Do black holes contain dark matter?

so a black hole is just a region of space where you have so much stuff it's just small amount of space then it's so dense that the escape velocity is greater than the speed of light and it shouldn't matter what kind of matter you have in that small amount of space it could be normal matter it could be dark matter right well it turns out that it might be a little bit more complicated than that so the very first and only Eureka moment I've ever had was when I was writing up my PhD thesis and I had this result that showed that the supermassive black holes at the Centers of galaxies might be proportional to the amount of dark matter that was also in a galaxy what we call a Dark Matter halo that surrounds the actual galaxy itself and I basically had this like moment where I was like can black holes grow by accreting dark matter and I didn't know the answer and I basically has just wander the physics department asking anybody that I could find that I thought might be an expert on this so I have some piece of the puzzle so I could work out you know what to say in my thesis about this possible result so we don't know a lot about what dark matter is but there is one thing we know about it and that is that it's something we call collisionless matter so you can have a collisional matter and you can have collision less matter so collision all matter is like what you would expect normal particles and stuff to do if they were in like a cloud or like a box right so you can imagine lots particles pinging around and they're basically just gonna hit into each other and when they hit into each other they exchange energy and momentum and they basically communicate a little bit about you know what the environment is like that they're in so if you think about water in a pound of stove you're playing is heat to the bottom of the pan and the water molecules close the bottle of hands start to get more energy and they start vibrating more and more and they let in small particles and the nail exchange energy and momentum and those will heat up and gain an energy and so particle in the pan basically know about the local conditions they know what the temperature and pressure and density is in other areas of the pan depending on whether they have more or less collisions so compare that with collision less particles you know literally particles that just do not collide it's not that they can't collide it's just the probability of a collision is extremely extremely low so we say that their mean free path so that's sort of the length that they can go without experiencing another collision is much greater than the size of the fluid container that they're in for example and so the particle barely interact and so they have no idea about the local conditions of the fluid that they're in so they're very dependent on initial conditions and your starting point for how they behave so that might sound quite a weird concept but an example of a collisionless fluid of particles is actually stars in a galaxy so if you imagine the stars are the particles in that sense then really there on orbits around the center of the galaxy there's no collisions between stars so the Sun for example has no idea what speed the other stars around it in the galaxy are going around the center of the Milky Way all it knows is that it's trapped in an orbit around some central black hole which is not a local condition at all it's a very very far away object and it's also based on the initial conditions of whatever cloud of gas formed our Milky Way because that kind of gas must have had some angular momentum then and then as it condensed down and it stayed everything's spinning that's the still the condition that the Sun knows about but it doesn't know about anything else in the system so you might be wondering how in the universe we even know that dark matter is collisionless because you know very little about it and also we don't know what it's made out of but it's actually something we've observed in the dark matter properties when we look out into the universe so it's a very famous collision of two clusters of galaxies going on that we call the bullet cluster so galaxies don't tend to be found by themselves they're found in big clusters where they grouped together under gravity and all of the space between the galaxies tends to be taken up by this big hello and hot hydrogen gas so when two clusters collide it's unlikely that the actual galaxies themselves in the cluster are going to collide because you know they'd have to be on extremely extremely tight paths you know like threading a needle kind of thing when reality sort of they tend to pass each other by so we look at the collision between these two clusters of galaxies we use x-ray emission to trace where the hot gases and then we use what we call gravitational lensing to determine where all the mass weight of all the matter is in the image as well so this is when we know that wherever masses in the universe it's going to curve space-time by Einstein's theory of general relativity I mean any light behind that big cluster of galaxies is going to get curved along its path away on its way to earth and soil acts as like a gravitational lens and so any little lensing events we see of galaxies in the background behind that cluster can tell us where all the masses in the cluster and so when we do that we see that all the x-ray emission is coming from the gas that's collided and basically be left behind is the two ghosts or galaxies passed through each other on this collision all the stars are obviously had passed through each other and then all of the mass is found tracing the Stars and the galaxies in that collision and so we know the dark matter must therefore also be collisionless just like the stars in the galaxies but what is dark massive being collisionless mean for supermassive black holes trying to grow by accreting dark matter so if you imagine a black hole just sitting there happily and you imagine some normal masses some gas like falling onto the black hole what's gonna happen is that as it falls onto the black hole it's not gonna be on the perfect trajectories you just take it straight into the center gravity's gonna get involved a little bit it's gonna travel in a curved path and settle down into what we call an accretion disk so a disk of material that eventually the black hole will accrete and so in that accretion disk the outer regions will actually be traveling slower than the stuff closer in just because of the bat that it's got a longer path to go and it's not as affected by the gravity of a black hole as much but this up closer into the black hole as long as it's not past the event mirai than that point of no return where it can't escape anymore it's just going to be a really fast circular orbit it has enough energy to just stay there what needs to happen is it for it to lose energy somehow and to reduce its orbit enough so that it passes that point of no-return how it does that is by transferring energy to other particles in that accretion disk by collisions so instead of gaps if you can imagine a clump of dark matter coming in that same process is not going to happen it's not collision or it's collision less and so the process by which you need to remove that energy or the angular momentum it has from going around in a very fast circular orbit it can't have those collisions to transfer the energy outwards and so it will never reduce its orbit in order to be accreted by the black hole so that way that accretion happens around a black hole is obviously all purely theoretical but what we've managed to do is confirm that idea with our observations as well this has been done quite recently by hopkins richards and her inquest in 2007 where they looked at something called the quasar luminosity function so quasars are basically growing supermassive black holes so the hydrogen gas the normal matter that's in that accretion disk around them gets very hot due to friction and then it starts to glow optical x-ray UV that'll cross all the spectrum and then we can see that and we can see the accretion disk around the black hole so we know that it's currently active and growing and so a quasar luminosity function is basically looking at the numbers of extremely bright quasars and the numbers of more faint equators are all different times or redshift in the universe and so we know how bright they are we know how much mass there actually going to accrete and then we can compare that with how massive we know supermassive black holes have grown to be and so we can say does that amount of brightness account for how big we now know chuckles ah and so what happened in his collaborators found that actually yeah most of them are supermassive black holes can be explained by the brightness that we see and so most of the mass being increasing by supermassive black holes must come from normal matter that we can see because it heats up and glows as it's being recruited the thing is all those observations are obviously gonna have some uncertainties about them especially because the more distant things you look at you know that fainter so observations are a little bit harder to do that less precise and so what we need to do to confirm that result is also to run simulations of this very thing happening on computers and the cool thing about simulations is it can try and put some percentage or a number on how much normal nothing and how much dark matter is greeted by a black hole over its lifetime so one group to do this was pay Ronnie and Deb Rita's Pacheco in 2008 and they basically said in the ideal circumstances when everything is so rosy you can get up to ten percent of all the matter accreted by a black ball in its lifetime is dark matter only up to ten percent in reality it's probably going to be less than that because that idealized the whole situation because this is just how black holes grow you also have to consider what happens when they all as dwell so normal black holes that just form from after when a massive star runs out of fuel and then this will core collapses down into a black hole you know that is going to be mostly normal massive human mostly normal matter in the star itself as well but what about the first black holes the first black holes that we think vary the universe so some of you might remember my video on direct collapse black holes in the early universe so instead of making sort of a black hole it was maybe ten times the mass of the Sun after a big supernova in a star they run out of fuel you could make something that was ten thousand times the mass of the Sun to see the formation of supermassive black holes that could grow to a billion times the mass the Sun at the center of galaxies and so people have been wondering when that process happens do you get Dark Matter collapsing into the center of the black hole but you also have to consider the properties of dark matter no matter again in that scenario so normal Maps so when it hunts together under gravity also has electromagnetic forces acting on it so magnetism and electric forces they're going to be positive and negative they basically make it a little bit sticky making it more likely to clump together more and more in collapse down at it into a star into a black hole dark matter though doesn't have that so it can clump but then those constant disperse and so it's a lot more diffuse that's like a diffuse halos around galaxies as well but obviously if you have that baryonic matter collapsing down then perhaps also you're going to trap some dark matter in the center of it as well but considering how dense that normal matter has to get full of collapse to occur it's more likely that there's going to be a much higher percentage of normal matter than there is dark matter in that very first collapse of material down into a black hole so I'm gonna let you in on a little astrophysics secret here and that is it doesn't astrophysics paper in published in a science journal and that paper has a question for its title the answer is usually always no and I think that sphere of YouTube videos as well especially ones on my channel but here I think the case is more of a maybe we don't know until we understand what dark matter is and what it's actually made of then we can't model this or simulate this properly or understand the observations fully that we're seeing either so we really need particle physicists as hurry up and figure out what dark matter is already because we all really want to know but lend out both of my mics are out of battery and I don't know how this happened at the same time but basically Conor hates me that it's so dense that it's escape velocity is though is great in there today is that by running simulations on computers of this very thing happening oh my god what is it with most of us in the streets and so we think da Matta is collision the little original collision let's go jellal collision with original English you

Channel: Dr. Becky

Views: 156,876

Rating: 4.9044409 out of 5

Keywords: dr becky, astronomy, astrophysics, space, black holes, dark matter, accretion, super massive black holes, blac hole accretion, dark matter accretion, collisional, collisionless, fluids, bullet cluster, direct collapse black holes, early universe, cosmic redshift 7, CR7, fluid dynamics, quasars, quasar luminosity function, womeninSTEM, womeninscience, female scientist, female, women, science, dr becky smethurst, rebecca smethurst, becky smethurst

Id: 9Qis5VDOd18

Length: 13min 17sec (797 seconds)

Published: Wed Mar 20 2019

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she needs a lav mic