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Comment by Twisol | Hacker News Reader

> Rather it's that the rate of expansion of the universe is accelerating, so that we're moving away from parts of it faster than its light can cover the distance to us.

From my understanding, it's not that we're moving away from parts of the universe, but that the distance between us is growing so fast that light sent from one part will find that after traveling toward us for some amount of time, the remaining distance to travel is actually more than than when it started.

One way for the distance between two objects to increase is indeed for those objects to literally be moving in opposite directions through space. But the expansion of the universe itself causes the distance between two otherwise-stationary points to nonetheless increase. Put differently, it's the cosmic yardstick that's shrinking, not the entities that must necessarily be moving.

(This is also why two points can be "moving apart" faster than light speed, the cosmic speed limit.)

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That rate is "only" approximately 68 kilometers per second per megaparsec.

68 kps isn't that fast (it's about the same speed as the Helios 2 solar probe) and a mega parsec is big distance (3.2M light years).

Its that there's a lot of megaparsecs between here and there and the sum of all of those 68 kps is more than the speed of light.

The relevant Kurzgesagt : TRUE Limits Of Humanity – The Final Border We Will Never Cross - https://youtu.be/uzkD5SeuwzM

Yes, true -- the distance between two points doesn't grow linearly, but rather proportionally to itself :D

So such an expansion phenomenon should affect everything uniformly right? Are we (people /animals) then getting "bigger"? Over time innthe extreme will it cause problems in signal transmission in our own nervous systems (since apparently we have a problem seeing light beyond the observable limit)

You don’t get larger - just like you don’t dissolve in water and wind doesn’t spread parts of you all over the land. There are interactions of matter keeping you together.

With the current rate of expansion, no. We're still bound together more tightly than the rate. This extends all the way out to a fair distance (galactic distance).

If the rate does get to the point where it is noticeable the "galaxies can't hold together" you get into the Big Rip end of the universe situation.

Gravitational attraction still dominates on even fairly large scales, and chemical bonds absolutely dominate over anything expansion could possibly do -- it would contribute an incredibly light force opposing any bonds. So no, we (and our galaxy) are in no danger of being inflated from the inside by space itself.

Thanks for the explanation!

The strong nuclear force, electromagnetic force, and even gravity are strong enough to overwhelm the expansion within individual galaxies and even local groups of galaxies.

In other words the expansion of spacetime very very very slightly tries to shift the atoms of your body apart but we can't even detect it because ordinary forces like chemical (electromagnetic) bonds are exponentially stronger - enough to pull things back where they should be. Actually in the current epoch I'm not sure if the expansion is strong enough to shift an electron by 0.01% of its own width let alone move an atom.

Space is really really big so that tiny amount of expansion adds up over long distances.

thanks for the explanation!

I don't think that's true. Expansion is not very weak around matter, it just doesn't exist. It sounds a little counter intuitive to think that expansion "shutdowns" as we get closer to galaxy. But that's because expansion and gravity are a little more complicated that just a force.

We over simplify what expansion really is, which leads to this type of thinking.

Expansion and gravity are results of the same equations.

Einstein equations are difficult to use, so we usually split the results in two models, FLRW (empty space) and Schwarzschild metrics (around matter). Computing the equations leads, respectively, to expansion and gravity. And it's not like there's one and the other, with gravity fighting expansion. It's "one or the other".

Around matter (in Schwarzschild metric), solving Einstein questions, we see zero expansion drifting. If there is matter, there is gravity, and no expansion.

Quoting Wikipedia [1]:

> Once objects are formed and bound by gravity, they "drop out" of the expansion and do not subsequently expand under the influence of the cosmological metric, there being no force compelling them to do so

[1]: https://en.wikipedia.org/wiki/Expansion_of_the_universe#Effe...

That makes a lot of sense -- thanks for contributing that nuance!

> Einstein equations are difficult to use, so we usually split the results in two models, FLRW (empty space) and Schwarzschild metrics (around matter). Computing the equations leads, respectively, to expansion and gravity. And it's not like there's one and the other, with gravity fighting expansion. It's "one or the other".

Arguably splitting a complex model of reality in two for convenience and saying that it’s “one or the other” is also an over simplification.

By the way, the same wikipedia entry also says things like “gravity binds matter together strongly enough that metric expansion cannot be observed on a smaller scale at this time.”

> So such an expansion phenomenon should affect everything uniformly right?

Apparently not. As far as I can tell (IANAC) cosmic expansion affects the empty space between galaxies, but not concentrations of mass. Galaxies (and everything in them) are immune to cosmic expansion.

I understood that cosmic expansion is a consequence of General Relativity; DE is supposed to explain accelerating expansion. Is that right? But wouldn't expansion result in there being more empty space and less nearby stuff; and therefore in accelerating expansion?

I wish I understood this stuff.

(I commented on the parent comment... and came back to this one...)

The question of "is the rate of acceleration accelerating"... PBS Space Time

Dark Energy Explained https://www.youtube.com/playlist?list=PLsPUh22kYmNAv1_8MA9-U...

> Want to understand what we know about Dark Energy: the hypothetical form of energy that exerts a negative, repulsive pressure on the universe that effects the energy on the largest scales? Then enjoy this Dark Energy playlist!

And the question is "is the rate of acceleration accelerating?" If so, then make sure you watch "Could the Universe End by Tearing Apart Every Atom?"

(a follow up...)

If that question is of interest to you, PBS Space Time - Could the Universe End by Tearing Apart Every Atom? https://youtu.be/gEyXTQ9do-c gets into the "what if" of dark energy and its influence on matter.

You'll note that it isn't until the very end of the instant before the Big Rip that that make it so that the expansion of the universe that it overcomes the strength of chemical bonds.

> but that the distance between us is growing so fast that light sent from one part will find that after traveling toward us for some amount of time, the remaining distance to travel is actually more than than when it started.

Good clarification, that's what I meant, I guess I didn't say it accurately. I don't actually think of us as moving, but more like the scale of the entire universe is increasing while the ability to traverse it - light speed - remains a constant.

You have it right. My cosmology teacher at UVA a couple decades ago used the analogy of points marked on the surface of a balloon that's being inflated. (maybe more intuitive than other shortcuts to understanding?)

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