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  • We're used to thinking of space as the emptiness in which things happen, like an empty warehouse

  • ready to be filled, or a theater stage on which the events of the Universe play out.

  • But General Relativity predicts that space is not just emptiness, it's a physical,

  • dynamic thing, and that prediction has been borne out by many, many experiments.

  • Space can bend because of matter and energy, curving the paths of objects that move inside

  • of it.

  • It can ripple with gravitational waves And it can expand, creating more and more space

  • between two objects.

  • All of these phenomena can be described by one idea: curvature of space (or spacetime).

  • In flat regions of spacetime (like, if there's no energy or matter nearby), objects traveling

  • along parallel paths stay along parallel paths.

  • In positively curved regions of spacetime (like near planets or black holes), parallel

  • paths converge, and in negatively curved regions of spacetime parallel paths (or even paths

  • pointed at each other!) diverge.

  • But what about space as a whole ? If space is positively curved everywhere, then there's

  • only one shape space can be: a giant hyper-potato.

  • If you went in one direction for long enough, eventually you'd end up in the same place

  • you started.

  • If space is flat everywhere, its shape could be simple: just extend out straight to infinity.

  • Or it could loop around in a periodic way, like in some video games:

  • And if space is negatively curved everywhere, sports would be impossible

  • So which is it?

  • There are basically two ways to measure the large-scale curvature of the Universe.

  • One is to measure the angles inside of triangles.

  • If the space is flat, then the angles will add up to 180 degrees.

  • But if the space is curved, those angles will add up to more or less than 180 degrees depending

  • on the type of curvature.

  • Cosmologists have done the equivalent of measuring our Universe's triangles by looking at a

  • picture of the early Universe, and studying the spatial relationship between different

  • points on that picture.

  • The second way to measure curvature is to measure the thing that causes space to curve

  • in the first place: the density of energy and matter throughout the Universe.

  • Which cosmologists have also measured.

  • It turns out that in both cases, measurements show the Universe to bepretty much flat

  • (within 0.4% margin of error).

  • But before you get disappointed that we don't live in a cool cosmic hyper-potato, let me

  • tell you one big problem

  • The fact that we live in a flat Universe appears to be a GIGANTIC, COSMIC-LEVEL COINCIDENCE.

  • If the Universe had just a little bit more mass and energy, space would have curved one

  • way.

  • And if it had just a little bit less mass and energy, space would have curved the other

  • way.

  • But we seem to have just the right amount to make space perfectly flat as far as we

  • can tell.

  • This perfect amount is the equivalent of five hydrogen atoms per cubic meter of space, on

  • average.

  • If instead there were six hydrogen atoms per cubic meter of space on average, or four,

  • the entire Universe would have been a lot more curved or a lot less .

  • And we so far have no idea why our universe has the density that it does.

  • When it comes to the curvature of the universe, our knowledge falls flat.


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B1 space universe curved curvature spacetime flat

What Is The Shape of Space? (ft. PhD Comics)

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    Summer posted on 2020/11/03
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