The particle credited with giving others mass, the Higgs boson, may also be to blame for the universe flying apart ever faster. That’s because the Higgs boson could, in principle, be giving rise to dark energy.
The standard model of particle physics encompasses the fundamental particles that make up matter, as well as associated fields. The photon, for instance, is tied to the electromagnetic field. Discovered last year, the Higgs boson also comes with an associated field but, unlike others of its class, the Higgs field is scalar – it does not act in a specific direction.
Taken together, the known particle fields create a certain density of energy permeating the universe. Before the discovery of dark energy, particle physicists were worried that the simplest versions of the standard model predicted an enormous, possibly infinite energy density that would force the universe to expand at an ever-increasing rate.
That seemed improbable until observations of distant supernovae showed that galaxies are not only moving away from each other, but accelerating. The discovery seemed to resolve the issue, but it turns out that the culprit, which we now call dark energy, is much weaker than the standard model indicates.
“It’s very different from what we would predict,” says Frank Wilczek of the Massachusetts Institute of Technology. “This is the profound embarrassment of this fundamental feature of the universe.”
Spurred by the appearance of the long-anticipated Higgs boson, physicists Lawrence Krauss of Arizona State University in Tempe and James Dent of the University of Louisiana at Lafayette may be on the trail of why dark energy is so wimpy.
“What we show is, if the Higgs exists – which it appears to – it can be a portal to new physics and in principle be associated with a new field, which could give an energy density in the universe that’s of the right order of magnitude,” says Krauss.
Even before the Higgs was discovered, Krauss was wondering if other scalar fields could couple to the Higgs field, offering links to new physical phenomena. But he was actually a Higgs sceptic until the very end.
“I was preparing papers on why the Higgs doesn’t exist, expecting them not to see it at the LHC. Then when they did, it hit me that, my God, all of these possibilities that I’d long discounted involving the existence of new scalar fields in the universe might actually be right.”
Krauss and Dent had devised a new scalar field that would exist outside the standard model. Without the Higgs, this field would have zero energy density. But the standard model says that all the fundamental forces and their associated fields should merge at extremely high energies, meaning there is a unified, high-energy field already out there. If the new scalar field can use the Higgs to link up to this high-energy field, it could acquire some energy of its own.
The amount of energy would be determined by a seesaw mechanism: if the value of one field goes up, the other goes down. Since the unification field is so energetic, the new scalar field would be at very small energies. Krauss and Dent found that it would be the same order of magnitude as the observed dark energy.
“For the very first time for me, it shows that it’s not unnatural to at least produce this really, really small energy scale, which otherwise is inexplicable in particle physics,” says Krauss.
Wilczek, who was not involved in the new work, notes that although it offers a way to produce the amount of dark energy that we observe, it doesn’t explain where the rest of the energy predicted by particle theories went.
“It’s a question of what do you buy and at what price? It does not buy you an answer to the big question, which is how did everything else cancel out to get zero?” he says. “If it’s right, it’s a remarkable thing. But if it’s wrong, I don’t think anybody should be terribly upset.”
Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.111.061802
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