Do Black Holes Grow? Investigating the Lifecycle of a Galaxy

[plainstupid]

I know that this can not be true because if it was, the universe would be mainly black hole by now but I can't seem to fathom how if something has an increase in mass (law of conservation of mass) from an external object, it does not increase in size.

My next thought would be, if gravity is proportional to mass (growth or not in size), then the more massive the black hole becomes, the greater effect that gravity would have on surrounding stars etc and then a theory popped into my head.

Considering that the Milky Way has a SMBH at its centre, the galaxy would, through growth in mass and therefore gravity of the SMBH, it will eventually draw in the outer spiral arms in toward the middle thus forming an eliptical galaxy akin to M87. This then would explain the lifecycle of a galaxy from rotating spiral to rotating eliptical to eventually nothing.

I don't know - my name is not plainstupid for nothing.

 

[atyy]

Black holes do grow when matter falls into them.

But they don't suck things outside their event horizon, and most of the universe is outside the event horizon of a black hole (unless we are all inside the event horizon of a really big black hole), which is why the universe is not mainly black hole by now.

 

[plainstupid]

Black holes do grow when matter falls into them.

But they don't suck things outside their event horizon, and most of the universe is outside the event horizon of a black hole (unless we are all inside the event horizon of a really big black hole), which is why the universe is not mainly black hole by now.

But then wouldn't the event horizon grow in relation to the new size of the BH? It would have to, otherwise you would need an infinite EH to start off with or your BH would at some stage exceed your EH.

 

[bcrowell]

But then wouldn't the event horizon grow in relation to the new size of the BH?

The horizon will only grow if the mass increases. The mass will only increase if something falls in. If nothing falls in, nothing changes.

 

[JesseM]

But then wouldn't the event horizon grow in relation to the new size of the BH? It would have to, otherwise you would need an infinite EH to start off with or your BH would at some stage exceed your EH.

The size of a black hole is the size of the event horizon, according to general relativity all of the actual matter in the black hole is compressed to a point of zero size and infinite density, the "singularity" (this would probably change in a quantum theory of gravity but the matter making up the black hole would still likely be compressed to a very tiny size) So, it doesn't really make sense to talk about the size of a black hole exceeding the size of the event horizon.

 

[Dale]

If the sun suddenly collapsed into a 1 solar mass black hole the Earth's orbit would not be altered. Specifically, the Earth would not be pulled into it. Similarly for the black hole that may be at the center of the galaxy, the rest of the galaxy is in orbit around it and will not be pulled in unless the orbits change.