THE Higgs boson has been moonlighting. Not content with its day job of giving other particles their mass, it may also have driven the expansion of the early universe, given a little tinkering, according to two separate studies.
Soon after the big bang the early universe is believed to have undergone a period of rapid expansion, known as inflation. The idea is that hypothetical particles, aptly named 鈥渋nflatons鈥�, drove this expansion by pushing space apart.
But there鈥檚 a problem. 鈥淚f you ask cosmologists what the inflaton actually is, they will stumble,鈥� says Anupam Mazumdar at Lancaster University in the UK. 鈥淲e need a connection between the inflaton and particles that we know about.鈥�
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That鈥檚 where the Higgs boson comes in. Although yet to make an appearance, it would seem to be the perfect candidate for the inflaton because the Higgs is the only particle with the 鈥渘egative pressure鈥� required to push space apart. But no matter how you adjust the Higgs鈥檚 properties in calculations, it either drives the universe鈥檚 expansion too quickly or creates huge ripples in space-time. Such ripples are nowhere to be seen in the cosmic microwave background (CMB), the radiation left over from the big bang, points out Mikhail Shaposhnikov at the Swiss Federal Institute of Technology (EPFL) in Lausanne.
鈥淭he Higgs is the perfect candidate for the inflaton: only the Higgs has the negative pressure to push space apart鈥�
Now Shaposhnikov and Fedor Bezrukov, also at EPFL, have found a way to rein in the Higgs. Their calculations show that if gravity interacted with the Higgs in a different way to other particles, it would damp down the Higgs鈥檚 explosive effect, slowing down inflation enough to fit with observations of the CMB, says Shaposhnikov ().
Andrew Liddle, a cosmologist at the University of Sussex in the UK, is impressed. 鈥淭his is definitely a very positive move,鈥� he says. But he points out that messing with our understanding of gravity 鈥� even if only in the early universe 鈥� might be too high a price to pay for this. General relativity predicts that gravity鈥檚 effects are solely determined by a particle鈥檚 mass, but in Shaposhnikov and Bezrukov鈥檚 model, the Higgs becomes a special case, with gravity apparently tugging on it less than on other particles of similar mass.
Shaposhnikov points out that other physicists, including Nobel laureate Richard Feynman, have also found that they needed to tweak the interaction between the Higgs and gravity in attempting to unite particle physics and general relativity. 鈥淲e didn鈥檛 invent this interaction, but we are the first to try and solve this specific problem with it,鈥� says Bezrukov.
Mazumdar, however, does not want to compromise general relativity. Instead, he and his colleagues are using a different tactic to control inflation, based on a theoretical extension to the standard model, known as supersymmetry (SUSY).
SUSY predicts that every standard particle has a heavier twin. Mazumdar鈥檚 team says that the inflaton鈥檚 behaviour would be modified if it was made up of the Higgs plus the 鈥渟neutrino鈥� 鈥� the neutrino鈥檚 heavier twin 鈥� plus any one of a range of other SUSY particles, none of which has so far been seen experimentally. In their model, the rate of inflation is controlled by the masses of the SUSY particles and of regular neutrinos, which have been detected. By using the measured masses for the neutrinos and predicting masses for the sneutrino and other SUSY particles, the team found they could get the right amount of inflation ().
What鈥檚 more, their model could also explain the origin of the mysterious dark matter thought to make up most of the matter in the universe. The sneutrino is already a good candidate particle for dark matter and Mazumdar鈥檚 team says the number of inflatons needed to drive inflation would decay after the universe鈥檚 rapid expansion, leaving behind just the right density of sneutrinos to match the amount of dark matter.
鈥淭hey鈥檝e successfully explained three seemingly unconnected things at once 鈥� inflation, dark matter, and the neutrino mass 鈥� and that makes it compelling,鈥� says Liddle. But he adds that the group needs to check if their model fits with CMB observations.
Both teams are also eagerly awaiting data from the Large Hadron Collider, the particle accelerator being built at CERN near Geneva in Switzerland, to be switched on later this year.
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