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The Question
(Submitted January 14, 1998)
I am a high school senior. I have little familiarity with astronomy.
I recently saw a program on PBS, where they discussed a way of
measuring the mass of the entire cosmos. I am wondering how this
could possibly be done, and how long it would take. I also wonder
what effect knowing the mass will have on science.
The Answer
There are a variety of ways of attempting to measure the mass of the
universe. All are based on assumptions which are somewhat analogous
to the assumptions used in public opinion polling. That is, we measure
the mass in some region of the universe which we think is typical, and
then assume (!!) that this can be applied to the universe as a whole.
Clearly this is a tricky thing to do, and the answers become believable
only when it is done by a variety of people using a variety of techniques
which all give consistent answers. For example, we can measure the
density of material in the earth's interior, and if we assumed that this
were typical of the universe as a whole we would go very wrong. More
plausible estimates come from measures of the mass in galaxies
(by measuring the speeds of stars and then using Newton's laws to
infer the mass), and from measuring the mass in clusters of galaxies
(by measuring the temperature and density of hot gas from its X-ray
emission and then assuming that it is confined by the gravity of the cluster).
You can read more about this at:
http://imagine.gsfc.nasa.gov/docs/introduction/dark_matter.html
I hope this helps,
Tim Kallman
for the Ask a High-Energy Astronomer Team
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