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Did the Higgs boson puff up the universe?

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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 "inflatons", drove this expansion by pushing space apart.

But there's a problem. "If you ask cosmologists what the inflaton actually is, they will stumble," says Anupam Mazumdar at Lancaster University in the UK. "We need a connection between the inflaton and particles that we know about."

That's 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 "negative pressure" required to push space apart. But no matter how you adjust the Higgs's properties in calculations, it either drives the universe's 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.

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's explosive effect, slowing down inflation enough to fit with observations of the CMB, says Shaposhnikov (Physics Letters B, vol 659, p 703).

Andrew Liddle, a cosmologist at the University of Sussex in the UK, is impressed. "This is definitely a very positive move," he says. But he points out that messing with our understanding of gravity - even if only in the early universe - might be too high a price to pay for this. General relativity predicts that gravity's effects are solely determined by a particle's mass, but in Shaposhnikov and Bezrukov's 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. "We didn't invent this interaction, but we are the first to try and solve this specific problem with it," 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's team says that the inflaton's behaviour would be modified if it was made up of the Higgs plus the "sneutrino" - the neutrino's heavier twin - 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 (Physical Review Letters, vol 99, p 261301).

What's 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's team says the number of inflatons needed to drive inflation would decay after the universe's rapid expansion, leaving behind just the right density of sneutrinos to match the amount of dark matter.

"They've successfully explained three seemingly unconnected things at once - inflation, dark matter, and the neutrino mass - and that makes it compelling," 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.

Period22/01/2008

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 "inflatons", drove this expansion by pushing space apart.

But there's a problem. "If you ask cosmologists what the inflaton actually is, they will stumble," says Anupam Mazumdar at Lancaster University in the UK. "We need a connection between the inflaton and particles that we know about."

That's 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 "negative pressure" required to push space apart. But no matter how you adjust the Higgs's properties in calculations, it either drives the universe's 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.

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's explosive effect, slowing down inflation enough to fit with observations of the CMB, says Shaposhnikov (Physics Letters B, vol 659, p 703).

Andrew Liddle, a cosmologist at the University of Sussex in the UK, is impressed. "This is definitely a very positive move," he says. But he points out that messing with our understanding of gravity - even if only in the early universe - might be too high a price to pay for this. General relativity predicts that gravity's effects are solely determined by a particle's mass, but in Shaposhnikov and Bezrukov's 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. "We didn't invent this interaction, but we are the first to try and solve this specific problem with it," 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's team says that the inflaton's behaviour would be modified if it was made up of the Higgs plus the "sneutrino" - the neutrino's heavier twin - 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 (Physical Review Letters, vol 99, p 261301).

What's 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's team says the number of inflatons needed to drive inflation would decay after the universe's rapid expansion, leaving behind just the right density of sneutrinos to match the amount of dark matter.

"They've successfully explained three seemingly unconnected things at once - inflation, dark matter, and the neutrino mass - and that makes it compelling," 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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References

TitleDid the Higgs boson puff up the universe?
Degree of recognitionInternational
Media name/outletNew Scientist
Date22/01/08
Producer/AuthorZeeya Merali
PersonsAnupam Mazumdar