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4-6 Big Rip

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从大爆炸到暗能量

4.8（1,534 個評分） | 40K 名學生已註冊

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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4.8（1,534 個評分）

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LF

Feb 29, 2016

This was an excellent course! It was very interesting, well-explained, and easy to follow (even for someone like me, who isn't necessarily up-to-date on mathematics). Can't recommend enough!

VK

Jan 02, 2017

Maybe the best course I have taken so far. Very interesting subject great professor and perfect distribution of content. Would like to see another course by professor Murayama in the future.

從本節課中

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.

4-4 Fate of the Universe5:41

4-5 Dark Energy9:26

4-6 Big Rip6:57

4-7 Future Observations11:45

教學方

Hitoshi Murayama
MacAdams Professor of Physics, University of California, Berkeley

腳本

So before getting into the actual measurements we would like to do, we

need to first discuss what is the thing we actually would like to measure.

So what we would like to understand is

what is causing this accelerated expansion of the Universe?

We call that dark energy, as I mentioned.

But we don't really know what it is.

So we need to parameterize it somehow, and for

that purpose, we need something called equation of state parameter.

So what

is it?

So this is the Friedman equation we have seen before.

This is telling you how quickly the Universe expands.

If you know what's inside this roe, it's the energy density inside the Universe.

But the key to this is that how does this energy change, density change over time?

If you remember the discussion of inflation,

if the energy density is constant then Universe

would expand exponentially.

That's the consequence of a constant energy density.

On the other hand, energy density for most

things would go down as the Universe expands because

as the volume gets bigger things are thinned

out and you have less and less energy density.

So that is described by a parameter equation state so energy density s related

to pressure. By an oval proportionality constant, or W.

And this W is the equation of state parameter.

For the gas of photons, which we call radiation, W is 1 3rd.

For usual matter, like atoms, or also for dark matter.

W is actually zero.

On the other hand, if the vacuum has energy,

a constant energy density, from these bubbling quantum effects

we talked about before, then w would be minus 1.

And depending on the values of w, you may either decelerate.

Or accelerate, expanding the Universe.

And if you work out the math, you'll find that W needs to be

less than minus 1 3rd for the Universe to get accelerated, or speed up.

And that's

shown from this equation, this is called the first law of thermodynamics.

And by solving this equation you'll find that energy density goes with

the size of the volume which has this exponent, so if W is

minus one, that accounts as one exactly so the exponent will be zero which

means the energy density is constant and that respond to vacuum energy.

On the other hand

if W is zero, you'll find minus three in the exponent.

Mainly this is one over volume and that's what you exactly expect for matter.

You could make the Universe bigger and matter thins out as one over the volume.

So this equation state parameter, W, really tells you

how quickly a component of energy would thin out.

And that in turn,

combined with your Friedman equation, tells you if

the Universe is going to accelerate or decelerate.

And this second derivative, namely acceleration of the Universe,

goes with 1 minus 3w with an oval minus sign.

So if w is minus 1 3rd, this combination vanishes.

Then there's no acceleration.

If w is less than minus 1 3rd, this combination's positive with

a minus sign in front. So the Universe would accelerate.

So that's what happens as a consequence of these equations.

In the case where the Universe is going to

end would correspond to w less than minus 1.

So if w is less than minus 1, then what happens with this is that, well,

this is less than minus 1, it's less than, zero.

So then minus

with this exponent is positive now. So as the Universe expands, energy

density actually grows and that would be the consequence of w less than minus 1.

So is, if the energy density grows the Universe expands you just

keep feeding it more and more energy that pushes the expansion even further.

And, that will eventually lead the Universe into

a Big Rip, so that expansion becomes so fast,

that even the galaxies, or stars, eventually

even atoms and nucleus get ripped apart,

and the Universe becomes totally ripped apart

and completely empty and then boom, it ends.

You can't even think about what happens next.

Universe ends with infinite speed of expansion, and so

that is sort of like the end of the Universe.

On the other hand, if w were slightly bigger than

minus one, this is a situation very much like

what happened at the beginning of the Universe, namely, inflation.

So by measuring this equation state parameter this holds the key to predict

the fate of the Universe and that's what we would like to measure.

So let me show you one video clip here

which is the computer graph, animation that shows what

a Big Rip might look like.

So many, many billions of years from now, if we head to a

Big Rip, then this might what happens 200 million years before the big rip.

Dark energy becomes so important and so multiplying

That it's whipping effect will start whipping the

galaxy as a whole, so the galaxy would

no longer be together despite the pull provided

by dark matter.

Dark energy wins over dark matter, and

then universe, the galaxy gets ripped apart into

individual stars, and even the solar system, the

planets started getting whipped away from the sun.

Once we head even closer to the big rip. So individual planets are now

ripped off from the sun, and if you get to only a few hours before the Big Rip,

that energy becomes so large and important that it

even rips the sun apart into smithereens, into individual atoms.

And of course it's not supposed to put use of sound like this.

And end, in the end even the planets would also be ripped apart.

And the individual atoms would separated from the given planet like the Earth.

Of course, by this time the solar system should

not exist anymore because the sun has blown up to a

big giant, a red giant, that will swallow up the entire Earth.

But this may happen in some other solar systems elsewhere,

and the planet would also break apart into individual atoms.

And finally, the atoms would also be ripped apart into nuclei

and electrons, because electromagnetic force will also lose against dark energy.

Nuclei also will be ripped apart.

And the Universe gets completely thinned out, and

everything is just dark energy in the Universe.

And that's it. And that is the picture of the Big Rip.

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