How big is the Universe?

I recently heard that the distance between the furthest galaxies in our Universe is calculated to be 125 billion light years.Yet to my simplistic mind this is not possible. If the Big Bang occurred 13.7 billion years ago and nothing can travel faster than the speed of light, surely the furthest apart any 2 objects could be (moving in completely opposite directions) is less than 28 billion light years.I don't for one minute imagine that I can be right so either I'm missing something obvious or I guess it's a lot more complicated than that.Any explanations, anyone?

Thread copied here from the How big is the Universe? thread in the Proposed New Topics forum.

It's a bit more complicated than that.The expansion of the universe is not primarily due to movement, but to the expansion of space itself. This expansion is not limited by the speed of light. Further there is nothing to say that at the big bang the size of the universe was limited.

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Further there is nothing to say that at the big bang the size of the universe was limited.

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Can you explain what you mean by that Mr Jack? Under current theories, 'our' universe did indeed start out very small indeed.

Not quite. Under our current theories every gets closer and closer together as time goes back approaching a theoretical point where everything is infinitely close together. However, the actual science of the big bang begins something like 10^-30 seconds after this point. At that point all the distance we measure now were compressed to a incredible degree: points billions of light years apart now were nanometres apart. But an infinite number of nanometres is still an infinite distance, so it's quite possible for the universe to be infinite at this time.That's as I understand it anyway. Hopefully, CaveDiver will be along in a bit to explain why I've got everything completely wrong.

Nope, I think that's a pretty good way of explaining it - I'll just add that there is no distance measure at the singularity, so it is meaningless to talk of how "big" it is, but in an open and flat Big Bang cosmology, the Universe is inifnite in extent for any T>0. Most relativists/cosmologists don't realise this at first, and watching their eureka moment can be quite amusing

Hello tuffers, an excellent question for starting an interesting path into the generally non-intuitive area of cosmology. I say generally non-intuitive, because it is non-intuitive to those of us regular-folks who hold so dearly to Newtonian mechanics. However, to those who study such things, I believe that the mechanics of cosmology are kind of intuitive? I dunno... I'm not that smart Anyway, just thought I'd take the time to add one more piece of information that made me sad when I heard it: Mr Jack is very correct in his description. The favourite analogy at this point is to think of ants walking around on a rubber skin. The ants moving around at top-ant-speed can represent photons moving around at the speed of light (other objects can move around at slower ant-speeds). However, if we stretch the rubber skin (space expanding), the ants get even farther apart. Even two ants who aren't moving will get farther apart because of the stretching space between them.One of the following conclusions is that two top-speed ants (representing light speed) may be moving directly towards each other, however the expansion of the rubber skin between them is so much that they are still getting farther apart.Therefore, without faster-than-light travel (seemingly impossible at this point) it is quite likely that there are parts of this universe that are even theoretically impossible for us to ever see or reach.My heart saddened a bit when I realized that Not sure why my heart was set on whizzing around the universe to places billions of light-years away... Also considering that even if faster-than-light travel is possible, it's highly unlikely that it would ever be developed in my life time (therefore, there's no real difference to myself).But, just the knowledge that even theoretical travel to certain areas of the universe may very well be impossible just makes me feel a bit... left out As far as the number for the size of the universe goes, I think it's important to remember that it's not simply a best-guess at the actual size of the universe. It's more a best-guess at the lower-bound for the actual size of the universe. That is, it's our best guess that the universe must be at least that big. But it's impossible (at the moment, I think?) for us to know, or even guess at, an upper-bound for the size of the universe.

Wikipedia gives a pretty clear response to your question in its article on the universe. Keep in mind that 93 billion light years is the size of the observable universe. We see the most distant galaxies, those that are maybe 12 or 13 billion light years distant, as they were 12 or 13 billion years ago, and in that time the expansion of space has carried them a much greater distance further away. When we look at these distant galaxies we're also looking far back in time, but cosmologists can calculate how far away they are today, and that turns out to be around 46 billion light years, which is the radius of the observable universe as opposed to its 93 billion light year diameter.Wikipedia also has an article on the observable universe that contains a section on the size of the observable universe, so you might want to give that a look, too.--PercyEdited by Percy, : Grammar.

It's much much more complicated First off, the Universe, whether finite or infinite, is massively larger than the observable Universe. If there are galaxies throughout the Universe, then the furthest apart could be 10¹⁰⁰ lyrs from each other!! And that would be for a very small Universe...I'm not sure about 125 billion ligt years figure - it should be around 78 billion, unless there has been a recent update. This is how far apart two galaxies are *today*, that we can currently see at either side of the visible edge of the Universe (so as they were 12-odd billion years ago)As MrJack has already mentioned, the speed of light has nothing to do with the Universal expansion - this is simply a case of distances getting bigger. There is no actual motion involved. Think of ants on a ballon skin. The balloon skin expanding will carry the ants further away from each other, without them actually moving. It is there own motion across the skin (by walking) that is restricted by the speed of light.

CavediverThanks to you and the others for your explanation.I have heard of this "balloon skin" analogy before, and it kind of makes sense in that respect, although how space can be like a balloon skin or how the distance between objects can grow without them actually moving still defeats my imagination!

ANother analogy I've heard is with baking bread. Let's make it raisin bread. The raisins don't actually move (much) bu as the bread bakes, it expands, carrying the raisins with it. If you have two raisins very close to each other at the start, they'll move little in relation to each other, bu if you have two raisins that are at the edge of the bread (space doesn't have an adge, but as an analogy, it works) they'll end up quite a bit more distant. This is because every millimeter of dough is expanding, amking a lot of expansion when added together, even if each particle of bread only moves apart a small distance.

In which case, Stiles may get his wish to whiz around the stars billions of light years away if we could find some way to warp space so that it crunches up shrinking the distances that would need to be traveled bypassing the limitation of the speed of light.Don't give up hope yet, Stiles! You may yet get to meet the blue-skinned alien of your dreams!Edited by Izanagi, : No reason given.

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It's just some things you never get over. That's just the way it is. You go on through... best as you can. - Matthew Scott
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Marge, just about everything is a sin. (holds up a Bible) Y'ever sat down and read this thing? Technically we're not supposed to go to the bathroom. - Reverend Lovejoy
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You know, I used to think it was awful that life was so unfair. Then I thought, wouldn't it be much worse if life were fair, and all the terrible things that happen to us come because we actually deserve them? So, now I take great comfort in the general hostility and unfairness of the universe. - Marcus Cole

Like all analogies, it can get conceptually confusing.Think of the skin, and the entire balloon, as the universe at this moment in time. And, think of the balloon expanding as time passing from moment to moment; the increase of the balloon in size is time progressing forward.If you can piture that, now imagine placing dots all over the balloon. These dots represent galaxies in our universe. Now, as you increase the size of the balloon - (as it increases remember that's time progressing forward) - you'll notice that the dots, while not moving from where you drew them, are getting further apart from each other. The space between each dot is expanding due to the balloon increasing in size.Likewise, the space between the galaxies is increasing due to the accelerated expansion of the universe.That's why there's no speed of light restriction, because nothing is really moving at any speed. It's just the space between it that's increasing. Edited by onifre, : No reason given.

Hi OniThanks for your input.Yes, I understand the idea of dots on a balloon (or raisins in a cake), although to my mind the dots are moving, because they are moving apart from each other as a consequence of the balloon enlarging. In other words, they are not moving under their own steam but they are hitching a ride on something else that's moving (the surface of the balloon in this case). That's why I still find it hard to get my head around the concept.

It's more that space itself is stretching or expanding. For example, take a standard X,Y grid. You put dots at certain intersections, say (5,2) and (5,5). The dots are 3 units apart. Now, if you stretch the grid, the dots are still at the same coordinates, but the distance between them is now bigger.