Did the cosmos have a beginning? The Big Bang theory seems to suggest it did, but in recent decades, cosmologists have concocted elaborate theories – for example, an eternally inflating universe or a cyclic universe – which claim to avoid the need for a beginning of the cosmos. Now it appears that the universe really had a beginning after all, even if it wasn’t necessarily the Big Bang.
At a meeting of scientists – titled “State of the Universe” – convened last week at Cambridge University to honor Stephen Hawking’s 70th birthday, cosmologist Alexander Vilenkin of Tufts University in Boston presented evidence that the universe is not eternal after all, leaving scientists at a loss to explain how the cosmos got started without a supernatural creator. The meeting was reported in New Scientist magazine (Why physicists can’t avoid a creation event, 11 January 2012). I’ve quoted a few brief highlights below.
In his presentation, Professor Vilenkin discussed three theories which claim to avoid the need for a beginning of the cosmos.
One popular theory is eternal inflation. Most readers will be familiar with the theory of inflation, which says that the universe increased in volume by a factor of at least 10^78 in its very early stages (from 10^-36 seconds after the Big Bang to sometime between 10^-33 and 10^-32 seconds), before settling into the slower rate of expansion that we see today. The theory of eternal inflation goes further, and holds that the universe is constantly giving birth to smaller “bubble” universes within an ever-expanding multiverse. Each bubble universe undergoes its own initial period of inflation. In some versions of the theory, the bubbles go both backwards and forwards in time, allowing the possibility of an infinite past. Trouble is, the value of one particular cosmic parameter rules out that possibility:
But in 2003, a team including Vilenkin and Guth considered what eternal inflation would mean for the Hubble constant, which describes mathematically the expansion of the universe. They found that the equations didn’t work (Physical Review Letters, DOI: 10.1103/physrevlett.90.151301). “You can’t construct a space-time with this property,” says Vilenkin. It turns out that the constant has a lower limit that prevents inflation in both time directions. “It can’t possibly be eternal in the past,” says Vilenkin. “There must be some kind of boundary.”
A second option explored by Vilenkin was that of a cyclic universe, where the universe goes through an infinite series of big bangs and crunches, with no specific beginning. It was even claimed that a cyclic universe could explain the low observed value of the cosmological constant. But as Vilenkin found, there’s a problem if you look at the disorder in the universe:
Disorder increases with time. So following each cycle, the universe must get more and more disordered. But if there has already been an infinite number of cycles, the universe we inhabit now should be in a state of maximum disorder. Such a universe would be uniformly lukewarm and featureless, and definitely lacking such complicated beings as stars, planets and physicists – nothing like the one we see around us.
One way around that is to propose that the universe just gets bigger with every cycle. Then the amount of disorder per volume doesn’t increase, so needn’t reach the maximum. But Vilenkin found that this scenario falls prey to the same mathematical argument as eternal inflation: if your universe keeps getting bigger, it must have started somewhere.
However, Vilenkin’s options were not exhausted yet. There was another possibility: that the universe had sprung from an eternal cosmic egg:
Vilenkin’s final strike is an attack on a third, lesser-known proposal that the cosmos existed eternally in a static state called the cosmic egg. This finally “cracked” to create the big bang, leading to the expanding universe we see today. Late last year Vilenkin and graduate student Audrey Mithani showed that the egg could not have existed forever after all, as quantum instabilities would force it to collapse after a finite amount of time (arxiv.org/abs/1110.4096). If it cracked instead, leading to the big bang, then this must have happened before it collapsed – and therefore also after a finite amount of time.
“This is also not a good candidate for a beginningless universe,” Vilenkin concludes.
So at the end of the day, what is Vilenkin’s verdict?
“All the evidence we have says that the universe had a beginning.”
SJD
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