigm shift can occur. “Most of us will stick to something until forced to give it up,” he said.
But even if these new ideas about the origin of the known universe are correct, they cannot address the ultimate questions: What came before all of those previous universes and Big Bangs? When did it all begin?
“We’re not going to actually answer the question of where the beginning was,” Bojowald said. “We’re just pushing the beginning back in time.”
Related on Scienceline:
[VIDEO] A new experiment uses NYC’s rooftop water tanks as cosmic ray detectors.
Can the universe expand faster than the speed of light?
How did the entire universe come from the Big Bang?