Black holes and universe expansion

[Geo]

As we know the universe is expanding. Could this accelerating expansion contribute or cause black hole evaporation given that the strength of the gravitational force does not depend directly on time, while the distance of two given points in space increases with time?

Sorry if my approach is silly i don't have a formal training in physics i am an engineer who tries to educate himself through susskind's online lectures.

Thank you in advance!

 

[mfb]

Could this accelerating expansion contribute or cause black hole evaporation

No. Black holes don't care about the expansion of the universe at all.

while the distance of two given points in space increases with time?

That does not apply to gravitationally bound objects.

 

[bapowell]

As we know the universe is expanding. Could this accelerating expansion contribute or cause black hole evaporation given that the strength of the gravitational force does not depend directly on time, while the distance of two given points in space increases with time?

Sorry if my approach is silly i don't have a formal training in physics i am an engineer who tries to educate himself through susskind's online lectures.

Thank you in advance!

Gravitationally-bound objects like galaxies, stars, and black holes are not affected by expansion. Black hole evaporation is a hypothetical quantum mechanical process unrelated to expansion.

 

[nikkkom]

Gravitationally-bound objects like galaxies, stars, and black holes are not affected by expansion.

They are. If space expands uniformly (I'm not sure we have a solid data to be 100% sure about it on small scales, but let's assume), then it does expand even within gravitationally-bound objects.

How does look for e.g. a planet orbiting a star? As a very small additional force pushing it away from the star, linearly proportional to the distance to the star.

If its orbit is close to circular, then this additional force is almost constant. Which, for the purposes of calculating the orbit, is equivalent for its star being a liiiiitle bit less massive. Shape of the orbit does not change. In fact, the correction is so teensy, in almost all cases calculations of star's mass based on observed orbital periods of planets will simply disregard it.

Now, for a very elongated orbit, this repulsive force, since it's not proportional to 1/r^2, but proportional to r, would distort the orbit (cause it to precess, probably). But the effect would be still extremely small.

On a scale of a large galaxy like ours, for a star orbiting far from the galaxy center, the effect is no longer that small. However, orbiting the galaxy is a slow process - we made what, only ~25 orbits? At this few orbital trips, orbit's precession won't be significant yet - and probably dwarfed by other deviations caused by non-uniform matter density along orbital path.

 

[bapowell]

They are. If space expands uniformly (I'm not sure we have a solid data to be 100% sure about it on small scales, but let's assume), then it does expand even within gravitationally-bound objects.

How does look for e.g. a planet orbiting a star? As a very small additional force pushing it away from the star, linearly proportional to the distance to the star.

If its orbit is close to circular, then this additional force is almost constant. Which, for the purposes of calculating the orbit, is equivalent for its star being a liiiiitle bit less massive. Shape of the orbit does not change. In fact, the correction is so teensy, in almost all cases calculations of star's mass based on observed orbital periods of planets will simply disregard it.

Now, for a very elongated orbit, this repulsive force, since it's not proportional to 1/r^2, but proportional to r, would distort the orbit (cause it to precess, probably). But the effect would be still extremely small.

On a scale of a large galaxy like ours, for a star orbiting far from the galaxy center, the effect is no longer that small. However, orbiting the galaxy is a slow process - we made what, only ~25 orbits? At this few orbital trips, orbit's precession won't be significant yet - and probably dwarfed by other deviations caused by non-uniform matter density along orbital path.

This sounds entirely wrong to me. Why should the universe expand on local scales? Where's the requisite homogeneous energy density? Do you have a reference for the various claims in your post?

 

[PeterDonis]

If space expands uniformly

This has no meaning. "Expansion" does not mean "space expands". It means commoving observers--observers who see the universe as homogeneous and isotropic--are moving apart. Such observers cannot be gravitationally bound to each other; if they were, they would not be moving apart and at most one of them could see the universe as homogeneous and isotropic.

then it does expand even within gravitationally-bound objects

No, it doesn't. See above.

This is an excellent example of why physics needs to be done using math, not ordinary language; trying to do physics using ordinary language leads you to faulty logic, because the ordinary language words don't have the same meaning as the math they are supposed to refer to.

 

[nikkkom]

This has no meaning. "Expansion" does not mean "space expands". It means commoving observers--observers who see the universe as homogeneous and isotropic--are moving apart.

And that is exactly what I informally described as "space expands".

 

[PeterDonis]

that is exactly what I informally described as "space expands".

No, it isn't, because you concluded from your version of "space expands" that it does so within gravitationally bound objects. And that conclusion is wrong. Gravitationally bound objects are not moving apart.

 

[nikkkom]

No, it isn't, because you concluded from your version of "space expands" that it does so within gravitationally bound objects. And that conclusion is wrong. Gravitationally bound objects are not moving apart.

I did not say that gravitationally bound objects are moving apart. You are objecting to something I did not say.

 

[PeterDonis]

I did not say that gravitationally bound objects are moving apart.

Then I am unable to understand what you meant by this (in response to @bapowell saying that gravitationally bound objects are not affected by expansion):

They are. If space expands uniformly (I'm not sure we have a solid data to be 100% sure about it on small scales, but let's assume), then it does expand even within gravitationally-bound objects.

 

[Arman777]

If gravitationally bounded objects effected by expansion then, we should have seen this effect in milky way. The affect of expansion can be seen in large scales approximately (100Mpc) meanwhile even the radius of the milky way is (15kpc).

 

[jocarren]

That does not apply to gravitationally bound objects.

Can you recommend any resources to read about this? expansion keeps getting weirder the more I read about it.

 

[Martin0001]

As we know the universe is expanding. Could this accelerating expansion contribute or cause black hole evaporation given that the strength of the gravitational force does not depend directly on time, while the distance of two given points in space increases with time?

Sorry if my approach is silly i don't have a formal training in physics i am an engineer who tries to educate himself through susskind's online lectures.

Thank you in advance!

There is a possible scenario of space expansion by one of forms of dark energy, so called *phantom energy*, which would split/evaporate/rip apart BH in final moments before the end of time.
Scenario is called Big Rip and it is considered seriously as our Universe's possible future. It is estimated that at least 22 billions of years are left to Big Rip.

 

[phinds]

Scenario is called Big Rip and it is considered seriously as our Universe's possible future.

No, I don't believe it is. Do you have any reference for that statement?

 

[Martin0001]

No, I don't believe it is. Do you have any reference for that statement?

www.mdpi.com/2218-1997/3/2/36/pdf

 

[phinds]

www.mdpi.com/2218-1997/3/2/36/pdf

I did not read the article but nothing in the abstract supports your statement that it is "it is considered seriously as our Universe's possible future" only that it is a hypothetical scenario being examined.

 

[Martin0001]

I did not read the article but nothing in the abstract supports your statement that it is "it is considered seriously as our Universe's possible future" only that it is a hypothetical scenario being examined.

But any predictions like heat death, big crunch, big bounce, big rip etc are stil hypothetical.
We know far to little to be sure of any of these.
However there are more and more indications for Big Rip to be plausible.
For example below you have discussed research suggesting Big Rip as an outcome:

<unacceptable reference deleted>

 

[Arman777]

However there are more and more indications for Big Rip to be plausible.

Where exactly I find very "If"s in the text here are some;

"So which type is the real one? We don’t know, but if the researchers are correct about Bulk Viscosity (a big IF at this point), then the evidence could point towards Phantom Energy."

"If the researchers are correct and Phantom Energy holds away, then this shrinking universe will continue to shrink, but at ever faster rates until, at the very end, it goes into nutso overdrive."

"
The authors themselves admit at the end of their paper that:

“…a single fluid with viscosity could account for both dark matter and accelerated expansion, although the latter idea faces severe difficulties”

Yes, there has been quite a flurry of articles published recently about this on the popular-science news sites "

Also the paper doesn't seem to be much scientific but rather then a blog.

https://www.physicsforums.com/threads/dynamical-dark-energy.927401/#post-5854518

 

[Martin0001]

@Arman777,
All theories dealing with "fate of the Universe" problem are speculative and there is no conclusive evidence to prove any of of these to be correct.
Chances for Big Rip base on existing evidence are about as high as for Big Freeze and already higher than for Big Crunch.
At the moment we are at "choose your temple as you like" situation.
And remember that cosmologists are "rarely correct but never in doubt". Forgot who said that...

NB. It is more and more common situation that you cannot give a reference to proper scientific work because these are usually behind paywalls. You may ask government why it is the case, eg why public money from taxes are used to prop up private publishers. After all results of publicly funded research should stay in public domain and remain publicly available with no fee or with handling covering symolic fee but it is not the case. Hence more referrals to popular sources.
Like it or hate it but not much can be done about it.

 

[Arman777]

All theories dealing with "fate of the Universe" problem are speculative and there is no conclusive evidence to prove any of of these to be correct.

and

Chances for Big Rip base on existing evidence are about as high as for Big Freeze

I agree. Well It depends on the models and how they well define the obervational data.

 

[mfb]

All theories dealing with "fate of the Universe" problem are speculative and there is no conclusive evidence to prove any of of these to be correct.

True, but that doesn't make them equal.

You can also speculate about the spontaneous creation of unicorns everywhere in the universe in 3.9216821 billion years. That is speculative as well. No one does it because there is nothing that would point to such a scenario. In the absence of measurements to distinguish theories, theories without additional assumptions are usually preferred - for the future of the universe this means a slow heat death.

 

[Martin0001]

True, but that doesn't make them equal.

You can also speculate about the spontaneous creation of unicorns everywhere in the universe in 3.9216821 billion years. That is speculative as well. No one does it because there is nothing that would point to such a scenario. In the absence of measurements to distinguish theories, theories without additional assumptions are usually preferred - for the future of the universe this means a slow heat death.

We are not speculating about unicorns here, but who knows, even these can live somewhere as Universe is vast, possibly infinite.
And if life spontaneously emerged (as we are assuming), then why not Unicorns?
Finally, if our Universe is infinite and life (means also a unicorn type of life) can spontaneously emerge then we can imagine a Hubble radius somewhere, where 3.9216821 billions of years from now on unicorns will emerge everywhere. Observers there will be truly baffled - unicorns wherever they look.

However regarding *phantom energy* which would imply Big Rip there is already >2 sigma evidence of it based on experiment (observations of far away 1a type supernova):
http://www-personal.umich.edu/~huterer/Papers/Shafer_wle-1.pdf
2 sigma is of course far from proven but anyway an indication.

Mind you, Big Rip can be considered as some special case of heat death.
After all in this scenario entropy is reaching everywhere absolute possible maximum, so time will end.
Very much like in heat death at t=infinity.
Differece here is that in heat death situation is asymptotic and in Big Rip there is a specific end of time.

 

[mfb]

2 sigma is of course far from proven but anyway an indication.

Not if (a) you can choose where to look for deviations and (b) you have to add a parameter (or even multiple parameters). Under these conditions you would be surprised if you can't find 2 sigma anywhere.