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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According to the Theory of Relativity, it’s possible for spacetime to have an intrinsic curve. One way to think of it is even if you travel in what appears to be a completely straight line to you, for an outside observer it would appear that you were travelling a curved path.
A sort of analogy would be how we generally view the Earth. Ignore the oceans and topography for a second, and imagine walking directly forwards across the entire Earth. From your perspective you’re walking in a straight line , but in reality you’re walking a big circle around the planet. The Earth has a curvature, even if it’s slight enough that at our human scale on the surface, we don’t notice it. But you still follow it when you travel across the planet.
The universe could potentially be kind of like that, with an intrinsic curve to spacetime that we don’t notice locally. According to careful studies of various cosmological data (most notably the Cosmic Microwave Background), we can put some limits on the amount of curvature to spacetime, and at least as far as our instruments can see, the universe seems to be flat or at least very close to flat. We can’t really see any noticeable curvature. That doesn’t mean for sure that’s it’s flat, but if our spacetime is intrinsically curved, the curve is extremely gradual.
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