ASK physicists what they would most like to find lurking in the debris created by experiments such as the Large Hadron Collider, and they’ll likely say “sܱ⳾ٰ”. That’s because it is the best candidate for solving several niggling problems in today’s theories. But if Nathaniel Craig of Stanford University in California is right, there could be another way to find evidence of supersymmetry. It could be there all around us, in the form of ripples in the fabric of space-time.
The standard model of particle physics, which explains all known particles and their interactions, suffers from several problems. For instance, it cannot explain why the weak nuclear force is 1032 times stronger than gravity. Supersymmetry – in which each known particle has a “super-partner” – has been the favoured fix for such problems.
These particle partners would only have existed in the high temperatures of the infant universe, before the hot supersymmetric vacuum of space-time changed to one without this symmetry. In many models, there are a number of different possible states for the vacuum in which the symmetry vanishes: a consequence of this would be that the universe underwent “phase transitions” from one state to another, before settling down to the way it is today.
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During these transitions, bubbles of a new vacuum would appear in the old vacuum. These bubbles would then rapidly expand and merge, converting all of space-time to the new vacuum state.
Craig has shown that the collision of these expanding bubbles during each phase transition would have generated ripples in the fabric of space-time called gravitational waves (g-waves). The collisions would also have churned up the plasma of particles that made up the universe at the time, which would have generated more g-waves ().
“Collision of expanding bubbles as supersymmetry collapsed would leave ripples in space-time”
Craig’s calculations show that these waves should have a telltale distribution. “It is not a single-frequency signal that you get from astrophysical processes,” he says. Instead, the process would have generated g-waves at all frequencies, with their intensity peaking at a frequency that depends on the temperature at which supersymmetry was broken. As the universe expanded, these waves would have been stretched, so that the peak frequency today would fall between about 0.01 and 10,000 hertz. If these waves exist, it may be possible to see them with , the Laser Interferometer Gravitational-Wave Observatory, once an upgrade now under way has been completed.
The possibility of seeing such waves is “exciting, if it is realised”, says Michael Dine of the University of California, Santa Cruz. “If there is a detectable signal, features of the signal provide unique information about these dramatic cosmic events, in part because gravitational waves can pass freely through space over cosmic history.”
