what does ‘the universe is flat’ mean? How can it be flat when there are stars, planets, galaxies etc. everywhere we look?

Question

what does ‘the universe is flat’ mean? How can it be flat when there are stars, planets, galaxies etc. everywhere we look?

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Answer 1

You probably know about the Pythagorean theorem which says that if you measure out a triangle you get s^2 = x^2 + y^2. This only works on a flat surface, so if I draw a triangle on a sphere it doesn’t work out that way.

The 3D version of this is s^2 = x^2 + y^2 +z^2. When they say the universe is “flat” they mean this equation holds true.