A particle proposed 50 years ago may be masquerading as a fraction of a neutrino. If so, it could help to explain the origins of dark matter, the mysterious, invisible stuff that makes up more than 80 per cent of the universe鈥檚 matter.
Proposed in the 1960s, the Goldstone boson is supposed to be part of the Higgs mechanism, which gives mass to some other fundamental particles. But no hints of it have been seen in nature so far.
Enter the cosmic microwave background (CMB), relic radiation from about 380,000 years after the big bang. New high-resolution maps of the CMB, made using data from the Planck satellite, show patterns based on temperature variations in the early universe.
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Cosmologists think these patterns arose from interactions between the various types of particles that should have existed. This includes the three known types on neutrino, near-massless particles that flit through the universe while barely interacting with ordinary matter.
Goldstone to the rescue
Even ghostly neutrinos with their minuscule masses can affect how matter clumps in the early universe. Strangely, the CMB maps show the early universe as being smoother than we expected, and analyses of them make the most sense if there are between three and four types of neutrino, rather than just three.
鈥淐learly there鈥檚 no such thing as half a neutrino,鈥� says Nobel-prizewinning physicist of the University of Texas, Austin. 鈥淪o what could it be?鈥�
The Goldstone boson could fill the gap, he says. It is its own antiparticle, so Goldstone bosons that collide with each other would annihilate. That means the boson would have half the effect of a more traditional particle pair on the maps. The boson is also expected to interact less with other matter as the universe expands, which means it could look like 0.39 of a neutrino now. 鈥淭hat seems to match what鈥檚 being seen in observations,鈥� says Weinberg.
The other option is that the extra bit of a neutrino is no more than noise in the data, says astrophysicist David Spergel of Princeton University.
鈥淥ne of the things I鈥檝e been watching since the relatively early WMAP days is this evidence for extra neutrino species,鈥� he says. 鈥淥ne thing I would note is that, as the data improves, the best fit is heading closer and closer to the standard model.鈥�
If the fractional neutrino persists, though, Spergel thinks the Goldstone boson is a plausible and potentially very exciting interpretation.
For one thing, the type of process that would have brought Goldstone bosons into being is the same one that, theoretically, would give rise to dark matter. The bosons are massless, so they can鈥檛 actually be dark matter. But Weinberg suggests that, if it is real, understanding the Goldstone could help reveal the nature of dark matter from the earliest days of the cosmos.
That鈥檚 more speculative, he says. 鈥淏ut you have to leave this as an open possibility.鈥�
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