Trying to wrap my head around some concepts involving the very large gravitational fields of black holes, and what the gravitational gradients look like on a cosmic scale.

I'm familiar with the Great Attractor, and wondered if it were possible if the Milky Way and it's neighbors are already "doomed" to fall into whatever the anomaly is.

Per recommendation, I've moved the second half of my question to a new Question here: Assuming a sufficient amount of mass above the density threshold, does the actual concentration of the mass matter in creating a black hole?

Original Text:

Extrapolating further, I've heard that the actual density of a black hole (within the volume of the event horizon) is pretty low, on the order of thin atmosphere - does that mean that any sufficiently large volume of mass over that density is also a black hole? Or does the actual concentration within the event horizon matter?

  • $\begingroup$ I think that this is a partial duplicate of astronomy.stackexchange.com/questions/6057/…. I say "partial" because a) only part of your question (i.e. title) fits that description, and b) the other question has several close votes against it because it's pretty darn unclear. I don't know what others think of this one, but I'm just going to say that I think it should stay open. On [what has now been reduced to] a side note, the second part of your question seems to be unrelated. Could you relate it to the first part, or post it separately? $\endgroup$
    – HDE 226868
    Commented Sep 9, 2014 at 23:02
  • $\begingroup$ Also (second comment because I hit the exact character limit for the first!), I would argue that this should stay open because it is not a personal theory, and, in fact, asks how it could be proven wrong. I'm just adding all of this because I'm worried that this question could be closed, and I don't think it deserves that. $\endgroup$
    – HDE 226868
    Commented Sep 9, 2014 at 23:04
  • $\begingroup$ Indeed, large black holes are relatively low in density: mass/volume. Supermassive black holes are often said to have the density of a cup of water. My understanding is that mass estimates put us at just under the density for a black hole the size of the observable universe. On the other hand, the cosmological horizon is in many aspects equivalent to a black hole event horizon. I lack a handy reference right now, otherwise I'd be posting this as an answer. $\endgroup$ Commented Sep 10, 2014 at 1:57
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    $\begingroup$ Updated question to fork the second half to a new Question. $\endgroup$ Commented Sep 10, 2014 at 16:12

2 Answers 2


This isn't anything near a proof that we don't live inside a black hole, but it's a bundle of evidence that certainly goes against it, and that the Great Attractor is not, in fact, the singularity.

First off: Expansion of the universe.

As you no doubt know, the universe is expanding. In fact, the expansion is accelerating. Do black holes expand? Yes. As they suck in more matter, they can get bigger. But if that was the case, we should notice more matter coming into the universe (well, I suppose it could be from outside the visible universe, but we should still see lots of matter coming towards us). Also, the universe's expansion is driven by dark energy, and it pushes (on a large scale) everything away from each other. In an expanding black hole, there would be no reason for the matter inside the event horizon to move away from each other; only the event horizon expands.

You also made a good point in a comment below about Hawking radiation. Eventually, in the far future, when there is nothing left in the universe but black holes, black holes will evaporate via Hawking radiation (okay, they do that now, but they can still take in more matter). If our universe is a black hole, it should then contract. But we see no reason why it should. In fact, the theory that predicts the universe's eventual collapse into a singularity (i.e. the opposite of the Big Bang), the Big Crunch theory, predicts that the universe's contraction will match its expansion. Contraction due to Hawking radiation wouldn't necessarily mirror the black hole's growth. Also, the Big Crunch theory is supported only by a minority of scientists due to the evidence against it.

Second off: The motion of galaxies by the Great Attractor.

First off, see When will the Milky Way "arrive" at the Great Attractor, and what all happen then? (and not just my answer! I dearly wish @LCD3 would expand his/her comment into an answer!). The general gist of things there, as pertains to your remark about the Great Attractor, is that the galaxies aren't all moving towards it. There are doubts (see the papers I mentioned) that the galaxies previously thought to be moving towards it are, in fact, moving towards more distant objects - other superclusters. If the Great Attractor was indeed the singularity, a) all the galaxies in the universe should be accelerating towards it, which is not the case, and b) we should be moving directly towards it, and due to its gravity, not that of the superclusters beyond.

Like I said in my comments, I don't quite think your last part relates to your first part, but I'll try to address it. First, I'm not sure where you got your sources, but I can say that we don't really know what goes on inside a black hole, and I don't now how someone got that density figure (I am, of course, not the authority on black holes - see @JohnRennie for that, on Physics SE, and I could be wrong about this). The density inside the event horizon would not be very thin, however, in regions where there is lots of matter. For example, in a black hole with an accretion disk, the material that is being absorbed might not have that low a density. Also, a large volume of that density would not necessarily collapse to become a black hole, because it would not be compact enough.

Hope it helps.

  • $\begingroup$ Awesome, this definitely help point me in a new direction to research. I'll leave the question open a few more days, in hope that it garners more answers, but +1 to you (at least when I have 15 rep. :p) $\endgroup$ Commented Sep 10, 2014 at 16:15
  • $\begingroup$ With respect to the second law, wasn't that amended with the "discovery" of Hawking radiation? If so, wouldn't that cause the boundary to recede over time? $\endgroup$ Commented Sep 10, 2014 at 16:25
  • $\begingroup$ Yes, it would, but very slowly. And the universe is expanding. I think I made that part a bit confusing; I'll try to edit it and better address your comment. $\endgroup$
    – HDE 226868
    Commented Sep 10, 2014 at 22:45
  • $\begingroup$ I made the edit. $\endgroup$
    – HDE 226868
    Commented Sep 10, 2014 at 23:27
  • $\begingroup$ " In an expanding black hole, there would be no reason for the matter inside the event horizon to move away from each other; only the event horizon expands." what about Tidal Forces? In other words, wouldn't object A entering the event horizon before object B; mean that the two objects, once both inside the event horizon, would appear to accelerate away from each other? $\endgroup$
    – Glurth
    Commented Apr 13, 2019 at 15:01

If Great Attractor were the center of a black hole to which we belong, we would approach it at speed greater than speed of light and there would be no way to move away from it, even temporarily.

Also we would observe it in any direction.


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