# Is there a finite amount of mass in the universe?

So, I'm not too physics savvy but I am curious to ask. Is there a finite amount of mass in the universe? or is there more and more being created from somewhere or something? If the universe is infinite, and there's a finite amount of mass, that just seems kinda weird I guess.

Hopefully this isn't too dumb a question...

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 This was also asked here. – Qmechanic♦ Feb 7 at 2:44

To start with we don't know whether the universe is infinite or finite but unbounded (i.e. closed on some length scale much larger than the visible universe). If the universe is infinite the amount of matter is probably also infinite, while if the universe is finite the amount of matter is definitely finite.

Also when you say "matter", you need to bear in mind that matter is being converted into energy in stars, so it would be better to ask about combined matter and energy, where we treat the two as the same and link them with Einstein's famous equation $E = mc^2$.

Having made these two points, with one exception we know of no mechanism for matter/energy to appear from nothing i.e. the amount of matter/energy is conserved. In fact this conservation law follows from a fundamental symmetry that the laws of physics are time invarient. So whether the universe is infinite or finite, the amount of matter/energy in it is constant with time.

I did say there was one exception: you've probably heard of dark energy, which is a fundamental property of spacetime. The thing is that because the universe is expanding the amount of spacetime is increasing and therefore the total amount of dark energy is increasing. Since we don't know much about dark energy, it's hard to comment on the significance of this.

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John, could you elaborate on this: "...because the universe is expanding, the amount of spacetime is increasing..." Increasing in what sense? – William Dec 24 '12 at 8:46
Hmm, rereading my answer I think that phrase was a bit careless. Dark energy is an energy per unit volume so if you define a volume bounded by e.g. some galaxies too far away to be gravitationally bound, the amount of dark energy in the region bounded by those galaxies increases as the Hubble expansion moves the galaxies apart. You could say that the amount of spacetime bounded by our selected galaxies is increasing, but I wouldn't attach any great physical significance to the statement. – John Rennie Dec 24 '12 at 9:02

The proper way to view this question is whether there is a finite amount of mass in the visible universe. The answer to this question is yes. The key component to this answer is the positive energy theorem and an understanding of asymptotically flat spacetime with the dominant energy condition. The notion of an asymptotically flat space is very similar to our understanding of the equivalence principle and the effects of tidal forces. In principle, there is always some frame of reference where gravitational effects can be ignored, either in making our laboratory very small, or by separating classical objects to great distance. Since we can imagine ourselves in a visible universe that is asymptotically flat, we can convince ourselves that the universe has mass and it is finite. There are in fact some crude approximations as to the actual mass.

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I would say that we don't know, and even have no clue, and the reason (beside the dark matter, energy/mass equivalence and other staff mentioned in the other answers) there is a very strange and mysterious thing which is Vacuum, we till now don't know exactly what happens there (and probably will never know) we know basically how do deal with it (more precisely how to get rid of it in our calculations) but still not sure about it's contribution to total amount of energy in the universe, and this despite that our universe is finite or note, because vacuum (although most physicists against this, but that depending on intuition only not experiment) vacuum may have infinite amount of energy, anyway that not very probable.

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