Dark Energy's Effect on Black Holes

[DarK_OmEgA]

If dark energy causes spacetime or things in the galaxy to expand (acceleratingly), then won't black holes die due to expansion past the Schwarzschild radius, rather than simple radiation evaporation?

(When black hole die, I mean, we have the ability to look into the space because it is no longer a black hole). I know this is pretty elementary thought, but this is the internet, and the quickest way to find truth is to post something wrong on the internet!

Pat

 

[PeroK]

If dark energy causes spacetime or things in the galaxy to expand (acceleratingly), then won't black holes die due to expansion past the Schwarzschild radius, rather than simple radiation evaporation?

(When black hole die, I mean, we have the ability to look into the space because it is no longer a black hole). I know this is pretty elementary thought, but this is the internet, and the quickest way to find truth is to post something wrong on the internet!

Pat

There is a common misconception that universe expansion applies to the space within gravitationally bound systems such as solar systems and galaxies. The expansion of the universe does not affect the internal dynamics of these systems.

The universe expansion is modeled by the Friedmann equation, which assumes a uniform distribution of matter (with very low overall density) dominated by very large regions of vacuum - this is on the very large cosmic scale of the universe). You put this observed data into the equation and you get the large-scale behaviour of the universe: accelerated expansion.

But, the Friedmann equation and the overall cosmic data do not apply to a black hole, galaxy, solar system, planet or atom. So, none of these things is affected in the way that might be supposed from the concept of accelerated expansion.

If, far in the future, the overall universal expansion has increased many times, this will still have no effect on the dynamics of the solar system: which will continue as an isolated system, evolving due to its own internal distribution of mass in a small region of space.

 

[DarK_OmEgA]

Ahh I see, so I guess dark energy is vacuum energy, whose pressure is exerted, or best approximated, by the distance between objects, and so, if objects are already receding quickly from each other, they will recede faster and faster, because of the dark energy between them.

Ugh, and here I was thinking I saw a solution in the rough, an alternate end to the black hole's lifespan, rebirth. Thank you Perok

 

[PeroK]

Ahh I see, so I guess dark energy is vacuum energy, whose pressure is exerted, or best approximated, by the distance between objects, and so, if objects are already receding quickly from each other, they will recede faster and faster, because of the dark energy between them.

Without dark energy (vacuum energy), the expansion would have continued to slow gradually. With dark energy, however, as the universe expands dark energy becomes more and more dominant. That's the current model. It's not quite as simple as a pressure, but it has a similar effect.

If we take the solar system, however, it is far more dense than the universe on average, so matter dominates and dark energy will never be more than a negligible factor in its dynamics.

 

[DarK_OmEgA]

Yes I understand, but you say there is a difference from pressure. I am wondering then what is that difference?

Now I am imagining that all regions of space have a certain "darticle" dark energy particle like a sphere of vacuum, that is expanding. Everywhere, all of space is expanding. It looks like its accelerating because there are more expanding spheres between the two objects as time progresses because their distance is increased. Is that how to think of it? Sorry to take up your time

 

[PeroK]

Yes I understand, but you say there is a difference from pressure. I am wondering then what is that difference?

The only way to understand the universe expansion is through the mathematics of the Friedmann equation. Technically, the vacuum has a negative pressure: it takes energy to expand space.

You can't understand cosmology by inventing your own explanations that are not based on solid mathematics.

 

[DarK_OmEgA]

ahh but solid mathematics are based on ideas, and my friend, I lie with mathematics all the time, but ideas have a much surer foundation for me that is

 

[DarK_OmEgA]

I would say, that if the Friedman equation is what you would have me look at, then that would be wonderful, however, I have looked at it, and again it is hard for me to understand. My language is in my head unfortunately, so I require lots of back and forth clarification, even when the equation is right infront of me. But let me pull that up, and yeah I hear about negative pressure - it reminds me of gravity, I've heard that gravity has negative energy at the very least, because we can derive work from the construction of the energy field.

 

[Ibix]

It's worth noting that a spacetime containing a black hole and a non-zero cosmological constant has been studied - Schwarzschild-deSitter spacetime. It turns out to impose an upper limit on the mass of a black hole, but does not cause them to explode as far as I'm aware.

S-dS spacetime is empty apart from the black hole and the cosmological constant. I don't know if anyone has studied an FLRW model with a cosmological constant and a black hole. My guess would be that it doesn't cause disruption of black holes unless you posit the more extreme (hypothetical) forms of dark energy type phenomena that lead to the Big Rip. Even then I'm not sure.

Black holes are not expected to last forever anyway, as Hawking radiation will start to decrease their mass in a few trillion years.

 

[PeroK]

I would say, that if the Friedman equation is what you would have me look at, then that would be wonderful, however, I have looked at it, and again it is hard for me to understand.

There is a series of Insights here, which may be accessible. Ultimately, however, mathematics is the language of modern physics. There are serious limitations on how much you can really understand without it.

https://www.physicsforums.com/insights/journey-cosmos-friedmann-equation/

 

[DarK_OmEgA]

Well, I mean let's just take a very simple one, which is C^2 appearing in everything, like the Friedman equation, like Einstein's mass equation. Why do we square it? I get it its the speed of light, but like why? Is there a geometric reason? Because when I see squares I think of surfaces. Do you see what I'm getting at, even if I can do the calculations, it doesn't mean that I have formed a novel insight - and to me, that's a big problem. But thank you guys so much, but for real, is there any geometric reason for these squares of light?

 

[PeroK]

is there any geometric reason for these squares of light?

The quantity $c$ is fundamentally a conversion factor between the units of length and time. It appears in equations as $c = 3 \times 10^8 m/s$ when you measure length in metres and time in seconds.

From that point of view it is related to the concept of spacetime geometry. It appears as $c^2$ when you need to convert units of metres squared to seconds squared.

It also follows from the theory of Special Relativity that light (massless radiation) propagates through the vacuum at the speed $c$.

 

[Ibix]

Well, I mean let's just take a very simple one, which is C^2 appearing in everything, like the Friedman equation, like Einstein's mass equation. Why do we square it?

This has nothing to do with the cosmological constant. $c$ is the natural conversion factor between units of time and units of distance, so it pops up everywhere.

 

[lomidrevo]

I would say, that if the Friedman equation is what you would have me look at, then that would be wonderful, however, I have looked at it, and again it is hard for me to understand.

Indeed, it is a very long journey to get good understanding of this equation(s), and to see how it came to be.