Behaviour of black holes

Question

This question could be strictly linked to my another question related to black holes.

It’s known that the horizon event is a place where dust,star components are ready to be incorporated to the black hole*.

Actually, my question can be divided in few points:

• Might be reasonable to believe that ,if it’s true that in many spiral galaxies there is a black hole in the middle, could these objects have so a powerful gravitational force to consider them the creators of the galaxies?

• (*) when the black holes are sucking up dust from the horizon event, are they increasing their mass?

  • If so, it’s reasonable to believe that their gravitational force will increase in a way to attract farther objects?
  • Could they eject the dust or anything else if they are “eating” so much? This can be in contradiction with above point.

Answer 1

1. No, it seems that the formation of a supermassive black hole is a consequence of galaxy formation, not a cause of it. The mass of the black hole is a very small proportion of the mass of the matter and dark matter in the galaxy. The details are not yet worked out but it is the distribution of dark matter that forms the galaxy, and then galaxy mergers contribute to the growth of a large black hole in the centre.

2. As indicated in the answer to your previous question, "sucked" is entirely the wrong metaphor for how black holes work. Black holes have lots of mass in a small space, so they "pull" gravitationally.

   • Now gravity gets weird when it is very strong, as it affects the time. From the perspective of a person outside the black hole, a clock on a body falling towards the horizon will slow down, and will appear to stop as the body reaches the black hole, This means that we will never see something fall into the black hole. However the mass of the body that falls towards the black hole will increase the effective mass of the black hole, and so increase the radius of the event horizon.

3. For objects further from the black hole, the acceleration towards the black hole is proportional to its mass. So it is not the case that an object will "start to feel the pull", but instead the pull will increase. Note that mass that is orbiting close to the black hole will act almost like part of the black hole from the point of view of someone further out. This is not a special property of black holes, just ordinary gravity.

4. Black holes form an accretion disk around themselves, a disk of matter orbiting the black hole at very high speeds and getting very very hot. They also produce high energy jets of particles moving close to the speed of light from their poles. These are produced from the accretion disk, not from the black hole itself. Objects that fall towards the black hole don't come back.