WHY was the big bang so very big? It has been a struggle to explain why the infant universe expanded so rapidly. But now Stephen Hawking at the University of Cambridge, and colleagues, think they are close to perfecting an answer – by treating the early cosmos as a quantum object with a multitude of alternative universes that gradually blend into ours.
The idea that the universe expanded at a blistering rate in the first 10-34 seconds after the big bang was proposed to explain why regions of the universe separated by vast distances have such a similar background temperature: before inflation occurred, these regions would have been close together with similar properties. But just why the universe inflated in the first place remains a mystery.
“There’s no fundamental theory that can explain why inflation happened in our universe – it’s just proposed as an ad hoc solution that explains some particular observations,” says Thomas Hertog of Denis Diderot University (DDU) in Paris, France.
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What’s worse, one of the best attempts to explain how the universe came to be – string theory – has arguably complicated matters. It suggests that more than 10500 variations of the universe, each with different physical parameters, could exist side by side in a vast “landscape”. “You will have all kinds of universes, some with no inflation, and others where there is a long period of inflation – and our universe could be any one of those,” says Hertog.
In 2006, Hawking and Hertog floated an idea that they hoped might explain why inflation happened and embrace all of string theory’s alternative universes. They did this by treating the early universe as a quantum object. According to quantum mechanics, when a particle travels between two points, it doesn’t simply take one path – it takes every possible path between the two locations simultaneously, although some paths contribute more than others.
The pair suggested that, in a similar manner, there was no unique beginning to the universe. Instead, the wave function of the universe encompassed a multitude of alternative paths up to today (żěè¶ĚĘÓƵ, 20 April 2006, p 28).
Instead of starting with a set of initial conditions and calculating how the universe evolves, Hawking and Hertog started with current observations and worked back to narrow down what the initial set of possibilities might have been.
“Instead of starting with a set of initial conditions, they took today’s observable universe and worked backwards”
They began by choosing the most fundamental characteristic that they believe defines our present universe – that we largely experience the world classically. In other words, Newtonian physical laws hold sway over our everyday lives rather than weird quantum effects. The pair then calculated all possible early histories that would produce a classical universe.
Here they encountered a problem: their calculations predicted that the early universe would have experienced only a tiny amount of inflation. This conflicted with observations of the pattern of temperature variations in the cosmic microwave background (CMB) – the radiation left behind by the big bang – which suggest that inflation lasted for a significantly longer period. “This has been recognised to be a problem for some time,” says Hawking.
Now, though, they claim to have solved it. Working with James Hartle at the University of California, Santa Barbara, their solution involves the fact that we can only observe a finite portion of the whole universe. This observable region is called the “Hubble volume”. First time around, they had assumed that the Hubble volume could only fit within the universe in one way, like a jigsaw piece.
Their original model predicted only a tiny minority of universes that could have inflated enough to fit the Hubble volume, so they discounted those universes as improbable. In fact, there are potentially millions of ways that our observable patch could sit within each of the universes in that minority of possibilities. Taking this “volume weighting” into account massively increases the probability that our universe developed from one of that minority (Physical Review Letters, vol 100, p 201301).
“We have shown that this proposal, with volume weighting, can explain why the universe inflated,” says Hawking, who presented the work at . “We found that you cannot have a classical universe without inflation,” adds Hertog. What’s more, the theory explains what “path” through string theory’s predicted universes we took to get where we are today, he says. “That’s a striking and powerful result.”
Alex Vilenkin, a cosmologist at Tufts University in Medford, Massachusetts, is impressed that the theory now matches observations of the CMB. “This is interesting work, by a talented group. They are finding intriguing connections with inflation,” he says. However, he adds that “for now, this is a theory that is still under construction”.
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