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When we study the universe as a whole, with its millions of galaxies, we have reached the largest scale of space and time; and again, at that cosmic level, we discover that the universe is not static-it is expanding! This has been one of the most important discoveries in modern astronomy. A detailed analysis of the light received from distant galaxies has shown that the whole swarm of galaxies expands and that it does so in a well orchestrated way; the recession velocity of any galaxy we observe is proportional to the galaxy\u2019s distance. The more distant the galaxy, the faster it moves away from us; at double the distance, the recession velocity will also double. This is true not only for distances measured from our galaxy, but applies to any point of reference. Whichever galaxy you happen to be in, you will observe the other galaxies rushing away from you; nearby galaxies at several thousand miles per second, farther ones at higher speeds, and the farthest at velocities approaching the speed of light. The light from galaxies beyond that distance will never reach us, because they move away from us faster than the speed of light. Their light is – in the words of Sir Arthur Eddington – \u2018like a runner on an expanding track with the winning post receding faster than he can run\u2019. <\/p>\n\n\n\n
To have a better idea of the way in which the universe expands, we have to remember that the proper framework for studying its large-scale features is Einstein\u2019s general theory of relativity. According to this theory, space is not \u2018flat\u2019, but is \u2018curved\u2019, and the precise way in which it is curved is related to the distribution of matter by Einstein\u2019s field equations. These equations can be used to determine the structure of theuniverse as a whole; they are the starting point of modern cosmology.\u00a0<\/p>\n\n\n\n
When we talk about an expanding universe in the framework of general relativity, we mean an expansion in a higher dimension. Like the concept of curved space, we can only visualize such a concept with the help of a two-dimensional analogy. Imagine a balloon with a large number of dots on its surface. The balloon represents the universe, its two-dimensional curved surface representing the three-dimensional curved space, and the dots on the surface the galaxies in that space. When the balloon is blown up, all the distances between the dots increase. Whichever dot you choose to sit on, all the other dots will move away from you. The universe expands in the same way: whichever galaxy an observer happens to be in, the other galaxies will all move away from him. <\/p>\n\n\n\n