The size of the universe - a question.

[Animavore]

Yeah nah. Circular argument is circular.

It's not circular at all. The universe is observed expanding. And it expands the same in all directions. Further galaxies appear to move away faster than nearer ones. Exactly what you would observe if the universe was expanding the same in all directions from all points.

QED.

That's a reasonable conclusion. But it's not an explanation. What is expanding, and what is it in relation to, given the universe is everything in existence?

Some scientists prefer to say the universe gets less dense than expanding.

[Animavore]

The light from the big bang would have past everything at time 0+1. We'd never see it.

Never see what?

[JacksSmirkingRevenge]

<snip>
The other point to consider is that every point is expanding away from every other point. But what does "expanding" mean? This is the bit I was really referring to in your post. Is it spacetime expanding? If it is, then the speed of light relative to points in space wouldn't change as spacetime expanded. in which case we would never see the occurrence of 'going dark' as you described.

From what I've gleaned from documentaries, etc. Spacetime is expanding at an accelerating rate (driven by some hitherto not-understood 'force' that has been labelled 'Dark Energy', for now.
The speed of light seems to be a (unbreakable?) natural speed limit. The analogy in my head is one of an ant trying to get from one end of an infinitely stretchy piece of elastic to the other. If the elastic expands at an accelerating rate then there will come a point where the ant can no longer keep up with the expansion and it will never reach the other end.
The upshot is - 1. an 'observable' universe...A perfect sphere around an observer from any point in the universe.
2. Everything outside that. Here be dragons.

[pErvinalia]

Yeah nah. Circular argument is circular.

It's not circular at all. The universe is observed expanding. And it expands the same in all directions. Further galaxies appear to move away faster than nearer ones. Exactly what you would observe if the universe was expanding the same in all directions from all points.

QED.

That's a reasonable conclusion. But it's not an explanation. What is expanding, and what is it in relation to, given the universe is everything in existence?

Some scientists prefer to say the universe gets less dense than expanding.

Your just shifting to a different abstraction. What is getting less dense?

[pErvinalia]

The light from the big bang would have past everything at time 0+1. We'd never see it.

Never see what?

Light from the early universe, which is a claim regularly made in pop science.

[Brian Peacock]

This will happen until the expansion starts moving faster than the speed of light. At which point the galaxies around hours will be moving so fast the light will never reach us, plunging the sky into darkness.

Expansion of what? In relation to what? I suspect you are one of the millions of lay people who think they understand how this stuff works, but really don't...

Although, I'm happy to be proved wrong.

And how would you judge if Ani got it right?

The balloon analogy is helpful and unhelpful. Its helpful because it can stand in to represent a certain idea about expansion - the balloon has a comprehensible topology and a defined boundary, and as it expands we can see how it's radius is increasing relative to a imaginable central point, and by that understand how some nominal point within the boundary in one state can be said to have moved away from all other nominal points in the next state. That's where the analogy reaches its limit. The universe has no balloon-like topology, boundary or centre - the balloon is only there to show how eveything can move away from everything else in every direction.

[Animavore]

The light from the big bang would have past everything at time 0+1. We'd never see it.

Never see what?

Light from the early universe, which is a claim regularly made in pop science.

Why?

[pErvinalia]

The light from the big bang would have past everything at time 0+1. We'd never see it.

Never see what?

Light from the early universe, which is a claim regularly made in pop science.

Why?

Because nothing can travel faster than the speed of light. So if the light originated in the centre of the universe then it must have passed every piece of matter at plank time after the big bang. That's not what happens supposedly, but I've never heard a sensible explanation of why this does not happen.

And you still haven't explained what is expanding / getting less dense.

[pErvinalia]

<snip>
The other point to consider is that every point is expanding away from every other point. But what does "expanding" mean? This is the bit I was really referring to in your post. Is it spacetime expanding? If it is, then the speed of light relative to points in space wouldn't change as spacetime expanded. in which case we would never see the occurrence of 'going dark' as you described.

From what I've gleaned from documentaries, etc. Spacetime is expanding at an accelerating rate (driven by some hitherto not-understood 'force' that has been labelled 'Dark Energy', for now.
The speed of light seems to be a (unbreakable?) natural speed limit. The analogy in my head is one of an ant trying to get from one end of an infinitely stretchy piece of elastic to the other. If the elastic expands at an accelerating rate then there will come a point where the ant can no longer keep up with the expansion and it will never reach the other end.
The upshot is - 1. an 'observable' universe...A perfect sphere around an observer from any point in the universe.
2. Everything outside that. Here be dragons.

If spacetime is expanding then what is considered a standard metre and a standard second would be expanding too (by the same ratio). So the speed of light would remain the same in relation to spacetime. In relation to distance (i.e. just space), that means that the time it takes light to travel between two points would remain the same regardless of how much the space between them was expanding. Therefore, no 'going dark' as Ani described earlier.

This is why I keep asking what is expanding (and what is it expanding in relation to). If it's spacetime, then the explanations never make any sense to me. If it's something else, then I'd like an explanation describing that process. It sounds an awful lot like it's expanding in relation to a metre, say. But a metre is dependent on spacetime itself. So you end up with the circular description of spacetime expanding in relation to itself. Which tells us nothing.

[pErvinalia]

This will happen until the expansion starts moving faster than the speed of light. At which point the galaxies around hours will be moving so fast the light will never reach us, plunging the sky into darkness.

Expansion of what? In relation to what? I suspect you are one of the millions of lay people who think they understand how this stuff works, but really don't...

Although, I'm happy to be proved wrong.

And how would you judge if Ani got it right?

The same way I'd judge if anything made sense to me. Take note, I haven't suggested that Ani isn't right. What I've suggested is that all explanations I've ever heard on this topic never make sense to me, and I suspect that's because most people don't actually understand how it works, and are just parroting pop-science. If Ani can explain it to me in a way that makes sense, I'll of course judge that he understands how it works. If he can't, then the possibility remains open that he doesn't really understand it, but there's also the possibility that he's making perfect sense and the problem is at my end.

The balloon analogy is helpful and unhelpful. Its helpful because it can stand in to represent a certain idea about expansion - the balloon has a comprehensible topology and a defined boundary, and as it expands we can see how it's radius is increasing relative to a imaginable central point, and by that understand how some nominal point within the boundary in one state can be said to have moved away from all other nominal points in the next state. That's where the analogy reaches its limit. The universe has no balloon-like topology, boundary or centre - the balloon is only there to show how eveything can move away from everything else in every direction.

Ok, you've said what it isn't, but can you now explain what it is?

[Animavore]

The light from the big bang would have past everything at time 0+1. We'd never see it.

Never see what?

Light from the early universe, which is a claim regularly made in pop science.

Why?

Because nothing can travel faster than the speed of light. So if the light originated in the centre of the universe then it must have passed every piece of matter at plank time after the big bang. That's not what happens supposedly, but I've never heard a sensible explanation of why this does not happen.

And you still haven't explained what is expanding / getting less dense.

There is no centre. Also matter had not yet coalesced. Also in the early universe the universe was so dense that photons bounced around creating an opaque universe. It's not until recombination, over 300,000 years after The Big Bang that these photons escape. This is what we see in the CMBR. We can't penetrate this period through to the earlier universe.

And finally, although nothing can travel faster than the speed of light within the universe, the universe itself is not bound by this constraint. The early expansion of spacetime was faster than lightspeed.

[pErvinalia]

Can you explain the concept of the universe not having a centre? And of course, what is spacetime expanding in relation to?

[Animavore]

<snip>
The other point to consider is that every point is expanding away from every other point. But what does "expanding" mean? This is the bit I was really referring to in your post. Is it spacetime expanding? If it is, then the speed of light relative to points in space wouldn't change as spacetime expanded. in which case we would never see the occurrence of 'going dark' as you described.

From what I've gleaned from documentaries, etc. Spacetime is expanding at an accelerating rate (driven by some hitherto not-understood 'force' that has been labelled 'Dark Energy', for now.
The speed of light seems to be a (unbreakable?) natural speed limit. The analogy in my head is one of an ant trying to get from one end of an infinitely stretchy piece of elastic to the other. If the elastic expands at an accelerating rate then there will come a point where the ant can no longer keep up with the expansion and it will never reach the other end.
The upshot is - 1. an 'observable' universe...A perfect sphere around an observer from any point in the universe.
2. Everything outside that. Here be dragons.

If spacetime is expanding then what is considered a standard metre and a standard second would be expanding too (by the same ratio). So the speed of light would remain the same in relation to spacetime. In relation to distance (i.e. just space), that means that the time it takes light to travel between two points would remain the same regardless of how much the space between them was expanding. Therefore, no 'going dark' as Ani described earlier.

This is why I keep asking what is expanding (and what is it expanding in relation to). If it's spacetime, then the explanations never make any sense to me. If it's something else, then I'd like an explanation describing that process. It sounds an awful lot like it's expanding in relation to a metre, say. But a metre is dependent on spacetime itself. So you end up with the circular description of spacetime expanding in relation to itself. Which tells us nothing.

The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 second.

https://en.m.wikipedia.org/wiki/Metre

The speed of light does not change. So no. A standard metre will not expand as the universe does because otherwise it would have to increase in speed to cover a larger distance in the same period.

[pErvinalia]

That's a circular argument. You are saying a metre won't change because the speed of light doesn't change therefore a metre doesn't change.

The real question, again, that you keep avoiding, is what is spacetime(?) expanding in relation to? In relation to that, distance (i.e. a metre) and time will change also. If the proposition is that spacetime expands (and in relation to something).

[Animavore]

Can you explain the concept of the universe not having a centre? And of course, what is spacetime expanding in relation to?

Well it's common in pop science to think of the early universe as point, but this point exists nowhere except on a graph drawn by people.

The truth is we don't actually know.

Sean Carroll posits the mind-bending scenario that the early universe could've been infinite, and infinitely dense. Then something set it off from this high entropy state to a lower one. In the process it loses density. Becoming less tightly packed. In all places at the same time. Infinity expanded is still infinity.

Something which is infinite can have no centre.