Do Black Holes Grow? Investigating the Lifecycle of a Galaxy

In summary, the conversation discusses the growth of black holes and how it relates to the law of conservation of mass and gravity. It is mentioned that black holes do grow when matter falls into them, but they do not suck in objects outside of their event horizon. The possibility of the universe being mainly black hole is also discussed, with the conclusion that the universe is not in this state due to the majority of objects being outside of the event horizon. The concept of the size of a black hole and its event horizon is also mentioned, with the understanding that the size of a black hole is equal to its event horizon. Finally, the conversation touches on the idea that the Earth's orbit would not be altered if the Sun suddenly collapsed into a black hole
  • #1
plainstupid
2
0
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.
 
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  • #2
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.
 
  • #3
atyy said:
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.
 
  • #4
plainstupid said:
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.
 
  • #5
plainstupid said:
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.
 
  • #6
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.
 

1. How do black holes grow?

Black holes grow through a process called accretion, where they pull in and consume surrounding matter, such as gas and stars.

2. Can black holes grow indefinitely?

No, black holes have a maximum size known as the Schwarzschild radius, beyond which the gravitational pull becomes too strong and nothing, including light, can escape.

3. Do black holes grow in size or mass?

Black holes primarily grow in mass as they accrete matter, but their size may also increase due to the stretching of space-time around them.

4. How do black holes impact the lifecycle of a galaxy?

Black holes play a crucial role in the evolution of galaxies. As they grow and consume matter, they release large amounts of energy, which can shape and influence the structure and dynamics of their host galaxy.

5. How do scientists study the growth of black holes?

Scientists use various methods, such as observing the effects of a black hole's gravity on surrounding objects, measuring the X-rays emitted from the accretion disk, and studying the motion of stars around the black hole, to understand and track the growth of black holes.

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