When they say “flat”, they mean “not curved”. And by “curved”, they’re talking about a kind of 3D curvature that’s difficult to picture.
Like, ever see a picture or video of a simulated black hole, perhaps in a film like *Interstellar* or in a video game? Notice how images of stuff behind the black hole get all funhouse mirror distorted? Like that. That’s 3D space curvature. When astronomers say the universe is flat, they mean the universe on a broad scale *doesn’t* look all funhouse warped like the black hole. As far as they can tell, it appears to just be the plain, boring, infinite(?) straight 3D grid line shape you’d intuitively expect it to be.
When they say “flat”, they mean “not curved”. And by “curved”, they’re talking about a kind of 3D curvature that’s difficult to picture.
Like, ever see a picture or video of a simulated black hole, perhaps in a film like *Interstellar* or in a video game? Notice how images of stuff behind the black hole get all funhouse mirror distorted? Like that. That’s 3D space curvature. When astronomers say the universe is flat, they mean the universe on a broad scale *doesn’t* look all funhouse warped like the black hole. As far as they can tell, it appears to just be the plain, boring, infinite(?) straight 3D grid line shape you’d intuitively expect it to be.
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