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The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 2.

Next: Spiral Galaxies Up: Abstract Previous: Introduction


Physical Evidence for Dark Matter


Evidence for dark matter (DM) exists on many scales, and it is

important to remember that the dark matter on different scales

may be different - the dark matter in dwarf spirals may not be the

dark matter which contributes tex2html_wrap_inline89 ; in fact, the tex2html_wrap_inline89 dark matter

may not exist. This consideration is especially important when

discussing dark matter detection, since detection is done in the

Milky Way and its environs, and evidence for dark matter outside

the Milky Way may not be relevant. So, let me start with an

inventory of dark matter in the Universe.


The cosmological density of dark matter on different scales is

quoted using tex2html_wrap_inline93 , where tex2html_wrap_inline95 is the density of some material

averaged over the Universe, and tex2html_wrap_inline97 is the critical density. Most

determinations of Omega are made by measuring the

mass-to-light ratio tex2html_wrap_inline99 of some system and then multiplying this by

the average luminosity density of the Universe:

tex2html_wrap_inline101 [67]. Here h = 0.4 - 1 parameterizes

our uncertainty of the Hubble constant. There are methods, such as

tex2html_wrap_inline105 from big bang nucleosynthesis, and potential reconstruction

from bulk flows, which do not depend upon tex2html_wrap_inline107 , but methods which

involve taking an inventory of material depend upon it. For

example, the mass-to-light ratio in the solar neighborhood is

tex2html_wrap_inline109, giving tex2html_wrap_inline111 . If the solar

neighborhood is typical, the amount of material in stars, dust and

gas is far below the critical value.



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