STEPHEN HAWKING is something of a gambler when it comes to physics, placing bets on everything from the action of black holes to the discovery of gravitational waves. The bad news for Hawking is that a touch of “quantum smearing” could significantly lower his chances of winning his latest wager.
In 2002, Hawking bet his University of Cambridge colleague that cosmologists would soon discover primordial gravitational waves and so verify the theory of inflation. Our universe is thought to have undergone inflation – a period of exponential expansion – a fraction of a second after the big bang, generating ripples in the fabric of space-time called gravitational waves.
NASA’s Wilkinson Microwave Anisotropy Probe found inflation’s footprints in the cosmic microwave background, radiation emitted about 370,000 years after the big bang. But WMAP was not sensitive enough to spot signs of gravitational waves. The Planck satellite was launched in May 2009 to get a much more detailed picture of the CMB. It is looking for the imprint of such waves by studying the tiny variations in temperature of the CMB from point to point in the sky. Hawking is betting that the strength of the waves will be above a certain value. If he is right, Planck should spot them.
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However, the chances of Planck seeing signs of gravitational waves depend on exactly what happened during inflation, according to of the University of Delaware in Newark. “There is a chance that Planck may miss it,” says Shafi.
Inflation was triggered by a field in the early universe called the inflaton, whose energy density fell slowly, like a ball rolling down a gentle slope. According to Einstein’s equations of general relativity space-time expanded exponentially, the process only stopping when the inflaton reaches the bottom of the slope. The simplest models assume that the slope – also called the inflaton potential – resembles a very shallow parabola.
Now Shafi is arguing that the inflaton potential should be modelled on another field that physicists think exists in nature: the Higgs field, which gives all elementary particles their mass. The Higgs potential is shaped like a Mexican hat (see diagram).
“If nature chose it for the Higgs field, then maybe it also chose it for the inflaton,” says Shafi. He modelled inflation using the Higgs potential, and also added one more variable, which dictates just how much the inflaton interacts quantum mechanically with other fields when inflation ends. This “coupling” would have transferred energy and created the radiation that led to the formation of matter, argues Shafi.
The calculations show that the higher the degree of coupling, the lower the strength of the gravitational waves generated by inflation. Also, the possible values for the strength of gravitational waves will be spread out over a much wider range than predicted in simpler models. That may mean that their actual strength may turn out to be below the threshold that the Planck satellite is capable of detecting. “The quantum [couplings] smear the predictions,” says Shafi, who will present his work at the at the University of California, Los Angeles, in February.
Hawking, however, remains optimistic. he reiterated his prediction that gravitational waves will be observed at strengths Planck can observe. Hawking has yet to name his stake though. “So far, Stephen hasn’t named an amount,” says Turok, now at the Perimeter Institute in Waterloo, Ontario, Canada. “I was willing to take it at even odds for any amount.”
Hawking’s wagers
- In 1975, Stephen Hawking bet Kip Thorne that the X-ray source Cygnus X-1 does not harbour a black hole. Thorne was to get a subscription to Penthouse if he won, while Hawking asked for a subscription to British satirical magazine Private Eye if he won. Hawking lost the bet.
- Hawking and Thorne bet John Preskill in 1997 that black holes destroy everything that falls into them, and that no information can escape black holes. Hawking conceded he was wrong in 2004, giving Preskill a baseball encyclopaedia. Thorne has not admitted defeat.
- In 2000, Hawking bet Gordon Kane $100 that the Higgs boson will not be discovered by the Tevatron collider at Fermilab in Batavia, Illinois. The collider is still searching.
