Evolutionary theories. The Belgian George Lemaitre put forward the idea that about 20000 million years ago all the matter in the universe—enough, he estimated, to make up a hundred thousand million galaxies—was all concentrated in one small mass, which he called the "primeval atom". This primeval atom exploded for some reason, sending its matter out in all directions, and as the expansion slowed down, a steady state resulted, at which time the galaxies formed. Something that upset the balance and the universe started expanding again, and this is the state in which the universe is now. There are variations on this theory: it may be that there was no steady state. However, basically, evolutionary theories take it that the universe was formed in one place at one point in time and has been expanding ever since.
Will the universe continue to expand It may be that the universe will continue to expand for ever, but some astronomers believe that the expansion will slow down and finally stop. Thereafter the universe will start to contract until all the matter in it is once again concentrated at one point. Possibly the universe may oscillate(摆动) for ever in this fashion, expanding to its maximum and then contracting and over again.
The steady-state theory. Developed at Cambridge by Hoyle Gold and Bodi, the steady- state theory maintains that the universe as a whole has always looked the same and always will. As the galaxies expand away from each other, new material is formed in some way between the galaxies and make up new galaxies to replace those which have receded. Thus the general distribution of galaxies remains the same. How matter could be formed in this way is hard to see, but no harder than seeing why it should all form in one place at one time.
How can we decide which of these theories is closer to the truth The method is in principle quite . Since the very distant galaxies are thousands of millions of light years away, then we are seeing them as they were thousands of millions of years ago. If the evolutionary theory is correct, the galaxies were closer together in the past than they are now, and so distant galaxies ought to appear to be closer together than nearer ones. According to the steady-state theory there should be no difference.
The evidence seems to suggest that there is a difference, that the galaxies were closer together than they are now and so the evolutionary theory is partially confirmed and the steady- state theory—in its original form at least—must be rejected. According to Hoyle and Iris friends at Cambridge ______.