It is one of those questions only children and professors ask - what was there before the Big Bang?
The world-famous physicist Stephen Hawking liked to dismiss this particular cosmic riddle by insisting it made no more sense than asking what is north of the North Pole. As time itself began at the moment of the Big Bang, he argued, it is simply meaningless to ask what came before.
But now, a former colleague of the late Prof Hawking has not only asked the same question, but claims to have glimpsed the answer.
According to Professor Sir Roger Penrose of the University of Oxford, our universe still carries the scars of the events of our universe’s predecessor, which vanished 14 billion years ago.
Prof Penrose, one of the world’s most distinguished theoretical physicists, claims evidence suggests our universe is just the latest in an infinite series, each emerging phoenix-like from its predecessor in a Big Bang.
Needless to say, it is a claim that is provoking strong controversy in some scientific circles. Yet even critics of Prof Penrose’s theory concede there are some serious problems with the textbook account of the birth of our universe.
According to accepted wisdom, our stars, planets and galaxies burst into existence literally out of nowhere, the result of the weird laws of quantum theory which govern the sub-atomic world.
Initially far smaller than even the tiniest sub-atomic particle, the newly-formed universe came with a kind of anti-gravitational force-field which made it rapidly expand. Almost immediately, this force-field vanished in a huge explosion — the Big Bang — whose energy turned into the matter we now see, courtesy of Einstein’s famous equation E = Mc^2 .
But you do not need to be Einstein to have issues with all this.
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For a start, where did that force-field come from — or, come to that, the space containing it?
This “inflation model” of the birth of the cosmos was first put forward in the late 1970s, and was initially hailed as a major breakthrough. But now it is increasingly seen as raising as many questions as it answers.
In the search for alternatives, some theorists — including Prof Penrose — are looking again at the ancient idea that our universe is just the latest of a never-ending series.
According to this “cyclic model”, the answer to what came before our universe is simple: another universe.
The possibility of such endless cosmic cycles is hinted at by Einstein's theory of gravity. Even so, the idea was dealt an apparently fatal blow more than 80 years ago.
Richard Tolman, an American theorist, argued that each universe would contain an ever-greater amount of radiation. By now the universe should be infinitely old and infinitely hot — making our very existence impossible.
Yet theorists now think there is a loophole in this argument. Put simply, Einstein’s equations describing the universe go haywire at the birth of each new universe, and so cannot be trusted.
Recent work combining Einstein’s law of gravity with quantum theory suggests Tolman’s argument against cyclic universes is also unreliable — putting the notion of the cyclic universe back on the agenda.
Prof Penrose and colleagues in the US and Poland have been investigating the implications, and now think they have found telltale signs of the universe that existed before our own.
They base their claim on studies of radiation left over from the Big Bang.
First detected in the mid-1960s, this radiation permeates the whole of space in the form of microwaves. Detailed studies by orbiting satellites have shown, however, that the radiation is not spread evenly across the sky.
That is partly because of the turbulence that existed in our universe at its creation. But now, according to Prof Penrose and his colleagues, the radiation also shows patterns consistent with events that took place in the universe before ours.
They argue that this cosmic predecessor would have contained giant black holes, objects whose gravity is so strong not even light can escape their clutches.
Over countless trillions of years, these black holes would have consumed all other matter in the earlier universe.
After countless more trillions of years, these too would vanish in bursts of so-called Hawking radiation, predicted by the eponymous theorist in the 1970s.
According to Prof Penrose, this radiation can seep through the Big Bang creating “hot spots” detectable in the microwave radiation permeating today’s universe.
To test their idea, they have examined data from the Planck satellite, launched by the European Space Agency in 2009, which gives the best chance of revealing these hot spots.
And in results being circulated within the scientific community, they claim to have found hard evidence for what they call “Hawking points” consistent with their theory.
So is our universe really just the latest in a never-ending sequence? Perhaps unsurprisingly, the reaction from other experts has so far been cautious.
Some are concerned Prof Penrose and his colleagues have fallen into the trap of seeing patterns in what is really just randomness.
The only way to distinguish the two is to show that the chances of a fluke result are vanishingly small. The team currently puts the probability of getting what they have seen in the Planck data by chance alone at about one in 1,000. While impressive, that is still far higher than the one-in-several million level normally used by physicists to take a claim seriously.
Undaunted, Prof Penrose is looking for other potential telltale signs of the existence of an earlier universe. His aim is to build a compelling case for the cyclic universe based on multiple sources of evidence.
Whether he and his colleagues will win over the sceptics remains to be seen. But whatever happens, they have succeeded in giving the rest of us something truly mind-boggling to contemplate on the drive into work.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK