This graphic shows a timeline of the universe based on the Big Bang theory and inflation models.
The Big Bang was the beginning of the universe as we know it, most scientists say. But was it the first beginning, and will it be the last?
A popular picture of the early universe imagines a single Big Bang, after which space blew up quickly like a giant bubble. But another theory posits that we live in a universe of 11 dimensions, where all particles are actually made of tiny vibrating strings. This could create a universe stuck in a cycle of Big Bangs and Big Crunches, due to repeat on loop.
Which scenario is closer to the truth remains to be seen, but scientists say new experiments underway could provide more answers soon.
The Big Bang
According to the Big Bang theory, the universe began extremely hot and extremely dense. Around 14 billion years ago, space itself expanded and cooled down, eventually allowing atoms to form and clump together to build the stars and galaxies we see today.
On this, most scientists are agreed.
"I would say that there is 100 percent consensus, really," University of Pennsylvania particle physicist Burt Ovrut said of the Big Bang theory. "There is overwhelming evidence ? all of the predictions are true."
For example, this theory predicted that the universe today would be filled with pervasive light left over from the Big Bang. This glow, called the cosmic microwave background radiation, was discovered in 1964, almost 20 years after it was forecast.
However, what caused the Big Bang, what happened at that exact moment, and what came immediately after it, are much more open to debate.
A giant bubble
A dominant idea that connects the dots between the Big Bang and the universe we find today is called inflation. This is the notion that during the first roughly 10 to the minus 34 seconds (0.0000000000000000000000000000000001 seconds), the universe underwent exponential expansion, doubling in size at least 90 times. During this early stage, matter was in a much different state than it is now.
This theory could explain some of the main conundrums posed by the Big Bang: Why does the universe appear mostly flat, with roughly the same amount of stuff spread smoothly throughout it in all directions?
"If you imagine the matter was in that different state in the early universe, the whole story changes," said Andreas Albrecht, a theoretical physicist at the University of California, Davis and one of the initiators of inflation. "The physics can create the smoothness for you. Inflation also makes it flat. It all fits beautifully into this story told by inflation."
But Albrecht and others admit the theory doesn't yet explain the whole picture.
"Inflation is easily the most popular theory in cosmology," said theoretical physicist Neil Turok, director of the Perimeter Institute for Theoretical Physics in Ontario, Canada. "It's a good theory, but it has some weak points. It can't describe the moment of the Big Bang."
The Big Bang theory envisions the universe beginning from a singularity ? a mathematical concept of infinite temperature and infinite density packed into a single point of space. But scientists don't think this is what actually happened.
"It wouldn't really be infinite," explained physicist Paul Steinhardt, director of the Princeton Center for Theoretical Science at Princeton University in Princeton, N.J., and another architect of inflation. "Infinity just means a mathematical breakdown. It's a statement that you shouldn?t have extrapolated your equations back that far because they just blew up in your face."
Neither Big Bang theory nor inflationary theory can describe what happened at that moment.
And inflation has other problems, in some people's view. Because of quantum fluctuations, different parts of the universe could inflate at different rates, creating "bubble universes" that are much larger than other regions. Our universe may be just one in a multiverse, where different scales and physical laws reign.
"It means everything and anything that can happen, will," Steinhardt told SPACE.com. "So basically everything could be a prediction of inflation. This to me is a fundamental problem and we don?t know how to get away from it."
Others say that while inflation may not be complete yet, it's still the most useful thing we've got to describe the origin of the universe.
"Even if all things are possible it could turn out that some things are much more possible than others, and you could still have a prediction," Albrecht said. "The real excitement to me is that there's so much data supporting inflation that it really makes it seem worth thinking about these questions."
Cycles and cycles
In 2001, Steinhardt and Turok proposed an idea called the cyclic model, based on an earlier concept called the ekpyrotic universe that they'd conceived with Ovrut.
In this scenario, the universe undergoes an endless sequence of "bangs" and "crunches" ? i.e. periods of expansion followed by periods of contraction. At each transition, the universe would have some finite temperature and density, rather than the infinity of the singularity, and the expansion and contraction would be relatively slow, as opposed to the exponentially quick expansion proposed by inflation.
The idea is based on M-theory, a version of string theory which suggests that every particle is in fact a tiny loop of string whose vibration pattern determines what type of particle it will be. However, M-theory requires the universe to have 11 dimensions. So far, we can only detect four dimensions ? three of space and one of time. But maybe the other seven are hidden, proponents say.
Scientists call the four-dimensional part of the universe we can see a brane, and suggest that other four-dimensional branes may also exist inside this 11-dimesnional space.
"If you have another brane living in higher dimensions, it's extremely likely to move and slam into our own brane," Ovrut said. "You have a brane with exactly the structure of our real world, and other branes that are likely to hit us, and all of the energy of colliding universes would come into play. Gee, that sounds a heck of a lot like the Big Bang to me."
Advocates of the idea say it offers an exciting way of addressing the issue of what prompted the Big Bang, and it avoids some of the pitfalls of inflation.
"In the cyclic theory you are not only describing the last bang, but the ones before it," Turok explained. "It?s a bigger picture, more complete and hopefully more logically consistent."
But other researchers say the cyclic model just hasn't come far enough to offer a real alternative to inflation.
"Inflation runs into trouble when you try to make the big picture work, but I don?t think the cyclic people have really worked that hard to make the big picture work any better," Albrecht said. "I think they have their hands full with technical issues."
Testing the models
Luckily, scientists may not have to wait too long to learn which theory is a better bet. The models make different predictions about certain aspects of the universe that are measurable today.
For example, inflation could have created gravitational waves ? distortions of space-time caused by gravity ? that should be observable.
Some new instruments, like the Planck satellite which launched in 2009, and an instrument called a polarimeter being built at the South Pole Telescope in Antarctica, could measure these waves.
"If we were to observe those gravitational waves, that would kill the cyclic-ekpyrotic-bouncing theories," Steinhardt said. "It would be very consistent with the inflation idea."
However, not finding the waves wouldn't really blow a death knell to either theory, since some versions of inflation don't require gravitational waves. Either way, it should be exciting, scientists say.
"The quality of astronomical data is just shooting up," Albrecht said. "The data will probably be collected in the next five to 10 years, and you can just see what happens."
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