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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From the course by University of Rochester

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.

From the lesson

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.

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

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

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

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

Â So first we want to talk about go back to

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

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

Â was not space and time that was separate but there was

Â one unified entity, that we call space time.

Â And that has as the possibility of having a shape.

Â That there's the fabric of space time.

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

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

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

Â think about cosmologies what we're going to be very

Â interested in is the curvature of the Universe.

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

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

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

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

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

Â And the first one is what we call positive curvature.

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

Â a ball.

Â and, imagine like the surface of the Earth.

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

Â What would be at, if it was a large

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

Â that triangle would be very different from the geometry of

Â drawing a triangle on on a flat sheet of paper.

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

Â up to something that was larger than 180 degrees.

Â Whereas we know about on a flat sheet of

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

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

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

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

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

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

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

Â So the really interesting thing about positive curvature

Â is that the Universe would be bounded, but finite.

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

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

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

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

Â And eventually I'd reach San Fransisco again

Â from the east.

Â So it's a nice way of thinking about

Â Universe that can be finite but have no boundaries.

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

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

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

Â It would go on in all possible

Â all, all three spatial dimensions would continue forever.

Â And there's also the possibility of the Uniiverse

Â being flat as well.

Â And in that case it would also be

Â an infinite Universe The space would continue on forever

Â and if you took your spaceship and headed in

Â that direction, you would keep heading in that direction.

Â So these three different possibilities for space are an

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

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

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

Â And the only way to really understand the

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

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

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

Â in the, in universe with positive

Â curvature, no curvature, or a negative curvature.

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

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

Â so,

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

Â filled with, we've talked about matter, but

Â really it, space is filled with, fields.

Â Fields of energy, electromagnetism, gravity, etcetera.

Â And all these fields ultimately have to be quantum

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

Â We can think about the Universe being filled with energy

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

Â for creating particles, or particles are the discreet

Â manifestation of these fields filling space and time.

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

Â And you can get what are called virtual particles.

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

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

Â However, they do have an effect.

Â So the vacuum is not empty space,

Â but is actually a seething Tumult of virtual particles appearing

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

Â And you can actually measure this.

Â There's experiments you can do.

Â Where you can put two plates next to each other.

Â And measure the force between the plates.

Â And it turns out that the vacuum energy is actually

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

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

Â is going to be very important to us as we through

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

Â [BLANK_AUDIO]

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