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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Inflation and Dark Energy

At the very beginning, the Universe exponentially expanded during a period known as the cosmic inflation. Recent studies suggested that the Universe has entered into another stage of expansion, considered to be caused by 'mysterious' dark energy. In this module, we will learn about inflation, dark energy, and the possible fates of our Universe.

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

So what is the way to test this idea? This really, nearly crazy idea.

That this macroscopic structure we see today

in the universe actually came up from

this tiny, tiny quantum noise at a 10th of minus 26 centimeters or less.

So one thing we know about this quantum noise.

When we actually looked at this experiment of shooting the

guns through the walls[ holes in the wall to a screen.

Everything was, indeed, random, right? So randomness means that probability

should follow a simple bell curve. And this is the bell curve.

And when you, and when people talk

about, say, population growth, and many other statistics.

Large statistics always means that you have this kind of Bell curve.

And that represents the total randomness. So can we test that?

And, indeed, we can. So what

you can do is look at this map of temperature.

Looking in all possible directions you can measure the

temperature of for are coming from the big bang.

And then just count the numbers of okay, how many of them

have a temperature 10 to minus 5 higher, 10 to minus 5 lower.

And you keep plotting the numbers as a function of the temperature.

So this horizontal axis here tells you how much temperature is different

from the average.

So here, this is twice as big, compared to the typical variation.

Here, it's twice as less than the typical variation you have among the temperatures.

And indeed you see a beautiful bell curve.

So what this is telling you is that this idea that the variation

of temperature in the cosmic micro

background really does come from the quantum

noise when universe was incredibly tiny.

When the microscopic laws of physics, namely, quantum mechanics, was at work.

So at looks like we have, have a group,

very good evidence now that inflation produced this quantum noise.

And quantum noise eventually led to the

temperature variation in the cosmic micro background.

And that temperature variation eventually led to

the formation of stars and galaxies, so everything

is now tied together.

So we are born out of that quantum noise, it seems.

But of course, we would like to test that further.

now the reason coming from the points light I mentioned the

first day, is really the beginning of that kind of test.

And that case is getting stronger and stronger.

Now, you would like to understand yet another question though.

So how could it possibly be true that energy density was constant for a while?

And the idea is this, and again these slides are slightly

technical, so if you don't want to follow through it, that's okay.

So it's like the ball just falling down this hill.

So let me show this once again.

So, if you have a ball sitting on a hill, slightly rolling

down from up here, Then it starts doing this, but the same time, the

universe actually expands exponentially. Now, why is that?

Well, if the ball is falling down very, very slowly.

Then for a while, we have this energy sitting

there without changing very much, assuming it's very slow.

So that's the situation.

You have

more or less a constant energy density. Not exactly constant.

Because the ball keeps going down the hill.

Energy keeps getting smaller and smaller.

But only very slowly. So to a good approximation, the energy

density is actually constant when the ball is slowly rolling down the hill.

And indeed that's actually what happens.

So if you assume this potential is shallowing up.

So it's nearly flat. The slope is more less flat.

Then the ball wouldn't be pulled down very quickly,so it rolls down hill very slowly.

And if you put that into an equation.

You find this equation here which shows that the

expansion of the universe itself actually acts like a molasses.

And it doesn't actually let the ball fall down quickly.

It sort of tries to stop it on the way. It acts like a friction.

So when the ball keeps going down the

hill because of friction, it can't go very fast.

It doesn't get accelerated.

It just keeps going downhill very, very slowly.

So energy density remains small as constant.

And if you put those equations together, this equation and the freedom

equation we talked about a few minutes ago, Then you find this result.

That energy density is just given by this height of the potential and

then and, and the universe expands exponentially.

And to make sure that we understand why the

entire universe looks so smooth and flat these days.

Then you need to have this exponential explanation more than a 60 N exponent.

Well once you achieve that, then you get

this the explanation of everything we talked about today.

Namely how come different parts of the universe seem to have

almost exactly the same temperature.

How come the universe is so smooth and flat.

But at the same time the universe seems to have these wrinkles.

And all of these things can be tied together

with this very slowly rolling downhill of a single ball.

And that ball, or particle, is named inflaton, and

that's the particle that caused inflation of the universe.

But there

are still many mysteries, so what exactly is this inflaton?

Who caused inflation?

When did it happen? How much did it inflate?

Can we come up with a definitive proof of this idea?

So we'd like to solve these mysteries too.

And one clue it turns out, is that when the inflation

happened, not only that it cause this fluctuations of the vacuum.

It also created the wrinkles and ripples in space

time itself.

So that is the, what a property called gravitational waves.

Space and time itself has a wrinkles and that propagates like a wave.

And so that wave is something we can still detect today if we try hard enough.

So when you have this cosmic micro background,

it's just a form of light as you remember.

It's a radio wave but the same thing as light.

And one thing

we benefit from is the kind of sunglasses with the polarized lenses.

And that shuts out the glare from the

surface very efficiently, because the glare as the polarization.

Meaning that electric field oscillates on the sideways.

So if you block the sideways motion, and

only let only the vertical motion get through.

That's a particular kind of sunglasses you can

buy, then you've shut off the glare almost completely.

But you can see everything else, because most

of them have both horizontal and vertical polarizations.

And you use these polarizations to separate different

kinds of modes of light, or radio waves.

One of them is called E-mode.

It looks like something is sort of emerging from a point.

The other one is called B-mode.

It looks like something is sort of curling around.

And it turns

out that this E-mode can be

generated without having these ripples in space-time.

But B-mode cannot.

So if you manage to detect this mode of polarization, that

would be a very good proof, that indeed inflation produced ripples.

Not only on overall energies, but also on the space and time itself.

And that would be a definitive proof that inflation indeed happened.

And you can even measure at what time in,

inflation has happened at the same time.

And there are many experiments that try to do

so and here is a partial list of those experiments.

So just to repeat everything gets this quantum noise

including gravitons which is the fluctuation of spacing time.

And that will lead into this

B mode polarization of Cosmic microwave background.

And how big that fluctuation is, how big this B mode

is, is directly tied to the energy scale when the inflation happens.

Which is the same as saying when it happened.

And these experiments are gearing up to do so.

So I'm going to tell you, just one experiment called LiteBIRD, with

the Japanese community trying to launch sometime in the next few years.

And this will be a satellite on board, and this is

really dedicated to study this B node polarization of cosmic micro background.

And may have a sensitivity to really see the definite approval of inflation.

So this is again the angular scale we talked about before And so, if you

go this way, you're looking at sort of a globally on the entire space time.

If you look at this way then you're looking

at the different spots and a smaller angular scales.

And many inflation theories predict the size the B-mode in

this range and the LiteBird experiment can go down to this part.

So if the prediction of these theories are right, then the LiteBIRD

explorer should be able to show that beam prioritization g/ indeed exists.

And that would be a very good and strong proof that inflation indeed happen, not

only produce these ripples in energies, but also ripples in space and time itself.

So that's the way we try to understand the spirit of inflation.

It's better, and we can now go all the way

back to the point when the universe was incredibly tiny.

And the time scale is something like 10 to minus 334 seconds.

We don't know exactly what it is.

As I told you, we'd like to measure that now, but

it happened very, very early on in the history of the universe.

Where the individual stretched by an incredible

amount in a very short period of time.

And planting this quantum noise on space and time

and energies throughout the stars and galaxies and us.

Could have gone much, much later, 13.8 billion years later.

So that's what we think is what happened at the beginning of the universe.

Now we switch our gear to think about what may happen in the future.

And you might

notice that here is expanding, but now sort of picking up speed.

So something is going on here, rather recently in the

big story of the universe that universe is actually speeding up.

So what's causing this speedup of the universe

is called dark energy and that's the subject next.

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