Working with a flat and infinite model of the universe (which seems to be the most popular serious model these days), the density of the universe is 3e-28 kg/m^3 [1]. When added to the cosmological principle, it would imply that the mass of the universe is infinite.

Yet, the same umass.edu link [1] reports "...that the number of atoms in the Universe is at least about 4e78, but perhaps as many as 6e79." What am I missing here? Note that, before this statement, no-where am I referring to the observable universe. If you know where I have erred, it would be great if you could restrict it to the flat and infinite assumption and also talk in the context of the entire universe (and entirely skip references to the observable universe).

  • $\begingroup$ Your link talks about the number of atoms in the observable universe (see also this question). If the total universe is infinite then obviously its mass would also be infinite. $\endgroup$ – Pulsar Mar 29 '14 at 4:41

I think you are missing that the source is actually referring to he observable universe explicitly - just somewhat indirect:

It is not obvious as it is talking about "visible/observable universe", but about "total mass of the visible matter" and "mass density of visible matter". As far as I can see, any references to mass etc in the universe are covered by this.

So it's basically talking about the visible matter in the whole universe; But that's just the same as the whole matter in the visible universe, and all we care about here is the matter.

  • $\begingroup$ I added the last line and accepted the answer. Yeah, I guess I misread the source, but still it is very difficult to find a link which says that the mass of the universe is infinite. The word "Big Bang" in itself gives rise to so many misconceptions, that I think a simple statement saying that the mass of the universe (not the observable universe but the whole thing) is infinite, will help erase some of those misconceptions. $\endgroup$ – Shashank Sawant Apr 1 '14 at 19:10

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