5. Nonbaryonic Candidates
Nothing Puny about WIMPs
Current theories estimate that, for the universe to be flat, 30% of the universe consists of nonbaryonic dark matter, whereas only 5% would consist of baryonic matter. For such a sizeable chunk of the universe, cosmologists are still unsure exactly what is this nonbaryonic “stuff” in the universe. Baryons are protons and neutrons, the particles that form the type of matter with which we are acquainted. Nonbaryonic matter, therefore, is exotic and difficult to imagine compared to baryonic dark matter. The general name given to nonbaryonic dark matter is WIMPs, but we’re still don’t know what WIMPs are, though we have some good ideas. Possibilities include electron-neutrinos, muons, taus, neutralinos, and other light supersymmetic particles (LSPs).
“Capturing and studying neutrinos on Earth will help scientists understand how stars are born. This image shows the early formation of a neutron star as it radiates billions of neutrinos per second.” (The University of Wisconsin) Photo courtesy of Oak Ridge National Laboratory
Understanding the nature of neutrinos can help scientists form a grand unified theory of the four forces, which could help explain why the differences between forces are so tremondous.
Neutrinos are difficult to detect because they hardly interact with regular matter. The existence of a partilce similar to the neutrino had been theorized for decades, but it was not until 1969 that Raymond Davis Jr. discovered the existence of the particle. If a beam of neutrinos were shot through a light-year lead black, almost all of them would leave the block at the far end without having interacted with a single atom. Every second, around 60 billion of these things travel through every square centimeter of your body! Neutrinos are theoretically massless, but particle physicists speculate that they probably have a very small mass. All the same, they are so tiny that even a very small volume would contain a great mass of them.
diagram of a neutrino detector — note how it is deep underground Photo courtesy of Sudbury Neutrino Observatory
However, small particles like the neutrino and neutralino can be be observed though a device called a scintillator, which allows scientists to to look at the light that is emiited when particles such neutralinos collide with regular atoms. Scientsits also study the vibrations WIMPs caus when they stike the nuclei of atoms.
“One seldom sees what one is not looking for, and theory tells experimenters where to look: Sometimes they find something unanticipated.” —Timothy Ferris, A State-of-the-Universe(s) Report
The three kinds of neutrinos are the electron-neutrino, the muon-neutrino, and the tau-neutrino. Not only are there different kids, but some scientists think that in “mid-flight,” a neutrino can transform from one kind to another. Scientists have noticed that neutrinos leaving our sun change forms on a random, or quantum, basis. Neutrinos are worth studying as a good example of WIMPs, but they probably aren’t the best candidates for dark matter because they aren’t consistent with theories of large-scale galaxy and supercluster formation.
Today, cosmologists feel that dark matter is partly WIMPs and partly MACHOs, as strange as this concept once seemed to many scientists. Cosmologist Jospeh Silk of UC-Berkeleyonce once said, “I personally think the organizer of the universe would really be playing a very mean trick on us if he had several different kinds of dark matter around.” For the time being, it appears, then, that the Almighty is quite a prankster.
Posted by rbernste on March 27, 2005 at 10:36 PM