SOMEWHERE in our universe, a new big bang is waiting to happen. It probably won鈥檛 go off during our lifetime, or even during the lifetimes of all the stars in our galaxy. But then again, it could happen tomorrow. It could happen on your kitchen table while you鈥檙e eating breakfast.
We have never seen any evidence that our universe has given birth to another one, and we are not even sure what the event would look like. But that has not stopped Sean Carroll and Jennifer Chen of the University of Chicago in Illinois from calculating the probability that it will happen at any particular point in space and time. Their result provides a new number for physicists to contemplate. It is 1 divided by 10 to the power of 1056; that is, 1 divided by 1 with 100 million trillion trillion trillion trillion zeros after it. 鈥淚t is probably the smallest number in the history of physics,鈥� Carroll says.
It is so small, he points out, that its units are unimportant. Call it what you want: the chance of a new universe per cubic metre per day, or the probability of a big bang per cubic light year per second. It doesn鈥檛 really matter which you choose. There are only 40-odd zeros between a cubic metre and a cubic light year, for instance: who鈥檚 going to quibble about a few dozen zeros among 100 million trillion trillion trillion trillion of them?
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It may seem like a fantastical figure plucked out of the air, but Carroll and Chen used well-established physics to find it. They started from the fact that the galaxies in our universe are flying apart. Trace them back, and it seems they came from one small, hot and dense patch of space-time. The widely accepted theory of inflation, first proposed by Alan Guth at the Massachusetts Institute of Technology, suggests that this patch blew up immediately after the big bang to become everything we see today.
Most of the energy that powered this period of inflation was converted into matter and radiation. But not all of it: some of the inflationary energy remains as 鈥渧acuum energy鈥�, the energy that is inherent in empty space. This causes space to push outwards on itself, a phenomenon that astronomers believe is responsible for the apparent increasing acceleration of the expansion of our universe (快猫短视频, 11 April 1998, p 26).
It is this vacuum energy that can give rise to a new universe. The vacuum energy is not a fixed quantity; quantum mechanics says it fluctuates at every point in space-time. If there is a sudden, sharp increase in this energy at some point, that point will blow up faster than its neighbours. And if the difference is dramatic enough, it will create a new big bang.
Carroll and Chen took the measured value of the vacuum energy of our universe 鈥� deduced from observations of how much the universe鈥檚 expansion is accelerating 鈥� and used it to calculate the chance that a quantum fluctuation in the energy of some patch will happen in just the way needed for inflation to begin. The uncertainty principle in quantum mechanics says that fluctuations in the vacuum energy are random, but that they occur around the vacuum energy鈥檚 average value (the value astronomers measure). The pair calculated the probability that one of these fluctuations would be high enough to make the point expand exponentially fast relative to the rest of the universe, and so kick off a new universe. Their result provides a measure of the probability that a big bang will begin at any particular moment and location. And, though ridiculously small, it is not zero: eventually, in an infinite universe, it will happen.
So what will the new big bang be like? Unsurprisingly, the experts are not sure. But if you are unlucky enough to have a new big bang go off inside you, it seems a fair bet that you won鈥檛 survive. That is because the high energy needed to kick start an inflating universe might well be hot enough to cause a fatal injury. But if it happens next to you, on the other hand, you might survive. You would probably see a flash of energy as the fluctuation happens. As the region of the fluctuation begins to inflate, it will expand at enormous speed, creating more and more space of its own. Most likely, the frontier of the new universe will appear to rush away from you incredibly quickly, moving the wavelength of its emitted light towards the red end of the spectrum before it vanishes. 鈥淵ou will see the light redden, go faint, and disappear,鈥� says Alexander Vilenkin, an expert on inflation at Tufts University in Somerville, Massachusetts.
However that would only be the case if the outer boundary of the new universe is expanding much faster than ours. But it might not be, Vilenkin says; it is possible that the new universe has less vacuum energy than ours, and thus will appear to be a real slowcoach. That would make its formation look like the collapse of a section of our universe 鈥� much like the formation of a black hole. 鈥淵ou will be sitting there guessing if there is something inside or not,鈥� Vilenkin says.
Long wait
Of course, it is unlikely any of us will have to ponder such questions. Although the quantum fluctuations are entirely random 鈥� and so defy prediction 鈥� it is fair to assume that the next big bang won鈥檛 happen while we鈥檙e around to appreciate it. We are living during a very brief epoch in the history of our universe, one in which the matter and radiation still exist. Calculations show that the mysterious 鈥渄ark energy鈥� that drives the expansion of our universe will eventually drive all the matter and energy far apart, diluting it so much that the universe becomes a cold, dark and very lonely place. Just a few hundred billion years from now, our universe will be unrecognisable to us 鈥� not least because we won鈥檛 be able to see most of it (快猫短视频, 20 October 2001, p 36).
And, because we now know the probability of a new big bang at a point in space and time is so low, we can be fairly sure that our universe will have to wait until the number of space-time points in existence is much higher before it happens. That almost certainly means the big bang won鈥檛 happen until long after we have disappeared.
But don鈥檛 let that put you off looking for it. Although you鈥檙e unlikely to open your refrigerator and catch a new big bang melting the butter, you shouldn鈥檛 dismiss the possibility entirely. 鈥淚t鈥檚 a probabilistic process,鈥� Vilenkin says. 鈥淚t can happen any time.鈥�