- The event which according to most modern cosmogony began the universe we now know, perhaps some 18 billion years ago. At that time, it seems, the whole of the matter now distributed throughout the universe was in a single compact mass whose properties can only be described in relativistic equations. Since then it has formed into galaxies, stars, planets and everything else we see, expanding constantly. From the point of view of the cosmologists, all these are details, and the basic shape of the universe in terms of the energy and matter it now contains was established in the big bang itself. It would be wrong to think of a small volume at the centre of the universe containing the whole of the mass we now see around us. Instead, the cosmos takes space with it as it expands. The problem of just what the initial fireball of material did before, during and immediately after the big bang is an elaborate one, touching on the nature of the subatomic particles and atomic forces now acting in the universe. However, there is powerful experimental proof that the big bang was a real event. The cosmic background radiation, which is made up of the heat left over from the big bang, is one proof, and its very homogeneous nature, allied to the shape of its spectrum, makes any other explanation of it implausible. In addition, the abundance of helium in the universe, making up about 23 per cent of its mass, with hydrogen making up almost all of the rest, is accounted for by the conditions of the big bang. The age of the universe is derived from measures of the expansion of the universe, which is determined by the measurement of galactic redshifts. These are now being revised because of the availability of data from the Hubble Space Telescope and there have been some indications of a much younger universe than had been thought.
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