How can black holes coexist with the expanding universe?

[Grajek]

It is said that black holes exist and are therorized to be very very abundant throughout the universe. These black holes are said to be sucking everything up around it, including light. It is also said that the universe is ever expanding, and that expansion is accelerating. How can the universe expand while at the same time billions and billions of black holes are sucking everything up around them? At the very least should there not be a De-acceleration of the universes' expansion?

 

[Jimmy]

It's a common misconception that black holes "suck up" everything. Objects can orbit a black hole just like they orbit a star or planet. A 5 solar-mass black hole will no more devour everything than a 5 solar-mass star.

http://hubblesite.org/reference_desk/faq/answer.php.id=61&cat=exotic

 

[maxverywell]

Check this out:

Researcher shows that black holes do not exist
http://phys.org/news/2014-09-black-holes.html
http://www.sciencedirect.com/science/article/pii/S0370269314006686

 

[maxverywell]

Anyway, black holes cannot be formed by the gravitational collapse of a massive star and this is well known.
For an outside observer the in-falling matter never crosses the event horizon (it takes "infinite time").
The only possible black holes are the hypothetical primordial black holes that existed from the very beginning of the universe.

 

[ViperSRT3g]

Google the term "Planck Star." You may find it very interesting when describing black holes in a new fashion.

 

[PAllen]

Anyway, black holes cannot be formed by the gravitational collapse of a massive star and this is well known.
For an outside observer the in-falling matter never crosses the event horizon (it takes "infinite time").
The only possible black holes are the hypothetical primordial black holes that existed from the very beginning of the universe.

This is nonsense. Classically, this false argument has been answered hundreds of times, and is now well accepted as a 'crank' point of view.

Your prior post links to serious new research which addresses the question semi-classically rather than classically. The question of event horizon formation considering quantum corrections without an accepted theory of quantum gravity is an on-going debate. It is worth watching how these papers stand the test of validation by others. However, it is likely that no consensus will be reached without consensus on a quantum gravity theory. [Edit: of particular interest, the authors agree with judgment I give above - that classically BH must form, and that this is well known and accepted.]

 

[maxverywell]

This is nonsense. Classically, this false argument has been answered hundreds of times, and is now well accepted as a 'crank' point of view.

Please explain why it's nonsense...
From the point of view of an external observer, the collapsing/infalling matter never crosses the event horizon because of the infinite time dilation at the horizon. So for the outside observers the star never collapses completely and BH isn't formed.
What is more interesting is that the outside observer will detect Hawking radiation from the BH and the BH will eventually evaporate, before it was created!

 

[D H]

Please explain why it's nonsense.

Much of what you wrote in your opening post is nonsense.

It is said that black holes exist and are therorized to be very very abundant throughout the universe.

That part is correct.

These black holes are said to be sucking everything up around it, including light.

This part is at best misleading. A planet orbiting a solar-sized black hole at Mercury's orbital distance and Mercury's orbital velocity would have almost exactly the same orbit about that black hole as Mercury's orbit about the Sun. There's (almost) nothing magical about the gravitational influence of a black hole. Some weird things do happen to an object that happens to venture very, very close to a black hole as a consequence of tidal gravity, but note that Mercury's orbit does not qualify as "close"; it's not even close to "close".

It is also said that the universe is ever expanding, and that expansion is accelerating. How can the universe expand while at the same time billions and billions of black holes are sucking everything up around them?

This is at best a misunderstanding. The expansion of the universe has nothing to do with black holes, or solar systems, or galaxies, or even groups of galaxies. At least not yet, and not for a long time to come. For example, our solar system formed about 4.6 billion years ago, and the Earth has been orbiting the Sun at about 1 AU from the very beginning. Our solar system isn't being torn apart by the expansion of the universe. The Milky Way galaxy was one of the first galaxies to form, and it too isn't being torn apart by the expansion of the universe. It's only at very, very long distances where the expansion of the universe becomes apparent. Locally, gravitation is a much stronger effect that easily overcomes the expansion.The first article you subsequently posted is a pop-sci article with a very misleading title that badly attempts to explain a serious scientific article, the second article you posted. That second article is a serious scientific journal article that tries to resolve the long-standing conflict between general relativity and quantum mechanics. That scientific article does not say that black holes can't exist. It says that the singularity at the center of a black hole can't exist. There's a big difference between those two statements. The scientific article suggests that the collapse predicted by general relativity stops before the collapsing material has a chance to cross the event horizon. The black hole still exists, just not in the form predicted by general relativity.

In addition to misstating the claims of that serious scientific article, the pop-sci article at phys.org does yet another disservice to science: It portrays that scientific article as fact. Having an article published in a scientific journal represents where science starts, not ends. The journal article was just published. We need to give other physicists time to rebut or strengthen the claims made in that article before one can say they are correct.

 

[PAllen]

Please explain why it's nonsense...
From the point of view of an external observer, the collapsing/infalling matter never crosses the event horizon because of the infinite time dilation at the horizon. So for the outside observers the star never collapses completely and BH isn't formed.
What is more interesting is that the outside observer will detect Hawking radiation from the BH and the BH will eventually evaporate, before it was created!

First, Hawking radiation is purely a quantum effect, so leave that out altogether, classically. With Hawking radiation (and other quantum issues, e.g. information paradox, unitarity, etc.) the question of horizons and their formation has been hotly debated for many years.

Classically, there is no Hawking radiation, and the statement 'for the outside observer it never forms' is a physically meaningless statement because it is true only for certain coordinate choices or simultaneity conventions. For different simultaneity conventions, one can give a precise statement to 'when' a horizon formed. Two physically meaningful statements about classical collapse are:

1) Someone on the surface of the collapsing body crosses the horizon in finite time [per their watch, for example], continues to receive information from the outside for finite time after horizon formation and reaches singularity in finite time. Further, if such an observer is watching the outside, there is last time they would see on some distant clock. Finally, note that if you require matter to locally behave consistently with classical SR requirements (dominant energy condition) then collapse with horizon formation + singularity are required under a broad range of conditions.

2) Someone outside would see the collapsing body red shift to relatively quickly to a state blacker than CMB. Further, the result would quickly be distinguishable even from a black body of matter 1 angstrom larger than the horizon. That is, a star, that for an observer riding the star, remains just short of ultimate collapse, remains distinguishable, externally, from a star that catastrophically collapsed (per an infalling observer).

As I noted, the authors of these new papers not only don't dispute the above, they discuss precisely where their quantum analysis predicts the behavior will diverge significantly from the classical picture.

 

[maxverywell]

Much of what you wrote in your opening post is nonsense.

Sorry but your answer is useless.

the statement 'for the outside observer it never forms' is a physically meaningless statement because it is true only for certain coordinate choices or simultaneity conventions. For different simultaneity conventions, one can give a precise statement to 'when' a horizon formed.

Of course the observers we are talking about are humans on the planet Earth, so there is only one possible choice of coordinate system in describing a BH or its formation. No need to complicate things. Consider that the humans on Earth are observing by a telescope a massive star, far far away from us, that has run out of nuclear fuel... Will we ever see it becoming a black hole?

2) Someone outside would see the collapsing body red shift to relatively quickly to a state blacker than CMB. Further, the result would quickly be distinguishable even from a black body of matter 1 angstrom larger than the horizon. That is, a star, that for an observer riding the star, remains just short of ultimate collapse, remains distinguishable, externally, from a star that catastrophically collapsed (per an infalling observer).

I don't understand completely what are you trying to say here. Yes there will be red shifting and gradually we will cease to see the infalling matter. Although we won't be seeing it later, theoretically the infalling matter is there (outside) and it never passes the event horizon.

 

[PAllen]

Of course the observers we are talking about are humans on the planet Earth, so there is only one possible choice of coordinate system in describing a BH or its formation. No need to complicate things. Consider that the humans on Earth are observing by a telescope a massive star, far far away from us, that has run out of nuclear fuel... Will we ever see it becoming a black hole?

What??! A fundamental principle of GR is that anything that depends on choice of coordinates is physically meaningless. Thus, people on Earth can use any coordinates we want. It is very easy to choose coordinates with Earth as the origin such that a horizon forms at a specific time. I must emphasize, that someone who says "so there is only one possible choice of coordinate system" has exactly zero understanding of GR.

As for seeing, please try to read carefully what I wrote before. That you don't see matter reaching a horizon doesn't mean it doesn't exist. What you can ask is: Can one distinguish from a distance, per classical GR, a body that has stopped collapsing arbitrarily short of the horizon (per a surface observer) from a body that catastrophically collapsed, per a surface observer. The answer is YES. For the body that collapsed catastrophically, there is a precise last signal that will never be returned, no matter how long you wait. For the body that stopped just short of the horizon, there is no such final signal. Every signal, even one a billion years after beginning of collapse, can be ultimately reflected back, if the collapse is non-catastrophic.

 

[maxverywell]

Of course there is infinite number of coordinate systems that you can define, but what I meant is a "natural" coordinate system defined by the observer itself, i.e. with coordinate t as proper time, x as distance of the star from the Earth (or from the observer) etc. I thought it was clear by my simple question which It seems that you cannot answer:

Consider that the humans on the Earth are observing by a telescope a massive star, far far away from us, that has run out of its nuclear fuel... Will we ever see it (in finite time) become a black hole (actually, disappear after a full collapse)?

 

[PAllen]

Of course there is infinite number of coordinate systems that you can define, but what I meant is a "natural" coordinate system defined by the observer itself, i.e. with coordinate t as proper time, x as distance of the star from the Earth (or from the observer) etc. I thought it was clear by my simple question which It seems that you cannot answer:

Consider that the humans on the Earth are observing by a telescope a massive star, far far away from us, that has run out of its nuclear fuel... Will we ever see it becoming a black hole?

I can very easily construct a coordinates system as you describe, which gives a precise time for when the event horizon forms. Suppose I am watching a star one light year away, collapse. My time coordinate is my proper time on earth. However, I assign time coordinates to events near the star as follows: If I send a signal at time t0 that is received on the surface of the collapsing star, I declare that reception has a time coordinate of t0+1 year. I don't worry about getting a signal back. Then, by this convention, there is a precise time coordinate that corresponds to reception by a surface observer when they cross the horizon, and another (short time later) time coordinate that corresponds to the surface observer approaching the singularity. By this same coordinate system, if the collapse halts per a surface observer, there is, instead, a specific time I assign to when the collapse stopped.

I have answered your other question in multiple ways which you obstinately reject. YES, you can see something form that is, in principle, distinguishable from any collapsed object (per the predictions of classical GR).

 

[maxverywell]

I have answered your other question in multiple ways which you obstinately reject. YES, you can see something form that is, in principle, distinguishable from any collapsed object (per the predictions of classical GR).

Ok, so we agree... Something distinguishable from any collapsed object (black hole) is certainly not a black hole.
Black hole is black i.e. you cannot see it, so the object simply disappears after its collapse.

 

[PAllen]

Ok, so we agree... Something distinguishable from any collapsed object (black hole) is certainly not a black hole.
Black hole is black i.e. you cannot see it, so the object simply disappears after its collapse.

No, we disagree, because you are using BH differently than virtually all physicists use it. At least per classical GR, it is a BH, and I can (as I demonstrated) assign a time coordinate (infinite number of ways) to when the event horizon formed. That is, if the theoretical model is that it is a BH, and it is distinguishable from anything that is not a BH, we say it is a BH. Further, a BH is visually as black as possible, if it is isolated - much blacker than the CMB (classically).

 

[phinds]

Sorry but your answer is useless.

The truth is not useless. Disagreeable, perhaps, but not useless.

I'm not even going to try to dispel your numerous and serious misconceptions because others here have done that very well but you continue to refuse to listen to the truth.

 

[Dale]

Closed pending moderation.

EDIT: The thread is reopened. Please keep the topic focused on the OP's discussion, not a referendum on the Mersini-Houghton paper.

 

[maxverywell]

Back on topic. This is a well known fact from classical GR that an static external observer sees the star shrinking asymptotically to its gravitational radius, thus he never witnesses the formation of an BH. This led to the idea of the "frozen star", which is now abandoned because eventually, in finite time, the observer will stop seeing the collapsing star due to the gravitational redshift. So for practical purposes we could call this object a BH. However it is not a real BH, it is a fundamentally different object and should be rather called "black star". Check this: http://arxiv.org/abs/0706.1203

For the body that collapsed catastrophically, there is a precise last signal that will never be returned, no matter how long you wait.

Why it will never be returned? This is nonsense.

 

[PAllen]

Back on topic. This is a well known fact from classical GR that an static external observer sees the star shrinking asymptotically to its gravitational radius, thus he never witnesses the formation of an BH. This led to the idea of the "frozen star", which is now abandoned because eventually, in finite time, the observer will stop seeing the collapsing star due to the gravitational redshift. So for practical purposes we could call this object a BH. However it is not a real BH, it is a fundamentally different object and should be rather called "black star". Check this: http://arxiv.org/abs/0706.1203

This paper is considered a fringe point of view, the opposite of 'very well known'. The author is, in fact, very well known for advocating for this fringe point of view. Here is an example of the opinion of his view by the mainstream:

http://arxiv.org/abs/0707.2450

In most cases, this fringe point of view is ignored as not worth responding to. It is dispuiting with, misconceptions, what is considered settled physics since the work by Hawking and Penrose in early 1970s.

Why it will never be returned? This is nonsense.

Actually, trivially true and extremely well known. If you follow the trajectory of the null rays sent by some external static observer to the collapsing star, there is a precise event on the external observers world line such that:

- for signals sent before that event, the null rays reach the surface outside the event horizon
- a signal sent from that event reaches the surface at the moment it intersects the event horizon [and can never be returned]
- a signal sent later than this event, reaches the the collapsing surface inside the event horizon. [and can never be returned]

Note, these are coordinate independent facts in classical GR.

 

[PAllen]

For the well known, mainstream view of BH properties and dynamics, the interested reader should explore:

http://www.black-holes.org/

specifically for what really happens with black hole collisions per classical GR:

http://www.black-holes.org/explore2.html

 

[PAllen]

And for analysis of BH collisions per accepted physics, published in prestigious peer reviewed journal, rather than not published in any peer reviewed journal (as was the case for the fringe paper given in #18):

http://arxiv.org/abs/0907.0869v1

 

[PAllen]

Perhaps better than looking at signal properties is looking at causal structure of collapse to BH. What the true claim "from outside a BH, you will never see matter at or inside the horizon" says causally is that the horizon is never in the causal past of external observer. However, there is a precise event on an external observer's world line such that the horizon ceases to be all in the causal future. Once it is no longer in the causal future, its separation from you is spacelike and can be considered 'present', depending on simultaneity convention chosen.

 

[berkeman]

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