The Formation of Heavier Atoms ,Edwin Hubble

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The Formation of Heavier Atoms
In 1929, the American astronomer Edwin Hubble, by measuring the Doppler shift of the spectral lines from many stars and galaxies, discovered that the universe is expanding. He showed that the lines are shifted to the red end of the spectrum and hence that these bodies are moving away from the Earth.
The effect is often known as the red shift. Hubble also showed that the further the objects are away, the faster they are moving. One explanation for this effect was that there was continuous creation of matter balancing the expansion, the steady-state theory of the universe. However, the presently accepted theory is that everything started about 13.7 billion years ago with a gigantic explosion known as the Big Bang. The idea of an expanding universe was proposed in 1927 by the Belgian cosmologist Georges Lemaître, and the model was described in detail by George Gamow (Figure 3.4), Ralph Alpher, and Hans Bethe in 1948 in their famous “Alpher, Bethe, Gamow” paper.

'FIGURE 3.4 George Gamow (1904–1968). Gamow was a Ukrainian, born in Odessa and educated
in Leningrad, but in 1934 he emmigrated to the US, where he was a professor of physics,
first at George Washington University and then at the University of Colorado. He was a prolific
writer of books on science for the layperson, particularly on cosmology; many of his works are
still in print. '





Their model predicted that there should be observable radiation left over from the Big Bang. This radiation, now known as the Cosmic Microwave Background Radiation (CMBR) (Box 3.3), was first observed by Arno Penzias and Robert Wilson in 1964 and was found to be close to the predicted level. The predictions of the background radiation and of the correct abundances of hydrogen,
helium, and lithium, which are now observed by spectroscopy of gas clouds and old stars, are the major successes of the Big Bang theory and are the justification for taking it to be the most likely explanation for the origin of the universe.

At the inconceivably high temperatures in the primeval fireball, mass and energy were continually interchanging. As the fireball expanded, it cooled rapidly, and at this stage the energy was converted permanently into mass—first as the underlying subnuclear building blocks were formed and then as these building blocks themselves combined to form protons and neutrons. Some deuterium and helium nuclei were formed, via the fusion reactions discussed earlier, when the conditions were suitable. As the universe expanded, it became too cold for these initial fusion reactions to continue, and the mix of different nuclei that had been produced was “frozen.” It is calculated that this stage was reached only 4 minutes after the initial Big Bang. At this point the universe consisted of an expanding cloud composed mainly of hydrogen (75%) and helium (25%), with small amounts
of deuterium and lithium. The universe today is known to contain 92 different elements ranging in
mass from hydrogen to uranium. The theory of the Big Bang is quite explicit that nuclei much heavier than helium or lithium could not have been formed at the early stage. The obvious next question is: What is the origin of all the other elements, such as carbon, oxygen, silicon, and iron? Advertisement