APOD: Time Spiral (2024 Jul 01)

[Chris Peterson]

"Dark energy overtakes gravity."

Is this because gravity was universally weak at this stage due to less matter? I am not sure what exactly this statement really implies. What would be the difference between the Universe with no dark energy and the Universe after dark energy "overtook" gravity? I know that gravity is far, far, far weaker than the strong force, yet virtually nothing - perhaps absolutely nothing - overtakes gravity at a certain point. The term "overtakes" in the quote has me.

This is the point where the expansion rate of the Universe transitioned from slowing to speeding up. Where the "force" of dark energy became greater than the "force" of gravity.

So this question just occurred to me: how much weaker (or stronger) is dark energy compared to gravity? Say, given two protons separated by a meter, how does their mutual attraction due to gravity compare to the dark energy expansion force due to the spatial separation between them?

"Stronger" and "weaker" is probably not the best way to think of it. The force of gravity falls of with distance, while the force of dark energy, while very small, is uniform across all of space. So it dominates when space is nearly empty of mass, which only occurs over cosmological distances.

[johnnydeep]

This is the point where the expansion rate of the Universe transitioned from slowing to speeding up. Where the "force" of dark energy became greater than the "force" of gravity.

So this question just occurred to me: how much weaker (or stronger) is dark energy compared to gravity? Say, given two protons separated by a meter, how does their mutual attraction due to gravity compare to the dark energy expansion force due to the spatial separation between them?

"Stronger" and "weaker" is probably not the best way to think of it. The force of gravity falls of with distance, while the force of dark energy, while very small, is uniform across all of space. So it dominates when space is nearly empty of mass, which only occurs over cosmological distances.

Sure, but it must be possible to compare the two opposing forces operating between my two protons. But I suppose in retrospect, that in isolated space, it would take much more than a 1 meter spatial separation for dark energy to overcome the mutual attraction between them. I'm still interested in the quantities though.

[Chris Peterson]

So this question just occurred to me: how much weaker (or stronger) is dark energy compared to gravity? Say, given two protons separated by a meter, how does their mutual attraction due to gravity compare to the dark energy expansion force due to the spatial separation between them?

"Stronger" and "weaker" is probably not the best way to think of it. The force of gravity falls of with distance, while the force of dark energy, while very small, is uniform across all of space. So it dominates when space is nearly empty of mass, which only occurs over cosmological distances.

Sure, but it must be possible to compare the two opposing forces operating between my two protons. But I suppose in retrospect, that in isolated space, it would take much more than a 1 meter spatial separation for dark energy to overcome the mutual attraction between them. I'm still interested in the quantities though.

Well, to further complicate things, the "force" of gravity is the product of a distortion of spacetime created by mass. That makes two masses want to "fall" towards each other. But dark energy isn't a distortion, because it is uniform. It doesn't care about mass (so it isn't like anti-gravity). It is just a steady expansive pull on all of spacetime. But spacetime can't expand against strong gravitational attraction (which is why galaxies and galaxy clusters are not getting larger or separating). Spacetime is held together locally by gravity. The gravitational attraction of a couple of protons (lets make them neutrons, okay... don't need to add EM forces to this problem) is extremely small, so if you could somehow get those particles isolated enough (or just take it as a thought experiment where they are the only things in the Universe), there will be some distance where dark energy dominates gravity. I wouldn't be surprised if it was a very small distance. But I don't know.

[johnnydeep]

"Stronger" and "weaker" is probably not the best way to think of it. The force of gravity falls of with distance, while the force of dark energy, while very small, is uniform across all of space. So it dominates when space is nearly empty of mass, which only occurs over cosmological distances.

Sure, but it must be possible to compare the two opposing forces operating between my two protons. But I suppose in retrospect, that in isolated space, it would take much more than a 1 meter spatial separation for dark energy to overcome the mutual attraction between them. I'm still interested in the quantities though.

Well, to further complicate things, the "force" of gravity is the product of a distortion of spacetime created by mass. That makes two masses want to "fall" towards each other. But dark energy isn't a distortion, because it is uniform. It doesn't care about mass (so it isn't like anti-gravity). It is just a steady expansive pull on all of spacetime. But spacetime can't expand against strong gravitational attraction (which is why galaxies and galaxy clusters are not getting larger or separating). Spacetime is held together locally by gravity. The gravitational attraction of a couple of protons (lets make them neutrons, okay... don't need to add EM forces to this problem) is extremely small, so if you could somehow get those particles isolated enough (or just take it as a thought experiment where they are the only things in the Universe), there will be some distance where dark energy dominates gravity. I wouldn't be surprised if it was a very small distance. But I don't know.

(Yeah, neutrons, except isolated neutrons are unstable. But I guess that doesn't matter for this thought experiment.)

I'm surprised you'd think that (in blue). But surely, this should be calculable based on the Hubble constant?

And one more thought: if dark energy content depends on the "amount" of space, what about when space is compressed in a black hole - does that concentrate the dark energy and result in a greater repulsive force?

[Chris Peterson]

Sure, but it must be possible to compare the two opposing forces operating between my two protons. But I suppose in retrospect, that in isolated space, it would take much more than a 1 meter spatial separation for dark energy to overcome the mutual attraction between them. I'm still interested in the quantities though.

Well, to further complicate things, the "force" of gravity is the product of a distortion of spacetime created by mass. That makes two masses want to "fall" towards each other. But dark energy isn't a distortion, because it is uniform. It doesn't care about mass (so it isn't like anti-gravity). It is just a steady expansive pull on all of spacetime. But spacetime can't expand against strong gravitational attraction (which is why galaxies and galaxy clusters are not getting larger or separating). Spacetime is held together locally by gravity. The gravitational attraction of a couple of protons (lets make them neutrons, okay... don't need to add EM forces to this problem) is extremely small, so if you could somehow get those particles isolated enough (or just take it as a thought experiment where they are the only things in the Universe), there will be some distance where dark energy dominates gravity. I wouldn't be surprised if it was a very small distance. But I don't know.

(Yeah, neutrons, except isolated neutrons are unstable. But I guess that doesn't matter for this thought experiment.)

I'm surprised you'd think that (in blue). But surely, this should be calculable based on the Hubble constant?

And one more thought: if dark energy content depends on the "amount" of space, what about when space is compressed in a black hole - does that concentrate the dark energy and result in a greater repulsive force?

I don't think it involves the Hubble constant. It involves some kind of calculation of energy density. And we don't have any physics that describes what happens "inside" a black hole. So who knows?

[Avalon]

So, so many questions. What IS outside of time, like before the BB? How is it that humans existed for millions of years and the formation of cities only during the last 10,000 years? Were they without any intelligence or social skills before that? How convinced are we about the Theia event? How are events taking billions of years even calculated time-wise? Etc., etc.

[Chris Peterson]

So, so many questions. What IS outside of time, like before the BB? How is it that humans existed for millions of years and the formation of cities only during the last 10,000 years? Were they without any intelligence or social skills before that? How convinced are we about the Theia event? How are events taking billions of years even calculated time-wise? Etc., etc.

"Before" need not have any meaning in the context of the Universe. Modern humans have existed for two or three hundred thousand years. Cities made sense with large populations and agriculture. Those didn't exist for most of human existence. It's not about intelligence, but about need. Support for the Theia event is pretty solid.

There are many questions. But we are fortunate to live in this brief moment of human existence when we actually have so many answers!

[zendae]

Congrats johnnydeep on your 3001 posts. That's quite an odyssey!

[Fred the Cat]

When considering the vast amount of information we have at our fingertips, we should all follow the Ten Commandments.

Lest we spiral down the rabbit hole.

[Avalon]

What kind of explanation is "before" not having any meaning in the context of the universe? The entire illustration is a timeline of the universe.

[Chris Peterson]

What kind of explanation is "before" not having any meaning in the context of the universe? The entire illustration is a timeline of the universe.

That doesn't mean there's a "before". That word is pretty hard to define when time doesn't exist.

[Christian G.]

What kind of explanation is "before" not having any meaning in the context of the universe? The entire illustration is a timeline of the universe.

That doesn't mean there's a "before". That word is pretty hard to define when time doesn't exist.

I'm reminded of the "What did God do before creating the Universe? He was preparing Hell for those who would ask!"
We can’t ask what was going on during the time when time did not exist - obviously. My question is: How do you ask the question? What is the correct choice of words?
Before, there was no before, fair enough. But that doesn’t mean it was nothingness.... Did the Big Bang result from a quantic fluctuation, was it a violent transition between two different states of the universe or whatever other speculation misguided or not, whatever the unknowable answer, - how do you merely formulate the question of the "upstream"of the Big Bang?

[Chris Peterson]

What kind of explanation is "before" not having any meaning in the context of the universe? The entire illustration is a timeline of the universe.

That doesn't mean there's a "before". That word is pretty hard to define when time doesn't exist.

I'm reminded of the "What did God do before creating the Universe? He was preparing Hell for those who would ask!"
We can’t ask what was going on during the time when time did not exist - obviously. My question is: How do you ask the question? What is the correct choice of words?
Before, there was no before, fair enough. But that doesn’t mean it was nothingness.... Did the Big Bang result from a quantic fluctuation, was it a violent transition between two different states of the universe or whatever other speculation misguided or not, whatever the unknowable answer, - how do you merely formulate the question of the "upstream"of the Big Bang?

"Nothingness" is just as difficult to define when "something" doesn't exist. There's no requirement that the Big Bang emerged from anything. There really is no correct choice of words. Words will likely always inform an analogy at best. If these questions are answerable, they will be in the form of mathematical expressions.