The Expanding Universe

[Old_Geezer]

Ok. This is the thread suggested in the Alien Astronauts thread. I'm not asking so much for what the expansion is (I'm willing to accept for now the idea that space growing/being created is the main driver, and actual motion of "stuff" is a lesser effect and occurs within the existing universe) I'm more into trying to get to understand of the idea that the universe isn't really growing after all, that it's all just measurements, and may have always been infinite.

So what is this idea that the universe started from a small area (maybe singularity) if everything to do with expansion is merely to do with spatial measurement on the largest measurable scales of our universe ?

Either,

a) ~14 billion years ago, the universe really was small, hardly any space/volume, in which case there has been a real expansion, rather than just a measurement thing, (regardless how that occurs - space being created is fine, although the cause would be a nice thing to know (but wiki adventures lead to "false vacuums" and "instantons" (!)¹)). In which case the issue regarding how a finite volume can become finite in the elapsed time since, is still an unexplained issue.

Or,

b) the universe was always infinite, which then leads us to two more options.
i) Either we have an infinite universe that started out extremely (infinitely ?) hot and dense throughout it's infinite size
ii) Or the "Big Bang" was some kind of local event within an infinite universe.

(I'm assuming ii is fairly unlikely).

Am I meant to accept an infinite universe, all of which is in an explosive state, bereft of even quarks etc. which sorted itself out throughout afterwards ? An infinite universe coming into existence seems a little far fetched.


¹ Sometimes I think it'd be nice to sit in on the early first year or whatever university physics lectures. They ought to have a public gallery ;-)

[Old_Geezer]

We may not have defined 'outside a universe' but Occam's Razor and all that, I don't feel we need to assume it needs to be changed. Nothing means nothing exists which is necessary if the universe isn't physically expanding into something. Otherwise all we do is change the idea of an infinite universe into the idea of a finite universe inside an infinite undefined something.

Brainstorming ideas is good though. Who knows where it may lead ?

[woofgang]

we don't "need" to assume anything. I do think its interesting though to think about what other options might be and what ramifications those options might have

[jim360]

Yes, of course!

I'm not sure what the ramifications would be, although I suppose my point is that the "intermediate step" of creating a nothingness to grow into is probably not needed and not really different from just growing in the first place. Maybe someone will be able to take the idea further at some point, though.

[Rev. Green]

Don't forget that, after a certain distance, the Universe gets SMALLER as we go further away. i.e. the amount of space within 2000 light years of us is nearly eight times (2 cubed) the amount within 1000 light years of us, but the amount 10,000 light years from us is much less than eight times the amount 5000 light years from us because we are looking at the distant past when the universe was smaller. In the limit, the outer region of the universe has no size at all, it is just the fuzzy point where the big bang is happening. Please don't reply that I'm imagining I'm at the centre of the Universe. I am, but the same logic is true for any observer.

[Old_Geezer]

Took me a while to grasp that. It may appear a certain way due to the speed of light, but I don't think it is actually smaller... I want to say 'now' .. well you know.

I don't think we can see all the way to the start: that's beyond the point where light can ever reach us, due to the 'treadmill effect' of space creation.

[jim360]

Where did you read that, Rev Green?

(At the very least, I think you would probably need to pick bigger numbers than ~10,000 light years for it to be true).

[Rev. Green]

Sorry. I meant 10,000,000 light years, or 13,700,000 if you like. My point was that the universe can be finite but not have a boundary. I didn't read it - it is obvious.

[jim360]

It's true that the Universe can be finite without a boundary (that we were aware of), although I am not sure that the rest of your point is correct.

Observationally, at least, the Universe is larger than for any such effects you describe to be seen. For example, although the Universe is about 13.7 billion years old, the Universe we can see is about 90 billion light years across; and in that region there is (as yet) no sign of the effect you're talking about.

At least, not so far as I'm aware. If you did have a specific source in mind that prompted your comment I'd love to be made aware of it, but if there weren't then it may be a result of the rule of thumb that "obvious" things about the Universe are usually also wrong.

[Rev. Green]

Or even 13,700,000,000! The number of 0's always confuses me. I thought almost all of the universe was visible - just the last 300,000 light years invisible, or are my 0's wrong again? Objects near the edge should be spread over large visible angles, but there wouldn't be recognisable galaxies at that time so perhaps there is nothing to see.
I've no sources. Anything I've said is only based on musings.

[Sipowicz]

My brain hurts..........again!

[jim360]

Yeah, it gets confusing doesn't it?

"I thought almost all of the universe was visible - just the last 300,000 light years invisible, or are my 0's wrong again? Objects near the edge should be spread over large visible angles, but there wouldn't be recognisable galaxies at that time so perhaps there is nothing to see. I've no sources. Anything I've said is only based on musings."

Nah, your 0's were right that time :)

It does sound like you're mixing a few pieces of information up. We can see back to about 380,000 light years of the Universe's history, but I don't think that's the same as saying that we can see all of the spatial extent of the Universe. Quite the opposite, if anything. Current observations are more consistent with the Universe being infinite, or certainly larger than we can see.

[Rev. Green]

Thanks Jim, Can you give me any pointers to the Universe being infinite? I can just about cope with photon action travelling at infinite speed in a finite universe, but infinite speed in an infinite universe would explode what is left of my brain. (Can I ask anyone who thinks photon action is limited to the speed of light to desist from replying?)

[jim360]

I'm not sure what you mean by your last remarks, but perhaps it's wiser not to limit responses to people who already agree with your preconceived notions if those notions are either wrong, debatable or at least in need of clarification.

As for a reference to an infinite universe, I am not sure how many formal papers will address the question directly, but it's certainly implicit in the definition of the "FLRW" metric, that appears to match well the observed "shape" of the universe. The point is that the range of the coordinate r, that stands for the radius in some sense, extends from 0 to infinity -- so that any paper discussing this metric will automatically include that assumption, that the universe is infinite.

No observation can do more than confirm the idea that this metric is a good fit to the data, but the fit would perhaps be equally good if the universe were finite, but of a size far greater than we can ever hope to see (ie, the range of r is huge, but has some upper limit after all; or perhaps the metric is only "good enough" for what we can observe, but beyond that breaks down and is wrong).

The question, then, of whether or not the Universe is truly infinite is likely to remain unanswered. I don't think that most scientists would bother to even try, at least not directly: "Here's some finite, bounded shape the Universe could take: does this work?" If not, then you could try another shape, and so on, but the ultimate possibility of an infinite universe would only be sorted if you did manage to find a finite shape that worked.

[jim360]

Also just while I am thinking about it, an infinite Universe can still be expanding even though it remains infinite throughout. Infinity is weird -- but to try to understand how this can be possible, consider the following true statements:

1. There are as many positive even numbers as there are positive whole numbers, and as many prime numbers as there are cube numbers.

2. if x is allowed to be any real number, then 2x also ends up describing every real number, and both x and 2x both reach infinity even if it seems that 2x should always be bigger than x.

So that's to say that in an expanding infinite universe, things simply get further apart than they did before. The universe as a whole doesn't exactly get bigger because it makes no sense to write "two times infinity is greater than infinity"

[Bazile]

So if the universe is expanding and there are parallel universes - what's between these universes - and wont they collide at some
point ?

[jim360]

Any parallel universes as exist necessarily imply a further dimension is needed to accommodate them. So our universe would be, say, as coordinates (t,x,y,z,0) and a parallel universe at coordinates (t,x,y,z,1).

[Rev. Green]

Apparently, there hasn't been much progress in the almost 100 years since FLRW. I like the idea of a finite universe because you can map it onto its boundary (holography?) which can make action at a distance more credible. Are all coordinates in (t, x, y, z, 1) real, so that there could be a (t, x, y, z, 0.000001) ?

[jim360]

Possibly. But in the notation I used I've said nothing about the nature of that "extra" dimension. I only made use of it to illustrate the point. Maybe it would be a bizarre "discrete" dimension, or some other property that forces universes to occur at only specific "points" within it, or it's limited in size in some way, etc etc.

The only thing I really want to say is that it's not difficult to imagine the possibility of parallel universes, as long as you have somewhere to put them (ie, an extra dimension -- or more than one extra dimension). But you can have extra dimensions within our universe too. I think that's what arises from (Super)String Theory, for example.