Exploring Curved Spacetime

Speaker 1
What's
Speaker 2
the difference between flat space and asymptotic desider space? So flat space is just an approximation of like the world we live in. So like desider space, asymptotic, I wonder what that even means, meaning like asymptotic over what? Okay, so for
Speaker 1
thousands of years, you know, until the last half of the 20th, well, sorry, until the 20th century, we thought space-time was flat. Can
Speaker 2
you elaborate on flat? What do we mean by flat? Well,
Speaker 1
like the surface of this table is flat. Let me just give an intuitive explanation. Surface of the table is flat, but the surface of a basketball is curved. So the universe itself could be flat like the surface of a table, or it could be curved like a basketball, which actually has a positive curvature. And then there's another kind of curvature called the negative curvature. And curvature can be even weirder because that kind of curvature I've just described is the curvature of space. But Einstein taught that we really live in a space-time continuum, so we can have curvature in a way that mixes up space and time. And that's kind of hard to visualize.
Speaker 2
Because you have to step, what, a couple of dimensions up? So it's hard to... You
Speaker 1
have to step a couple... but even if you have flat space and it's expanding in time, you know, we could imagine we're sitting here, this room, good approximation, it's flat, but imagine we suddenly start getting further and further apart, then space is flat, but it's expanding, which means that space-time is curved. Ultimately,
Speaker 2
it's about spacetime. Okay, so what's the sitter and anti-desitter space? The three simplest spacetimes are flat
Speaker 1
spacetime, which we call Minkowski spacetime, and negatively curved spacetime, anti-desitter space, and positively curved spacetime de-sitter space. And so astronomers think that on large scales, even though for thousands of years we hadn't noticed it, beginning with Hubble, we started to notice that space-time was curved. Space is expanding in time, means that space-time is curved. And the nature of this curvature is affected by the matter in it, because matter itself causes the curvature of space-time. But as it expands, the matter gets more and more diluted. And one might ask, when it's all diluted away, is space-time still curved? And astronomers believe they've done precise enough measurements to determine this. they believe that the answer is yes the universe is now expanding eventually all the matter in it will be expanded away but it will continue to expand because well they would call it the dark energy Einstein would call it a cosmological constant. In any case, in the far future, matter will be expanded away and will be left with empty de Sitter space.

#359 – Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics

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