The Creation of the Universe

Before we can discuss the basic problem of the origin of our universe, we must ask ourselves whether such a discussion is necessary. Could it not be true that the universe has existed since eternity, changing slightly in one way or another in its minor features, but always remaining essentially the same as we know it today? The best way to answer this question is by collecting information about the probable age of various basic parts and features that characterize the present state of our universe.

For example, we may ask a physicist or chemist: 'How old are the atoms that form the material from which the universe is built?' Only half a century ago such a question would not have made much sense. However, when the existence of natural radioactive elements was recognized, the situation. became quite different. It became evident that if the atoms of the radio-active elements had been formed too far back in time, they would by now have decayed completely and disappeared. Thus the observed relative abundances of various radio-active elements may give us some clue as to the time of their origin.

We notice first of all that thorium and the common isotope of uranium (U²³⁸) are not markedly less abundant than the other heavy elements, such as, for example, bismuth, mercury or gold. Since the half-life periods of thorium and of common uranium are 14 billion and 4.5 billion years respectively, we must conclude that these atoms were formed not more than a few billion years ago. On the other hand the fissionable isotope of uranium (U²³⁵) is very rare, constituting only 0.7 % of the main isotope. The half-life of U²³⁵ is considerably shorter than that of U²³⁸, being only about 0.9 billion years. Since the amount of fissionable uranium has been cut in half every 0.9 billion years, it must have taken about seven such periods, or about 6 billion years, to bring it down to its present rarity, if both isotopes were originally present in comparable amounts. Similarly, in a few other radio-active elements, such as radio-active potassium, the unstable isotopes are also always found in very small relative amounts. This suggests that these isotopes were reduced quite considerably by slow decay taking place over a period of a few billion years. Of course, there is no a priori reason for assuming that all the isotopes of a given element were originally produced in exactly equal amounts. But the coincidence of the results is significant, inasmuch as it indicates the approximate date of the formation of these nuclei. Furthermore, no radio-active elements with half-life periods shorter than a substantial portion of a billion years are found in nature, although they can be produced artificially in atomic piles. This also indicates that the formation of atomic species must have taken place not much more recently than a few billion years before the present time. Thus there is a strong argument for assuming that radio-active atoms, and along with them, all other stable atoms were formed under some unusual circumstances which must have existed in the universe a few billion years ago.

As the next step in our enquiry, we may ask a geologist: 'How old are the rocks that form the crust of our globe?' The age of various rocks - that is, the time that has elapsed since their solidification from the molten state - can be estimated with great precision by the so-called radio-active clock method. This method, which was originally developed by Lord Rutherford, is based on the determination of the lead content in various radio-active minerals such as pitchblende and uraninite. The significant point is that the natural decay of radio-active materials results in the formation of the so-called radiogenic lead isotopes. The decay of thorium produces the lead isotope Pb²⁰⁸, whereas the two isotopes of uranium produce Pb²⁰⁷ and Pb²⁰⁶. These radiogenic lead isotopes differ from their companion Pb²⁰⁴, natural lead, which is not the product of decay of any natural radio-active element.

As long as the rock material is in the molten state various physical and chemical processes may separate the newly produced lead from the mother substance. However, after the material has become solid and ore has been formed, radiogenic lead remains at the place of its origin. The longer the time period after the solidification of the rock, the larger the amount of lead deposited by any given amount of radio-active substance. Therefore, if one measures the relative amounts of deposited radiogenic lead isotopes and the lead-producing radio-active substances (that is, the ratios: Pb²⁰⁸/Th²³², Pb²⁰⁷/U²³⁵, Pb²⁰⁶/U²³⁸) and if one knows the corresponding decay rates, one can get three independent estimates of the time when a given radio-active ore was formed. By applying this method, one gets results of the kind shown in the following table.

The last two minerals are the oldest yet found, and from their age we must conclude that the crust of the earth is at least 2 billion years old.

A much more elaborate method was proposed recently by the British geologist, Arthur Holmes. This method goes beyond the formation time of different radio-active deposits; and claims an accurate figure for the age of the material forming the earth. By applying this method to the relative amounts of lead isotopes found in rocks of different geological ages, Holmes found that all curves intersect near the point corresponding to a total age of 3.35 billion years, which must represent the correct age of our earth.

How old is the moon? As was shown by the work of the British astronomer, George Darwin, the moon is constantly receding from the earth, at the rate of about 5 inches every year. Dividing the present distance to the moon (239,000 miles) by the estimated rate of recession, we find that the moon must have been practically in contact with the earth about 4. billion years ago.

Thus we see that whenever we inquire about the age of some particular part or property of the universe we always get the same approximate answer - a few billion years old. Thus it seems that we must reject the idea of a permanent unchangeable universe and must assume that the basic features of the universe as we know it today are the direct result of some evolutionary development which must have begun a few billion years ago.

(From The Creation of the Universe by George Gamow.