Frontiers | Pages 196-197 | (back to unlinked version) | ||||
MACHO passing directly in front of a distant star creates a tiny gravitational lens. We see the star's light spike upward brightly for a few hours or days. These events are so rare that special telescopic systems must monitor millions of stars at once to catch one in action. As technology improves, astronomers are seeing a growing number of minilensing events. However, the jury is still out on what's causing them. Even if MACHOs are common, they might add just a percent or two to the total mass of the universe. Far more important, most astrophysicists believe, are the contributions of tiny particles. Neutrinos are a strong choice. These flecks, members in good standing of the standard model , stream from all nuclear reactions. That includes fusion in the core of the Sun and the collapse of matter in supernovas , which unleash Neutrinos in whopping numbers. Long presumed to be massless, Neutrinos may possess the barest whisper of mass , according to recent experiments. The evidence comes from detecting subtle changes in a few Neutrinos as they pass through Earth. A tiny fraction oscillates between two types of matter described by the standard model . That transformation is possible only if a neutrino carries some heft--but, mind you, perhaps 100,000 times less than electrons. Even with such tiny scale readings, all the Neutrinos in the universe might outweigh the total mass of stars and galaxies. Other dark-matter candidates have earned the dubious honor of being called WIMPs , for "weakly interacting massive particles." String theory predicts supersymmetric particles as partners to every bit of matter and every force-carrying particle in the universe. If that's true, those "sparticles " (as they are known) exist all around us. They may begin to interact with the matter we know only at very high energies, perhaps beyond the reach of existing accelerators. It's a compelling goose chase. Many physicists suspect that much of the dark matter will consist of completely unknown particles, thus extending the standard model in new and exciting ways. All this fuss about dark matter is driven by two factors. One simply is our insatiable curiosity about the nature of our universe. Ignorance of 99 percent of the cosmos doesn't sit well with most people, especially astrophysicists. The other is our obsession with fate. And dark matter , like it or not, controls our fate. Specifically, the amount of matter in the universe determines whether space will continue to expand forever or collapse back in on itself from the force of gravity . We call these (continued) |