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Three evidences of Dark Matter – the first evidence

Why do individuals believe there is "dark matter" in our universe?
In this video, we will show you the first evidence of the existence of dark matter!
Last time, we talked about that 24 percent of the total energy budget of the universe is taken by so-called dark matter. Then today we’re going to talk about how then how we know there are dark matter, and how we know if they are taking 24 percent of the energy budget of the universe. There are three major evidences, today we are going to talk about the first one. Let’s get started. The first one is galaxy rotation curve. Typical, so this is a typical spiral galaxy, uh, like our Milky Way galaxy. It’s in a disk shape, and these spiral galaxies are rotating, like this.
And this view graph is showing the rotation velocity, as a function of radius, or distance from the center. In the center, around here, galaxy rotation velocity is 50 kilometer per second. That’s actually a very fast speed, it’s much faster than your running speed, and the speed goes up, up, up, and then flattens out at the outside of the galaxy. Okay, um, but, eh, still stars at the outside of the galaxies are not flying away. If I take something like this cable, then, if I rotate it, then this part is not going to fly away, right? Uh, that’s because there’s a pull from this string, this string is keeping this part together.
If there’s no string, and this part is prime rotating like very high speed, of course, this part is gonna fly away, or like something like this, and if I cut the string all of the sudden this part is gonna fly away, right? So, in terms of spiral galaxy, what’s happening? Is, it’s gravity, gravity from the spiral galaxy is keeping the stars together. Although they are rotating very fast, but there’s a gravity to keep them in one structure. However, mysterious part is here. If we can measure how much stars are in this spiral galaxy from the picture, you take a picture and then it’s bright part is star, so we know how much light are there.
And then we can calculate the gravity from stars. And what you expect is this curve here. Because outside of the star graph, actually there are not much stars. You don’t see many stars right, there are a lot of stars here, but not many here, so the gravity is weaker at the outside of the spiral galaxy. So, um, the gravity is weaker so the rotation velocity is expected to be weaker. However, on the contrary, the data show a very high velocity of 100 kilometer per second, and so at the outside of the spiral galaxy. So what’s going on?
That means, this very high velocity means there’s additional gravity, additional strong gravity is keeping stars within a spiral galaxy, and the way is this additional gravity is coming from, that’s coming from dark matter. So, this very fast rotation velocity outside of spiral galaxies telling us, there are a lot of dark matter at the outside of the galaxy. And actually, in reality, so here’s the simulation. Uh, there are two simulations, the left part is without dark matter, and the right part is with dark matter.
Without dark matter galaxy cannot rotate very fast at the outside, speed is slower here, right, compared to here, but with the strong gravity of the dark matter, galaxy can still rotate very fast at the outside, so here’s it’s faster right? That’s the evidence of the dark matter. Um, and then this amount of dark matter is actually ten times more than the mass in stars. So there are ten times more mass in dark matter than stars in a spiral galaxy. So, if someone asks you to write a picture of a spiral galaxy, you might write something like this. There’s a bulge, and then spiral arm here, but this is only ten percent of the mass of the spiral galaxy.
The actual picture of a spiral galaxy should look like something like this. The bright part starts at only ten percent, and the vast majority of the mass is in the so-called dark matter halo. Of course, because this is dark matter we cannot see them, but it’s there and we know from its gravity. Does this one evidence that we know, dark matter is occupying 24 percent of the energy budget of the universe. In the next lecture, we’re gonna see the second and third evidence of the dark matter. Okay, see you next time.

“Just because you don’t see it, doesn’t mean it’s not there!”

This is the case for dark matter, a mysterious object that we haven’t detected, yet we believe that it’s there. If not for dark matter, it would be impossible for our Universe to form. What are the evidences that it’s there? In this two-part video, Prof. Goto will discuss the three main evidences that dark matter is indeed real.

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Mysteries Of The Universe

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