OUR universe has been around for nearly 14 billion years, but it could vanish in the blink of an eye.
That鈥檚 because the fabric of space-time might be in a state of precarious stability 鈥� what physicists call a false vacuum 鈥� and could collapse at any moment, taking us with it. 鈥淚f a bubble of true vacuum appeared near us in the universe and expanded to include us, we鈥檇 all be very, very dead,鈥� says Sean Carroll, a theoretical physicist at the California Institute of Technology in Pasadena.
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The key to understanding how stable the vacuum is rests with the Higgs boson and its associated field, which pervades all of space-time and gives elementary particles their mass. In July 2012, physicists at the Large Hadron Collider (LHC) in Geneva, Switzerland, finally sighted the Higgs boson and pegged its mass to somewhere between 125 and 127 gigaelectronvolts.
Although elementary particles get their masses by interacting with the Higgs field, the mass of the Higgs boson depends on those particles as well. The heaviest of these, the top quark, has the biggest impact on the Higgs mass. And based on the most recent measurements of both their masses, physicists can now use the properties of the Higgs field to deduce the state of the vacuum of space-time. The news isn鈥檛 great: our universe could be on the brink.
Like a ball rolling down a hill, the vacuum will eventually come to rest in the lowest possible energy state. If there鈥檚 a small trough halfway down, however, the ball could get caught. The ball would have a kind of stability, but also the potential to roll further down.
It seems we鈥檙e in just such a trough (see diagram). 鈥淚f the Higgs mass and the top quark mass were a little bit different, we would either be in a completely stable vacuum or in an unstable vacuum that would have decayed a long time ago,鈥� says Carroll. 鈥淚nterestingly, the world seems to be on an edge. We don鈥檛 have enough precision to say whether our vacuum is stable or not.鈥�
The root of the problem lies in the equation used to predict the Higgs mass, which gives a higher value than has been measured. To fix this, physicists must make the terms relating to the elementary particles鈥� interactions with the Higgs cancel out to a remarkable degree of precision 鈥� remarkable, that is, unless something unobserved is pulling the strings. That鈥檚 one rationale behind the theory of supersymmetry, a long-favoured way of avoiding such fine-tuning. Supersymmetry doubles the entries in the standard model of particle physics by giving each particle a heavy superpartner, cancelling out the terms in the equation as required. But the LHC has seen no hints of these superpartners, casting serious doubts on the theory.
These doubts have reignited interest in another hypothetical particle called the axion, which new calculations show can interact with the Higgs and the top quark so as to prevent the Higgs mass from ballooning. The axion was originally proposed to solve other problems with the standard model, but its ability to multitask makes it a hot target.
If no axion turns up, however, we might be forced to turn to the multiverse for salvation. In a reality filled with infinite universes, each with its own set of particles of arbitrary masses, living in an implausibly fine-tuned universe becomes a lot more likely.
This article appeared in print under the headline 鈥淭he universe is on the brink of catastrophe鈥�
