The size of the universe - a question.

[Animavore]

Sounds wrong, bro. There's no reason for us to be moving faster.

[pErvinalia]

It's simple maths. If the universe expands x2, then the gap between us and both other points doubles. Hence why they both recede at the same rate from us. Draw it out on paper. It's simple stuff.

[pErvinalia]

Maybe Brian can do a funky javascript animation.

I could do a really lame one, but it's the middle of the night, and I've got arguing to do with 42...

[Animavore]

It's simple maths. If the universe expands x2, then the gap between us and both other points doubles. Hence why they both recede at the same rate from us. Draw it out on paper. It's simple stuff.

No it doesn't. Not in an explosion where everything moves together away from the blast. The universe expands in concentric spheres in that case. You would only see the type of expansion you describe for all galaxies which share the same circumference as the one you're on. Like all dots that share a circumference of a balloon. The galaxies perpendicular inwards and outwards move with you. At the same speed. Into a presumed void, and leaving a presumed void behind.

[pErvinalia]

We are talking about the expansion of space, not things moving from an explosion.

[Animavore]

We are talking about a universe which expands rapidly from a singularity centre, no? For all intents and and purposes such a thing could be treated like an explosion.

The universe doesn't look that way.

[pErvinalia]

No, I'm not talking about an explosion. I've always been referring to the expansion of spacetime. You even make the same point as me in this post, which I somehow forgot to address.

[pErvinalia]

[post redacted; a bit more thinking].

[Animavore]

No, I'm not talking about an explosion. I've always been referring to the expansion of spacetime. You even make the same point as me in this post, which I somehow forgot to address.

That only works if the universe expands at the same rate everywhere at once, again like a 3-D version of the balloon skin, or the infinite raisin cake. In a sphere which is expanding you have different lines of expansion from the centre along the radius than you do the circumference. In this case its more like the balloon as a whole. Growing in volume at a cubic rate while its surface area increase at a squared rate.

[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.

I was thinking about this earlier and I wanted to correct myself. The gravity-well would presumably expand with the expansion of space-time, which would result in stronger gravity (helping matter to stay together and not elongate). Which then raises the seeming contradiction of the inverse square law of gravity. If metres aren't growing, but gravity is, then the inverse square law would break, wouldn't it?

[Animavore]

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.

I was thinking about this earlier and I wanted to correct myself. The gravity-well would presumably expand with the expansion of space-time, which would result in stronger gravity (helping matter to stay together and not elongate). Which then raises the seeming contradiction of the inverse square law of gravity. If metres aren't growing, but gravity is, then the inverse square law would break, wouldn't it?

How would gravity grow stronger unless the mass of things was increasing?

[pErvinalia]

No, I'm not talking about an explosion. I've always been referring to the expansion of spacetime. You even make the same point as me in this post, which I somehow forgot to address.

That only works if the universe expands at the same rate everywhere at once, again like a 3-D version of the balloon skin, or the infinite raisin cake. In a sphere which is expanding you have different lines of expansion from the centre along the radius than you do the circumference. In this case its more like the balloon as a whole. Growing in volume at a cubic rate while its surface area increase at a squared rate.

I'm not following. Volume doesn't come into it. We are talking about linear distances. When I say expansion x2, I literally mean that the distance between two points doubles.

[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.

I was thinking about this earlier and I wanted to correct myself. The gravity-well would presumably expand with the expansion of space-time, which would result in stronger gravity (helping matter to stay together and not elongate). Which then raises the seeming contradiction of the inverse square law of gravity. If metres aren't growing, but gravity is, then the inverse square law would break, wouldn't it?

How would gravity grow stronger unless the mass of things was increasing?

Because gravity is a function of spacetime. As spacetime grows, so too must gravity. But as you identify, this leads to a nonsensical outcome (which is why I mentioned the breaking of the inverse square law). So how does spacetime expand without effecting gravity?

[Animavore]

No, I'm not talking about an explosion. I've always been referring to the expansion of spacetime. You even make the same point as me in this post, which I somehow forgot to address.

That only works if the universe expands at the same rate everywhere at once, again like a 3-D version of the balloon skin, or the infinite raisin cake. In a sphere which is expanding you have different lines of expansion from the centre along the radius than you do the circumference. In this case its more like the balloon as a whole. Growing in volume at a cubic rate while its surface area increase at a squared rate.

I'm not following. Volume doesn't come into it. We are talking about linear distances. When I say expansion x2, I literally mean that the distance between two points doubles.

If you blow up a balloon does the distance between the inner surface and outer surface double as it expands to twice its former size?

[Animavore]

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.

I was thinking about this earlier and I wanted to correct myself. The gravity-well would presumably expand with the expansion of space-time, which would result in stronger gravity (helping matter to stay together and not elongate). Which then raises the seeming contradiction of the inverse square law of gravity. If metres aren't growing, but gravity is, then the inverse square law would break, wouldn't it?

How would gravity grow stronger unless the mass of things was increasing?

Because gravity, in this case is a function of spacetime. As spacetime grows, so too must gravity. But as you identify, this leads to a nonsensical outcome (which is why I mentioned the breaking of the inverse square law). So how does spacetime expand without effecting gravity?

Gravity is dependant on the mass of an object.