(7 of 9)
An early theory was that the missing matter is composed of commonplace particles called neutrinos. One problem with this is that dark matter is massive, and no one knows if neutrinos have mass. Even if they have, in computer simulations they do a poor job of making a recognizable universe. Cold dark matter was another possibility ("cold," in physics jargon, means slow-moving; neutrinos, by contrast, are "hot"). Also known as wimps, for weakly interacting massive particles, these are purely hypothetical particles derived from speculative theories. They perform somewhat better in computer models, but WIMPS can't account for such newly discovered features of the cosmos as Great Walls, Great Voids and Great Attractors.
Physicists hoping to observe dark matter directly have searched for objects both large and subatomic. On the theory that the dark stuff is made of some as yet undiscovered particle, they have built all manner of sensitive detectors. On the chance that it is composed of very dim stars or large planet-like objects (known collectively as MACHOS, or massive compact halo objects), they have studied stars for telltale flickers that might indicate a MACHO has passed by.
Physicists and astronomers have looked for all of the above and more, but results have been inconclusive. WIMP searches are barely getting under way; MACHO hunts have turned up disappointingly few flickers at the outer edges of the Milky Way (but surprisingly many toward the galaxy's core). The latest teaser came last month when news leaked out that researchers at Los Alamos National Laboratory had seen evidence for what could be a slightly massive neutrino.
If it's true, the finding is of literally cosmic significance: there are so many neutrinos in the universe that they alone could account for some 20% of the dark matter that inflation theory requires. Just add in another 80% worth of WIMPS and you've got it, says Joel Primack of the University of California, Santa Cruz. With this recipe, Primack has used supercomputers to produce synthetic universes that look almost identical to the data gathered by real-life astronomical observers. But some theorists think Primack is grasping too quickly at a "discovery" that is still controversial.
Neutrinos with mass might help solve the dark-matter problem and thus provide support for the inflation theory. But in some ways that would just make the crisis in cosmology worse. The more dark matter there is in the universe, the harder it is to explain the new findings made by Freedman's group about the age of the cosmos. When they say the universe is between 8 billion and 12 billion years old, their vagueness reflects uncertainty about how much matter the cosmos contains. If there's a lot, as inflation suggests, its gravity would be slowing down the universe's expansion, making the universe younger than it looks. If, on the other hand, there is relatively little matter, the slowing has been minimal, and 12 billion is more like it.
If inflation is correct, then, the age crisis is as bad as it can pos-sibly be. No amount of theory adjustment can bring stars down to 8 billion years of age. So if Freedman's initial attempt to date the universe holds up, Primack and plenty of other theorists may have to begin prying themselves away from an idea they have held dear for more than a decade-unless they can think of some clever way out.