Quote:
Originally Posted by JimJast
What is the ratio of mass of the universe to its volume and how this number is calculated by the astronomers. My non professional attempt calculated from the Hubble redshift gave me (6.0+/-0.5)x10^{-27} kg/m^3 and I'd like to know how far it is from the real number. Thanks in advance.
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The latest estimates have the average baryonic matter density at ~4.2x10^-28 kg/m^3. If you also throw in dark matter, which amounts to a factor of ~5x greater, one finds a total matter density about 2.6x10^-27 kg/m^3.
The latter is smaller than your estimate by about a factor of 2.3.
The numbers are arrived at both theoretically (GR, Friedmann-Walker-Robertson equations) and observationally (baryon surveys, dark matter surveys, Cosmic Microwave Background fluctuations, etc).
The critical matter density (now) is calculated as: 3H_o^2/(8*pi*G), where H_o is the "local" Hubble parameter.