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• WHAT IS THE SHAPE OF SPACE?

• 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.

WHAT IS THE SHAPE OF SPACE?

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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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