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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,, and]

Koji Mukai
with helps from Drs. Chen, Loewenstein and Snowden
for Imagine the Universe!

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