At the University of California, Santa Cruz, cosmologist Joel Primack leads a team of researchers who simulate the evolution of the structure of the universe based on various new concepts about its content that may account for the invisible or dark matter. They employ a supercomputer as a cauldron to brew their cosmological stew and then peer into it, using scientific visualization, to see if they have achieved the right textures.
Primack's pot is square. He uses supercomputers, such as the IBM POWERparallel System (SP) at the Cornell Theory Center (CTC), to model a random cube of space as it evolves from just after the Big Bang to the present--approximately 15 billion years of history. Such simulations provide a universe-in-a-box.
Tracking the glowing matter in this box as it evolves into galaxies that cluster and clump, the team can examine the spatial structure of the visible universe. But in fact, they are more interested in the unseen matter that they believe makes up as much as 99% of its mass--all but a few percent of which they believe to be exotic hot and cold dark matter. The gravitational pull of this unknown quantity is thought to control the evolutionary process.
Little is known about dark matter; it has not been seen directly, but there is a great deal of evidence in the observed behavior of galaxies and clusters of galaxies to suggest its existence. Without it, researchers such as Primack are hard pressed to make a virtual universe evolve properly. Many possible forms of matter have been suggested--from black holes to weakly interactive massive particles (WIMPS), and most recently to neutrinos, which are abundant, but usually presumed to be virtually massless.
Primack's simulations suggest that both hot and cold dark matter are essential to the mix. A universe with just cold dark matter is clumpy. The addition of neutrinos improves the blend. These elementary particles account for the hot dark matter. Although previously believed to have no gravitational pull, if neutrinos had only 1/100,000 times as much as the electron's mass, neutrinos would add 20% to the density of the model universe. The evolution of the simulated cosmos responds to this addition by clustering more slowly, allowing long filaments to stretch between clusters of galaxies and larger voids to form in space. At the cosmic scale, Primack's work suggests that hot matter behaves differently than cold, and is associated with these outer reaches. This new simulated universe is much more similar to what can be found in the 3D data currently available from red-shift surveys of slices of the sky, such as those by the Harvard/Smithsonian Center for Astrophysics. These surveys locate galaxies based on their location in the dome of the night sky, esti mating the distance from Earth of each galaxy by measuring the amount that the light we observe from them has shifted toward the red end of the spectrum on its journey to Earth.
Particle physicists are scrambling to contribute to this picture by detecting a neutrino with mass. However, it may prove to be an elusive prey. Primack's team is now experimenting with mixes of matter in the primordial universe. They are also differentiating among the neutrino species--three have been identified by particle physicists--in the recipe. In this way, cosmologists are exploring not only the large scale structure of the universe but also the characteristics of some of its tiniest components. "It used to be said that cosmology is a science where the ratio of theory to data is almost infinite," Primack says, "but with wonderful new data streaming in from new telescopes in space and on the ground, the ratio is reversed. Any cosmological theory that can survive the gauntlet of new data for even a few years may actually be true!"
A Look at the Universe
The Square Pot
The Right Mix