Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.02.3 What is the dark matter? Next Document: H.03 What is the Hubble constant? What is the best value? See reader questions & answers on this topic! - Help others by sharing your knowledge There are many searches now underway for the dark matter. For MACHOs, the most promising method is "gravitational microlensing," where we wait for a MACHO to pass between us and a distant star, and the gravity of the MACHO bends the starlight into two images. These images are too close together to resolve, but add up to more light, so the star appears to brighten and then fade back to normal as the MACHO passes by. The shape is quite distinctive, and the brightening happens only once so does not look like a variable star. The probability of such a close-enough approach is very low, so millions of stars must be monitored to have a chance of finding these events. The Large Magellanic Cloud is the most popular target. A number of groups---MACHO, EROS, OGLE, among others---have been doing this for several years, and have found a number of good candidate microlensing events. At the moment, it is too early to say that MACHOs have definitely been discovered, but it looks as though the "brown dwarf" objects are just about excluded, while perhaps as much as 50% of the dark matter could be in larger objects roughly 0.5 solar masses, e.g., white dwarfs. There is an axion search recently started at Lawrence Livermore Labs, which uses a huge superconducting magnet to convert axions (if they exist) into microwave photons. For the big bang neutrinos, there is currently no hope of detecting them because they have far less energy than the well-known solar neutrinos (see FAQ entry E.01). However, if a neutrino mass could be measured by lab experiments, we could calculate their contribution to the dark matter. For the supersymmetric particles, there are broadly three ways at detecting them: i) Direct detection by watching a crystal down a deep mine, and waiting for a WIMP to bounce off a nucleus in it with observable results such as scintillation or heating of the crystal. Very roughly 1 WIMP per day should hit each kg of detector, but the tricky part is discriminating these from natural radioactivity. The WIMPS should have a preferred direction (due to the orbit of the Sun around the galaxy), but we'll have to wait for next-generation experiments to measure this. ii) Indirect detection, whereby WIMPs get captured in the Sun, and then a WIMP + anti-WIMP annihilate into super-high energy (GeV) neutrinos which could be detected in huge volume detectors, e.g., Antarctic ice or ocean water. iii) Create WIMPs directly at next-generation accelerators like LHC, measure their properties and then calculate how many should have been produced in the Big Bang. With all these searches, there is a good chance that in the next 10 years or so we may find out what constitutes dark matter. Further reading: Astronomy magazine, Oct. 1996 issue contains many dark matter articles. The Center for Particle Astrophysics home page at <URL:http://physics7.berkeley.edu/> has several links including the Question of Dark Matter page. The MACHO home page at <URL:http://wwwmacho.mcmaster.ca/> has info on the MACHO project and links to many other dark matter searches. For cosmology background, see Ned Wright's Cosmology Tutorial at <URL:http://www.astro.ucla.edu/~wright/cosmoall.htm>. A more technical conference summary is at <URL:http://xxx.lanl.gov/abs/astro-ph/9610003>. Krauss, L., _The Fifth Essence_, Basic Books, NY 1989. Silk, J., _The Big Bang_, Freeman, San Francisco, 1988. Peebles, P.J.E., _Principles of Physical Cosmology_, Princeton, 1992 (advanced) User Contributions:Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.02.3 What is the dark matter? Next Document: H.03 What is the Hubble constant? What is the best value? Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
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