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Comment by denton-scratch | Hacker News Reader

> it would eventually push all the stars and galaxies so far apart that even atoms could be torn asunder

If that's what expansion driven by dark energy does, then why hasn't it already happened? And where did the reporter get this factoid?

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From what I've read, that is entirely wrong. Expansion is very weak compared to gravitational forces, which is nothing when compared to the electromagnetism that binds atoms into molecules, which is nothing when compared to the strong force holding the nucleus together.

Everything from atoms to galaxies are safe in regards to expansion. Galaxy clusters (maybe) and up would be disrupted by expansion.

There are some theories that the expansion force can grow in unbounded ways.

But it's as speculative as any other explanation of Dark Energy.

In a big rip scenario, the scale factor goes to infinity in a finite time. The author is correct on this one. Whether we are in a big rip scenario or not is not entirely settled, even though it's more on the unlikely side and just became a tiny bit more likely, considering the new measurements.

Big rip scenarios are inconsistent with dark energy having a constant value of w=-1, which is what the author is discussing in that paragraph. The article is wrong.

In that case, true. I couldn't get past the paywall.

This isn't quite how I understand the expansion of the universe. The interesting thing is it affects space itself, so the more distance between objects, the more space there is to expand. This does try to pull stuff apart, but it's quite slow so it doesn't take much to overcome it. Even gravity (the weakest force) is enough to keep stars and galaxy clusters bound together. Atoms are way to small to be affected.

This changes if you allow Dark Energy to vary with time, but the paragraph in the article assumes it's constant.

> If the work of dark energy were constant over time, it would eventually push all the stars and galaxies so far apart that even atoms could be torn asunder...

Depends if the expansion is accelerating or not.

https://www.youtube.com/watch?v=gEyXTQ9do-c

Agreed, and I've edited my comment to include this.

That's not necessarily true. If Dark Energy is a property of the vacuum state of space and it's a force acting opposite of gravity, then the further apart those objects become then the more those objects will accelerate from one another. That's what leads to an accelerated expansion of the universe.

That doesn't change the fact that gravity can overcome Dark Energy. To wit, the Andromeda and Milky Way galaxies are on a collision course. Yes, Dark Energy is working to drive those galaxies apart, but gravity is winning and the closer those galaxies get to each other then the strong gravity is going to become.

Gravity gets stronger as the distance between objects decreases, Dark Energy gets stronger as the distance between objects increases. That is if Dark Energy is a property of spacetime. Since we don't know what Dark Energy is we can't say for certain, but it appears to be a property of spacetime and if that's the case then this statement is true.

I believe they're refering to the big rip: https://en.wikipedia.org/wiki/Big_Rip

That wikipedia article might be the worst physics article that I've read on WP.

Which edits did you add to improve it?

Have you ever tried contributing to a Wikipedia article? It's a nightmare due to the other contributors who'll revert all your changes and fight you on everything and it's not worth the effort.

Yes, I've been contributing for two decades as a drive-by editor. I'm occasionally reverted; I suck it up, I don't own these articles. I don't get into fights. I avoid editing articles about politics, especially about nationalist hot buttons. I also move gingerly around some history articles, which are often patrolled by fanatical revisionists.

I don't get nightmares.

How well do you know the subject? I will deal with it for you.

You can post a diff and I will find someone who will get it in.

None. I know nothing about the Big Rip. Judging from the (brief) talk page, the article doesn't get a lot of love from editors.

When the universe was dense, gravity dominated. As it expands, dark energy gets more and more important. From Ars Technica's article:

"If dark matter gives rise to the gravity that holds the universe together, then dark energy is the counter-force pushing the universe apart. Very early in the universe’s existence, dark matter dominated. Everything was closer together, so its density was higher than that of the dark energy, and its gravitational pull was stronger so the early galaxies could form. But as the universe continued to expand, the dark matter density, and hence the gravitational pull, decreased until it was less than that of the dark energy. So instead of the expected slowdown in the expansion rate, the now-dominant dark energy began pushing the universe apart at ever-faster rates."

Still hoping for micro-black holes as the solution as the early universe density must have been enough for some to form. Also, where's the anti-matter gone?

Both of those are pressureless, so they could be dark matter but not dark energy.

Except that primordial black holes are ruled out (as 100% of dark matter) by various observational bounds, and antimatter would be just as visible as normal matter and not "dark" at all.

Well, if we suppose all the anti-matter is in those black holes, we kill two birds with one stone. It’s ridiculous, I know, but your remark made me realize that an anti-matter black hole is essentially indistinguishable from a ‘normal’ one.

Could just be the anthropic principle at work : in a ridiculously large universe, we live in a tiny fraction of it where local fluctuations in the respective distributions of matter and antimatter have happened to favor matter. (And the observable universe is even smaller than that.)

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