1-10 Cosmic Expansion

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Del curso dictado por The University of Tokyo

From the Big Bang to Dark Energy

1473 calificaciones

The University of Tokyo

From the Big Bang to Dark Energy

1473 calificaciones

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.

De la lección

From Daily Life to the Big Bang

Understanding the Universe in which we live and how to probe it can begin with simple daily life experiences such as night and day and the four seasons. Starting from these observations, we will take a journey from our local Earth at the present time all the way to the edge of the universe and back to its birth, learning the techniques and findings in modern physics that provide us this understanding.

1-5 Beginning of the Universe Look Far into the Past I13:31

1-6 Beginning of the Universe Look Far into the Past II5:45

1-7 Beginning of the Universe Look Far into the Past III6:41

1-8 Beginning of the Universe Look Far into the Past IV8:54

1-9 Beginning of the Universe Look Far into the Past V10:20

1-10 Cosmic Expansion6:26

Conoce a los instructores

Hitoshi Murayama
MacAdams Professor of Physics, University of California, Berkeley
University Professor, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), Todai Institutes for Advanced Study (TODIAS)

So having learned that we can still see the

Big Bang as something really hot with a temperature.

And we can observe it with so called cosmic microbe background.

At least we would like to

know the mathematics behind this expanding universe.

And if you're not familiar with the calculus,

you can sort of skip this part entirely.

But if you do have some background in it, you can

follow through this with a fairly simple math, it turns out.

So as it turns out,

the expanded universe itself is governed by the

force of gravity, Einstein's theory of, general relativity.

So according to Einstein, it's actually a pretty simple thing.

It's like you keep a ball in your hand.

You stand on the surface of the earth and

throw it upwards, and initial thrust is the big bang.

The way a ball keeps going up is the expansion of the universe.

They follow exactly the same equation, it turns out.

And then as you can imagine, if I throw a ball myself, then I can throw it very

much with a high speed, so, it would eventually

go up, slows down and stop, and come back down.

Maybe the universe would do the same thing.

It may start expanding, but at some point stops, and would come back down, and

collapse.

So that's one possibility for the fate of the universe.

And we'll come back to this question later on.

But as far as we can tell, anything that goes up should slow down, at least.

So that's the equation for the cosmic expansion.

So imagine the entire universe is this yellow thing.

It has a certain mass big m. And it has the size, big r.

And you place a probe, a tiny mass, up here.

As a way of describing a motion of this whole universe.

And you put this initial thrust and that's the big bang.

You try to separate everything away from each

other and that's how universe got started to expand.

And then you look at these equations.

If f equal m a, Newton's equation and this is universal gravitation f is

given for this size of object.

You put that together, and acceleration, if you know some calculus,

is the second derivative of the position with respect to time.

And just by looking at these three equations together, you find the last one.

And as we remarked before, it doesn't depend on the mass of

the object you're using to study it, because of the gravity, is actually

the nature of space itself, it doesn't depend

on the particular mass of your particular object.

So that's the answer you get.

And this negative sign here is telling you that universe must be slowing down.

And given this, you have three possible fates.

So if you use this equation, which is

the same thing as the throwing the ball upwards,

from the surface of the Earth.

You can immediately see there are three possibilities.

One of them is I said already the ball keeps going up

to some extent but slows down and stops and then comes back down.

So that's this green curve here.

As time goes on, the universe gets bigger and bigger but slows down, stops

here, and starts to come down, and then eventually ends up with

the big collapse. And this is called the big crunch.

The universe starts with the big bang, and may end up with the big crunch.

But if you managed to throw your ball with

incredible thrust, let's say you launch a rocket with it.

So then you have such a big speed to begin with...

That's beyond escape velocity we talked about before.

Your rocket may escape the gravity of the Earth, and

just may keep going farther and farther away from us.

That possibility would be in this red curve.

Initial thrust is so big, it keeps going away from the

Earth with, pretty much, constant speed at the end of the day.

So that would be a universe that keeps expanding forever.

And that that would be this red curve here.

In between the green curve and red curve,

there is a possibility that the initial thrust

is just right to escape the force of gravity.

Then it just keeps going upwards, but slows down and down and down.

And, and, and it almost stops.

And that would be this blue curve.

Either way universe keeps expanding forever, but with

this choice, universe would have an end to it.

So these are the three possible fates to

the universe.

And that's what we use to talk

about since the expanding universe had been discovered.

So that leads to this interesting question,

is there an end, to the universe?

And that would be a subject of my last lecture, so

you should hold on, to wait for the answer about it.

But one thing, if you are, are, well-versed enough with the

mathematics, and this is something that would be useful to know.

So with this equation.

You can show immediately that this combination doesn't

depend on time, it's a so called conserved quantity.

If you take it times zero to both sides of

the equation, you'll find it's zero using that equation in green.

And this looks so much like the energy of the universe so to speak.

And if this energy is positive,

the corresponding situation that you can escape

the gravity It keeps expanding in red curve.

If energy is negative, it looks like you are trapped by gravity.

If, if you go up, would eventually

come down, that would correspond to green curve.

And the blue curve corresponds to

situation when this energy is exactly zero.

So that's the way you can mathematically

describe three possible fates of the universe.

But either way,

the universe seems to be slowing down.

What would mean to us, if you're actually an

observer looking into telescope, is a bright future for you.

Because it is, universe is, is, is getting more and, slowly as time goes on.

More things will come into your view as time goes on.

And you will be able to observe more and more things

as time goes on, so observation becomes more and more fun.

If that's the right

future for us, well, we'll talk about that in later lectures.

So that's the end of my first lecture today.

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