# There could be an infinite amount of matter, right?

Last I heard, if the universe is unbounded, then the “Big Bang” doesn’t consist in an expansion of space in the sense of a growing region, but instead in the sense of lessening density. So, it would appear to me, there could be (and could always have been) an infinite amount of matter. This infinite amount of matter used to be much denser than it is now, but that has nothing to do with whether the amount is finite or infinite.

Am I right? Or has it somehow been ruled out that there might be an infinite amount of matter in an unbounded universe?

As far as I know, we don’t currently know for definite whether or not the universe is infinite, although we have a number of potential models. As far as we can tell, the universe has a flat geometry; but we are unable to see the whole thing.

http://map.gsfc.nasa.gov/universe/uni_shape.html

“The density of the universe also determines its geometry. If the density of the universe exceeds the critical density, then the geometry of space is closed and positively curved like the surface of a sphere. This implies that initially parallel photon paths converge slowly, eventually cross, and return back to their starting point (if the universe lasts long enough). If the density of the universe is less than the critical density, then the geometry of space is open (infinite), and negatively curved like the surface of a saddle. If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper, and infinite in extent.”

Like you, my mind was blown by that thread a couple of months ago that explained this. I’m glad you started this thread.

An infinite space filled with an infinite amount of matter that somehow gets less dense isn’t what I thought the Big Bang was at all, and I thought I had a pretty good non-technical grasp of these things for a non-scientist.

I don’t even see how you can define density for an infinite amount of matter and an infinite amount of space, and I would have thought an infinite amount of matter was impossible, or rather, that “an infinite amount of matter” was a meaningless phrase. I know aleph-null is a “countable infinity” but I rather thought you couldn’t actually have “infinity objects” (and as far as I know, all matter is composed of particles, which are countable objects).

This concept blew me away as well. I first heard it in the thread I started here: http://boards.straightdope.com/sdmb/showthread.php?t=614581. It may be the one Alan is referring to.

…Of course, in an open universe, we could never rule out the possibilty that there is a finite amount of matter either. The amount of matter in the universe may be an unanswerable question if the universe is open…

Though the “we’re not special” axiom of physics may require a working assumption that matter is infinite in number