January 31, 1995 Contact: Robert Irion (408/459-2495)

LIGHTWEIGHT NEUTRINOS FIT PERFECTLY INTO LATEST MODELS OF THE UNIVERSE, ACCORDING TO COMPUTER SIMULATIONS

Paper to appear in Physical Review Letters explains the cosmological consequences of neutrinos with mass. It also predicts the most likely scenario: two species of neutrino with the same mass.

FOR IMMEDIATE RELEASE

SANTA CRUZ, CA--The discovery that the ghostly particles called neutrinos may have a small mass supports a special class of theoretical models of how the universe has evolved, according to a group of scientists led by cosmologist Joel Primack of the University of California, Santa Cruz.

An article in today's edition of the New York Times reveals that physicists at Los Alamos National Laboratory have found strong evidence that neutrinos have mass. For now, the physicists have disclosed only a range of possible masses. But if their claim holds up, says Primack, "This would be one of the great discoveries of physics. It would change our whole picture of particle physics and cosmology."

In particular, neutrinos with mass would have deep implications for models of the universe that probe how its structure is influenced by "dark matter"--the mysterious material that appears to make up most of the universe's mass. The gravitational pull of dark matter, cosmologists concur, controlled the evolution of galaxies and will dictate the fate of the universe. However, they have disagreed over whether neutrinos play an important role. The experiments at Los Alamos could turn the tide.

Primack's group will publish its latest results on the implications of neutrino mass for cosmology in an upcoming issue of the journal Physical Review Letters. Primack, professor of physics at UCSC, is first author; coauthors are Jon Holtzman of Lowell Observatory, Anatoly Klypin of New Mexico State University, and David Caldwell of UC Santa Barbara.

The preliminary range of neutrino masses cited by the Los Alamos team is vanishingly small, from one-half to five electron volts. That translates to between a millionth and a hundred- thousandth of the mass of another minuscule particle, the electron. However, neutrinos are ubiquitous: There are about 100 of them in every chunk of space the size of a sugar cube, and about a billion of them for every electron and proton in the universe. Even a tiny mass, then, could mean that neutrinos are a crucial element of dark matter.

According to the detailed computer simulations by Primack and his colleagues, neutrinos should compose about one-fifth of the mass of the universe. That fraction is consistent with the early results from Los Alamos.

Moreover, neutrinos represent a special kind of dark matter, called "hot" dark matter. Some cosmologists have argued that all of the universe's dark matter is "cold"--massive particles that moved sluggishly one year after the Big Bang. Others maintain that some of the dark matter must consist of "hot" particles, such as neutrinos, which zipped near the speed of light in the same era. Both the early Los Alamos findings and astronomical observations strongly support a dark-matter recipe with cold plus hot ingredients, Primack says.

In its Physical Review Letters paper, Primack's group states that a model using two types of neutrino, each with a mass of about 2.4 electron volts, offers the best theoretical solution yet to the dark-matter puzzle. The researchers call on several lines of evidence to support this view:

* Results from the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos. Those results, the authors emphasize, are preliminary and unpublished, but they are consistent with those reported today in the New York Times. (Caldwell is a member of the LSND team.)

* Results from another neutrino experiment, at the Kamiokande detector in Japan. Those data, published last year, do not point to a specific neutrino mass. However, they indicate that two species of neutrino, the "muon" and "tau" neutrinos, may have about the same mass.

* Attempts to match the results of computer simulations with observations of galaxies, clusters of galaxies, and distant objects that formed within a billion years or so of the Big Bang. To match the newest observations most closely, the team concluded that its computer models require a neutrino mass of about five electron volts. That corresponds to about 20 percent of the total mass of the universe. Further, the most recent model by Primack's group shows that splitting the mass evenly between two species of neutrino produces an even tighter fit to all available data from telescopes, satellites, and experiments in particle physics.

The researchers make other key assumptions, some of which are under debate. For instance, the team's model features a universe with just enough matter to slow down its expansion perpetually, but not enough to make it recollapse. Other cosmologists claim that the universe contains less than this critical density of matter. They invoke some other factor, such as Einstein's "cosmological constant," to complete their models. In addition, Primack's group assumes that the universe is older and is expanding more slowly than some recent studies indicate. Further measurements of this rate of expansion, known as the "Hubble constant," should help settle the issue.

"Neutrinos with mass will force us to zero in on a small class of models as acceptable," Primack says. "Even the existence of only one species of neutrino with a mass of two or more electron volts is essentially fatal for models that rely upon a small density of cold dark matter plus a cosmological constant. This probably is the most popular class of models today, but the Los Alamos results may rule it out.

"The purpose of our paper is not to claim that we now have the right model," Primack says. "It is not yet clear that two equal-mass neutrinos is the best model for the hot dark matter. But it makes a nice consistent picture, and it is highly predictive--it can be shot down by observations being done right now. If it survives for a few years, it may even be right."

Primack continues: "We still don't know what the dark matter is, and we don't know how old the universe is. But this is the wonderful moment in history when we might be able to solve these problems, perhaps even by the end of the present decade."

The researchers considered many models with various kinds of neutrinos of different masses, using computer codes developed mainly by Holtzman. They ran a simulation of the most promising model on the CONVEX C-3880 supercomputer at the National Center for Supercomputing Applications in Illinois.

The simulation, created primarily by Klypin, tracks the paths of 50 million particles in a random cube of the universe about 300 million light-years across. The cube contains 512 million resolution elements through which the particles move. The program uses the cold plus hot dark matter model with two equal-mass neutrinos as a starting point for the simulated universe. It then lets the particles interact by standard laws of physics until gravity pulls them together--over billions of computer "years"--to form galaxies, clusters, and superclusters. Various analytical techniques allow the team to gauge how closely its results match the structures found to date in the real universe.

This research was supported in part by the National Science Foundation and the U.S. Department of Energy.

Editor's note: Contact information for the research team: Joel Primack: (408) 459-2580 (office), or joel@lick.ucsc.edu Jon Holtzman: (602) 774-3358 or holtz@lowell.edu Anatoly Klypin: (505) 646-1400 or aklypin@nmsu.edu David Caldwell: (415) 926-2547 or caldwell@slacvx.slac.stanford.edu

For a copy of the forthcoming article in Physical Review Letters, which was accepted for publication on December 22, 1994, call Robert Irion at (408) 459-2495 or send e-mail to irion@ua.ucsc.edu.

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