Video Transcript: Dark Matter

We're starting to get close to the end of our course and as we get there, we're going to start encountering some of the most mysterious and bizarre discoveries that have been made in our universe.  And in this video, we're going to look at one of those discoveries, which is the discovery of dark matter.  You know, to understand how dark matter was discovered and why we think it's there, we have to kind of go through some background, some background physics.

Let's start with the Doppler effect.  The Doppler effect is something you've all experienced, and you can see a picture in the gallery, the first picture, illustrates how we've experienced it.  We often experienced the Doppler effect with sound.  Sound is a wave just like light is a wave.  So some things that affect light, like having wavelength and frequency, are also true of sound.

When you hear a siren go by, or someone honking their horn while they're driving past you, you can hear a change in the pitch of the sound, and that's a change in the wavelength of the sound. So the pitch will go like, it usually sounds something like this (makes sound of sound as it passes and fades away).  Sound like that, right?  It goes ‘high…low’.  So there's a change in pitch.  So that's something… and it depends on the speed.  If a car is going slower, it won't be as significant… it will be ‘high…low’.   But if it's a really fast will be like, ‘high…low’; It'll be a ‘higher…lower’.  So the amount that the pitch changes depends on the speed.  All right, that's the Doppler effect. 

Now, why does that matter for astronomy?  We don't deal with sound in astronomy.  Well, it matters because light is a wave, and if something is giving off light, it also can have the Doppler effect.  So for example, I have my hand right here, and if I move my hand toward you really fast, then the light that's coming off my hand is a wave and it will have a slightly higher pitch; higher pitch in terms of light.  If the wavelength is shorter which means it'll look a little bluer, so the color will change.  You can't detect this with your eyes, but if you had a spectrum, where you are precisely measuring the lines, the atomic transition lines, these emission lines, and you could see the shift in the spectrum towards the red or towards the blue.  So when I move towards you, it’s blue shifted.  When I move away from you, it's red shifted, and that's the Doppler effect.  And the faster I move, the more it's shifted.  

So this is beautiful, because let's say my hand is hydrogen gas.  Well, I can take some hydrogen gas in the lab, the lab here on Earth, and I can very precisely measure the wavelength of, say, the red line in hydrogen gas very precisely.  And now I go look in space, and I see that same red line, which should be in the exact same spot, but now it's shifted over here.  Well, depending on how much it has shifted, I know I can measure directly the speed that that object is moving toward me or away from me.  That's a powerful tool that we're going to come back to.  That tool, the Doppler effect, is used studying galaxies.  We'll see how it's used to discover dark matter.  It's used to understand the universe as a whole.  It's even used to discover planets around other stars.  So it's a powerful tool that astronomers use.

Let's look at galaxies in particular.  Now, how does the Doppler effect help a galaxy?  Well, one of the questions we want to try to understand, this is the second picture in the gallery, one of the questions we want to try to understand is how galaxies rotate.  Now, when you look at a galaxy, we can't watch it rotate, because even though it's moving quickly, it's so big that we can't watch it.  Over thousands of years, even if we had cameras that could take pictures over a thousand years, we wouldn't really detect any motion, because it's so big.  So how can we tell how it's moving?  

Well, the Doppler effect, because with a single observation, you can tell how much it has shifted, the spectrum has shifted. And with that observation, you can measure the speed either toward you or away from you.  Now, this only works if the paper plate of your galaxy is kind of tilted a little bit so that some of that motion is toward you and some of its away from you.  If it's a face on Galaxy like this, then all of the motion is in a circle, none of its toward you or away from you, and it doesn't really work.  But there's so many galaxies that have some components tilted towards us and away from us that we can do this.  So what would you expect?  Well, when you look at the spectrum of a galaxy then, you'd expect that if the galaxy is rotating around like shown in the picture here, that on one half of the galaxy, you'd expect to see the spectrum shifted towards the blue, because it's rotating towards you, and on the other side of the galaxy, you'd expect to see it shifted towards the red because it's rotating away from you. 

Now, you can do this for all kinds of galaxies because, in practice, what this means is the spectrum gets kind of smeared out.  Some of it’s shifted red, some of it’s shifted blue, and the amount that it's smeared is the speed.  Because if it's traveling really fast, rotating really fast, it would smear a lot.  If it’s rotating just a little bit, it would smear a little. Okay, and you can do this. Well, what's cool is you can do this for lots of different places on the galaxy.  So you could check how fast is it rotating right near the middle?  How fast is it rotating around the edges of the galaxy?  How fast is the gas moving way on the outer edges of the galaxy, the gas, we can barely see?  Because there's really cold gas out there that glows in radio light and you could look at it with a radio telescope and see that shift.  So we can map out the rotation of these galaxies very accurately.

So then the question becomes, well, what would you expect the rotation of a galaxy to look like? How fast should it be moving and how would the rotation change depending on where you are in the galaxy?  Well, here's a couple of possibilities.  

This is the third picture in the gallery.  One possibility is that the whole galaxy is like a giant wheel and the whole thing is held together by some invisible like spokes that are holding this wheel together so that if the inside part turns around once, the outside part turns around once.  And, as you can imagine, with a wheel, what that means is that if you're on the inside, like you’re a little bug riding on the inside of a bicycle wheel, and you're on the you're close to the center of the rotation, well, you might do a small little loop, maybe once a second, you're doing a small loop.  But if you're a bug riding on the very outside of that wheel, you have to do a rotation once every second, you got to travel a lot faster, to get all the way around that huge circumference in one second. 

So what you would expect then, if it's like a wheel, if the rotation is like a wheel, you would expect a graph... now this graph in the upper right corner is showing the speed on the vertical axis, how fast you have to move, and then the horizontal axis is showing how far are you from the center.  And so you can see it's showing if it's a wheel rotation, then the further you are away from the center, the faster you have to be rotating to keep up.

Now, there really aren't any objects like this, that orbit like that in the universe, because things aren't solid disks like that; they're made up of lots of little particles.  So maybe another possibility would be a planet-like rotation.  The planets in our solar system, for example, are all orbiting the sun, but they orbit at different speeds.  If you're really close to the sun, you orbit much faster because the gravity is stronger, and if you're further away, you orbit much more slowly because the gravity is weaker.  And so you would see a graph like the one shown in the bottom right where the further you get from the center, the slower you go. 

And this is what we would expect the galaxy to look like.  We'd expect that in the very middle there's a lot of material, so you'll probably all be going about the same speed.  But then as you get further and further out, the rotational speed, the orbital speed, should go down, down, down because that's the law of gravity.  That's how gravity works.  And this whole galaxy is dominated by gravity; the gravity between the middle of the galaxy and all of the stars.

So here's a problem.  We can measure this for galaxies and we see something rather bizarre. So this is the fourth picture in the gallery.  As you measure what's called a rotation curve for a galaxy, this is what it ends up looking like.  The first part is kind of what astronomers expect.  The speed gets really like, increases, goes faster and faster, and levels off for a little bit in those regions in the middle of the galaxy where there's a lot of material.  But as you get further away, astronomers would expect that the rotational speed should decrease.  Kind of that same idea with the planets - the further away you are, the slower the rotational speed should be.  

The problem is that what astronomers expect to see is different from what they actually see.  For basically all the galaxies we observed, instead, the rotation curve stays flat out to a tremendous distance like beyond what you can even see stars.  We see the edge of the galaxy?  Well, stuff is still rotating really fast even further out.  This is really bizarre.  The way you can make sense out of this though, the way you get a flat… we saw two examples in the previous picture.  We saw one where it increases, and one where it decreases dramatically, so how do you get a flat one?  

Well, with gravity, the way you can get a flat rotation curve, is if you have a uniform amount of matter throughout the whole thing.  Now, think about with a solar system.  In the solar system, the vast majority of the mass is at the very center because that's where the Sun is.  And that's why it gets less and less over time; it’s because all the mass is in the center.  And you might think the same would be true of a galaxy because it's the brightest in the center, that seems to be where the most stars are.  If you follow the light, it's certainly way more light in the middle of the galaxy.  And so if that's where all the mass is, then you'd expect to be slower and slower as you get further from that mass.  But if it's flat, that suggests that the mass is evenly distributed throughout the whole galaxy, that every time you go a little further, there's still more mass, and more mass, and more mass.  So there's this fundamental contradiction.

If we look at the light from the stars of these galaxies, then we get one idea of where all the matter is, where all the stuff is, and it's concentrated towards the center.  If we look at the gravity, the gravitation inside the galaxy, we get a very different picture.  It suggests that there's material throughout the galaxy that we can't see; material that's completely dark.  And so that's what astronomers gave the name dark matter. Basically, to say, this is matter because it interacts with gravity, but it's dark, because it doesn't interact with light.

And there's lots of possibilities of what dark matter could be.  I mean, people suggested maybe it's black holes.  Maybe it's these theoretical particles that should be there but we don't know.  When they do these kinds of calculations, like black holes as an example, you can figure well, okay, how many stars should there be in a galaxy?  How many of those should turn into black holes?  If you do those kinds of calculations, you say, that's impossible.  It's not possible that black holes can account for all that dark matter because it's something like 90% of the mass of the galaxy is dark matter, the vast majority of the matter inside the galaxy is dark matter.

So we're only seeing 10% of what's actually there?  Even when we look in all the different parts of the spectrum.  We're not just talking in visible light, here.  We're talking infrared, in radio light, and all the different parts of the spectrum, all of that represents only 10% of the material that's there?  Well, this was a problem and astronomers still don't really know what this dark matter is.  There's experiments trying to figure that out but we fundamentally don't know what dark matter is.

There are other ways though, of proving that dark matter is there. One such method is called gravitational lensing, and this is the last picture in the gallery.  So what you can do is you can, look at very distant galaxies, and the light from those very distant galaxies passes through space. And as it passes through space, it can actually be kind of warped, like the pictures can be warped by gravity, so that light is passing past other galaxies, or even groups of galaxies, and the light gets kind of distorted and warped.  And you can use that distortion to create a map of the gravity, of like the mass that's between you and those distant galaxies.  I don't know if that makes any sense.  But let's just say it, let’s try one more time, okay?  

So there's a galaxy way, way far away out here.  There's a group of galaxies that are here.  And then I'm looking at it. And as I look at the light from way far away, it gets distorted, like it's passing through a lens or even like passing through water, it’s distorted, and then I look at that light and I can kind of model on a computer and say, well, where must all the gravity be that caused that distortion?  And when you do that, it's a totally independent method, you also find an enormous amount of matter that is not interacting with light, like dark matter. It's interacting through gravity, but it's not shining or absorbing any light.

So this last picture is an illustration of, you can see it's like the surface.  It's showing where all the matter is in like a galaxy, and I show it to you as an example of saying, here's an independent way where you can map, kind of like in two dimensions, you can map out where the dark matter is inside of the galaxy.  So really, in all of astronomy, there's no question among professional astronomers, dark matter is real.  The question is, what the heck is it?  And at the moment, we don't really know.  But it's a big problem, because it is the vast majority of matter in our whole universe.  So with all these amazing things, we've discovered, what if all of this stuff we discovered only represents 10% of the universe that we see.

Wow, there's so much more that remains to be discovered then, and not only about our universe, but these were not… this material was not predicted in the laboratory.  No one has ever observed anything in a physics experiment on Earth.  So we have no way of recreating dark matter, or discovering it using ordinary situations, ordinary equipment here on Earth.  So that poses a problem.  It suggests that we must be missing something rather significant if we can't explain this.

Okay, cool. Now, as we move into the next unit, we're going to start studying the universe as a whole.  And we're going to see there's even other bigger questions.  Imagine that.  Bigger questions than this, that we're still just now starting to scratch the surface and try to understand. 

Very cool, we'll see you next time.