Measuring the minuscule mass of neutrinos has so far proved impossible—and not for lack of trying. Numerous laboratory experiments over the past few decades have succeeded only in placing loose limits on the three neutrino masses.
We have very compelling reasons to expect that the best way to measure the mass of these tiny particles is, surprisingly, to look for their influence at the largest scales of the universe. For although neutrinos are virtually massless and nearly invisible, their sheer numbers—some 1089 in the universe—make them very consequential players in the cosmos.
Our logic goes like this: Early in the history of the universe, when everything was very hot and dense, nuclear reactions forged helium from hydrogen, releasing huge numbers of neutrinos as a by-product. As the universe evolved, expanded and cooled, small fluctuations in the density of this primordial particle soup were amplified; in regions of above-average density, gravity tried to pull more material in.
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Dark matter, the essentially invisible stuff that accounts for a goodly portion of the universe's mass, collapsed into clumps first because it only interacts through gravity. These initial clumps of dark matter formed the seeds of the galaxies and the clusters of galaxies that we see today. Neutrinos, being extremely light, began to clump somewhat later on in the universe's development. In fact, by zipping so freely through the cosmos, neutrinos have actually slowed the clumping of dark matter—an effect that should be detectable today.
The more massive the neutrinos, the more they will have hindered the clumping of matter—in effect, blurring the edges in the large-scale structure of the universe. By measuring how matter is distributed in the universe, then, we can infer how massive neutrinos are.
Mapping the distribution of matter—most of which is dark matter—is hardly straightforward. Yet researchers have seen that the leftover radiation from the big bang, known as the cosmic microwave background (CMB), is slightly distorted because of the light-bending gravitational effects of dark matter clumps that fill the space between the CMB and us. Examining this “gravitational lensing” of the CMB is a very promising way of measuring the distribution of dark matter in the universe.
New precision measurements of the CMB now under way will enable us to measure the lensing distortions to very high accuracy, effectively mapping out the otherwise invisible dark matter. If the distribution of dark matter is confined to sharp-edged structures separated by voids, we can infer that the neutrino masses are small; if instead the edges are blurred, we will know that the neutrino masses are larger. The new generation of CMB experiments should allow us to pin down the combined masses of the three neutrino types to within one five-millionth of the mass of the electron.
That it may be possible to measure the mass of the lightest and most elusive of subatomic particles by observing the entire universe is just another example of how the study of physics, across all scales, continues to surprise and inspire astrophysicists to delve ever deeper into the workings of the natural world.