News from the Frontiers of Cosmology: A companion to the book The Edge of Physics
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Does physics win if Hawking loses bet?

A STORY this week in New Scientist (which I wrote) looks at the likelihood of the European Space Agency’s Planck satellite detecting gravitational waves generated during inflation. Stephen Hawking’s wager with Neil Turok adds a touch of spice to the tale. At a meeting in Cambridge last August on primordial gravitational waves, Hawking reiterated his 2002 bet with Turok, arguing “that primordial gravitational waves will be observed, with a ratio of tensors to scalars, of above 5%.”

Basically, Hawking is betting that inflation would have generated gravitational waves strong enough to be detected by Planck (not directly, but as imprints in the cosmic microwave background). Inflation is that episode in the history of the universe – just a fraction of a second after the big bang – when the universe expanded exponentially. Inflation is needed to explain why the universe is flat and homogeneous, and is thought to have occurred when a field called the inflaton suffused spacetime and changed very slowly (physicists talk of the inflaton rolling down a gently-sloping potential hill). This caused spacetime to literally blow up.  It was a very violent period, leading to the roiling of spacetime and the generation of gravitational waves.

Turok, on the other hand, argues that inflation never occurred. He and Paul Steinhardt explain the observed properties of the cosmos using their cyclic universe model, which posits that the universe cycles through a series of big bangs and big crunches. In their model, the gravitational waves generated in the early universe would not have been strong enough to leave a detectable imprint on the CMB.

But it’s possible that inflation could have occurred, and yet the gravitation waves would not have been strong enough for Planck to see. So, Hawking could still lose the bet, without Turok and Steinhardt being right about the cyclic universe model.

How could that happen? Well, the New Scientist story explains one physicist’s take on it. Qaisar Shafi of the University of Delaware worked out models of inflation in which the inflaton field rolls down a potential that’s similar to the potential of the Higgs field. Add to that the fact that at the end of inflation, the inflaton field has to interact with standard model fields to reheat the universe and give rise to the standard model particles. One consequence of this is that the inflation-generated gravitational waves would not be strong enough for Planck to detect.

The strength of gravitational waves could also be lower if the universe is supersymmetric, an idea that says that there is a super-partner particle for every known standard model particle (see New Scientist feature on supersymmetry). Say, the Large Hadron Collider detects supersymmetry at the teraelectronvolt (TeV) scale. What this would mean is that the gravitino (the super-partner of the graviton) is only about a thousand times heavier than a proton. And this has implications for inflation: a gravitino with such a mass means that the energy scale of the universe at the end of inflation was also small, and as such, any inflation-generated gravitational waves would not be within reach of Planck.

I suspect most physicists would rather find evidence for supersymmetry at the LHC and forego the detection of inflation-generated gravitational waves until a future, more sensitive experiment is ready.

Why? Well, supersymmetry is the most likely candidate for physics beyond the standard model (and we know there has to be physics beyond the standard model). Finding evidence of supersymmetry would help physicists chart a course towards a theory of quantum gravity.

Finding evidence of inflation-generated gravitational waves, while extremely exciting and genuinely gratifying, would probably not help physics in the same earth-shaking manner as the discovery of supersymmetry. It would help fix the energy scale of inflation and maybe even provide clues to whether models of inflation built using string theory are on the right track. But it would not have the same potential for steering physics in a new direction as the sighting of a supersymmetric particle at the LHC.

That’s a roundabout way of saying that it’s better for physics that Hawking loses his bet and the LHC finds supersymmetry.

The Edge of Physics


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