New Neutrino May Have Heated Baby Universe
. . .In separate papers, three groups of theorists now argue that when combined with other data, the Planck data show evidence of a new particle called a sterile neutrino, something that the Planck team had dismissed.Wake me up when the new data are published.
"At second glance, the universe might turn out to be less boring than initially thought," says Jasper Hasenkamp, a cosmologist at New York University (NYU) in New York City and co-author of one of the papers.
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Now, the three teams argue that, taken all together, cosmological data point to a sterile neutrino. Hasenkamp and Jan Hamann of CERN, the European laboratory for particle physics near Geneva, Switzerland, spelled out the argument last October in the Journal of Cosmology and Astroparticle Physics. In separate papers published on 6 February in Physical Review Letters, two teams—Mark Wyman, a cosmologist now at NYU, and colleagues at the University of Chicago in Illinois, and Richard Battye of the University of Manchester in the United Kingdom and Adam Moss of the University of Nottingham in the United Kingdom—make similar arguments.
The three papers all start with the same basic observation: With its parameters set by the Planck data, the standard cosmology predicts more clusters of galaxies than astronomers see. "All the cluster counting technologies agree with each other, and they all have a profound disagreement with Planck," Wyman says.
But clusters would be less abundant in a universe containing sterile neutrinos about a millionth as massive as an electron. That's because in the standard model, dark matter consists of cold, massive, slow-moving particles that tend to clump. But if some dark matter consisted of hot, fast-moving particles such as neutrinos, then clumping would occur more slowly, producing fewer clusters. A sterile neutrino would also add a dollop of radiation to the early universe and bump up Planck's estimate of the Hubble constant, solving that puzzle as well.
The case for the sterile neutrinos is tricky. To avoid adding too much radiation and running afoul of Planck's previous results, the new neutrino would have to be out of so-called thermal equilibrium with other particles in the early universe, says John Beacom, a physicist at Ohio State University, Columbus, who doubts the new claim. "That's possible, but not natural," he says.
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The issue could come to a head quickly, says David Spergel, a cosmologist at Princeton University. This year, the Planck team should release new results based on more data. And other researchers may improve their cluster-mass calibrations by comparing methods. Whether or not the discrepancy is real, Spergel says, "we should know soon."