Dark Matter Exists. Here's how we know.

hey crazies dark matter is 84 of the matter in the universe and it single-handedly explains a lot of stuff gravitational lensing cluster motion galactic rotation fluctuations in the cmb scientific consensus is that dark matter exists we have piles of irrefutable evidence let's go through it shall we [Music] the first hint of dark matter came to us all the way back in the 1930s to the timeline in 1933 fritz zwicky was studying a few galaxies in the coma cluster that's a cluster of over a thousand galaxies about 330 million light years away zwicky discovered galaxies in the cluster were moving way too fast they were moving so fast there wasn't enough mass to hold the cluster together oh hey man long time no see where's question clone i thought i could use a break plus this video has my name written all over it like literally okay sure that that makes sense yep so how does wiki guy take all these measurements anyway oh great question taking measurements at cosmic scales is tricky today let alone in 1933 first you have to measure that distance with some kind of standard candle that's a star with a predictable brightness like a cepheid variable or a type 1a supernova if we know the actual brightness we can compare that to the apparent brightness since light obeys the inverse square law we can get a distance from that knowing that distance gives us a sense of size in images like this next pull out your pocket hertzsprung russell diagram what you don't have a pocket hr diagram every respectable astrophysicist has one of these on hand at all times is it too starky no it's perfect okay that's snarky enough whatever you can use mine an hr diagram tells us almost everything we need to know about stars if we know its actual brightness and its color this diagram tells us its radius and the stage of life its in and ultimately its mass this diagram is kind of a big deal and not like big deal clone like an actual big deal dude again not cool anyway this diagram was around decades before zwiki entered the astronomy scene all he needed was the overall brightness of a galaxy and he could estimate the mass of its stars speed is a bit more difficult cosmic distances are vast so we don't actually see any of this stuff moving visuals you might have seen of galaxies colliding are simulations in extreme fast forward to figure out speed we need to measure something called doppler redshift that's when light changes frequency as it moves toward or away from us kind of like this these two stars are always emitting white light it's the relative motion between us and them that changes the color blue for tortoise and red for away from us that's highly exaggerated though you wouldn't be able to see the color shift with your eyes you have to split the light into its spectral colors then you look for gaps and compare those gaps to where we expect them to be based on what the stars are made of it's not easy but it's something we can do okay let me get this straight you get the mass from the brightness and the speed from the redshift yep now what now we apply gravity and see what happens this was originally about zwicky's observations of a few galaxies in the coma cluster galaxies in a cluster are moving purely under the influence of gravity gravity is in control solving a many body problem can be tricky though so zwicky tricky's wiki trickies wiki used a mathematical tool called the vreal theorem sparing you the gruesome details it relates the kinetic and potential energy in a system and gives us this based on the measured speed of the galaxy and the effective radius of the cluster we can figure out how much mass is needed to make the galaxy do that spoiler alert the measured mass wasn't enough tricky's wiki inferred there must be matter there that couldn't be seen there must be some kind of dark matter now i say inferred rather than discovered because he had entirely the wrong impression and his estimates were way off by dark matter zwicky meant matter that is dark essentially stuff that doesn't emit or reflect enough light to be seen from earth sure that could be weird stuff but a lot of ordinary things meet that definition too planets moons asteroids comets big dark clouds or even aliens if they exist zwicky was imagining giant clouds of cold diffused gas the mass in his universe looked like this a tiny bit of it was stars and the rest was mostly just gas he had opponents and for good reason he was making claims he didn't have enough evidence for that's bad science zwicky had the right idea but for the wrong reasons he thought the proportions were like this when they were actually like this but we wouldn't figure that out for a few more decades back to the timeline in 1951 harold eowyn and edwin purcell purcell in 1951 harold ewan and edward purcell discovered the 21 centimeter hydrogen line this allowed us to find all the cold hydrogen between stars and between galaxies wait wait wait coal hydrogen 21 centimeter line explain this stuff okay okay fine you know how electrons can emit light when they jump energy levels well sometimes they can emit light just by flipping over seriously it's a thing it's called a hyperfine transition the electron in a hydrogen atom has slightly more energy in one orientation than the other the difference is so tiny that the transition only emits a radio photon the wavelength of that photon 21 centimeters even if hydrogen is cold in a diffuse cloud of gas it'll still emit this 21 centimeter light which means we can see it and we see a lot of it but it still isn't enough to explain tricky's wiki's mass problem then in 1962 we discovered the first extrasolar x-ray source using detectors on rockets yeah you heard me rockets earth's atmosphere absorbs a lot of x-rays from space x-rays from space so our x-ray detectors have to be almost in space before they'll see it these days though we have space probes orbiting the earth anyway x-ray astronomy opened a whole new way for us to look at our universe and when we did we saw something new hot hydrogen so in the 50s we found cold hydrogen and in the 60s we found hot hydrogen that kind of sounds like all the hydrogen and it still wasn't enough to explain the mass problem progress on the dark matter problem wouldn't arrive until the early 1970s that's when vera rubin was studying galactic rotation using kent ford's spectrograph vera rubin was a big deal you should know her name anyway she mapped out the distribution of hydrogen in several galaxies but when she compared that to the rotation rates she noticed something strange see a galaxy isn't a solid thing the material doesn't all rotate together it's a bunch of separate gravitational orbits consider our own milky way galaxy our sun is about this far out and orbits around the galaxy you might think that orbit is controlled by the supermassive black hole at the center but that's only partially true the mass in control of the sun's orbit is actually all the mass inside this sphere that's everything from the center out to the orbit if the orbit were closer the sphere would be smaller the opposite is true farther out that's just how gravity works in this kind of situation which brings us back to vera rubin her maps of hydrogen in a sample of galaxies looked like this it consistently gets thinner near the outer edge because of the way gravity works she expected to see a similar drop in the orbital speeds of stars near the outer edge but the data showed something else entirely the speeds actually level off instead of a galaxy rotating like this it was rotating like this when data doesn't match your prediction it's time to reevaluate things and that's exactly what vera rubin did she asked herself a question what if there's more mass than we can see yes this should sound familiar she's talking about dark matter it was the same argument tricky zwicky made back in the 1930s but vera rubin was doing it with a proper amount of evidence vera rubin was doing good science vera rubin discovered dark matter this was the first time that dark matter was a legit scientific conversation so let's take an updated look at the breakdown for matter active stars represent about 1.5 percent of the matter in the universe planets moons asteroids comets and aliens percent altogether not even worth counting diffuse clouds of gas and dust 14.5 there's still 84 unaccounted for that's the stuff we call dark matter but you know what maybe you're still not convinced maybe the motion of galaxies and clusters and the orbits of stars and galaxies aren't enough for you well i've got more evidence for you back to the timeline in 1979 we discovered the first gravitationally lensed object that's when light from a distant object is bent around another and we see multiple images mass bends space-time and anything traveling through that space time will be affected by any bends there might be in it that includes light this lensing effect usually occurs around bright massive galaxies but occasionally we find gravitational lensing around seemingly empty space in 1998 we discovered the bullet cluster that's two clusters of galaxies that have collided uh collided might be a strong word they kind of just pass through each other but you get the idea observing with visible light this pair of clusters isn't much to look at most of the light being emitted is actually x-rays remember that's the hot hydrogen we started finding in the 1960s anyway these two pink blobs are what we actually see with x-ray telescopes this one on the right gives the bullet cluster its name but if you look closer you'll see gravitational lensing over here despite there not being enough mass in that spot for the lensing to occur the vast majority of the visible mass is in the pink blobs the gravitational lensing is caused by invisible matter by dark matter isn't dark matter supposed to be in galaxies what's it doing all the way over there the collision separated the regular matter from the dark matter before the clusters collided all the matter was mixed together as expected but as the collision occurs the regular matter interacts and slows down dark matter doesn't interact in the same way it doesn't slow down this allows the dark matter to get out ahead of the regular matter in the collision it separates them but if you're still not convinced let's take a look at this cosmologically the cosmic microwave background or cmb is arguably the greatest source of data we have available about our universe it's the oldest light that currently exists so what direction is it coming from all directions this is just a map of the spherical sky using the mall vita projection you've seen maps of earth using the same projection just like the earth the cmb is actually a sphere so this is what the cmb looks like it's mostly uniform but we can see tiny variations if we exaggerate them for clarity those variations tell us a crap tone about our universe here's the 2018 data from the planck space probe but just throwing the data at you like that isn't exactly user friendly we can do better starting with the standard model of cosmology known as lambda cdm that's dark energy plus cold dark matter we can plot out exactly what we expect those background variations to look like that would be something like this the first peak assumes a universe with zero curvature the second peak assumes all the regular matter that we observe that's all the stars and all the diffuse gas the third peak assumes the rest of the matter is dark matter so this is what the model would predict for an 84 dark matter universe the measured data from the actual cmb tells us this it matches perfectly with the prediction including the dark matter peak bingo bango huh that's kind of undeniable isn't it yep pretty much also this is just the highlights i'm holding back here because this video is already too long the point is we have almost a century of evidence backing this up it's not just some random idea and every time we collect more data it just supports the existence of dark matter when it was just the weird motion of galaxies we couldn't take it seriously dark matter wouldn't be taken seriously until vera rubin's measurements of galactic rotation then we discovered gravitational lensing around seemingly empty regions of space the data from the cosmic microwave background is just the most recent item on the pile dark matter exists the data is irrefutable consensus has been reached and scientists are pretty argumentative amongst each other if they reach consensus it's a big deal and you should take it seriously so which piece of evidence do you find the most compelling for dark matter please share in the comments thanks for liking and sharing this video a special thanks goes out to my patreon patrons and youtube members like sean k for making it possible for this to be my job don't forget to subscribe if you'd like to keep up with us and until next time remember it's okay to be a little crazy eric totally expected me to get electro boomed by my crazy circuit board that board is pretty dangerous isn't it all that exposed metal on 120 volts ac please don't ever do anything like that at home i'm a trained professional i've been shocked by this thing a few times but i'm still alive because i know what i'm doing anyway thanks for watching

Channel: The Science Asylum

Views: 337,305

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Id: AAhWLN2qHGs

Length: 15min 31sec (931 seconds)

Published: Fri Aug 13 2021