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Could we say the universe has an infinite number of galaxies?


mdg583
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I am wondering something about how cosmology doesn't imply an infinite number of galaxies in the universe, and was wondering what input people can give. Here is a description.

Consider if we look (using a telescope) into the sky and see a galaxy at 95% of the distance to the cosmological horizon, ie to the edge of the universe. And we say that in some sense there are N galaxies 'between' us and that galaxy. Looking at that galaxy, you are also looking 95% of the way back in time closer to the big bang, so the galaxy you see exists at a point in time much closer to the big bang.

Now if we change our point of observation to that galaxy, and go forward in time back to 13.82 billion years after the big bang, I think the universe from the perspective of that galaxy should look much the same as it is from our perspective. So you should be able to look in the opposite direction from the earth and again see another galaxy about 95% of the way to the cosmological horizon, with N galaxies 'between'.

And we can repeat this procedure over and over, each time discovering more galaxies.

So the idea would be that at 13.82 billion years since the big bang, an infinite number of galaxies must have been flung out into space if we can use all of the points in space as reference points for observation.

What do you think?

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Hmm, could you replace the [Physics] tag with something more appropriate like [semantics] or [Poetry]? With your argumentation, I have an infinite amount of money because I have some now and some lest Friday and some on every infinitesimal moment in between.

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Hi cfds, I'm not very sure what you mean. I'm trying to think of the number of galaxies that exist in the whole universe after a specific amount of time after the big bang.

I found some good discussion of this in places like here. It seems like it is accepted that there could be an infinite number of galaxies in the 'unobservable' universe.

I guess this may be a bit of a philosophy question. The possibility of an infinite number of galaxies seems a bit strange. A finite universe is much nicer to think about.

Edited by mdg583
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The idea you're describing is what's known as the Cosmological Principle, which is basically the assumption that there's nothing special about our location in the universe. In other words, the universe would not look significantly different at large scales if observed from any other location. However, it does not imply an infinite universe unless spacetime is flat - if it's curved, it can loop back on itself but still have no apparent edge.

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However, it does not imply an infinite universe unless spacetime is flat - if it's curved, it can loop back on itself but still have no apparent edge.

I wish I had a better idea what that means, but it probably would take a lot more reading.

There are also some other strange philosophical questions. If the big bang happened in the entire observable and unobservable universe 13 billion years ago, then there should have been a period of time in the entire unobservable universe where there were no galaxies, and then at some future point in time there were an infinite number of galaxies. And it's hard to imagine a way for there to be a first galaxy among an infinite set of galaxies.

But I guess with special relativity sequences of events aren't so hard set, so maybe you can only talk about a first galaxy from any particular perspective in an observable universe.

I don't know a lot about this - just some strange questions.

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We have no reason to assume that you wouldn't see a grand swath of galaxies stretching out endlessly from any point in the universe;

but neither do we have any reason to assume that you would. The Cosmological Principle is founded on the presumption that the laws of physics are the same everywhere, which doesn't necessarily mean that the universe is homogeneous forever. It may well be that the cloud of galaxies simply ends and yields to infinite empty space. Or, perhaps, "wraps around" like a game of Asteroids so you look out into the distance and eventually see Earth again.

And personally that makes me wonder this: If there are an infinite number of galaxies, then there must be an infinite amount of space for them to occupy. Thus the universe as generated in the Big Bang must have expanded to an infinite size in a finite amount of time, meaning it must have expanded infinitely fast. Were that the case, how would anything be able to condense into matter and form galaxies if it's all rocketing away from everything else at infinite speed?

I have thus concluded that while the universe is very large, the universe as we know it (space with galaxies and stars in it) can't literally go on forever.

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It may well be that the cloud of galaxies simply ends and yields to infinite empty space.

Yes, that's a good point - that there can still be a finite number of galaxies in an infinite space.

Thus the universe as generated in the Big Bang must have expanded to an infinite size in a finite amount of time, meaning it must have expanded infinitely fast.

I'm not sure about this, because I think the Big Bang theory only goes back to just when the universe was very very dense and very very hot. I think the idea is that it still could have been infinite then, and still could have experienced expansion the same as if the universe were finite.

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...However, it does not imply an infinite universe unless spacetime is flat - if it's curved, it can loop back on itself but still have no apparent edge.

It doesn't have to be flat to be infinite. It could also be infinite if the curvature is negative.

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Current cosmology takes it that the universe must be :

1. Isotropic. This is based on CMB observation, though we're now trying to figure if there's any anisotropies, and what does it says to the isotropic principle.

2. Homogeneous. Based on most notably faint galaxy surveys. And considering dark energy, it's going even more homogeneousbin later dates (because they'll dominate, at least in our universe).

One thing important : this is short of a simplification. Only universes governed by the Friedmann equation would these things holds true, and it's fine if you want to prove them wrong or anything - just ensure that you're using observation to do something like that. :wink:

Now, for real...

Figure a sheet of paper. How bad it could be ?

1. Plain flat, no curvature. In this case, if you draw a comic figure on top of the sheet, they'd figure that a triangle have its sums of inner angles as 180 deg and a circle will have it's circumference exactly pi times diameter. Applying cosmological principle, we must have no center (a preferred direction) - thus, no edge as well. Then it got to be infinite.

2. The surface of a sphere. This surface have positive curvature. Our comic figure will find out that he can make a triangle that have its inner angles greater than 180 deg, and a circle will have its circumference less than pi times diameter. Applying cosmological principle, there's no center for the surface already - therefore, it can be finite.

3. The surface of a saddle. This is, so far, the best representation of negatively curved surface. Our comic figure will figures out that a triangle he draws can have inner angles less than 180 deg and a circle will have its circumference larger than pi times diameter. A bad thing, though, that the negatively curved 2D surface cannot be represented infinitely in our 3D world. But we can infer from the finiteness of saddle and their sides that such surface needs to be infinite as well.

4. The surface of a bagel ! Nobody said we can't do that ! And a lot of other combinations ! You only need to figure the metric...

Infinite surface with homogeneous density ~ infinite density product (whether it's mass, energy, galaxy etc.) . And that's the best our current physics understanding tells us ! Nobody said it can't be wrong, but remember, in science, you need observational proof. Theories without observation is fake, observation without theories is fail.

Additional warning : Never, ever, ever, dream of curvature in the space you live in. I mean, the same as how the comic figure can't tell height, you'll never tell where's the curvature except of the proofs already given.

Edited by YNM
grammars. and warnings.
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Consider if we look (using a telescope) into the sky and see a galaxy at 95% of the distance to the cosmological horizon, ie to the edge of the universe. And we say that in some sense there are N galaxies 'between' us and that galaxy. Looking at that galaxy, you are also looking 95% of the way back in time closer to the big bang, so the galaxy you see exists at a point in time much closer to the big bang.

Now if we change our point of observation to that galaxy

That's the problem--we can't actually go to another galaxy and see what the universe looks like from there.

Yes, the universe does look approximately the same in every direction--from our current location. That doesn't mean it looks just like this from everywhere else. We naturally want to assume that it looks the same from everywhere else, because we're a self-hating species that abuses itself in its own movies and books. The idea that we might actually BE at the center of the universe doesn't square with our self-hate.

Bottom line? There's no way to know. But from the post-Big-Bang appearance of the universe--i.e. from the fact that it's expanding--we can infer that the universe expanded from a smaller and therefore finite space. If space is finite, then matter must be finite. That's pretty much the best guess we can make until we invent warp drive.

Catch ya later. Gotta head back to the garage and finish up that warp bubble thingy.

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