An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.

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來自 University of Rochester 的課程

Confronting The Big Questions: Highlights of Modern Astronomy

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An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.

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What is the Fate of the Universe?

The History of The Universe - Why the Big Bang? A History of Time, What Happened Before the Big Bang

- Adam FrankProfessor

Physics and Astronomy

Welcome back everyone.

So what we are going to do now start we are going to

start walking our way through the modern story of cosmology and to do

that we are going to think about the players that were going to

have and the stage on which the drama of cosmology is going to occur.

So first we want to talk about go back to

space-time and idea of of we are talked about this

a little bit Einstein's idea from general relativity that there

was not space and time that was separate but there was

one unified entity, that we call space time.

And that has as the possibility of having a shape.

That there's the fabric of space time.

So when we come to thinking about cosmology, what Einstein allowed us to

do was being to think about the shape of the Universe as a whole.

And what he was able to do through his equations was show that when we

think about cosmologies what we're going to be very

interested in is the curvature of the Universe.

And we want to think about this as sort of a, a rubber

sheet that can be bent in different ways and the response of that

of the sheet to the mass that's in the Universe, is going to determine

very much the fate and history, or the, the history of the Universe.

So there are three possibilities, for the curvature of the, of space-time.

And the first one is what we call positive curvature.

And it's very much like a sphere. You imagine the surface of

a ball.

and, imagine like the surface of the Earth.

And imagine drawing a triangle on the surface of the Earth.

What would be at, if it was a large

enough triangle what you'd find is that the geometry of

that triangle would be very different from the geometry of

drawing a triangle on on a flat sheet of paper.

For example, the angles would be, would be if you added up the angles, it would add

up to something that was larger than 180 degrees.

Whereas we know about on a flat sheet of

paper, the angles always add up to a 180 degrees.

So does, the space could be cog Universe could be like

at the surface of a sphere, an expanding sphere for, in particular.

Now, people really like this idea, at least to imagine, because it gives you

a way of answering that question of, What's at the edge of the universe, right?

Everybody has this trouble of imagining you're traveling in your space ship.

And you suddenly come to a brick wall at the edge of the Universe.

So the really interesting thing about positive curvature

is that the Universe would be bounded, but finite.

So what that means is, is I got in my space ship, and

I headed off in that direction I would eventually come back from this direction.

That's because, just like in the surface of the

Earth, I could start from San Fransisco and head west.

And eventually I'd reach San Fransisco again

from the east.

So it's a nice way of thinking about

Universe that can be finite but have no boundaries.

But there's other kinds of curvature which don't have that nice property.

For example, there's negative curvature where the Universe is sort of

shaped like a saddle and this would be an infinite Universe.

It would go on in all possible

all, all three spatial dimensions would continue forever.

And there's also the possibility of the Uniiverse

being flat as well.

And in that case it would also be

an infinite Universe The space would continue on forever

and if you took your spaceship and headed in

that direction, you would keep heading in that direction.

So these three different possibilities for space are an

important distinction because the, what you're going to get, which of

those possibilities you're going to get will depend upon the

matter, the amount of matter and energy in the Universe.

And the only way to really understand the

fate of the Universe is to actually try and go out and

measure that energy density, or directly try and measure the geometry of space.

As we've talked about, the geometry of, say, a triangle would look different

in the, in universe with positive

curvature, no curvature, or a negative curvature.

So if you can find some way to actually measure that geometry, then you'd also

have a sense of what the whether the Universe was infinite or bounded or not.

so,

now it's important to understand that also in space, that, that space is

filled with, we've talked about matter, but

really it, space is filled with, fields.

Fields of energy, electromagnetism, gravity, etcetera.

And all these fields ultimately have to be quantum

fields, and particularly when we think about the early universe.

We can think about the Universe being filled with energy

filled that are what we call quantum fields, that are responsible

for creating particles, or particles are the discreet

manifestation of these fields filling space and time.

And so even the vacuum, even empty space, is full of these quantum fields.

And you can get what are called virtual particles.

Particles that sort of emerge and then are, go back to the,

the background in a timescale so short that you can't actually see them.

However, they do have an effect.

So the vacuum is not empty space,

but is actually a seething Tumult of virtual particles appearing

and disappearing back, you know, into, into their quantum fields.

And you can actually measure this.

There's experiments you can do.

Where you can put two plates next to each other.

And measure the force between the plates.

And it turns out that the vacuum energy is actually

significant enough that you can actually make a measurement of it.

So we're going to have to, this idea of a vacuum energy

is going to be very important to us as we through

the narrative of cosmology. Okay, let's go on.

[BLANK_AUDIO]