The size of the universe - a question.

[Animavore]

No he didn't. He said the universe might be infinite. The only sphere he mentions is the observable universe.

Watch it again.

[pErvinalia]

He doesn't say the universe is a sphere. He says the observable universe looks like a sphere to the observer.

Ah yes, sorry, I misinterpreted that bit.

It seems to me there's two critical points:

1.The universe can't be a sphere if every part of the universe can see 13.7 billion years in any direction (how exactly have they determined this to be a fact?), and therefore must be infinite.

2. I accept the scientific fact that the speed of light isn't related to the dimensions of spacetime, and therefore at some point if acceleration of expansion continues we will 'go dark'. But I don't understand how that can be explained. From either of two perspectives:

• 2a. how the speed of light isn't related to the dimensions of spacetime. I.e. if spacetime causes the distance between two objects to grow, why doesn't it change the distance that light can travel in a unit of time by the same ratio?

• 2b. How does infinity expand? You can't have infinity x 2, in my understanding of infinity.

And, of course, the answer of "expansion in relation to what?" doesn't make sense to me. Expanding in relation to itself tells us nothing. If tomorrow I appeared to be 2 times the volume of myself today, how do I know I've expanded instead of the surrounding space contracting? Expansion makes no sense unless it is grounded in a reference frame. I can see that the reference frame is essentially unit metres, but then that just leads me back to the seemingly contradictory assertion that spacetime is expanding at the same time that space (i.e. the distance measured by a metre) isn't expanding. (and the related concept with time, and therefore the speed of light).

[pErvinalia]

see my edits, Ani.

edit: I'm still editing..

[Animavore]

1.The universe can't be a sphere if every part of the universe can see 13.7 billion years in any direction (how exactly have they determined this to be a fact?), and therefore must be infinite.

It's not a fact. We don't know the universe is infinite. It's just that if it is then this is how it would be to all observers.

2a. how the speed of light isn't related to the dimensions of spacetime. I.e. if spacetime causes the distance between two objects to grow, why doesn't it change the distance that light can travel in a unit of time by the same ratio?

Because why would it? You keep putting this assumption in but you haven't justified it. Spacetime is expanded by a repulsive force we don't understand. But matter is held together by different forces. When they universe expands the things within it don't. Humans and galaxies, for instance, aren't being elongated as the gravity of a galaxy holds it together. The space between galaxies, however, is. Light will need to travel farther between these galaxies as they move apart.

2b. How does infinity expand? You can't have infinity x 2, in my understanding of infinity.

This is why I prefer the "becomes less dense" analogy. Infinity is infinite. It doesn't get bigger.

[Animavore]

And, of course, the answer of "expansion in relation to what?" doesn't make sense to me.

The universe is expanding in relation to the observer.

Take a look at this diagram.

t1. A---B---C

t2. A------B------C

Let's call each dash a lightyear. At t1 the observer at B is 3 lightyears away from the observer at A.The observer C is 6 lightyears away. At t2 B is now 6 lightyears away while C is now 12. B has only moved away a distance of 3 lightyears while C has moved a distance of 6! For that to happen C would have to appear to be moving away from A at a rate faster than B to move a seemingly further distance in the same time. From C, B and A have moved away the same distances from the opposite direction. But from B, both A and C have moved the exact same distance. Of course the total distance from A to C has been doubled (ie. has expanded) at the same rate everywhere taken as a whole.

This is kind of how the universe looks. Nearer galaxies appear to move away slower than further ones. The ones really far away appear to be flying away from us compared to the imperceptibly moving nearer ones. From someone on one of those far away galaxies looking back we appear to be flying away. From our perspective an observer on the far end of the observable universe is moving away from us while we remain stationary. From their end it looks like it is they who remain fixed.

[pErvinalia]

Referring to your second to last post... Did you read my edits? The "less dense" analogy is no better, because you still end up at the "in relation to what" part.

I'll have a think about the rest of your post and get to a reply in a bit. But there's still a few problems. As you say, no evidence for the 13.7 billion light years for every observer. So why are we even making this assumption?? And why is Brian asserting that the universe isn't a sphere like a balloon. What's he basing this on? And why are you asserting that the universe doesn't have a centre? What are you basing that on? And there was something else, which I've temporarily forgotten, and which I'll edit in in a bit..

edit: oh yeah, the other thing was, how do we know that spacetime is expanding, and not that things are just getting further apart. Like my magnet analogy from earlier. Why must it be spacetime that is expanding, and not solely the distances between things?

[Animavore]

I'll have a think about the rest of your post and get to a reply in a bit. But there's still a few problems. As you say, no evidence for the 13.7 billion light years for every observer. So why are we even making this assumption?? And why is Brian asserting that the universe isn't a sphere like a balloon. What's he basing this on? And why are you asserting that the universe doesn't have a centre? What are you basing that on? And there was something else, which I've temporarily forgotten, and which I'll edit in in a bit..

The balloon analogy talks about the 2-D surface of the balloon. Not the 3-D dimensions of the balloon. The surface of a balloon, like the surface of a sphere, has no centre.

If the Big Bang happened everywhere at once at the same time then the observable universe would have to be the same for everyone because there is no place in the universe where observable light could've come from a further distance.

[Animavore]

edit: oh yeah, the other thing was, how do we know that spacetime is expanding, and not that things are just getting further apart. Like my magnet analogy from earlier. Why must it be spacetime that is expanding, and not solely the distances between things?

It would have to expand to accommodate things getting further apart if the universe has an equal distribution of matter throughout. Unless we have a small ball of matter in the middle of an infinite vacuum which is spreading out into that vacuum.

[pErvinalia]

2a. how the speed of light isn't related to the dimensions of spacetime. I.e. if spacetime causes the distance between two objects to grow, why doesn't it change the distance that light can travel in a unit of time by the same ratio?

Because why would it? You keep putting this assumption in but you haven't justified it. Spacetime is expanded by a repulsive force we don't understand. But matter is held together by different forces. When they universe expands the things within it don't. Humans and galaxies, for instance, aren't being elongated as the gravity of a galaxy holds it together. The space between galaxies, however, is. Light will need to travel farther between these galaxies as they move apart.

I've got a couple of thoughts on this:
The gravity thing first. That makes no sense to me as it implies that gravity is independent of spacetime. But it's a theory of spacetime. It appears independent, as you'd expect gravity to weaken as spacetime got bigger. But if gravity is maintaining the clumping of matter independent of the expansion of spacetime, then that doesn't really gel in my brain.

The lightspeed thing. This comes down to what I see as a contradiction. It would be good if you could directly address this "contradiction". Spacetime is expanding. Yet the length of a metre (and the unit of a second) aren't expanding. So you have a seeming contradiction that spacetime is expanding, but space and time aren't. Regarding the light speed aspect of that, if space and time are expanding by the same ratio as the expansion of spacetime, then the speed of light remains constant as the universe expands, contrary to your rebuttal of earlier where you implied that the speed of light would have to increase and therefore violate the most fundamental law of the universe.

[Animavore]

The contradiction only exists in the way you are framing your thoughts. You repeatedly mistake expanding for stretching.

[pErvinalia]

I'll have a think about the rest of your post and get to a reply in a bit. But there's still a few problems. As you say, no evidence for the 13.7 billion light years for every observer. So why are we even making this assumption?? And why is Brian asserting that the universe isn't a sphere like a balloon. What's he basing this on? And why are you asserting that the universe doesn't have a centre? What are you basing that on? And there was something else, which I've temporarily forgotten, and which I'll edit in in a bit..

The balloon analogy talks about the 2-D surface of the balloon. Not the 3-D dimensions of the balloon. The surface of a balloon, like the surface of a sphere, has no centre.

That seems like a counter-inuitive analogy to me, then. I always thought it referred to the volume of the balloon being the volume of the universe. That makes far more sense to me.

If the Big Bang happened everywhere at once at the same time then the observable universe would have to be the same for everyone because there is no place in the universe where observable light could've come from a further distance.

You can't hold the position that it's both unknown that this is the case, and also that there definitively isn't a centre to the universe. The two positions are incompatible.

[pErvinalia]

edit: oh yeah, the other thing was, how do we know that spacetime is expanding, and not that things are just getting further apart. Like my magnet analogy from earlier. Why must it be spacetime that is expanding, and not solely the distances between things?

It would have to expand to accommodate things getting further apart if the universe has an equal distribution of matter throughout. Unless we have a small ball of matter in the middle of an infinite vacuum which is spreading out into that vacuum.

So the answer is another assumption. Can you see how this is looking like turtles all the way down?

[pErvinalia]

The contradiction only exists in the way you are framing your thoughts. You repeatedly mistake expanding for stretching.

i don't know that I do. I see a metre as a measure of space (concomitant to the speed of light as a measure of spacetime). So I don't see how space can both expand and not expand at the same time. Not really anything to do with "stretching".

You also didn't address the seeming contradiction about gravity and spacetime.

[pErvinalia]

I kind of feel like we are getting nowhere again. There's still too many holes for me. I want to see a whole picture explanation that makes sense considering what we actually know about the universe. Not just a series of quotes out of New Scientists... . There seem to be a lot of 'just-so' stories being given. The expanding spacetime one is the obvious one. We could imagine two competing theories. The one that I assume is right, but poorly explained - that spacetime is expanding but gravity is somehow not affected by that, despite gravity being a theory of spacetime (in one common interpretation, at least); or another one that has gravity being ye olde gravity, and spacetime not expanding, but instead things just getting further apart for some other standard model reasons. The latter doesn't seem to contradict the theory of gravity. At a lay persons non-mathmatical level, it makes far more sense. Now I have to assume that it's wrong for a number of reasons (not least mathematically), but unless someone can explain in lay(ish) terms why the more convoluted explanation is correct and the more simple explanation isn't, I deem the whole to be inadequately explained.

[Animavore]

That seems like a counter-inuitive analogy to me, then. I always thought it referred to the volume of the balloon being the volume of the universe. That makes far more sense to me.

No. It refers to the surface. If you are on the surface of an expanding balloon which is poka-dotted the dots farther away will appear to move away faster than the ones nearer, just like on the diagram I made. Space is the same principle taken to 3-D.

You can't hold the position that it's both unknown that this is the case, and also that there definitively isn't a centre to the universe. The two positions are incompatible.

If the universe is infinite there's no centre. How could there be?

If the universe is a 3-D version of a balloon where you travel in the same direction long enough you get back to where you started there's no centre.