It doesn't seem like there's a limit to how big they can get just a limit to how quickly they can get bigger due to what's called the Eddington Limit which explains how matter falling into the black hole emits radiation and if enough radiation around the accretion disk builds up, it can overcome the pull of the black hole and push matter away, at least until enough matter is pushed away that the radiation levels fall back under the limit and matter starts falling in again.

Importantly, the Eddington limit does not apply to black hole mergers, theoretically allowing as much growth rate as you're able to feed in from smaller black holes.

This said, the final parsec problem isn't solved/understood. We know black holes do merge, but we don't understand what energy is being bled out of the system so supermassive black holes crash into each other in the timeframes we're seeing it occur.

So then the only theoretical limit on black hole mass would just be how fast you can put matter in black holes and/or merge existing black holes versus how fast the universe expands?

I'm 100% an armchair physician so take my words with a grain of salt but it seems like according to the math there is no limit to how massive a black hole can get. There are limits on the size of how big and small things can get and how hot or cold they can get, the second part is pretty cool, Physics Explained on yt has a good video on it (he's got a lot of good videos) but I enjoyed this one on what the maximum temperature is in the universe: https://www.youtube.com/watch?v=NVlEQlz6n1k

> I'm 100% an armchair physician

Not to be that guy, but a physician is a doctor.

But that's not important right now.

Just pointing out a simple mistake.

In the time that it took you to type that response, you could have learned 10 new words.

I do it because I appreciate it when people do it for me.

That was the purpose of my comment. What was the purpose of yours?

I heard a joke about a nerd who dies and finds himself in a very hot underground cavern. The devil is there, and says "Welcome to Hell! This over here is the lake of molten lava where you'll spend the rest of eternity". The nerd says "well actually, since it's underground it's called magma rather than lava". The devil replies, "um, you do understand why you're here, don't you?".

I try to remember that when I'm tempted to point out mistakes that are fine to overlook.

shirley you cant be serious

Not to be cet homme, but in French a physicien is a physicist.

Not to be that guy, but English is not French

PBS Spacetime had an episode somewhat recently about a black hole which is growing at many (hundreds? thousands? I forget) times the Eddington Limit. And, as far as I remember, it isn't the only one to exceed the Eddington Limit - just the one with the record for how much it exceeded it.

I'll try to dig it up when I'm not at work (or if I remember the exact episode through the day).

I remember this episode too. The answer is four thousand times bigger than the Eddington Limit. Blimey!

The episode is called “The NEW Ultimate Energy Limit of the Universe”. https://youtube.com/watch?v=0rzgYzbzq5Q

https://en.wikipedia.org/wiki/List_of_most_massive_black_hol... shows the maximal theoretical limit as 270B solar masses.

To expand on this, as stated in your source:

> [270B solar masses] is the maximum mass of a black hole that models predict, at least for luminous accreting SMBHs.

as well as:

> The limit is only 5×10^10 M [50B solar masses] for black holes with typical properties, but can reach 2.7×10^11 M [270B solar masses] at maximal prograde spin (a = 1).

However in the chapter before, it's stated:

> New discoveries suggest that many black holes, dubbed 'stupendously large', may exceed 100 billion or even 1 trillion M.

There's a theory that the universe we live in is itself inside a giant black hole. No idea how it is supposed to have gotten so biig.

It couldn't have, the theory is nonsense.

I know this article. It's citing a bunch of speculative hypothesis by mostly this one person which relies on something super exotic called Einstein Cartan theory. I stand by my statement. I even suspect the article was written by them.

I hate random links being thrown at me, because I don't know what you are trying to tell me. Perhaps you can spare a few key strokes.

For everyone else reading the thread, let me summarize. The article agrees with me:

> the entire observable universe exists within a black hole—except, that is, for all the evidence to the contrary

>....

> It does not seem likely that we live inside a rotating universe, let alone a black hole.

You have elsewhere in this thread objected to people providing links without giving context, so I hope you won't mind being asked to unpack this claim a little. Why is it nonsense? If, as you say, it's principally pushed by one person, who is that, and why does that argue against it?

(I'm not thinking this is too much to ask; saying it's wrong might require empirical support, but the claim that it's "nonsense" should be easier to justify.)

First of all, black holes have an interior and an exterior. Our universe only has an interior. Next, black holes have a singularity into which everything vanishes, or at least moves towards. Im our universe, everything moves away from a singularity. So if anything, it resembles a white hole more than a black hole. Also, our universe is expanding, whereas black holes shrink (unless matter falls into them, which can't happen to our universe because it has no exterior).

It really looks nothing like a black hole.

Agreed, how do you feel about our universe being some sort of post evaporated BH-like-thing from a previous universe-like-thing?

>Next, black holes have a singularity into which everything vanishes, or at least moves towards

I mean, everything in our universe does move towards something. The future.

The interior and exterior are isolated; we have no idea if are universe has an exterior or not and, according to present theory, never will.

As another comment pointed out, in GR our "future" is a singularity which everything moves towards (so what we see as a time dimension).

"Expanding" and "contracting" depend on your coordinate system. If your rulers are shrinking, you can't tell this from space expanding.

The common factor here is you are wanting to use our reference frame (somewhere in this universe, not near a black hole) to describe things as they would be seen from other reference frames.

> giant

How would we know the size? Relative to what?

If you assume constant density, anything becomes a black hole at certain volume. The question is: is our universe big enough to be a black hole or not.

So what happens if two such black holes collide?

Can black holes even collide? I guess their horizons can merge somehow... Probably a spectacular show.

Disclaimer: This is my own work

https://www.youtube.com/watch?v=doS85Mh78Vc

This is what they look like when they merge, its pretty darn cool

I love to contemplate galactic-scale synchrotrons that accelerate supermassive charged black holes to collide at relativistic speeds. The thought never really goes anywhere, but I'm sure it'd be a spectacle to behold.

It would be just about the only way we could get the data required to resolve the contradictions between the Standard Model and general relativity. The unification energy is simply stupendous.

That’s precisely what LIGO measures, the gravitational waves from black hole mergers (or neutron star mergers, etc).

>Cosmic Heavyweights Collide – LIGO Detects Largest, Fastest-Spinning Black Holes Yet

https://scitechdaily.com/cosmic-heavyweights-collide-ligo-de...

That could be a good question for AI to answer.

Given things like https://en.wikipedia.org/wiki/TON_618 and https://en.wikipedia.org/wiki/Phoenix_Cluster#Supermassive_b..., probably not. Seems like you can just keep shoving mass into it.

Poking around those articles (and knowing nothing really), it is interesting to note a couple references to a 50B solar-mass limit for “luminous accreting black holes hosted by disc galaxies.” (In your Phoenix cluster link). I guess these ones are easier to spot, based entirely on the word “luminous.”

There are other larger ones out there, looming in the darkness.

Those supermassive black holes are very old, from a time when the universe was much denser - they likely collapsed directly without any star formation

Yes - but it's basically the same as the total mass of the universe.

EDIT: I believe the above could be incorrect - if the universe has too much electrical charge or angular momentum. (And some other cosmological properties, so you couldn't get around the charge & spin issues.)

Might there be a black hole astrophysicist in the house, to comment on this?

There is this whole theory that the observable universe is inside a black hole.

So the upper limit is the weight of the universe.