# 100 billion galaxies?

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How do we know that there are 100 billion galaxies in the universe?

If the universe is expanding away from every point,rather than as a sphere from a central point,how do we no it's limits?

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I think you misunderstood what you read. We know that there are at least 100 billion galaxies from our view point. This is because there is a theory in cosmology called the cosmological principle.

In this, we assume, that the universe is expanding from every point, like you said. Now if that is true, we know that all matter in the universe is basically a mirror of each other, i.e, if you look into the sky, the matter there is going to be as dense as matter some were else in the galaxy, or the matter is going to be equally distributed.

Now, what the Hubble telescope did, is made a image called the Hubble deep field, and zoomed in as far as it could go, and took a bunch of pictures. Then from all of those pictures, they counted up the amount of galaxies in the picture, and because they were close to the same for all the pictures, the count of the galaxies, and they were able to take that data, then make an estimate, from that small region, to calculate how many galaxies we could see if scientists wanted to count them up, which they won't because we don't live that long. We don't know how big the universe is, but from our stand point, 100 billion is probably one of the best guesses that are made from data, but some scientists say it is more a lot more.

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Thanks for the replies.

So 100 billion is the minimum number?

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No, some scientists say more, others say less. 100 billion is a safe average that scientists like to use because it's "just right"

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I think derek w meant, "the minimum number in the entire universe"; in which case, yes, the total number of galaxies is almost certainly many orders of magnitudes larger, if not infinite. We don't (and possibly can't) know how big the whole universe is, or even whether or not it is finite.

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The Milky Way has been estimated to have 100-400 billion stars, and there are galaxies much smaller and much larger. What is the average size of (number of stars in) a galaxy?

Edited by EdEarl
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The Milky Way has been estimated to have 100-400 billion stars, and there are galaxies much smaller and much larger. What is the average size of (number of stars in) a galaxy?

About 100 billion. Do you get the impression these might be quite round numbers?

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About 100 billion. Do you get the impression these might be quite round numbers?

Seems they are guesstimates. And, know the number of stars in Andromeda better than the Milky Way.

Due to the large number of mini-galaxies (star clusters), I thought there might be fewer than 100 billion. But, the count of galaxies might not include star clusters, IDK what is visible in the Deep Field and Extreme Deep Field photos.

Edited by EdEarl
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If the universe was smaller than an atom before inflation occurred, then no matter how rapidly it expanded it cannot be infinite in size now. Does anyone disagree with this and why?

Edited by Airbrush
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If the universe was smaller than an atom before inflation occurred, then no matter how rapidly it expanded it cannot be infinite in size now. Does anyone disagree with this and why?

If the universe is infinite in size, then it must have always been infinite in size.

I assume "smaller than an atom" (if it is real) would refer to the size of what is now the observable universe?

Edited by Strange
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If the universe was smaller than an atom before inflation occurred, then no matter how rapidly it expanded it cannot be infinite in size now. Does anyone disagree with this and why?

I'm sure we've discussed this before but that relates to the observable universe and you could have an infinite number of adjacent observable universes, so, it's not illogical for there to be an infinite universe as well without invoking an infinite rate of expansion for it to be that way. Does that make sense?

Edited by StringJunky
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If the universe was smaller than an atom before inflation occurred, then no matter how rapidly it expanded it cannot be infinite in size now. Does anyone disagree with this and why?

And just to follow on: The portion of the universe we can observe (and potentially the only portion we will ever observe) is finite in size. That finite portion of the universe was once extremely tiny.

It is possible, however, that rather than the entire universe being tiny in size, it was already infinitely large (or even just much bigger than that tiny portion; who knows?) and merely very dense. As it expanded, that tiny portion became the volume of space we can observe and the rest lies outside the range of our detection because the expansion is too great.

In any case, the universe may or may not be infinite, but the observable universe (the thing packed into that tiny space before the Big Bang) is certainly not infinite.

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This might be complete nonsense but here goes:

An object flies past you at 90% SOL, from your POV it is shortnened in its direction of travel.

That is to say, a standard unit length (SUL) travelling at 90% SOL along the x coordinate of your frame of reference will appear to you shorter in its direction of travel than a SUL aligned in the same direction but stationary relative to you.

If it was the SUL that got shorter then it would be shorter in its own frame of reference which is clearly not the case, so it must be the frame the SUL is in that is shortened in which case it is space that is shortened.

So is it the case that an area of space, billions of lightyears away, that is receding from us at approaching the SOL will to us appear shorter in its direction of travel and will appear to show time running slower in it.

With that in mind back to the OP and the expanding universe

If the Universe is expanding at the SOL would not therefore the furthest observable parts be shortened to zero length in the direction of travel (i.e. away from us) and would not time in these places stop ticking as observed by us. So relative to our position the edge of the observable Universe is not moving.

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If the Universe is expanding at the SOL would not therefore the furthest observable parts be shortened to zero length in the direction of travel (i.e. away from us) and would not time in these places stop ticking as observed by us. So relative to our position the edge of the observable Universe is not moving.

No.an observer at the edge of our observable universe would probably see the same as us(100 billion galaxies expanding away from him in all directions)and he would see our part of the universe as it was 13.8 billion years ago.

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What they are really trying to say is that there are many, many galaxies out there

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