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View Full Version : What is the density of the universe?

JimJast
2008-Mar-23, 05:39 PM
Q1: Does anybody know whether the density of the universe equal 6x10^{-27} kg/m^3 looks reasonable or there are some serious problems with this number and if yes then what is the problem?

Q2: If someone answers how do I find the answer in this forum? (I'm new here).

Noclevername
2008-Mar-23, 06:40 PM
Without knowing exactly how big the Universe is, how can we calculate its density?

StupendousMan
2008-Mar-23, 06:54 PM
Q1: Does anybody know whether the density of the universe equal 6x10^{-27} kg/m^3 looks reasonable or there are some serious problems with this number and if yes then what is the problem?

The critical density -- that required to halt the expansion of the universe -- is about 1 x 10^(-26) kg per cubic meter, for a Hubble constant of 75 km/s/Mpc. Now, the best estimates from nucleosynthesis suggest that the density of ordinary baryonic matter is only about 3-5 percent of this critical amount. That would lead one to estimate only about 0.5 x 10^(27) kg/m^3, which is quite a bit smaller than the value you quoted.

If one includes non-baryonic dark matter, then current observations suggest about 30 percent of the critical amount, or about 3 x 10^(27) kg/m^3; that's about half of your quoted value.

There are many, many good books and websites which describe the observations by which we can derive these estimates. Go read some if you wish to know more.