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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:

I hope this helps,

Tim Kallman
for the Ask an Astrophysicist Team

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