Cosmologists have used measurements of some of the most massive objects in the universe to place a limit on the mass of the lightest particle in the cosmos. Using data from a survey of 700,000 galaxies, the researchers found that an elusive subatomic particle called a neutrino can have a mass of no more than 0.28 electron volts (eV), which is less than one-billionth of the mass of a hydrogen atom.
Neutrinos are the loners of the particle world; they hardly interact with other matter at all. For example, billions of neutrinos are passing through you right now, yet only one of them is likely to hit a nucleus in your body in your lifetime.
Still, neutrinos are critical to our universe. Cosmologists think that the universe consists of the ordinary matter that makes up galaxies, mysterious "dark matter" whose gravity holds the galaxies together, and some weird space-stretching "dark energy" that appears to be speeding up the universe's expansion. And even though neutrinos weigh almost nothing, they are so abundant that they make a significant contribution to the ordinary matter part of the recipe, says Shaun Thomas, a cosmologist at University College London (UCL) and a co-author of the new study. In cosmological models, the quantities of each of the ingredients influence the rest, so if researchers get the mass of the neutrinos wrong, they may miscalculate the amount of dark energy, he says.
Particle physicists long assumed that neutrinos were massless. That view began to change in the late 1960s, when physicists found that there were different types, or "flavors," of neutrinos and that it appeared that one type of neutrino could transform into another—something that can happen only if different neutrino types have different masses. Since then, studies of neutrinos, which use enormous underground particle detectors or particle accelerators, have shown that there are three types of neutrinos—electron, muon, and tau neutrino—and that they do [change flavor from one type to another. However, "current particle physics experiments don't tell us what the actual masses of the neutrinos are—just the differences between the masses," Thomas says.
That's where galaxies come in. If it's big enough, the weight of all those neutrinos could affect the evolution of the galaxies after the big bang. To look for this, the researchers first used measurements of the afterglow of the big bang, the so-called cosmic microwave background, from NASA's space-borne Wilkinson Microwave Anisotropy Probe (WMAP) to determine the statistical distribution of density fluctuations in the primordial soup of particles in the newborn universe. They then worked out the three-dimensional distribution of matter in the universe using data on 700,000 galaxies from the Sloan Digital Sky Survey (SDSS), a survey of one-quarter of the sky using a 2.5-meter telescope based in New Mexico.
With these two data sets in hand, the team then ran a computer simulation that takes the ingredients for building the universe—such as the amount of dark matter and the mass of neutrinos—and constructs theoretical mass-distribution plots based on those parameters. The researchers found that they could best match the simulation to the WMAP and SDSS data if the neutrinos had a mass of no more than 0.28eV, they reported at a conference earlier this week at UCL and in a paper in press at Physical Review Letters.
Not everybody is taking the result as the gospel truth, however. Astroparticle physicist Guido Drexlin of the Karlsruhe Institute of Technology in Germany congratulates the UCL cosmologists on an "impressive body of work," but he believes that traditional particle physics experiments in the laboratory are more robust. "Cosmology has achieved very impressive results on neutrino mass, but it relies on the validity of the underlying cosmological model," he says.
Looking to the future, the UCL team expects to be able to refine the mass of the neutrino even further, thanks to two new observing projects: the Dark Energy Survey, which will provide data on 300 million galaxies by 2016, and Europe's Planck spacecraft, which was launched in May 2009 and will make even more precise measurements of the cosmic microwave background. The team should then be able to set a limit on the neutrino mass of just 0.1eV—or actually measure the precise value if it happens to be larger than that, says study co-author and cosmologist Ofer Lahav of UCL.