cosmology: The Big-Bang Theory

The Big-Bang Theory

According to big-bang theories, at the beginning of time, all of the matter and energy in the universe was concentrated in a very dense state, from which it “exploded,” with the resulting expansion continuing until the present. This big bang is dated between 10 and 20 billion years ago, most likely c.13.799 billion years ago. In this initial state, the universe was very hot and contained a thermal soup of quarks, electrons, photons, and other elementary particles. The temperature rapidly decreased, falling from 10¹³ degrees Kelvin after the first microsecond to about one billion degrees after three minutes. As the universe cooled, the quarks condensed into protons and neutrons, the building blocks of atomic nuclei. Some of these were converted into helium nuclei by fusion; the relative abundance of hydrogen and helium is used as a test of the theory. After many millions of years the expanding universe, at first a very hot gas, thinned and cooled enough to condense into individual galaxies and then stars.

Several spectacular discoveries since 1950 have shed new light on the problem. Optical and radio astronomy complemented each other in the discovery of the quasars and the radio galaxies. It is believed that the energy reaching us now from some of these objects was emitted not long after the creation of the universe. Further evidence for the big-bang theory was the discovery in 1965 that a cosmic background noise is received from every part of the sky. This background radiation has the same intensity and distribution of frequencies in all directions and is not associated with any individual celestial object. It has a blackbody temperature of 2.7K (−270℃) and is interpreted as the electromagnetic remnant of the primordial fireball, stretched to long wavelengths by the expansion of the universe. More recently, the analysis of radiation from distant celestial objects detected by artificial satellites and orbiting observatories has given additional evidence for the big-bang theory.

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