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How large is the Universe?

Or do we even know? If we do know, I'd love to know how we estimate such, as there are presumably many more galaxies than we can see from our vantage point on earth. Essentially I'm interested in whether the number of stars in the Universe is estimated in billions, trillions etc.

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No one knows if the universe is infinitely large, or even if ours is the only universe that exists. We are surrounded by a "horizon" set by the distance that light can travel over the age of the universe. You can get more information at http://www.nasa.gov/audience/foreducators/5-8/features/F_How_Big_is_Our_Universe.html.

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Its constantly expanding

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Partially correct
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You will never know it is very big like expanding is duper easy for the universe

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Huge man huge

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Every Second the universe is expanding, there is no end...

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HUGE! I dont think that scientist know approximently how big it is

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The light sphere centered on Earth is estimated to have a radius of 46 billion light years.

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Size would determine that there is a limit, in which case, there is none. You humans always have a difficulty when it comes to the idea of infinity.
First off, What do you consider to be the Universe?
Is it all the matter in space, or is it both the Matter and the emptiness?
If you just count the Matter, then there's no way to know, because, for all you know there could be multiple big bangs happening far away in some distant area of space.
And for the latter, Emptiness is endless, there is no horizon, there is no limit to how far it reaches
Unless of course you go by the theory that the universe is round, that if you go far enough, you end up back where you started, but again, that is because humans have a difficult time with the idea of infinity.

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Wow mate, thanks, there I stood, totes convinced that oblivion is finite; You saved me.
Just joshin ya, yes, I am thinking in terms of either an estimate of the Diameter across which matter can be found in our Universe or an overall estimate of how much mass it contains.
If there are other big bangs as you suggest, we can assume there are other galaxy clusters than our "Universe" and that our Universe (if it is round, as some of us pesky humans assume) is in fact no more than a galaxy cluster, albeit one containing other smaller galaxy clusters.
For these alternate Universe - sized galaxy clusters to, A) not show up in the sky B) not exert gravitational pull on our cluster (albeit they might be doing just that, it's as plausible an explanation for Universal expansion as dark matter theory) and C) not appear in the sky during their initial big bangs, we can assume that they are incredibly distant -- so distant that not even the S.S. Enterprise could reach it at Warp speed, so distant that if NightCrawler teleported with the Hubble Space telescope as far as he could see through it repeatedly, for millions of Earth years, he wouldn't even see a speck. Thus I suppose I'd like to omit those for the purposes of this question. They will likely prove irrelevant to us human types.
And I seriously doubt that the Universe will, er, put me back where I started off, if I travel in a straight line, regardless of whether it is shaped like a Sphere, oval, boot or Octagon - as I'm sure you know that works in the Earth model only because we aren't actually traveling in a straight line - though it would be mega interesting if the inability to move in a straight line became a factor in light speed space travel.
So I've heard mathematics types speaking of "magnitudes of infinity". What do you think about this in terms of measuring, what, in light of these other potential big bangs I will address as the "relevant universe"?
I have no difficulty imagining limitless time or limitless space.
Limitless mass/energy, puts me off however.
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Huge dude!

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endless

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Size means limit. Technically, there is no limit right now. However, by the theory of the big bang, the universe is rounded, traveling outward in every direction, so it's limitless. Currently the universe moves extremely fast, seeming to have no end.