How do we know dark matter exists? | A century's worth of science history

before we get the science of dark matter this video is sponsored by audible because my audio book comes out in the UK next week the Thursday the 5th of September now I have narrated the audio book myself which is why I partnered with audible this week so that you can actually listen to the book as I intended it to be heard I absolutely love audiobooks for that reason also I spend about an hour of my day walking to and from train stations as part of my commute into work and so listening to an audiobook actually makes that part of my day really really fun so you could go to audible and find my audiobook and do the same thing or if you fancy something more like a fiction book I just finished listening to paced Browns red rising saga so particularly the dark 81 that's just come out it's so good if I had to explain it it would be like Game of Thrones but instead of like medieval it's like Roman and also set in space like a thousand years in the future when humans have like colonized the solar system it's amazing so you can start listening with a free 30-day trial with audible and your first audiobook and your first two audible originals are free and so to do that go to audible.com forward slash dr. Becky or text dr. Becky to 500 500 if you're in the u.s. only that's audible.com /dr ve CKY for your 30 day free trial Hey anyway back to actual science I figured since a book is coming out I'd give you a little sneak peek as to what one of the chapters is actually about and so I've picked chapter four just because you haven't seen it doesn't mean it doesn't exist now I may have lifted that line directly from the Christmas movie The Santa Clause just please you'll have it soon guess what doesn't exist but it's not just relevant for father Christmas it's also something that rings true for dark matter something that sort of been considered since sort of like pre nineteen thirty but only really accepted in as part of our best theory of the universe is on the end of the 1970s and it's a concept that a lot of people struggle with because if you can't see it how do we know that it exists so the first person to even conceive the idea of dark matter was Lord Kelvin back in 1884 and what he was doing was measuring the velocity of stars in the Milky Way so the velocity of stars in the sort of general neighborhood around the Sun and he was trying to work out if they were in like a stable system whether the velocity of the stars and therefore the kinetic energy they had with equal to the amount of gravitational energy they had if the gravity is too strong and we knew sort of collapses in on itself and if they've got too much kinetic energy then the whole thing flies apart and if you're in a bound system then the spread of your velocity is something that we call the velocity dispersion we use the Greek letter Sigma to represent it is proportional to your mass divided by your radius and with that velocities version he then got the amount of mass there should be in the system in order for it to be bound which we knew it was because all the stars weren't flying apart from each other and when he did that and compared that amount with how much you calculate is sort of trapped in stars that's glowing and we can see is that the two are very different there's not enough mass in stars to explain how much mass you have to have to stop the whole system flying apart and you'll see that that is a common theme going forward in the study of dark matter is that there's more mass there than we can see so Lord Kelvin said there must be dark stars that aren't shining that we therefore can't see that must be making up this missing matter so then in 1906 the French astronomer Kwang Clare was then discussing kelvins work in a big sort of a review discussion paper and he referred to that missing matter as Messier obstacle II which directly translates from the French as unknown matter for obscured matter but somehow that kind of got lost in translation and mixed with kelvins failure stuff where he said they were dark stars to give us in English what we call dark matter and it's kind of a German simply has ever since it's not that it's dark is that it is unknown and then it doesn't interact with light at all but the term stuck it was also in the 1920s that astronomers realized thanks to the work of Hubble and Shaklee and Kurtis that the Milky Way wasn't the entire universe there was actually galaxies outside the Milky Way that again.what islands of hundreds of billions of stars in their own right and they were found clustered together and sometimes alone in sort of the vastness of space and so then work turned away from the Milky Way to really studying these individual galaxies and the clusters they were found in so in 1930 loon mark was again measuring this difference of the mass that we calculate to be in a galaxy compared to the amount of mass suggested there by light it's only we call the mass to light ratio or M over L and he measured this mass-to-light ratio for five of these new galaxies that have been found and found that it was anywhere from a hundred times more mass than light you could see for a galaxy called Messier and 81 and all the way down to six times more mass than light you can see for galaxy called Messier 33 and again he concluded that the only thing this mass could be would be extinguished stars things like comets or meteors and clouds of gaps that you couldn't actually see then in 1933 Fritz Zwicky came along and if you don't know for its wiki just go and read about his life he was an amazing character in astronomy and what he did was he measured again this velocities version but instead of stars in the Milky Way he mention it for galaxies in clusters so he measured the speeds that galaxies are moving around these clusters and he found a master light ratio for a very famous cluster of galaxies called the Coma Cluster of four hundred four hundred times more mass in the cluster than you could see in stars now that actual estimate was off by about eight times because our estimate for how quickly universities expanding was off at the time but still he correctly concluded that there was more dark matter than there was visible matter in stars and again he concluded that that non luminous matter had a made of cold stars gases and other small bodies like planets asteroids comments etc then at the end of the decade in 1939 Babcock published his PhD thesis in which he'd measured the rotation curve of the andromeda galaxy now what that means is that you measure how fast the stars are rotating around the center of the galaxy at increasing radius in the galaxy and then you get a nice curve out for what is going on in the galaxy we could do the same thing for the solar system we could look at how fast the planets are rotating around the Sun and we found that the closer in you asked for some the faster a planet is going around and then the further away you are from the Sun the slower planets going around and that's because 99.9 percent of all of the maps or all the stuff in the solar system is in the Sun the majority of the mass is concentrated in the center so when you look at a galaxy you also see more stars in the center of the galaxy than the outskirts where it gets dimmer and so people expected that a similar thing would be found as what we see in the solar system where you get a drop-off of the rotation speed as you went further out but Babcock in 1939 actually found the complete opposite he found that it was increasing with radius now again his observations weren't the most precise at this time but still it's gonna be very hard to get an increasing rotation curve rather than a decreasing rotation curve just because your telescope isn't that great and again what this suggested was that there's more mass on the outskirts of the galaxy then there is in the center despite the fact that that's not what you see in stars at all so research on this obviously stalled in the 40s because of world war two but it did mean that by the 1950s galaxy surveys and cluster surveys really took off because a lot of the post-war technology was sort of left lying around and not used anymore everything from military radar to radio detectors that was no longer needed could then be used actually for astronomy and pointed at the sky to be used for radio observations so a lot of that was actually done by young ought in the Netherlands who really coordinated that whole big effort to do that so that by nineteen seven Van der Hulst Raymond and Van Borden were the first people to obtain a rotation curve for Andromeda not in optical light but in radio light they were actually detecting emission in radio wavelengths at 21 centimeters from hydrogen gas in the Andromeda galaxy and so the first time we were probing stuff that we couldn't actually see in stars and so they took this rotation curve out much further than the optical and found that it does rise in the center but then flattens out towards the outskirts giving you a flat mass-to-light ratio and so as Ricky's unseen Matter idea floats again but this time thinking that it might beat you to gas then in 1958 i'm Barsoomian brought the attention back to news galaxy clusters again whether they were these gravitationally bound systems and whether the kinetic energy balance the gravitational energy and he said there's no missing matter at all in fact these systems are very short list lived systems the velocities of the galaxies means that they're gonna fly apart in the space of about 10,000 years or so but actually Burbidge Burbidge had to step in 1959 and point out that if that was the case then the galaxies these clusters contained were younger than the clusters themselves so how did the galaxies even form in a cluster in the first place if they were billions of years older than the cluster so this led to a lot of arguments in the 60s particularly at a conference that was held in Santa Barbara in California in 1961 where people got together to sort of argue between zucchinis missing matter and embark simians they should just fly apart despite the fact that they're younger than the galaxies they contain but the conclusion from that meeting was essentially we don't have enough evidence to decide thankfully there was a couple of discoveries later in the 60s and also in the 70s they would do just that the first of which was that in 1963 Schmidt discovered the first quasar or qso quasi stellar object it was something that looked like a very very bright star but when you got a spectrum of that object I you split the light through a prism into its component wavelengths you realize that that looks nothing like a star at all and actually also had a redshift of 0.158 ie it was recessing away from us it's 16% of the speed of light that might not seem relevant to the problem at hand right now but eventually it did contribute in the 70s so bear that in mind for later then another of these results that's going to help in the long run in 1974 Penzias and Wilson accidentally discovered the Cosmic Microwave Background this radiation relic from the Big Bang that has been red shifted as it traveled across the entire universe to get to us into the sort of microwave radio regime it was the first direct proof that the universe started in a Big Bang so people now accepted the universe had a beginning and so their thoughts then turned to well what would the end of the universe be and people realized it all depended on the balance of gravity pulling all the stuff in the universe together and that expansion pushing outwards and so people were very concerned with measuring how much matter is there in the universe as a whole again if the expansion is stronger than that gravitational pull then will expand forever if it's not as strong as the pull of gravity pulling all that mass inwards them will eventually sort of end up in like a Big Crunch scenario or is it perfectly balanced again and we'll end up with a nice equilibrium suddenly this missing matter problem wasn't just a curiosity for people who study galaxies anymore it was a cosmology problem and so there was a real focus on measuring the mass in the universe in the next couple of decades in the meantime rood in 1965 was studying this problem of galaxy clusters again and this problem of missing matter and he was looking at how the galaxies move in the clusters and he determined that that missing massive zwicky's must be in between the galaxies themselves in the intergalactic medium and he concluded therefore that had to be gas the gas that went beyond the visible stellar light that we can see in galaxies except that Penzias in 1961 had already looked the amount of gas you can detect in between the galaxies in the Pegasus cluster and he found that it was only 10% of the mass that you measure must be gravitationally there from the speed that the galaxies are moving up that was confirmed two years later by Meekins who was looking at the gas this time in x-rays you get very hot gas between galaxies and clusters that glows in x-ray and again he worked out how much mass was there in this hot gas and found that it was only 2% of the gravitational mass of the system of cluster of galaxies so if it was true that there was all of this missing matter just pervading the space between galaxies in clusters then these results in the end of the 60s proved that it actually couldn't be gas alone so the 1970s was really the decade where dark matter came to the forefront of physics and it was all kicked off in 1970 by Vera Rubin who again was looking at the rotation curve of Andromeda this time in the optical again but using a much better instrument that was designed by Ford and so the two of them together published their results in 1970 showing a very accurate rotation curve andromeda and showing that it was again flat suggesting a flat mass-to-light ratio not just in gas but also in the stars as well now they did not give an interpretation of that flat mass-to-light ratio they left it for the reader is sort of an unsolved problem it took Freeman coming along later in the same year combining their optical rotation curve with the radio rotation curves that been done in the previous decades and then also theoretical modeling of what you see in the galaxy he said that the amount of mass there must be at least the amount of visible matter we could see and that it must be distributed very differently to how the stars were distributed then in 1974 there was two papers that came out within a week of each other the first byte of striker Peebles in your Hill then ina snow Kazakh and saw both of which brought together these two separate problems of them flat rotation curves in Andromeda and other galaxies and the missing matter in clusters and where the clusters should be flying apart or gravitationally bound with much more mass there than we could actually see and they said both of those problems would be solved if you assume that on average the mass-to-light ratio in galaxies was about 10 ie 10 times as much matter there than we could actually see these two papers though still attributed that sort of mass light ratio of 10 down to gas again though despite the results of the previous decade showing that it wasn't gas but no one really had a better explanation for it at the time so they took two very different problems on very different scales and brought them together to solve them it makes me wonder what unsolved problems we have now on like vastly different scales that could be brought together to help solve them things like unknown knowns so exciting now remember also the first quasar that was discovered by Schmidt back in the 60s well in 1979 Walsh Carswell and Wayman discovered the twin quasars using the telescope's at the Georgia Bank Observatory in Manchester New where I grew up and what they found was that these quasars were separated by a tiny amount on the size I mean like six arc seconds that's six 3600 of a degree through a tiny amount they were also the exact same brightness and when they took a spectra the spectra was exactly the same shape pretty much and also that they were at the same redshift are the same distance away from us and so what they were grudgingly concluded was that actually what they were observing was the same object but with two images and I really how does one object turn into two images well they said it might be caused by a gravitational lens if there was a galaxy in the way of the quasar they were seeing then what could happen is that the gravity of that galaxy could bend the light of the quasar and produce two images of it that we would see that's not a new concept Einstein it predicted it in theory general activity way back at the beginning of the century and then Eddington had used that in their clips of 1919 to show that actually the Sun so gravity like actually bends the light from stars behind it and allowed us to measure the mass of the Sun so even though it was a tentative conclusion it wasn't out of the realms of possibility there that little piece of information in mind for the next decade taking in the seventies though in 1979 at the very end of the decade Faber and Gallagher produced a review article that summarized the current view point on this missing matter problem in clusters and galaxies and brought everything together to conclude that the case for missing matter was strong and growing stronger before that article was published you know everyone sort of in the galaxies field and galaxy cluster field was kind of believing this idea of missing matter or dark matter but outside of the field it was sort of looked at with a bit of scorn or scoff to it and only after that article was produced summarizing everything that had been found in the past couple decades did people outside of that field of astronomy and in physics start to take the problem much more seriously so into the 80s with people starting to take this idea of dark matter much more seriously Milgram in 1983 then sort of said whoa hold your horses what if we've just got gravity wrong this entire time and what if we don't actually need all this missing matter it's just that we don't understand our laws of gravity he said all of the observations could be explained if very small accelerations that we don't really see on earth or in the solar system but you do see four stars in galaxies that actually Newton's laws were F equals MA squared instead of F equals MA and the F equals GMM over R rather than GMM over R squared and this was dubbed Mont or modified Newtonian dynamics and people in universities across the world are still today researching various different permutations of mond I'm not going to go into here though why dark matter is still the favorite theory and why Mon sort of fell out of favor but my book which comes out on Thursday September 5th in the UK does go into that if you are still interested in hearing or about it back to our pool quasars though in 1985 chakra discovered what was dubbed the Einstein cross very similar to the twin quasars that have been discovered last decade but now there was four quasars that looked identical had the same spectra shade were at the same redshift and at the same brightness as well and again the only way that people thought you could really describe that was if it was one object with four images caused by gravitational lensing then again two papers in a couple of weeks of each other came out one by su ko for Tamela and pika and another by governer that found lens galaxies in clusters of galaxies these huge big blue arcs which is a single galaxy in the background that has been stretched out by the galaxy cluster acting like a lens to produce this sort of arc and suddenly everyone is finding lenses in old new data and everyone is all of a sudden measuring the masses of these galaxy clusters from the shape that the lens is made because that suggests how much faster has to be there to actually make those shapes and once again they're finding that gravity ie Einstein's theory of general relativity suggests those ten times more matter there than we can actually see in stars in the 1990s more and more evidence for dark matter came through this time it was using that cosmic microwave background that had been discovered accidentally by Penzias and Wilson back in the 70s and cosmologists has said that the way matter clumped together in the early universe should be imprinted on that cosmic microwave background that relic of the Big Bang and the Coby satellite team in 1992 back when I was just a teeny tiny toddler discovered this variation in the Cosmic Microwave Background with place on the side something we call an isotropic and so that reveals the areas that there was more and less matter in the early universe and if you look at the differences between those fluctuations in the Cosmic Microwave Background and you look at the frequency of how often various different differences occur if dark matter exists you should find a peak at one of those frequencies this is why they wanted to try and look this anisotropic because they wanted to look for this specific peak that would say our models suggest that Dark Matter exists if we find this peak now in 1892 it was too coarse to detect with that data we actually had to wait till 2000 with the boomerang experiment which was actually just like a high-altitude balloon experiment to measure the Cosmic Microwave Background to get it in enough detail to actually detect that peak at that given frequency and say for definite that in our best model of the universe dark matter has to exist at the same time though astronomers are still trying to determine what this dark matter is and so if it's not gas then the focus turned to the idea of dark stars again of kelvins now that we know is sort of like the end process of a star's lifetimes things like white dwarfs neutron stars and black holes the thing is the number of those objects you need to give you that mass-to-light ratio of 10 of so is massive and so Strom is like well we should be able to detect those things because as soon as they pass in front of a star from our line of sight they're going to act as a gravitational lens again and then what they're going to do is brighten that star from our perspective and so people went in search for these things you have the macho collaboration which stood for massive compact halo objects which is how they referred to this sort of collection of what Dark Star objects and also the eros collaboration looking for these two and in 1993 they both announced a detection of one of these lensing events and so people got really excited being like oh it could be black holes because we've actually detected one of these but then over the next sort of seven eight years they continuously monitored for more and more of them and didn't find as many as they expect at all and so with the number of detections they did have they could put an upper limit on how many of these map shows there must be in our Milky Way and the macho collaboration came up with a number between eight and fifty percent of all of the missing matter in the Milky Way and the eros collaboration said actually it's got an upper limit of maximum eight percent of all the missing matter or dark matter so the results did differ a little bit but it was very clear to people by the end of the nine used that dark matter was not made of gas than it was not made of macho's ie black holes neutron stars either in the 2000s the first direct proof for dark matter was found so Clow Gonzalez and Markovic in 2004 discovered the bullet cluster this was two clusters of galaxies that were in the process of colliding again using the distortions of background galaxies through the clusters in two arcs they were able to get a map of where all the matter was in this collision of these two clusters of galaxies they could then see where all of the stars were in the galaxies in the clusters as well and then also they had x-ray observations that showed where all the hot x-ray gas was now in a collision of two classes of galaxies all the gas that's in between all of the galaxies collides in that collision heats up incredibly hot and that's why it glows in the x-rays but the galaxies they kind of just fly past each other and go out the other side so what you've got with the bullet cluster collision is you've got all the galaxies having to pass through each other but leaving behind all that hot x-ray glowing gas that actually turned out to be the dominant amount of mass that we can actually see with visible light there's more gas there in the x-rays than we actually see in stars in the optical the thing is when you look at where all the lens galaxies are which reveal where the majority of the actual mass is in terms of the gravitational mass ie the dark matter that you can't see actually traces where the galaxies are and you actually find that there's more mass there than there is in that x-ray hot gas that's been left behind in the middle despite the fact that that's brighter and that's where the most of the visible mass actually is as this was the first direct proof of dark matter it definitely wasn't gas because that's why the gas was is definitely not the stars that enough matter to be able to create that amount of lensing it had to be dark matter but it won't showed us was that dark matter wasn't normal matter ie what we call baryonic matter everything around you right now from this desk to me to this chair to my camera is made out of baryonic matter normal matter but dark matter can't be this normal matter because it doesn't interact with light it doesn't absorb it it doesn't block it it doesn't give out light at all and then it also doesn't behave the way that you would expect a collection of normal particles like in a big gas to behave either it had to be some form of exotic particle that we never detected before that particle physicists can even dreamed up before that existed neutrinos totally would have been the favored particle by particle physicists if for the fact that in 1998 the super-kamiokande experiment had shown that their mass was tiny like far too tiny to explain the amount of matter that astronomers were detecting had to be there so research is still ongoing into Dark Matter today it's one of the biggest unsolved problems in physics in particular astronomers and physicists have shown that it has to be that it has to exist but we still haven't actually detected any of it we don't know what it's made of and that's a problem for particle physicists and to try and figure that out that either trying to detect some through the collision of a Dark Matter particle with a normal matter particle in these big underground mines or by trying to make some in these big particle accelerators that they built like the LHC in CERN underground this huge big circular kilometer long track the problem is don't know the recipe to try and make some dark matter so theoretical physicists are also trying to work on what the possibilities could be for this unknown exotic dark whatever you want to call it material so remember just because you haven't seen it doesn't mean exist okay focus focus Becky we must talk doc mad announcing using an optical spectra designed by Henry Ford what Henry Ford doesn't anymore design cars not bad for it and so people are very interested to see if they saw any variation in the Cosmic Microwave Background and so the COBE satellite team in 1920 to 1922 neutrinos would have been the favourite particle by particle physicists to explain dark matter but for the fact that this super coming Clarke super-kamiokande you become your condo super-kamiokande cameo canned a cameo candy was a cameo of candy cameo candy so in the 2000s the first direct proof for dark matter was do you mind my back I am trying to prove dark matter here you

Channel: Dr. Becky

Views: 157,700

Rating: 4.8969984 out of 5

Keywords: astronomy, astrophysics, space, physics, science, milky way, hubble, great debate, van de hulst, vera rubin, history of science, dr becky, dr becky smethurst, rebecca smethurst, becky smethurst, infrared, curtis, shapley, lord kelvin, poincare, fritz zwicky, cosmic microwave background, CMB, big bang, dark matter, dark stars, andromeda galaxy, rotation curves, galaxy clusters, bullet cluster, quasars, gravitational lensing, twin quasars, einstein cross, ad

Id: nbE8B7zggUg

Length: 29min 49sec (1789 seconds)

Published: Wed Aug 28 2019

Reddit Comments

[u/SteakAppliedSciences]

Is this worth watching during my lunch break, or is it something that would be better to wait until home? I don't want to get absorbed into the video and go on a youtube spree after this one.