So, as I told you already, people wanted to know what kind of fate is awaiting us.

And, there were three possible curves people knew about, and the people tried

to measure the expansion of the universe far back in time and more recently.

By comparing them, we want to know in which curve we are.

But it led to a surprise in 1998,

when people tried to indeed precisely do this measurement.

You have found that the universe is,

rather than slowing down, it's getting faster.

It's accelerating.

And we don't really know the cause of it, but we actually named it,

what may be accelerating the expansion of the universe, and that is dark energy.

So, what people have done is really do what I told

you and by measuring the energy and causing the universe to

accelerate or the other way around by measuring how the universe is

expanding if you wanted to decide for what energy is causing it,

the only way we can understand it that dark energy is fitting

up the entire universe sort of like the heat exposing on the way.

But with really contributing to the expansion

rate of the universe, and it's everywhere.

So the way people actually came up with this observation

is by using a very clever trick.

So the astronomical object called Type-1a supernovae, which is an explosion of a

star at the end of it's life and it has to come up,

actually come from a binary system, namely that we have two stars rotating around

each other, one star actually feeding into the other star by providing material.

And once the other star becomes so heavy that it can't sustain

itself anymore, it just gives you a big explosion at the end of the day.

And that gives you a big flare of light for like a month or so.

And you can measure this type of a supernovae even

at a very very large distances because they are so bright.

One supernovae becomes as bright as an entire galaxy.

And that's great Because if it's,

if you can observe this when it is very far away, you can study

the galaxies when they actually are billions

of light years away, and that will

tell you some information about how quickly they are moving away from us, and

that tells you how the universe was

expanding, like, billions of years ago, right?

So, that way we can measure the expansion rate far away

from us In addition, there's one added bonus with Type-1a supernovae.

We pretty much know how bright they are.

So if you see one of these things, they're

like, okay, you know, it's a 100 watt light bulb.

So if this 100 watt light bulb, if that

looks bright to you, that means it's close to you.

If that looks very dark, or dim then it's far away from you.

Just, just by measuring its brightness, you can

tell how far away this light bulb is.

And same thing with Type-1a supernovae.

So, depending on how bright it looks, you can measure how far away it is, and that

in the case of the universe, of course, translates

to how far back in time it has exploded.

So, that's a very good feature of Type-1a supernovae.

On the other hand, if you measure its color,

as we talked about before, that color has to redden.

Because it had been expanded.

It stretched by the expanse of the universe itself.

That's a rubber sheet of the grid we talked about on the first lecture.

So by measuring the color, or spectrum to be more precise, you can

measure how much the universe has expanded

since the time of the supernovae explosion.

So you have an information about time. You have an information about expansion.

Putting that together, you have an

information on expansion history, and that's

how people figured that expansion of the

universe is actually getting faster, which has a huge surprise to all of us.

So here's the 100 Watt light bulb, and that gave us this following piece of data.

Namely, the universe would accelerate on this side of

the plot, and decelerate on that side of the plot.

What we used to think is that the universe

has only matter, that's 100%, and no dark energy in this direction.

And, that kind of universe, as we talked about before, should slow down the

expansion of the universe, and that's indeed

in this decelerating part of the plot.

But the data showed that you're up here.

You do have matter, which is mostly dark matter, like 25% of the universe.

But that can't explain

the accelerating expansion on its own if you're down here.

You need to have this 75% of the universe in this dark energy.

So that universe can accelerate.

So you can see that entire universe is basically

given in terms of the battle between two titans.

One titan being the dark matter, other titan

being dark energy, and that competition seems to

tell us how the universe has evolved.

And what is going to happen also in the future.

And this measurement gave you this incredible, interesting plot.

This shows how the universe has been

expanding since, called Hubble diagram, and by fitting

this data, you can tell the amount of dark energy we need in the universe.

If it was just a matter of dark matter we need in the

universe, and this was actually very clever

idea by measuring these supernovae on demand.

The supernovae you never know where and when it won't happen.

But keep covering a quitty, a pretty large portion of the

sky, at least there's a very good chance that spotting one.

And then you build a system, but once you spot one, then you

follow that up with other telescopes to

measure its spectrum and brightness very precisely.

And that's indeed what has been done by my

colleagues all permanent in Berkeley and other groups as well.

And by measuring how quickly the uni, supernovae fades, and

that time scale will tell you exactly how bright the

uni, how bright the supernovae was, and that is based

on some empirical data by studying the near by supernovae.

People learned a lot about other impacts of the environment, like

[UNKNOWN],

dust extension, and so on.

But they have been shown to be rather less important.

It's important at some level, but not that important to

change the conclusion that the universe is actually speeding up.

And this is the latest data, a compilation

of data which had been shown by the conference.

So the expansion is getting faster.

So what is wrong with this original idea by Einstein?

It told

us that universe should slow down, but now we know it's actually speeding up.

And this thought to happen actually rather recently.

towards the beginning of the universe, it was indeed

slowing down and there are data that shows that.

But it started to speed up recently, as recently as 7 billion years ago.

So, what does it mean?

So if you think of ball thrusted upwards it was slowing

down for a while but at some point it started to pick

up the speed and zoom, it just keeps going further and further away.

So somehow, energy is increasing.

Ball is picking up speed, picking up more energy.

We don't know what this energy is, that's why we call this dark energy.

It seems to be almost if as if it's an infinite source of energy.

And it became a huge impact on general science discussions.

Some people even doubted, well if you get

this kind of weird conclusion maybe what's wrong

that we trusted this guy Einstein that's why

we're left to this very, very Bizzarre conclusion.

Maybe he was wrong.

Well, a lot of people tried to

change Einstein's theory to accommodate his expansion.

the accelerated expansion of the universe.

Well, we all know Einstein was a pretty smart guy.

Nobody really managed to go beyond him so far.

So this is pretty

unlikely, but people are still studying that.

But, either way this is actually incredibly interesting.

Either we found a new paradigm of the universe

based on dark energy and accelerated expansion or you

find a new fundamental law with that goes beyond

Einstein, either way it's going to be very, very exciting.

And, also if this rate of increasing energy is very quick maybe the universes,

keep expanding and speeding up to the extent

that the expansion rate may become infinite at

some point, then universe gets infinitely ripped apart

at that stage, and the universe would end there.

And this scenario is called big rip.

The universe is totally ripped apart to infinite size

infinite speed, and there's no universe beyond that point.

So what can we do to find out?

Well the only thing we have seen so far

is that history of the expansion of the universe.

And the data is still not accurate enough to tell what kind of fate is awaiting us.

So what we need to do is to do a much better measurement

with a greater accuracy to measure how quickly dark energy has been increasing?

How quick the universe has been speeding up?

So having heard all this discussion so far, what is the best way to do that?

4-5 Dark Energy

From the Big Bang to Dark Energy

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