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3-2 Dark Matter I

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从大爆炸到暗能量

4.8（1,532 個評分） | 40K 名學生已註冊

We have learned a lot recently about how the Universe evolved in 13.7 billion years since the Big Bang. More than 80% of matter in the Universe is mysterious Dark Matter, which made stars and galaxies to form. The newly discovered Higgs-boson became frozen into the Universe a trillionth of a second after the Big Bang and brought order to the Universe. Yet we still do not know how ordinary matter (atoms) survived against total annihilation by Anti-Matter. The expansion of the Universe started acceleration about 7 billion years ago and the Universe is being ripped apart. The culprit is Dark Energy, a mysterious energy multiplying in vacuum. I will present evidence behind these startling discoveries and discuss what we may learn in the near future. This course is offered in English.

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4.8（1,532 個評分）

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LF

Feb 29, 2016

This was an excellent course! It was very interesting, well-explained, and easy to follow (even for someone like me, who isn't necessarily up-to-date on mathematics). Can't recommend enough!

VK

Jan 02, 2017

Maybe the best course I have taken so far. Very interesting subject great professor and perfect distribution of content. Would like to see another course by professor Murayama in the future.

從本節課中

Dark Matter and Anti-Matter

In this module, we will learn about dark matter and anti-matter -- some of the more mysterious sides of the Universe. We know dark matter played an important role in the formation of stars and galaxies, but much of properties are still unknown to us. In another strange mystery, we will see how the beginning of the universe started with equal amounts of matter and anti-matter, yet somehow only matter (which we are composed of!) survived.

3-1 Power Spectrum11:05

3-2 Dark Matter I6:24

3-3 Dark Matter II10:20

3-4 Nature of Dark Matter I9:38

3-5 Nature of Dark Matter II9:29

教學方

Hitoshi Murayama
MacAdams Professor of Physics, University of California, Berkeley

腳本

So as you have seen in this energy

[UNKNOWN]

of the universe, dark matter makes up about 25%, a quarter of the universe.

energy project it is now.

And also have learned that antimatter doesn't

seem to exist in the current universe.

So we would like to get started with the discussion on dark matter.

And we talked about this in, in the first lecture.

If you look at the solar system, all the planets revolve around the sun.

But it's velocity is fixed by the distance from the sun.

It goes down, like one over square root of the distance.

And we have also learned that each planet is

moving at a pretty high speed around the sun.

But the most important thing here is that as you

go farther and father away from the source of gravity.

Because the gravity gets weaker by inverse square

root, its reverse revolution velocity should become slower.

So that's what we see in this situation. In the solar system.

But if you look at the scale of the galaxy, and this is our own galaxy.

Our solar system is away from the center by 28,000 light years away.

It doesn't seem to be the case.

Our solar system is moving actually very fast, much faster than the

entire source of gravity coming from the stars combined would support us.

So something else beyond the stars exists in our own galaxy.

That is pulling us inside the Milky Way Galaxy.

So if we actually tried to measure this as a function of

distance from the center of the galaxy, this is our solar system.

We're moving at a speed of about 220 kilometers per second.

But if even if you go to farther and farther away outskirts

of our Milky Way Galaxy, the rotation speed doesn't seem to go

down at all. And that's strange, right?

What we learned from Newton is that if the, the center of

the gravity and the source of the gravity is centralized in some way.

Then as you go farther and farther away from

the source of gravity, we should be slowing down.

But clearly things are not slowing down towards the outskirts

of the galaxy, so there must be a lot more mass.

That would actually keep coming in as you go

farther and farther out and that's what

we call the dark matter of intergalactic halo.

So even when you go to the places of

the galaxy where you don't see any star at all.

Then rotation speed doesn't seem to go down, so that's where

we have a good evidence on the existence of dark matter.

So the way you actually see the rotation speed of

individual stars and even the gas in a given galaxy

is again by using the doppler shift. So if you look at a a galaxy like this.

We are looking at this galaxy edge-on; we are looking sideways on galaxy.

And this is a kind of ideal one we can study.

Because if you go away from the center you can see how

much the light coming from the stars or gas is dopplar shifted.

So that will tell you

if the, the star in this region is sort of coming towards us.

Then we will expect the light should be bluer than what it actually should be.

If the stars are going inwards then the light

should look reder to us than what it should be.

That's exactly what you see in this picture.

So this line here is the center of the galaxy.

And what

is plotted in the vertical axis is the wavelength of light.

And what you see is that on the left and the

right of the center of the galaxy, you see different wavelength.

And that's because this part of the galaxy is now pushed, being pushed in.

So that the wavelength looks longer while this part of

the galaxy is coming towards us, the wavelength looks shorter.

But the most important thing here is that the wavelength looked

pretty much flat as you go to the outskirt of the galaxy.

Which means that rotation speed again is pretty much flat.

It doesn't seem to go down, like what it did in the case of solar system.

And that is true in all the galaxies that

people have studied, this is the case of Andromeda.

Again, the rotation speed is more less constant

as you go outwards in the Andromeda galaxy.

While if you think that the source of gravity is only stars and nothing else.

Rotation speed should be going down along this yellow curve.

But clearly something is missing here.

Again, we attribute that to the existence of dark matter.

Here's yet another example of a galaxy.

We again see a flat rotation curve while the

source of gravity coming from just the stars in the

galactic disk.

Should give you this folding curve so that

something else should be making up the rest.

And that is the case with basically any

galaxies people have studied one way or another.

And this galaxy study had been pioneered by

an astromer named Vera Rubin back in the 70s.

So the true nature of galaxies now we know today is something like this.

So it's actually sort of an ocean

of dark matter.

Inside this ocean, the stars are embedded, so they are sort of sprinkled in.

They are actually a rather minor component of a given

galaxy, and the galaxy may spread over like 100,000 light years.

But the ocean of dark matter just keeps going, even beyond a million light-years.

So this is a true picture of galaxies.

We don't see it this way in a telescope, but this is actually the true nature

of galaxies.

Now here you see a very interesting picture.

So what you see is actually two galaxies here.

And there but you'll see something really, really stretched out.

And because of this picture, it sort of looks like a Cheshire

cat you may have seen in some movie from Disney, for example.

That appears in the Alice and the Wonderland.

This is a picture that shows something else is

extremely distorted. Because the force of gravity.

So now question to you is, how would

you think we can understand this really funny picture?

It can't possibly be a galaxy that is stretched like this.

It looks like that to us, but

there's, there can't be such an astronomical object.

So how do you understand this object?

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