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Comment by iszomer | Hacker News Reader

My dumb answer: the universe is an expanding balloon being inflated by The Big Bang. If you were a standing on one point on the surface of the balloon, objects relative to your observable distance would be moving away from you. This observation can be measured by a phenomenon known as the redshift.

I'm lost from here..

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I think more like you’re in the middle of a balloon being blown up and all points are accelerating away from you.

Except every point in the universe is like the middle of the balloon with space accelerating away from it in all directions.

I've read about the fruitcake metaphor. When you bake a fruitcake in the oven it expands and thus all the little pieces of fruit in it move away from each other.

But if that is the proper metaphor then my question is: Is there any difference between everything "moving away from everything else" (like pieces of fruit in expanding fruitcake) vs. "space expanding"?

Why do we say "space is expanding" rather than saying "all stars are moving away from each other"?

What is the difference between "space expanding" and "things moving away from each other"?

> What is the difference between "space expanding" and "things moving away from each other"?

Not an astrophysicist, but I think both are happening.

Space is the medium we live in. It’s the cake in the fruit cake. The fruit wants to stay in the same place, but the cake is expanding which is causing the fruit to appear to move away from each other even though the fruit wants to stay in one place. Same with the universe, space is expanding which is causing objects to become farther apart, even if they could somehow have 0 speed.

But in our universe the planets, stars, galaxies, etc. (the fruit) do have speed. Some are getting closer to us, like the Andromeda galaxy, while many others are moving away from us.

However, given the vast distances and how fast space is expanding (how fast the cake is rising), even those distant objects that want to move towards us are actually moving away from us.

Imagine a swimmer swimming against a tsunami. You can be swimming any direction you want but the power and speed of the tsunami will offset that and dictate where you end up.

Space is expanding at incredible speeds, so it’s quite the tsunami: https://www.forbes.com/sites/startswithabang/2020/06/12/ask-...

Let's assume that at Bing Bang all the matter was in a very small area. Then it exploded, in other words all the pieces of matter started moving away from the starting area, presumably at an accelerating speed. That would simply mean that everything started moving away from everything else and that movement seems to be even accelerating. Fine. That is a good and easy to understand description.

Why then do I have to add "BTW. space is expanding"? Things are moving away from each other I get it. Why do they need to add to that "Space itself is expanding"?

I don't doubt there is a reason since best scientist are thinking about it. I just haven't seen a good explanation for it, yet.

AIUI, “Started moving away from the starting area” is wrong.

The starting area is everywhere.

It isn’t that there is some point where the center of the Big Bang was.

The idea is that, stuff was basically evenly distributed everywhere , and very dense, and then later, without needing the amount of stuff to have decreased, stuff is nevertheless more spread out.

Now, you might just explain this with like, “well, space is infinite in extent, so you can just spread it all out, without running out of room, by moving things out towards infinity” but you can describe the same sort of situation even if space is compact, turning back around in itself (as in the analogy of “imagine space is the surface of a balloon”)

  > What is the difference between "space expanding" and "things moving away from each other"?

The difference is the distance, and that is measured by looking at light. Let us take as true the fact that light moves at a constant speed in a vacuum. We observe that light from very distant objects is more redshifted than light from closer objects, when looking at objects on the order of millions to hundreds of millions of lightyears away. The expanding universe theory explains this and other phenomenon as well.

There are other evidence as well, but the redshifted light I believe was the first hint.

FYI, redshift is a Doppler effect: https://earthsky.org/astronomy-essentials/what-is-a-redshift...

One difference is that space can expand at whatever velocity it wants to, but objects can't move faster than light.

I find it difficult to wrap my head around as well. One way you might think of it is not space expanding, but all objects shrinking. If everything shrank to half size, including wave-lengths and velocities, the size of the universe would double in size, and objects would appear to move apart from each other above the speed of light at large enough distances. However, you could never use this fact to travel above the speed of light.

Not sure if this comparison is valid though.

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