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Observations

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Cosmos Study Dashes Hope for New Neutrino

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Planck CMB intensity map

Planck's map of the cosmic microwave background, courtesy of ESA and the Planck Collaboration

First particle physicists discovered “a boring old Standard Model Higgs boson,” as my colleague Michael Moyer put it, meaning that the particle hewed closely to theoretical predictions and offered little in the way of guidance to new and exciting physics. This week the European Space Agency’s Planck satellite gave a significant boost to cosmology’s own standard model, the so-called lambda-CDM cosmology in which dark energy and dark matter rule the universe. But the situation here is hardly boring, given that the very nature of dark matter and dark energy remain a mystery.

The first batch of Planck cosmology studies arrived March 21, and it is formidable. Spread across 29 lengthy scientific papers, Planck’s precise measurements of the cosmic microwave background (CMB)—remnant light from the very early universe, just 370,000 years after the big bang—confirm that the universe is about 13.8 billion years old and is dominated by dark energy, with dark matter playing a significant supporting role and normal matter (the atoms of the everyday world) constituting just a few percent of the overall contents.

In short, the Planck data seem to contain no major surprises, although they confirm a few outstanding anomalies from Planck’s CMB-measuring predecessor, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). In an intriguing anomaly, the CMB appears somewhat asymmetrical, as if the universe were a little lopsided.

Neutrino physicists, however, may find themselves a bit disappointed with Planck’s results, which disfavor the possible existence of an extra, yet-to-be-discovered variety of neutrino. Particle experiments on Earth have shown that neutrinos come in at least three flavors—electron neutrinos, muon neutrinos and tau neutrinos—and that neutrinos oscillate between those flavors as they propagate through space. But some puzzling experimental results have hinted that a fourth neutrino flavor—the sterile neutrino—might exist as well. (This hypothetical neutrino is called “sterile” because, unlike the other neutrinos, it would not feel the weak nuclear force and would barely interact with other particles.) Now that possibility looks rather unlikely.

CMB experiments and neutrino physics would seem an odd partnership, but the number of neutrinos and the masses of the particles can indeed have large-scale effects that shape the appearance of the CMB. The total number of neutrino species, for instance, affects the rate at which the cosmos expanded in its earliest epochs: more neutrinos means a faster expansion. Recent WMAP data were consistent with the three-neutrino family portrait but easily accommodated—even hinted at—the possibility of a fourth particle. The new Planck results (pdf), however, favor the existence of just three neutrinos.

One potential bright spot for neutrino physicists: the Planck observations have better pinned down the combined masses of the three neutrinos, none of which can yet be individually weighed with any precision. Cosmology studies currently provide the best limits on the neutrino’s mass, and Planck is already tightening those constraints to clarify the picture of the elusive neutrino. The sum of all three neutrino masses as per the Planck collaboration is no more than 0.23 electron-volt (for comparison, a single electron is two million times more massive), roughly half of the upper limit set by WMAP.

 

The views expressed are those of the author and are not necessarily those of Scientific American.

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