Is dark matter made of black holes?

so about a year ago now i made a video on this channel about whether black holes could contain dark matter and nearly every question in the comments was what if it's either way around what if dark matter is made of black horse dark matter is matter that we know is there but we can't see it doesn't interact with light in any way or even interact with other matter in a normal way either that suspiciously kind of sounds like black holes but this isn't a new idea right this has been kicking around for 30 years or so now since the early 90s but this idea has gained a lot more traction recently as a viable hypothesis for to explain dark matter since the detection of gravitational waves yeah this video is going to have pretty much everything chalked into it black holes dark matter gravitational waves and even the mysterious fast radio bursts come into it at the end so let's just take a deep breath and dive right into this so dark matter is matter that we cannot see it doesn't interact with light in any way which is the main way that we receive information from the world and the universe around us and we've talked on this channel before about the bucket loads of evidence we have for dark matter that's built up you know over the past century it's not something that's just sprung out of the ground overnight as an idea to to fudge things you know everything from how galaxies rotate to what happens when galaxies collide to how much mass we measure must be there in galaxies compared to how many stars we can actually see we know it's there we know it's dark and we know how it's distributed in the universe we just don't know what it's made of i just wish i knew now the leading hypothesis right now for what dark matter is made of is that it is an elementary particle that we haven't discovered yet one of the fundamental building blocks of the universe and these elementary particles have been dubbed wimps weakly interacting massive particles because well they don't interact the same way that normal elementary particles do with light so that we can detect them now about 10 years ago you probably would have heard any particle physicists confidently state that well as soon as the large hadron collider is turned on at cern this huge big particle accelerator we will have evidence for a particle that can explain dark matter now sure the large hadron collider found evidence for the higgs boson which we talked before about on this channel but it hasn't found any evidence for a particle that can explain dark matter that's left particle physicists with experiments that search for incredibly rare collisions between normal matter particles and dark matter particles in sort of supercooled fluids underground in these huge big mines but those experiments haven't yielded any results yet either now a lack of evidence doesn't necessarily always mean evidence against but it has got people thinking what if dark matter could be explained by something else it's particularly brought people back to the idea of not wimps but machos massive compact halo objects i.e objects that don't shine that are dark they're just hanging around the galaxy unbeknownst to us so anything from black holes to neutron stars to even rogue planets that aren't orbiting a star anymore now we can tell when one of those objects is there if it passes in front of something that does shine that's in the background the dark object acts as a lens it bends space around it to focus the light from a background star essentially that does is it briefly brightens that background star from our perspective we call these events micro lensing events and if we compare the rate at which we see these micro lensing events happen to the way we think stars are distributed in our galaxy and the way that we think the macros these dark objects are also distributed that would give us sort of a predicted rate the rate that we see them at actually falls sure of the predicted rate and actually the rate that we see them at would only account for 20 of all the dark matter that we think has to be there that was done way back in the 90s which is why the idea of machos is dark matter fell out of favor quite quickly with people when they realized that it just wouldn't account for all of the dark matter that we know has to be there but since there hasn't been any detection of any elementary wimp particle in the meantime people have now begun to revisit the idea especially since 2015 when the ligo team announced the detection of gravitational waves almost a hundred years after einstein first predicted them in his theory of general relativity general relativity is a theory or a description of what gravity is like it essentially says that any massive object bends space around them and so any object or light moving around that object will travel along curved space and therefore take a curved path this is how we get orbits and these lensing events as well now you can imagine that if two massive objects are actually orbiting around each other then the amount that space is bent is going to change with time producing ripples that then travel outwards from those two objects through space and this is what was detected by the ligo collaboration back in 2015 and that collaboration was over a thousand people strong and all a thousand of them were shocked when they first saw that detection because it almost perfectly fit with a model of what general relativity would predict for two merging black holes and the collaboration spent months trying to explain that signal in any other way see if there was any other explanation for it before they announced it to the world and said what we have detected has got to be two merging black holes where the surprise came from with this detection was how heavy the black holes were that have merged they were both around about 30 times the mass of the sun and the question was how in the universe did two black holes 30 times heavier than the sun end up orbiting around each other okay that makes no sense now most black holes in the milky way except the supermassive black hole in the center are thought to form at the end of a star's life when a star goes supernova so something that's at least 20 times heavier than the sun when it runs out of hydrogen gas i.e it's fuel for nuclear fusion in its central core regions where it's actually hot enough for nuclear fusion to happen then the star will rebound outwards in a supernova throwing like the majority of its outer layers out whilst that core collapses back down under its own gravity into a black hole somewhere around you know one to three-ish times the mass of the sun depending on how big the star was before it went supernovae to a much bigger star will produce a much bigger black hole at the end now the sun is actually a rarity in our galaxy the milky way in that it is a solo star in the center of the solar system 85 of star systems in our galaxy are made of two stars or even more so something we call a binary system when there's two stars orbiting around each other so if you've got a binary system of two massive stars you know more than 20 times the mass of the sun then you're going to end up with a binary black hole system at the end after those two stars have run out of fuel and gone supernova but the black holes that have been formed will be again like one to three times the mass of the sun and not 30 times the mass of the sun like what ligo detected in fact the average mass of the black holes that ligo has been finding merging together is about 26 times the mass of the sun so this surprise at the ligo result was due to two things right it was firstly due to the fact that no one had considered these mass ranges of black holes before and second because no one really had any explanation for how these black holes could have formed okay maybe you could have had previous mergers of black holes that would have grown them and maybe they decreated a little bit more matter over the billions of years maybe you could have had some specific system of five stars that became two black holes in the end you could explain maybe one system in that way but not you know more than ten that ligo have now detected now because these black holes detected by ligo were so much larger than anyone had considered when thinking about machos back in the 90s that's why this idea of these black holes is dark matter was revisited here now we could just speculate away on the internet right about whether black holes are made of dark matter but what we need to make this a viable scientific hypothesis is some sort of test that we can do and the ligo detection gave people just that now after just a few months of the announcement of the detection of gravitational waves by the ligo team two separate research teams published papers with a similar experiment they said that if you assume that dark matter is made of black holes and they are distributed to give us the distribution that we know dark matter has to have in the universe then how often would you expect them to encounter each other at a close enough distance that they will eventually end up spiraling around each other and merging if you do that you get a number that's around about three per year per giga per sec cube that's a volume of space quite a large volume of space now that number is heavily dependent on what assumptions you make in terms of what mass the black holes are what the local density of those black holes you have rather than sort of the global density is as well but even changing those parameters you can see in this part there's many different model lines there but they all fall into that shaded purple region that purple region is the merger rate estimated by the ligo team for how many similar things that they should detect which is between two to 54 per year per giga per sec cubed now the fact that these two rates were very similar very coincident with each other is what had people questioning whether what ligo had detected was the black holes that make up dark matter now obviously that didn't explain still where the black holes of the actual mass actually came from but the leading hypothesis to explain that is that they are primordial black holes i.e they formed in the very early universe not from sort of gravitational collapse of stars down but from these tiny quantum fluctuations in the early universe that meant that you know there was slightly more particles in this area of the universe than that area of the universe and where there was more particles it just so happened that you created this tiny object that you know you would need to escape velocity greater than the speed of light to escape a black hole now their existence was first proposed by zeldovich and novokov back in 1966 and then stephen hawking came along and did the maths behind them in 1971 showing that they could be as small as you know 10 nanograms but could be as large as tens of millions of trillions of trillions of kilograms i.e tens of times the mass of the sun so there are now theoretical astrophysicists that believe that primordial black holes could be a viable hypothesis to explain dark matter but there's always a but there is probably a bucket load more evidence against primordial black holes being dark matter than there is for it from these coincident rates and the masses that ligo just happens to be detecting if you have these primordial black holes forming in the early universe they're obviously going to leave an imprint on the early universe and on the universe that we see today so for example one of the big pieces of evidence against them is the cosmic microwave background that echo of the big bang from the early universe it rules out the presence of so many massive primordial black holes because of the effect that they would have had on it that we don't see even the fact that dwarf galaxies exist at all pretty much rules out the presence of such massive primordial black holes because the presence of so many of them in a dwarf galaxy would have completely disrupted the orbits of the stars and basically just sort of poofed out the galaxy and dispersed them across the universe also if that many primordial black holes existed especially in the centers of galaxies where we think dark matter is at a quite high concentration there's also a lot of gas there as well so those black holes would start accreting that gas and feeding on it and when that happens gas heats up it gets excited and it starts glowing in x-rays and we don't see a lot of little points of x-rays towards the centers of galaxies or the center of our own galaxy when we look so this gives us a situation where the majority of astrophysicists myself included you know look at all this evidence against primordial black holes in dark matter and therefore still prefer the hypothesis that dark matter is made of an elementary fundamental particle which handily shifts the onus of responsibility to figuring out what dark matter is to the particle physicists but those astrophysicists that are still in favor of the primordial black holes as dark matter hypothesis have come up with you know one more test that they can do before they let the hypothesis go and that is using fast radio bursts these are these incredibly bright millisecond bursts of radio waves coming from other galaxies in space and as yet what's producing these signals is unexplained although if you see my night sky news from may a couple of months ago you will know that perhaps there's some evidence that it could be magnetized but as of yet still no consensus as to what's causing fast radio bursts but despite not knowing you know what they are yet they still provide us with a very useful test now if a primordial black hole got in the way of the signal from one of these fast radio bursts from another galaxy in space making its way across intergalactic space to us then that primordial black hole would once again act as a lens because it would bend the space and therefore the signal would get bent around the black hole in fact take two different paths around the black hole one that was slightly longer than the other and what this would create in the signal that we would actually detect is a slightly fainter echo of the fast radio bursts a short time afterwards as long as the primordial black hole was large enough to do that around about 30 times the mass of the sun now to test this we need a large sample of fast radio bursts but at the minute we know of around 40 or so sources of fast radio bursts which is nowhere near a large enough sample to do this within to get good statistics with however the chime telescope in canada is now actively surveying the sky littening out for these fast radio bursts every single night now the hope is that within a decade or so we should have about a thousand or so of these fast radio burst detections and a thousand is a sample we can definitely do statistics with and if dark matter does turn out to be made of primordial black holes at about 30 times the mass of the sun on average then we should see about 10 of those a thousand fast radio bursts having one of these faint echoes but that's not for another 10 years or so we have another waiting game just like the search with elementary fundamental particles these wimps and once again this search might not turn up anything either now i know a lot of people don't like waiting but for me it's the anticipation from waiting that'll make that result you know whatever it might be all the more exciting when we finally have it but those experiments haven't found it haven't found it now the leading hypothesis for this is that they're made of primordial black hole prime ordeal they've they've really gone through an ordeal they're just they've come through a terrible ordeal wondering what is dark matter [Music] even if we're just primordial black [Music] holes

Channel: Dr. Becky

Views: 126,173

Rating: 4.9438972 out of 5

Keywords: astronomy, astrophysics, space, black holes, dark matter, super massive black holes, dark matter accretion, early universe, gravitational lensing, primordial black holes, WIMPs, MACHOs, microlensing, science, physics, stargazing, cosmology, fast radio bursts, general relativity, womeninSTEM, womeninscience, female scientist, female, women, dr becky smethurst, rebecca smethurst, becky smethurst, dr becky

Id: d0wV5frSb6s

Length: 18min 13sec (1093 seconds)

Published: Wed Jul 15 2020

Reddit Comments

[u/Charyou-Tree]

A good high-level overview about why black holes were considered dark-matter candidates, why they were initially ruled out, why LIGO results reignited the debate, and how future observations could definitively tell us one way or the other.

[u/SirJekyll]

Dr. Becky is awesome. Just finished her book! Highly recommend.

[u/mpf1949]

Very informative... I subscribed to her channel immediately...

[u/mad_science_of_hell]

Hmm I will have to watch this just to see how we wouldn't be aware if dark matter was absorbing light like a black hole. I thought the one thing we knew about dark matter was it didn't interact with light.