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The Question
(Submitted November 30, 1996)
What is the present accepted value of Omega,the density of the
Universe and does this value include the missing mass recently
discovered by astrophysicists ?
The Answer
Many theorists prefer Omega=1.0. Observationally, many research groups are
trying to measure the value of Omega; although this is difficult and there
are many sources of uncertainties (including the current uncertainty over
the value of the Hubble constant), most recent published values fall far
short of 1.0. For example, a recent paper by a couple of scientists here
at Goddard (Loewenstein & Mushotzky 1996, Astrophysical Journal Letters, vol
471, L83) quotes the plausible range as 0.1-0.4. This particular measurement
is based on X-ray observations of clusters of galaxies, which is one of the
most powerful techniques available for the study of Omega, since the
distribution of X-ray emitting gas in clusters is believed to trace their
total gravitational potential (stars, gas, and dark matter), and the clusters
are believed to contain a major fraction of the mass of the Universe.
Dark matter, by the way, is the preferred name for what you are calling
the missing mass: it is not missing, we can detect its gravitational
influences (that's how astrophysicists can detect its presence), it just
doesn't shine like the stars do.
[Note added on 2003 March 4: There have been exciting developments in
this regard over the last several years. There appears to be a mysterious
substance called the dark energy, in addition to normal and dark matters;
together, they make up a total Omega of 1.0. See
http://imagine.gsfc.nasa.gov/docs/ask_astro/answes/990210c.html,
http://imagine.gsfc.nasa.gov/docs/ask_astro/answes/010104a.html, and
http://map.gsfc.nasa.gov/m_mm/mr_content.html.]
Koji Mukai
with helps from Drs. Chen, Loewenstein and Snowden
for Imagine the Universe!
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