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Introduction: How much we don’t know about the Universe.

This video presents motivation for the course, by explaining how much we do not know about our Universe.
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What would you say‘ if I say this is our current understanding of the universe? We…our…we live on the earth, and the earth is supported by three elephants on top of a turtle, which is on top of a big snake. Do you believe that? This is our current understanding of our universe. No, no, you don’t believe that, right? Then…of course, we know this is not our universe. But then, how much do we know about our universe? I’ll show you the current understanding…based… on our understanding of the universe, is like this. So, this is the total energy budget of the universe. Uh, seventy percent is so-called dark energy, which is pushing the expansion of the universe.
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But we don’t know what is dark energy. Twenty-four percent is so-called dark matter. We know that matter exists because it has gravity, but we don’t know what dark matter is. And the only thing we understand is the remaining 4.6 percent, 4.6 percent. These are our atoms, our stars, and our planets, and we of course, are made of atoms; also there is a huge amount of hydrogen gas in the universe. Adding all this together, it’s only 4.6 percent. The remaining 96 percent of the universe is something we don’t know. Then…so, at the beginning, when I say the universe is like this, people didn’t believe me, right? But then, how much more do we understand in the 21st century?
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Ah, we advance from there, we have made many improvements, but I’m not so sure. So, in this lecture, I want to tell you how much we don’t know, what we don’t understand about our universe. That’s the main purpose of this lesson. In ancient times Socrates said “I know that I know nothing,” so let’s try to understand how much we don’t understand our universe. I’m Tomo Goto, I’m an astronomer at National Tsing Hua University. So how do we know in this introduction section, I want to briefly introduce how do we know 70 percent of the energy budget of the universe is dark energy? It’s through this super-nova measurement.
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So this is a galaxy, a distant galaxy, a decent galaxy, and there’s a warm bright side. This is a super-nova explosion, and this super-nova explosion, it’s so bright, this is just one star, but it’s as bright as its own galaxy, which has a hundred billion stars inside. So, one star is as bright as a hundred billion stars. It’s so bright so we can see through to a distant universe, and what’s nice about this super-nova explosion is that we know its intrinsic brightness. If we know the intrinsic brightness, we can compare the brightness to the apparent, observed brightness, and then we can measure the distance, the so-called luminosity distance. If a super-nova looks faint, that means the super-nova is far away.
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And if a super-nova looks bright, that means the super-nova is close by. In this way, we can measure the distances to each super nova and then map out the expansion of the universe. The result is the famous plots here, ah, so these three gentleman, by using a super-nova, measured the expansion speed of the universe. This is the so-called Hubbell diagram. As a function of distance, how the faint super-nova becomes. The fainter, the larger number. And a surprise is that each individual points here, individual super-nova explosions, and these super-nova explosions, surprisingly, are fainter than expected. Fainter means the expansion speed is accelerating. So, they found the expansion of the universe is accelerating, because of the dark energy.
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That’s how they discovered dark energy, and that’s how they got the Nobel Prize in 2011. So, this is how we know that 70 percent of the energy of the universe is dark energy, here. And what about dark matter? One way to know about dark matter is through gravitational lensing. So here is one example of gravitational lensing. There’s a big galaxy here and then those arc-shaped galaxies are far behind this center of a bright galaxy, but their shapes are gravitationally lensed by the huge gravity of this galaxy. And the amount of the gravity needed to, this, bend these distant galaxies this far is far beyond, about ten times more than the stellar mass of this big galaxy.
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So, ten times more gravity than this galaxy is existing here. So, that which we call a dark matter is affecting, enforcing the gravity towards distant galaxies. So, that’s how we know that 24… so this is the schematic view graph, so a distant galaxy’s light is bent by the huge gravity from here, but it’s not this galaxy, this galaxy is fueled with dark matter, and this dark matter’s gravitational pull is bent to the light of the Earth. So, the amount of this dark matter is ten times more than the big galaxy, the mass of the big galaxy at the center. This is how we know 24 percent of the energy budget of the universe is dark matter.
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But, we don’t know what this dark matter is. So, at the beginning I showed this ancient, this is actually an ancient Indian view of the universe, and our current understanding is like this. But the problem is 96% of the energy, dark energy, dark matter, we don’t understand what they are. So, are we advanced from, much advanced from this Indian cosmology or not? Socrates said “I cannot teach anybody anything. I can only make them think.” So, through this lecture, let’s think about what we can do, how much we don’t understand the universe, and what we can do to understand this remaining 96% of the universe.
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So, cosmology is like a journey, you know we are like a small boat in the big ocean, and we want to understand where we came from and then where we are going in the future. So, let’s begin the journey together.

“I know that I know nothing” – Socrates

Many years ago, people have different concepts about our Universe. For instance, various mythologies from different countries (e.g. India) had believed that our planet Earth is supported by elephants which are on top of a huge “world turtle” surrounded by a snake (need references). Now, we know that Earth is part of the Solar System together with 7 other planets, and in addition, we also got to know that our universe is composed of “invisible” dark matter and dark energy, things that are yet to be discovered in the future. Just like what the famous philosopher Socrates said, “I know that I know nothing”, we have so much to unravel about our Universe, and our natural curiosity to know the unknown have always been our main driving force in studying what lies beyond our current human understanding.

The video covers briefly the things that we already know about our Universe, and the methods used to understand them. However, our main focus is to talk about the things that we still don’t know about our Universe. Our main goal in this class is not only to inform, but also to invoke thinking and curiosity, as that will be our important step in learning the mysteries of the Universe.

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Mysteries Of The Universe

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