Thread: Just how big IS the universe?

I'm gonna set the reference at the edge of any form of substance, matter/energy/dark matter, whatever, basically this ISN'T a "well the universe has an infinite amount of space and so is infinite" question
it also isn't a globular or any other theorized universe shape, I'm assuming the universal existance (stuff) has been expanding like a blob of dye suddenly appearing underwater (diffusing equally in all directions)



The BBT happened roughly 14.5billion years ago, giving everything a supersonic kick up the backside. now although for some reason I don't fully understand (dark matter I think) we have been steadly moving faster outwards since then

however 'nothing' can go as fast as light, which means the dimensions of the universe should be a rough sphere of LESS than 29Billion lightyears across in any given direction.

is this even remotely correct? if so, why do I see so many bizzare estimates? ranging from only a few billion to a few billion billion to infinite


also vaguely around the same subject, how does intergalactic gravity affect the whole mess? I suppose further away galaxies would pull further behind galaxies faster while further behind galaxies pulled further ones slower, meaning Galaxy A would be in front of Galaxy B, then at some point B would be in front of A, and so on

The universe may or may not be infinite. We don't know (to the best of my knowledge at least). However, anything that is far enough away that its light hasn't had time to reach us is outside of the visible universe and is causally disconnected from us. Also, the universe isn't expanding faster than light, at least not now. If it had been expanding that fast since the big bang, no light from anywhere would ever be able to reach us. I don't know how big the visible universe is after accounting for the expansion of the universe though.

For the second part, there's a problem with defining "in front of". The universe is mostly homogeneous. Gravity at that scale just tends to make dense areas denser, resulting in a web-like structure.

I cannot accurately input my opinion for the first part of the question, for I am not a believer in the BBT; however, in regards to the end part of your post, I have been led to believe that, in space, all directions are eliminated, but are only referred to as "front", "back", or "above" for the sake of identification.

does it really make any difference?

It matters not a whit what any person subjectively believes or not - the objective facts are incontrovertible.
The Big Bang did happen based upon everything we observe.
No better mechanism has been put forward.

As for the size of the observable universe, it would seem to be the outer edges estimated at the farthest objects observable. What does hubble show so far? We get an approximation based upon its resolution level in a finite number of viewing angles. From that we can approximate a roughly spherical shape of an estimated diameter.

Perhaps Chandra has a better approximation. Go do the sleuth work.

I still view it like the dog chaseing a car, what's it going to do with it when he catches it. actually I find all the thrashing about on the subject ammusing, infinite universes, microuniverses, open -closed universes,multi-dimensional universes. We haven't even solved interplanetary flight yet.
guess I'll have to hop in my inter-universe time and space bender and check it out.

Well, we do what we can with what we have. So far it's been relatively easy to figure out how much of what's out there compared to actually getting there. Basically, there's no point in not trying to figure it out, and the possibility that we'll find something interesting or useful during our attempts.

*sigh* I got bored and made a rough calculation


current estimated speed of our galaxy is 600kps or 2.16million kilometers an hour

rough age of the universe is 14.4billion years

using our galaxy as the average speed of mass in the universe and assuming acceleration has been constant since BBT


Distance = (initial velocity + final velocity)/2 X time

or 0 + 600000M / 2 (300000) X 45,441,973,430,784,000s (45 quadrillion 441 trillion 973 billion 430 million 784 thousand seconds since the big bang)

giving 136325920292352000000000 Meters radius or

27,265,184,058,470,000,000,000 (27 sextillion, 265 quintillion.....) Meters from any end to end point of the material universe

one lightyear is 9,460,730,472,580.8 kilometers

meaning the universe is roughly 2,881,392 lightyears wide

The observable universe is basically a sphere of things we can see surrounding us, we cannot see further than this because the light has not yet reached us. The fact that the observable universe is spherical tells us nothing about the shape of the actual universe.

Although the big bang happened roughly 13.7 billion years ago, this does not mean that the universe is a sphere with a diameter of 27.4 billion light years. Space is expanding, and so appears to have grown at a rate greater than the speed of light. According to wikipedia the edge of the observable universe is 46.5 billion light years away, this gives a sphere with a diameter of 93 billion light years. And this is just the observable universe, the universe itself is larger still!

Originally Posted by MagiMaster

The universe may or may not be infinite. We don't know (to the best of my knowledge at least).

I do not understand (at least if you believe the BBT) how you can state that the universe "may or may not be infinite" given the BB created space, time and matter around 14 billion years ago.
For me, to say the universe is infinite must mean it is infinite in extent: in other words space goes on forever in all directions. I do not see how this is possible given the fact space was created at a specific point, in the past, and has not always existed.
One could put forward the idea our universe was caused by some happening "somewhere else" such as an event in another universe. If there are "other places" (other universes) or branes then one could argue that space does go on forever.
At the moment we do not have any concrete evidence this is the case.

I'm assuming the universal existance (stuff) has been expanding like a blob of dye suddenly appearing underwater

Not really AFAIK. The BB is thought to be the origin of all space, so rather think about it as the 3D version of the 2D surface of a balloon that inflates from nothing.

however 'nothing' can go as fast as light

Not exactly true. A distinction is made between something moving through space, as opposed to with space. The former has C as an upper limit, but the latter, as only an apparent speed, has none AFAIK.

does it really make any difference?

It is interesting.

I do not understand (at least if you believe the BBT) how one can state that the universe "may or may not be infinite" given the BB created space, time and matter around 14 billion years ago.

Why can't the BB have been an event in an existing universe? Light from "outside" would never reach us as long as expansion continues, as the light would continually be stretched to beyond being observable. The expanding bubble of our universe would then indeed be roughly spherical and the BB would only be the origin of our little bit of space-time.

Originally Posted by KALSTER

Why can't the BB have been an event in an existing universe? Light from "outside" would never reach us as long as expansion continues, as the light would continually be stretched to beyond being observable. The expanding bubble of our universe would then indeed be roughly spherical and the BB would only be the origin of our little bit of space-time.

I suppose the BB could "have been an event in an existing universe" but, at the moment, that is pure speculation.
My point was simply that I would describe the BB as a fact, altho' a few others might not agree, and not a piece of speculation. This means that our universe has existed only for a certain amount of time. Given that, I cannot accept (altho' I am willing to be convinced) the universe is infinite in extent.

Originally Posted by Halliday

Originally Posted by KALSTER

Why can't the BB have been an event in an existing universe? Light from "outside" would never reach us as long as expansion continues, as the light would continually be stretched to beyond being observable. The expanding bubble of our universe would then indeed be roughly spherical and the BB would only be the origin of our little bit of space-time.

I suppose the BB could "have been an event in an existing universe" but, at the moment, that is pure speculation.
My point was simply that I would describe the BB as a fact, altho' a few others might not agree, and not a piece of speculation. This means that our universe has existed only for a certain amount of time. Given that, I cannot accept (altho' I am willing to be convinced) the universe is infinite in extent.

Well, my point was partly that we might never know one way or the other anyway. What we see might be exactly the same in either case IMHO. But I concede, it is pure speculation on my part.

Originally Posted by Halliday

My point was simply that I would describe the BB as a fact, altho' a few others might not agree, and not a piece of speculation.

Just to be somewhat pedantic... Expansion and/or inflation are facts. What we know about the universe coming out of a singularity and/or nothingness (what people call the BB) is still very much being reconsidered by serious cosmologists and quantum gravitationalists.

Originally Posted by inow

Just to be somewhat pedantic... Expansion and/or inflation are facts. What we know about the universe coming out of a singularity and/or nothingness (what people call the BB) is still very much being reconsidered by serious cosmologists and quantum gravitationalists.

I understand little is known about the physics of the singularity, where it came from, why it appeared, and agree expansion and inflation are facts.
So what exactly is being "reconsidered?" Can you go into some more detail?
The idea that the BB "is still very much being reconsidered by serious cosmologists etc." suggests you believe a major overhaul of the theory could well be on the cards.

Hi Halliday,

In the simplest possible terms, few (if any) serious cosmologists really think the universe ever existed in a singularity state. The predominant (AFAIK) viewpoint is that the singularity is merely an artifact of our math... That our models are not yet honed enough to adequately describe the situation. The same thing applies to the center of blackholes. Most people working on the idea don't think that the center is really a singularity. It's just that the math we use which works so well in other reference frames tends to break down at these levels (early universe, center of BH, etc.)... not that those points are truly singularities.

So, the math shows singularities, but most think that the math is limited and needs further work, not that the universe actually existed itself as a singularity.

Originally Posted by inow

Hi Halliday,

In the simplest possible terms, few (if any) serious cosmologists really think the universe ever existed in a singularity state. The predominant (AFAIK) viewpoint is that the singularity is merely an artifact of our math... That our models are not yet honed enough to adequately describe the situation. The same thing applies to the center of blackholes. Most people working on the idea don't think that the center is really a singularity. It's just that the math we use which works so well in other reference frames tends to break down at these levels (early universe, center of BH, etc.)... not that those points are truly singularities.

So, the math shows singularities, but most think that the math is limited and needs further work, not that the universe actually existed itself as a singularity.

Talking purely as a layperson, which probably means I should not be discussing this stuff, I tend to agree that "the singularity is merely an artifact of our maths" if a singularity is defined as being the physical equivalent of a dimensionless mathematical point, with zero volume and infinite density.
I still believe, however, that our universe began with a region of tiny volume and huge density which created space, time, energy and matter.

Just to add, there's no particular reason to believe that we can see everything created by the big bang. There's also no particular reason to believe that the universe has a border. It might be possible that if you go far enough, you'll end up back where you started, even if you might have to go beyond the edge of the visible universe for that to happen, in which case we'd have a very hard time figuring it out.

Originally Posted by MagiMaster

Just to add, there's no particular reason to believe that we can see everything created by the big bang. There's also no particular reason to believe that the universe has a border. It might be possible that if you go far enough, you'll end up back where you started, even if you might have to go beyond the edge of the visible universe for that to happen, in which case we'd have a very hard time figuring it out.

You stated in an earlier post that the universe "may or may not be infinite".
Maybe my idea of infinity is naive, simplistic or unscientific, but I have always taken the number system as one example, of infinity, because it really does go on forever.
One can say our planet has no border and "that if you go far enough you'll end up back where you started" but this does not mean that the Earth is, in any sense, infinite in extent.
This could be said to be a trivial argument over the meaning of words but I have noticed, even in fine popular science books, that the words "infinite" and "unbounded" can sometimes appear to be used as if they mean the same thing. I do not believe these words are interchangeable or have the same meaning.

No, you're right, but that was somewhat tangent to the point that it might be infinite. Really, it could be infinite, unbounded, bounded, or possibly something I can't think of at the moment. I'm just saying that there's a very good chance there's more to the universe than what we can see.

Originally Posted by MagiMaster

I'm just saying that there's a very good chance there's more to the universe than what we can see.

That I do agree with and also (as the man said) probably more to the universe than we can imagine!