How massive can black holes grow?

so there's massive black hole we've ever found is in a quasar galaxy called tio n61 8 and they found it by looking at how fast the hydrogen gas is spiraling around that black hole if you know how fast the gas is spiraling then you know how big the thing is that it's spiraling around just by the rules of gravity and this gas was moving at 7,000 kilometers per second which means that the black hole that it was spiraling around is 66 billion times the mass of the Sun compare that to the black hole in the center of the Milky Way which is like 4 million times the mass of the Sun so that is piddly in comparison how teeny tiny but is there anything to actually physically stop a black hole from growing any bigger like is there a physical limit at which point it could no longer grow any further so to answer that question we're gonna have to understand how black holes grow and that's the process that we call accretion essentially collect material under gravity and it's something that we see happening across all areas of astrophysics things like binary stars and x-ray binaries white dwarfs neutron stars things like that and essentially what happens is that in falling material onto a black hole gets spread out into a disc and that's because it has something we call angular momentum or rotational energy so in a sense what's happened is the materials come in with some rotational energy and so it's flattened out and form a disc the same way that when you you know throw a ball a pizza dough above your head it flattens out into a disc cause you've set it spinning I'm like and what happened is that the material in that disc has a lot of energy because it's moving very fast because it's the thing that it's moving around is so massive and so it has enough energy to stay on very well ordered orbits and just continually orbit the black hole rather than actually losing energy and falling beyond the event horizon of the black hole ie that point of no return when nothing else can escape so the reason the black hole then actually accretes that material and it doesn't just permanently stay in that disc is because energy is actually radiated a through collisions between particles that exchange momentum and then also because you end up generating lots of friction ie heat and so you end up with a lot of radiation coming off of that accretion disk as well and what we call a blackbody spectrum so depending on the accretion disk temperature you get a different shape of the spectrum out and you can end up getting higher and higher energy radiation up to like x-rays and gamma rays as well and that's one of the reasons that we actually know that these accreting supermassive black holes are there because of the huge amounts of high-energy radiation coming from the center of galaxies where they're found but actually it's this radiation that's going to cause a lot of issues in the question of how big black holes grow because if you have that radiation constantly pushing outwards away from the black hole then that's actually gonna form what we call a pressure a radiation pressure your photons are particles that actually have momentum so that if they hit into other things around the black hole they're actually gonna start pushing stuff outwards but of course you've also got the gravity of the black hole constantly pulling stuff inwards as well so the maximum rate at which a black hole could grow would be when gravity pushing inwards is perfectly balanced by the force of radiation pushing outwards and this has been known for over a hundred years since Arthur Eddington figured this out in 1916 I'll bet he used a lot of simplifications you know Newton's laws necessarily relativity and then also some assumptions that we now know not to be true around a black hole but it was a nice simple math to say take the equation that says this is the force and gravity and take the equation that says this is the force of your radiation and here you get the maximum accretion rate of the black hole and the main thing that it's dependent on is the mass of the black hole I the bigger the black hole is the more mass you should be able to recruit but the thing is black holes don't spend their entire lives accreting at that maximum level and that's because a you know you don't always have that amount of material available it's not being inflow to the center of a galaxy where the black hole is but then also that it's depend on the fact that your black hole is spinning and so the spin can actually affect how much matter it can accrete but then also spinning black holes have magnetic field lines which then get all tangled as the black hole spins and that actually funnels particles and energy away from the accretion disk in these big light year long Jets and then also these sort of winds as well which can actually sweep away material to clear out the area around a black hole so by setting up an accretion disk you can also clear the area around a black hole to add a load of energy to that surrounding material it gets really hot and until it's cooled down enough it won't sink back towards the center and reform that accretion disks the black hole can start growing again kind of like the black hole is shooting itself in the foot effectively now you can also merge two black holes together as well which can double the mass of your black hole and we know that happens for sort of stellar-mass black holes like what my gos detected out the gravitational waves and we think that it happens in the center of two galaxies merging together because you've got two black holes in the center that will eventually merge as well I'm not really going to cover that here but I do talk about it in my new book that's coming out on September 5th it's called space the ten things you should know and I'll provide a link in the description for you to preorder it but essentially if you're thinking about how black holes can grow by accreting material then really all you're limited by is time how long have you got to wait for the black hole to accrete material after doing its maximum rate for so many years and the really cool thing is that we can look at the population of supermassive black holes around us in the universe today and we know they've only had fourteen billion years to grow that big because that's how old the universe is so we can then work out well how efficient are these black holes on average ie what percentage of that maximum accretion rate are they actually accreting at across the whole age of the universe you find that it amperages out around about ten percent or so you know some are actually accreting at maximum right now some well below that but over time about 10% so that made me think I've been thinking that's dangerous if you're only limited by time then technically the biggest black hole you could ever make would be the entirety of all matter in the universe in a black hole which is difficult to estimate but we put it around about 10 to the 60 kilograms which is a 1 with 60 zeroes after it now this well it's very unlikely that will happen so it's not like a doomsday scenario at all it's a doomsday device gosh because the universe is expanding it's even actually accelerating expanding so all matter is actually moving away from everything else so it's unlikely this will ever happen but you know what I figured why not work it out back of the envelope calculation to say if a black hole is gonna start at say 10 times the mass of the Sun how long would it take at the maximum rate that it could accrete to reach 10 to the 60 kilograms and you find out that it's about 70 billion years which is shorter than the age of the universe but obviously we know that black holes don't increase at that maximum accretion level because they blow out all the material around them just by setting up the accretion disk in the first place so it's probably more likely that it would be hundreds of billions of years maybe even trillions because it's kind of exponential how fast you grow your black hole but still cool calculation to do on a Friday afternoon gotta say now Friday turns out though my Ultra massive super duper black hole which Webb decided I'm calling it now from now on trademarked is not actually physically possible for another reason that's actually to do with black hole accretion physics and essentially it's all to do with the radius at which you get materials so yes you have the event horizon around the black hole which is the point of no return but another radius around the black hole you have is called the inner most stable circular orbit or the Isco people often refer it to and that's essentially the last point at which you can have something in a very stable orbit around the black hole say and material like an accretion disk or stay in orbit of a star like what we've observed around our black hole in the center of the Milky Way and the thing is that in a most stable circular orbit gets pushed further and further out from the black hole as the black hole gets more and more massive and eventually it will actually pass what we call the self gravitational radius and what I mean by self gravity is how much stuff around the black hole is attracted to each other compared to how much it's attracted to the black hole so this is why you don't have one huge accretion disk around the black hole you actually have galaxies composed of stars because beyond the self gravitational radius the gas is attracted to each other and it collapses to form stars rather than just everything eventually you're just falling into the black hole so as your black hole grows and grows your innermost stable circular orbit as it pushes outwards and outwards is eventually going to pass that point at which stuff is attracted to each other or stuff is attracted to the black hole and so at that point you can't form one of these accretion disks if you've got material coming in if it's on a direct trajectory it's gonna go straight into the black hole but if it's not on a direct trajectory instead of being pulled into that nice orbiting accretion disk it's not gonna happen you're not going to be able to have those collisions and that sort of radiation of heat outwards that allows all of that material to be accreted by the black hole and direct trajectories of material is nowhere near the same amount as actually being able to accrete at that maximum level that accretion disk allows you to do so because we know the mass behind general relativity quite well we can work out what that inner most stable circular orbit will be for a given mass of black hole and we can also work out what that self gravitational radius will be given the mass of the black hole and that allows us to work out the maximum mass at which a black hole can grow - at which point those two things cross over and it does depend on your spin of your black hole again but on average it works out to be about 50 billion times the mass of the Sun pushing to higher spin gets you to the 66 billion times the mass of the Sun of that black hole that's seen in the quasar tio ends that one eight and so what that means is that because we're seeing black holes of that mass in our current universe it means that we're already pushing that limit after fourteen billion years of the universe's history at which point black holes can't grow by accretion anymore they can only grow by mergers and so that growth of black holes is gonna slow down but eventually what it means is that quasars will win count quasars is what we call these actively growing supermassive black holes that are giving off all this radiation in the radio and infrared in the x-rays in the center of these galaxies that appear as these huge bright point sources so that we know that they're there eventually they're all gonna wink out and all these quasars will go dark and will no longer be able to see that process of a black hole accreting material so that image from the event horizon telescope of the light from the accretion disk around the supermassive black hole in Messier 87 came at a really fortuitous time because it meant the black hole had grown big enough so that we could actually resolve the shadow of the event horizon on the accretion disk using the telescopes that we currently have on earth but also that it hadn't grown so big as to already have cut off that accretion process so that hadn't already winked out of existence then you're never going to be able to form that accretion disk of material around you little burp ebert there what here like the word Isco could be used as like a drama basement yes it's it looks like disco I wonder if discos to his brother was gold it's goes to achieving my goal mattered so ha oh oh hey favor one good meal boom one emesis at the minute is grass Oh you

Channel: Dr. Becky

Views: 112,148

Keywords: astronomy, space, astrophysics, black holes, eddington accretion, accretion, universe, physics, general relativity, dr becky, science, mass of the universe, dr becky smethurst, rebecca smethurst, becky smethurst

Id: 1ooL9cvvHdA

Length: 13min 9sec (789 seconds)

Published: Wed Jul 10 2019