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Comment by teekert | Hacker News Reader

Thank you, went to the comments to learn this. So the universe actually expands pretty fast then! Faster than the speed of light? Strangely...

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This is a well-known phenomena.

The usual image is to consider ants traveling on an expanding balloon. The speed of light is the limit of how fast the ants can walk. The expansion of the universe is the expansion of the balloon. And now you see how two distant ants can be carried away from each other faster than they can walk towards each other.

If you understand manifolds and general relativity, this analogy is surprisingly exact.

I've heard of raisins on a cupcake instead (I hate raisins on cupcakes btw), I assume since it's a 3D example. It's meant to show how it's possible for everything to be getting farther away from each other without anything being at the center.

Cupcakes have a center.

That wasn't the point. The cupcake represents space. The things are the raisins which are generally not in the center despite moving away from each other.

This is physics, so it is best to assume an infinite cupcake.

I haven't heard the cupcake version. It does have something to recommend it, though raisins don't crawl. So that may make it harder to visualize the difference between how fast you're expanding versus how fast you're moving versus local space. Which is the whole point.

Good point, I guess they're good for illustrating different things :)

What I find confusing is that the space inside the ants/raisins should expand as well. Why do we not experience this expansion locally?

Space is expanding everywhere, but the matter that resides within it is bonded to other matter, or gravitationaly attracted together, so it holds.

Back to the balloon analogy, the surface of the balloon is expanding, even under the ant's feet. But the ant doesn't grow.

And since the even the whole groups of galaxies are gravitationally bound, it seems that the galaxies, including our own galaxy, are not "stretching" but remain the same size. And, based on the same principle, everything inside of them also, including us.

So back to the cupcake analogy, the raisins are the whole galaxy groups.

Also:

https://medium.com/starts-with-a-bang/this-is-why-we-arent-e...

"As physicist Richard Price once put it, “Your waistline may be spreading, but you can’t blame it on the expansion of the universe.”"

https://www.newscientist.com/article/dn8082-cosmic-expansion...

https://arxiv.org/abs/gr-qc/0508052

Here "bonded to other matter" means "electrostatically attracted together".

It's not strong enough to overcome the forces keeping us together.

But that might not be the case for forever: https://en.wikipedia.org/wiki/Big_Rip

Yup! There is light moving towards us from the other side of the universe, but we'll never see it because the space between that light is stretching faster than thr light can travel (or something like that). Check out this page on the Observable Universe... https://en.wikipedia.org/wiki/Observable_universe.

I think it expands uniformly, so that, after time t, a distance of x becomes a distance of x * k, with k being very slightly greater than 1. Then there will always exist large enough x to outstrip the speed of light.

To illustrate, imagine the expansion factor is, say, 10^-20 per second. Then a distance of 10^30 meters will increase to a distance of 10^30 + 10^10 meters in one second, while light travels at 3 * 10^8 meters per second, so expansion would outstrip lightspeed at that distance. If the expansion factor were 10^-40 per second, then a distance of 10^50 meters would be increasing faster than lightspeed.

Which makes it then effectivelly infinite?

IANAP, but I think so. I vaguely recall that early inflationary expansion was very fast. Like several times the speed of light.

No, not really. One theory is that eventually things will slow and then collapse again. https://en.wikipedia.org/wiki/Big_Crunch

Taken to the extreme, this expansion and collapse has been happening over and over again in a cyclical fashion. https://en.wikipedia.org/wiki/Cyclic_model

The shape of the universe currently prevents this. For what we can tell the universe is flat, which means at no point will it collapse on itself, unless something major changes. That said, major changes have happened, around 8 billion years ago expansion appears to have sped up.

I'm no expert (so correct me if I'm wrong), but from what I'm read, flat, concave, and convex universes are all possibilities and they would all look flat to us because our measurements take place on such a small scale.

Our measurements occur across billions of light-years. By comparing redshift agaist the standard candles we measure relative speeds of vastly distant objects. And given the speed of light, we are also measuring what those speeds were billions of years ago. It is a very big ruler.

We've measured the flatness of the universe on the scale of the cosmic microwave background, which is essentially as big a ruler as could possibly exist.

https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.htm...

https://wmap.gsfc.nasa.gov/universe/uni_shape.html

Those darn flatspacers

Space itself is expanding. At small scales, it isn't really that "fast" (gravity can/does overcome the effects within our galaxy and the Local Group). However, at large scales, it can exceed the speed of light (google "Hubble volume") so at some point, light emitted from sources outside this range will not longer reach us because the space between here and there expanded faster than light travels.

Belated edit: More like ~10^81 times the speed of light.

> According to the theory of inflation, the Universe grew by a factor of 10 to the sixtieth power in less than 10 to the negative thirty seconds, so the "edges" of the Universe were expanding away from each other faster than the speed of light [0]

It seems that the universe was on the order of ~1 meter in diameter then.[1] So the "'edges' of the Universe were expanding away from each other" at ~10^90 meters per second.

0) http://curious.astro.cornell.edu/physics/109-the-universe/co...

1) https://www.forbes.com/sites/startswithabang/2017/03/24/how-...

Edit: Upon reflection, I get that "~1 meter in diameter" makes no sense. Maybe just say that the "edges" were ~1 meter apart.

So the universe is expanding at greater than 10^90 m/s now, since the expansion is accelerating?

No, "inflation" refers to a specific episode in the very early history of the universe. Or rather, in some models of this.

Much slower expansion continued and continues today, and if you look really carefully is slightly accelerating not decelerating right now.

m/s, or comparing to the speed of light, isn't really a great way to measure this. Points far away from us are moving away, and faster the further you go; we know of no limits to this.

> Belated edit: More like ~10^81 times the speed of light.

I have no grounding in physics on which to place this hunch but... doesn't this sound like a wee fudge factor to make the Big Bang theory fit evidence?

I am reminded of the concept of "aether" and "land bridge theory" where people had a workable hypothesis except for the niggly little problem that the hypothesis was contraindicated by some of the evidence.

I wonder about that too.

I think it means that it's expanding and traveling in the opposite direction of us. If we start at the same spot, then both travel in opposite directions at 1 mile/hour, at the end of an hour we will be 2 miles away from each other.

There is not really a center, galaxies are more like raisins in a rising bread. They all move away from each other with the same relative speeds. At least... Any physicists here to tell me whether or not space-time density get less closer to the edge? I mean... Is there an Edge?

There is no edge. It defies the normal way of thinking but every point in the universe is also the center of the universe. The Big Bang happened at every point and space-time is expanding at every point.

There could be an edge, but it's definitely not within the distance we can see out. The main mind-bender is that there's no need for an edge.

Well, not really. The distance we can see out is measured in time and we can very nearly see the Big Bang. We can observe the Universe's Recombination event and it is in every direction we look as the Cosmic Microwave Background. This happened everywhere in existence.

If there is an edge to the Universe it is in the fourth dimension when the Universe was a singularity.

Space still expands lower than light speed, the moment expansions goes faster night skies will be completely dark because other objects' light won't be able to reach our field of vision. I do not remember how that phenomenon is called.

Not really. The far edge of our perception can be at lightspeed, but then everything closer would remain slower. So night wont suddenly become dark. We will first loose distant galaxies, but 99.9999 of starlight comes from our own galaxy. By the time that is moving away at lightspeed, the view will be the least of our concerns.

Or the night skies light up completely because there was another big bang somewhere else and the light from that universe cluster just reached us

our observable universe may be around an unobservable area whose light hasn't reached us yet

so a parallel universe somehow interferes with ours? I don't think even most enthusiastic multiverse theorists will be able to grasp that.

not really parallel like alternate dimensions, maybe adjacent to not get distracted by colliqualisms and scifi tropes?

just another area in the vast void that also had a big bang which is outside of our observable area right now, but whose light may reach us one day.

You might be referring to the Big Rip?

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