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Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.02.2 How much dark matter is there? Next Document: H.02.4 Searches for Dark Matter See reader questions & answers on this topic! - Help others by sharing your knowledge Since it's detected in a negative sense---not visible in gamma rays, X-rays, ultraviolet, visible light, infrared, millimeter, or radio regimes, and it doesn't block light either---it's a theoretical happy hunting ground. First, let's list some things that can't make the dark matter. Most forms of gas are excluded, because atomic hydrogen would be seen in 21cm radiation, and hot gas would be seen in X-rays and/or distort the spectrum of the CMB. Cold molecular gas is a possibility, but it would tend to collapse into visible stars. "Snowballs" made of solid hydrogen would evaporate due to the CMB, and larger snowballs would leave too many craters on the Moon or be seen as high-speed comets. "Rocks" are unlikely because there haven't been enough stars to make the heavy elements. Faint red stars are excluded because they're not seen in deep images e.g., the Hubble Deep Field. This leaves two main classes of dark-matter candidate: large objects called MACHOs and subatomic particles, some of which are called WIMPs. MACHOs stands for Massive Compact Halo Objects; examples are "interstellar Jupiters" or "brown dwarfs," which are lumps of mostly hydrogen less than 0.08 Solar masses; objects this small don't get hot enough to fuse hydrogen into helium, and so would be extremely faint and hard to find. Other varieties of MACHOs are dead stars, such as old white dwarfs or neutron stars, and black holes. The second class is some form of sub-atomic particle; if so, there'd be millions of these passing through us every second, but they'd hardly ever interact with normal matter, hence the term "weakly interacting massive particles" or WIMPs. Many varieties of these have been suggested; the only one of these that certainly exists is the neutrino, but neutrinos may not have any mass. The number of neutrinos made in the Big Bang is similar to the number of CMB photons (few hundred per cm^3), so if they have a small mass (around 30 eV = 6 x 10^-5 electron masses) they could contribute most of the dark matter. However, computer models indicate that galaxies form much too late in a neutrino-dominated universe. Another possibility is the "axion" which is a hypothetical particle invented to solve a strange "coincidence" in particle physics (called the strong CP problem). The most popular WIMP at the moment is the "neutralino" or "lightest supersymmetric particle"; supersymmetry is a popular way to unify the strong and electroweak forces (also known as a Grand Unified Theory), which has some (tentative) experimental support. Supersymmetry predicts an unobserved new particle or "superpartner" for every known particle; the lightest of these should be stable, and lots of them would be left over from the Big Bang. These probably weigh about 30-500 proton masses. An important piece of evidence here is "primordial nucleosynthesis," which explains the abundances of He-4, Deuterium, He-3 and Li-7 produced a few minutes after the Big Bang; in order to obtain the observed abundances of these elements, the density of baryons (i.e., "ordinary" matter) must be Omega_baryon ~ 0.02--0.1. Since Omega_stars ~ 0.01, there are probably some dark baryons, but if Omega_total = 1 (as inflation predicts) most of the dark matter is probably WIMPs. User Contributions:Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.02.2 How much dark matter is there? Next Document: H.02.4 Searches for Dark Matter 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
Last Update March 27 2014 @ 02:11 PM
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