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.
Director, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), Todai Institutes for Advanced Study (TODIAS) MacAdams Professor of Physics, University of California, Berkeley
So now we come to this question, why do these distant galaxies look red?
And that is actually evidence that universe is getting bigger.
Universe as a whole is expanding, so
the distant stars and galaxies actually look red.
And that's the same phenomenon as something we're also familiar with.
So when you have an ambulance approaching you, the pitch of a siren,
sounds actually high. You are familiar with that.
Once the ambulance passes in front of you, all of a
sudden, the pitch of the sound of a siren actually goes down.
So that is something called a Doppler effect.
So approaching, object would give away a high pitched
sound, receding object would give away a low pitched sound.
So that's about the sound, but what about the light?
So, if you have a star, that say, that is supposed to be yellow.
But if this star is moving away from you, it would actually look red.
So the reddening, it's the same thing
here, as, the pitch going down, because the
object that is letting the sound or light away is actually moving away from you.
And if you're
familiar with the calculus, you might have seen this formula, for the Doppler effect.
The frequency of the sound.
It's different when you listen to it from the original frequency
of a sound that was let out by the object that's moving.
And here, this big V is the velocity of the sound.
Little v is the velocity of the object.
Well you see there are two different formulae here.
One of them is when the object that's letting the sound out is moving.
The other formula, when you are moving, relative to the air.
But there is no air that lets the light propagate in outer space.
So the correct formula for light turns out
to be the geometric mean of these two formulae.
So this is the correct formula in the case of light.
Again, this big
V is supposed to be the speed of light, little v is the relative speed
between object that let the light out. And object that receives the light.
If you do something called Taylor
expansion, if you're familiar with this calculus.
You can expand this formula out in the approximation that the
speed of the object is much much smaller than speed of light.
And using this expansion, you find the first term,
which is pretty much the only important, for most cases.
And that is actually going to look like this.
So, if the object is moving away from you, frequency is subtracted by an
amount proportional to its velocity, so the frequency looks less to you.
And for the sound, when frequency is less, the pitch
is low.
In the case of light, if the frequency
is less, the light is stretched and looks redder.
So that's what happens with this Doppler effect.
So, the way we understand now that this galaxy look red
to you, it means that it's actually moving away from you.
But then, do you think that we are sitting at the center of the universe?
And all the other galaxies know you are
the center, and you're trying to escape from you?
That clearly is not the right picture, right?
We have seen in the video, that, no place in the universe appears very special.
So that can't possibly be the explanation.
So what could it be?
So that's the idea that space itself is getting bigger.
So as Einstein told us, that gravity is, in some sense, an illusion,
that space bends, and warps. That is the true nature of gravity.
So if you think of the universe as a box, a container, kind of static.
That's not the right picture we should use anymore.
So according to Einstein, universe is in some sense a live box.
It can bend, twist, warp, expand, or even collapse.
So that's the way
we should view how the space of the universe should behave.
So, it's possible that universe as a whole,
can get bigger because it's a live box.
It's not a static box.
So that would allow us to understand, why all of this new galaxies look red.
So, if the universe is getting bigger,
then, lets look at actually this picture here.
So let's imagine that universe is sort of a grid system.
Each galaxy is actually on a grid point.
None of the galaxies thinks it's moving, because it's at the grid point.
But, let's imagine that this grid is sort of a rubber sheet.
You can just stretch it, and if you're watching, for
example let's say this is your galaxy; you're watching this other
galaxy over here.
At this moment you see the distance is only this much.
But, at a later time, you see the distance from
our galaxy, the other one looks like it's farther away, right?
So if the grid itself is getting bigger, the other galaxy seems
to be moving away from you. But it's not special to our galaxy.
If you happen to live
in this galaxy over here and watching that galaxy, again, later
time you see that that galaxy is farther away from you.
No matter on which galaxy you live in, all the other galaxies seem
to be moving away from you, because space itself is actually getting bigger.
So that's the idea of expanding space.
And if a galaxy lets out a, say, blue light, and light is
actually a wave, so it actually
repeats itself over certain distance called wavelength.
And this light, with this wavelength is traveling through space.
It's not doing anything other than just going speed of light, but before
it reaches the other galaxy, again, the underlying space is stretched.
So how quickly it repairs itself? The wavelength
of the light is also stretched together with the expansion of space.
And once the wavelength is stronger, then we actually observe a red light instead.
So that's how the original color and color you see are different.
And everything looks redder, because the light
is stretched by the expanding universe itself.
So, that's how we think the universe must
be expanding.
The fact that all the other galaxies look red
to you, must mean that universe itself is expanding.
Because you don't think you have a very special point in the whole universe.
And if the universe is getting bigger, let's say you imagine a, a helium balloon.
You might have your birthday party.
Let it just fly up, and it's going to expand.
And, as you can imagine, it actually gets cooler and cooler if you keep
going up that way.
So something expanding, would actually cool down.
So, as the universe expands, it must be getting cooler.
And, if you managed to tape the universe, and if you run
the video backwards, you'll find the
universe would start getting smaller and smaller.
And also getting, hotter and hotter.
So just base, based, on this fact
that you would think, universe must been much
smaller than it is today, in the past.
And it also must have been much hotter than it is today.
So universe started small, and hot, and there you go.
That's the idea of the Big Bang.
And if you're a little familiar with the way the wavelength of
light and the energy of the light is related, this is the formula.
This little h is called a Planck constant.
If you don't, are not familiar with this
question, it's completely fine, you can forget about it.
But this is really telling you, as the wavelength of
light lambda gets bigger energy of the light gets smaller.
So, as the universe becomes bigger,
wavelength is stretched and becomes longer.
And energy goes down, and therefore, universe gets cooler.
On the other hand
if you go back in time, the wavelength
was shorter, energy was higher, and universe was hotter.
So we start with hot, small, universe the Big Bang.
But now is a question to you.
So how do we know this is the right picture?
We did see that distant galaxies indeed look red.
Is that enough?
Is Big Bang just an idea or is there evidence
for it?
So, we'll come to the evidence in a moment.
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Hitoshi MurayamaDirector, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), Todai Institutes for Advanced Study (TODIAS)
