How do we know how big is the universe?
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We know how big the OBSERVABLE universe is, but we don’t know if that is the entire universe. So parts we can “see”, we can estimate the distance and therefore a size. Everything else is speculated but not known.
We don’t. Or at least, we don’t know the extent of the greater universe, which could very well be infinite in extent. What we do know is the size of the observable universe, and this comes from knowing the speed at which space is expanding. If you imagine all of space as a 3D grid of points, the space between points is constantly expanding in all directions. No matter where you (the observer) are, space appears to be expanding away from you, since every point in space is getting further away from every other point in space. This isn’t strictly true of gravitationally bound objects like galaxies, but we’ll ignore that for now. A consequence of space expanding everywhere is that the further away from the observer you look, the faster relative to that observer space appears to be expanding. At some point, far enough away from the observer, space is expanding at the speed of light relative to that observer. Beyond that distance, nothing is observable because light from events past that boundary will never enter the boundary, because the space through which the light travels is expanding faster than the speed of that light. Everything within that boundary is what we call the “observable” universe, and our measurements indicate that the boundary is about 46 billion light years away, making the observable universe a bubble about 92 billion light years across. The position of that bubble is always centered on the observer though, so if you instantaneously transported yourself to the edge of the observable universe, you would see earth at the limit on one side, and 46 billion light years of new stuff on the other side that you can’t see from here. The extent of the greater universe beyond the observable boundary is unknown. It very well may be infinite, but we do not know, and can not know from observation.
We believe the universe as a whole is infinite, because it seemingly has no intrinsic curvature (as opposed to local curvature, ie gravity caused by stars etc). Something like a ball with curvature can’t be infinite, but a flat piece of paper can.
We know the size of the *observable* universe because we can see the stuff at the edge.
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