I originally posted this query at Universe Today but did not receive any useful replies before the site merger - so here it is again.

As I understand black holes absolutely nothing (barring quantum evaporation) comes out of a black hole except for the net gravitational and electrical effects of the matter in the hole.

Also as I understand curved space time a straight line is (by definition as it were) the path followed by light in a vacuum.

So, if you imagine two particles on opposite sides of a chunk of matter about to become a black hole:

Before the mass becomes a black hole the shortest path between the particles is through the centre of the mass which is not yet a black hole. Each "sees" the other as a point mass. The gravitational effect of each on the other is determined by the straight line distance and their masses.

After the mass becomes a black hole the previous shortest path between the particles is no longer available and any light or gravity effects must take the longer path round the circumference of the black hole. In effect each particle now "sees" the other as a ring of matter at a greater distance (in the extreme case at a distance of half the circumference of the hole).

Since this increase in distance applies to any pair of masses on opposite sides of the new black hole the effect of the formation of the black hole would appear to be a slight reduction in the average density of the universe. For any particle, in calculating the gravitational effect of mass on the far side of the hole, the bigger the black hole the larger the area of sky affected by the black hole and the greater the reduction in density.

Is this analysis correct? If not, why not?

If correct it raises an interesting situation. In principle intelligent life is capable of diverting comets onto stars to alter their trajectories to make them either hit or miss the central black hole in many galaxies. This would make the density of the universe not a given but a (slightly) modifiable quantity with the wild speculation that the universe might just be poised at the density where intelligent life could make a difference.

No, it's not. The amount of space isn't increasing. It's just the length of the path through the universe is longer. Think of it this way. If your standing in the middle of a straight road running north and south. The shortest path to something north of you is to move north, straight down the middle of the road. If you move east and west of the middle of the road, as your move to the north, the street isn't getting bigger (it stays the same size), but the path has become longer than the straight down the middle path.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

I believe the gravity effects would continue to go straight through. Light would go straight in as well; it just wouldn't get to the other side any longer.

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Everything I need to know I learned through Googling.

If you think that the force of gravity is conveyed by particles which move at the speed of light, and are affected by gravity, then perhaps you think that these "gravitons" cannot escape a black hole?!?! in which case there would be no apparent gravity from said object. Similarly, you'd have to think that if an object were travelling away from a massive object, that it would feel less gravitons from it, and so the force of gravity would be dependent upon relative velocity between the objects.

Our current theories of gravity do not predict these phenomena, and in fact observations contradict these ideas as well.

It may be that there is some way for us to change the mass of the universe (we don't know of one), but this is not it.

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Forming opinions as we speak

The idea that the shortest distance between two objects being a straight line is slightly complicated because we are unsure what we mean by straight when we are dealing with an intense gravitational field, be it a black hole, neutron star, or even close to the Sun.

The breakthought that Einstein developed in 1916 or thereabouts in defining his general theory of relativity was that gravity warps, or bends, space and time. The phrase is bandied about now without much thought to the shape of that bend. I see it as space, and time, decreasing near a massive object. I visualize a shell of space near that object that is thinner the next moment than it is now. Because all the shells will be thinner, other objects near the massive one become close to the mass as each moment progresses.

The idea does not need gravitons, so if they are ever discovered, then some revission will be necessary.

As for light, the path of light changes its direction, as seen by an outside such as ourselves, as it passes the massive object. This is because the space the light traveled through thinned and therfore moved closer to the mass.

Inside the event horizon (EH), which has here been used for being inside a black hole, all directions are toward the singularity. It is not just that the shortest distance is through the center; all of them are. This is because no matter which direction you or a ray of light takes, the path will bend back toward this center point. Rather than being an abrupt change from outside to inside the EH, it is a gradual change. The qualitative separation is that once inside there is no possibility of changing direction.

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There are ten kinds of people. Those that understand binary, and those that do not.

I think part of the confusion with getting the answer you’re looking for is the interchanging the words ‘path’ and ‘distance’ in your analysis. The distance is the same in both scenarios and does not increase, just in one you have a super massive object between the particles. But you are correct, that if you want the two particles to meet, they WOULD have to take the longer path around the outside of the EH.

I’m not sure about your density reduction, mass is mass, it’s just more compact. Hopefully someone with a bit more experience in black holes can address that.

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Photons have mass? I didn't even know they were Catholic.