If we know the universe is expanding in whatever direction we look, can't we reasonably estimate where the 'center' of the Universe is?
Is the rate of expansions in all directions the same?
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The speed that a galaxy moves away from you is larger the further away the galaxy already is. This is most consistent with an expanding universe.
I find the following mental picture helpful: Imagine a balloon, with lots of little dots painted on it. If you inflate the balloon, then every point is moving away from every other point. Where would the "center" be? Pretty much everywhere.
Wikipedia kindly presents four models you can use to visualize the expansion of the universe. All of these are meant to explain why we think the universe is expanding, yet we think there is no center. Everything† moves away from everything else, with expanding velocity proportional to the distance.
- In the "ant on a rubber rope model" one imagines an ant (idealized as pointlike) crawling at a constant speed on a perfectly elastic rope which is constantly stretching. If we stretch the rope in accordance with the ΛCDM scale factor and think of the ant's speed as the speed of light, then this analogy is numerically accurate—the ant's position over time will match the path of the red line on the embedding diagram above.
- In the "rubber sheet model" one replaces the rope with a flat two-dimensional rubber sheet which expands uniformly in all directions. The addition of a second spatial dimension raises the possibility of showing local perturbations of the spatial geometry by local curvature in the sheet.
- In the "balloon model" the flat sheet is replaced by a spherical balloon which is inflated from an initial size of zero (representing the big bang). A balloon has positive Gaussian curvature while observations suggest that the real universe is spatially flat, but this inconsistency can be eliminated by making the balloon very large so that it is locally flat to within the limits of observation. This analogy is potentially confusing since it wrongly suggests that the big bang took place at the center of the balloon. In fact points off the surface of the balloon have no meaning, even if they were occupied by the balloon at an earlier time.
- In the "raisin bread model" one imagines a loaf of raisin bread expanding in the oven. The loaf (space) expands as a whole, but the raisins (gravitationally bound objects) do not expand; they merely grow farther away from each other.
† Bound objects—in particular galaxies—are held together by binding forces like gravitation and therefore do not expand internally.