Expanding universe. Computer artwork illustrating the spatial expansion of the universe, using the analogy of an expanding balloon. Here, the universe and its galaxies are situated on the surface of the balloon, so that when it is inflated (left to right), the galaxies all recede from each other. Because the 2D surface of the balloon is analogous to the 3D fabric of space, it helps to grasp the idea that the motion of the galaxies is not through space, but is due to the expansion of space itself. In addition, there is no centre to this expansion as the universe will appear to be expanding uniformly from the point of view of any galaxy. This analogy was used by the British astronomer Arthur Eddington (1882-1944) in his book The Expanding Universe (1933).
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