Comment by EnterpriseTell | Hacker News Reader

Comment by EnterpriseTell | Hacker News Reader

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>In short, even if galaxies weren't accelerating

-Galaxies are not accelerating, space is expanding.

> Imagine there are some objects in space all moving in random directions and speeds, relative to Earth...

-No, In your scenario then end result would be a most static average distance between all objects in the universe. As an infinite number of objects come from infinite distances, there would ALWAYS be objects in the neighborhood.

I think what you're imagining is a bunch of objects in a box, give them random vector and then remove the box. If they maintain course, all will eventually move outside the original box boundaries, and away from each other. (not the way the universe is).

They know space is expanding. The primary mechanism we know this is the speed with with we measure an object (moving away) is redshifted. Objects at the same distance from Earth, but opposite regions of space are moving at the ~SAME measured velocity.

There simply is no existing theory which can account for what we are seeing besides space expanding. I'm not big on thinking we understand it all, but in this particular measurement, there is basically zero doubt. Space is expanding, which has the affect of accelerating all objects in the universe away from you, with an acceleration relative to the distance. The more space between you, the more opportunity to expand.

What does "space is expanding" mean? That the distance between objects is increasing? How can you tell the difference between "space expanding" and "objects moving in a non-expanding space"? Is there any way to tell the difference or is it just that all objects are moving away at the exact speeds that satisfies the "space expanding" explanation and nothing else?

But then, I'm back to what does "space is expanding" mean? What is doing the expansion?

space expanding means what it literally says: there is more space everywhere at once. there was less a moment ago and now there's more. the longer the distance, the more space gets added in between, thus the effect is extreme on universe scale and undetectable on planetary scales.

Well yes, but did you read my post in detail? I asked what the difference was between "space expanding" vs "objects moving away from each other".

And secondly, what is the mechanism for "space expanding"? What is "space" in this context? What actually is expanding?

Spacetime is 4D array of points. It's like a movie file, but with 3D frames instead of 2D frames (pictures). Expansion of space means that coordinates of points in a frame changed to move away from us, to match movie (model) with reality.

An array of 4D points implies some sort of construct upon which the points exist. As far as I know, the that was the concept of "the ether" and that fell out of fashion long ago.

So again, how can you tell the difference between space expanding or two things actually just moving away from each other (to keep it to a very simple example of two bodies).

Ether now known as "physical vacuum". As I told you already, expansion of spacetime is the mathematical tool, like a shader in OpenGL.

Let me try to explain- galaxies moving would be like balls floating in a pool. The galaxies are moving across the water.

Space moving would be like balls placed on a bed sheet and the sheet expanding. The galaxies aren't moving- the sheet is.

What's actually happening is more like galaxies moving around on a sheet that's being pulled further at all sides.

How can you tell the difference?

things are moving away from other things the faster the farther away they are, uniformly across the universe. everything is moving away from everything, not just two particular points.

is there a difference if there isn't a difference?

Imagine a deflated baloon with two dots drawn close to each other. Now inflate it; the dots didn't move but the plane that they were drawn/positioned on did.

I understand that concept. I'm asking, how can you materially tell the difference?

Nobody can answer because nobody knows

I think it has to do with the observation that no matter in which directinn we look, galaxies are moving away. Its not like we look towards one direction and say, "aha, that must be the center of the universe". Its the same story in ALL directions - we wee galaxies moving away from us at increasing velocity.

Here is one difference: The "velocities" suggested by an expansion of space can be well above the speed of light whereas classical velocities cannot.

https://bigthink.com/starts-with-a-bang/universe-speed-of-li...

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