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Cosmology comes off age.


It is hard to think of any scientific endeavour that has changed more during the past century than cosmology. A hundred years ago other galaxies were thought of as gas clouds probably somewhere in our galaxy. The Universe was regarded as static and never-changing, with no beginning and no end. The birth of the Universe from a quantum sized object was never considered. The expansion of the Universe and the description of the effects of gravity as the curvature of space-time were still to come. The synthesis of elements in the stars was not understood and the Microwave Background Radiation was still to be discovered. It is not only cosmology that has been changed drastically by these discoveries but our view of the world and man's place in it has been altered beyond all recognition. Cosmology as a science has come of age and is no longer considered to be part of religion or philosophy. Let us look at what these discoveries were and why they changed cosmology so dramatically.

Einstein's general theory of relativity

To talk about cosmology and the Big Bang means to talk about general relativity. The period preceding general relativity was dominated by Newton's theory of universal gravitation. General relativity described the effects of gravity as the geometry of space-time. Space and time were united in a new dimension, space-time, which is no longer a passive background as described by Newton, but is an active player in shaping the Universe. General relativity improved Newton's theory, which could only predict motions up to galactic scale, to be able to deal with collections of matter on a cosmic scale. But Einstein's first solution described an unstable universe which is expanding or contracting. Einstein did not like this and he introduced a 'cosmological constant' describing a static, unchanging universe in line with his own beliefs.

Shortly after Einstein published his general relativity theory, the Dutch physicist, Willem de Sitter, solved the general relativity equations without Einstein's cosmological constant to describe what appeared to be a static unchanging universe. A breakthrough came when it was discovered that De Sitter's description was a misinterpretation; the de Sitter universe is indeed expanding.

The Belgian priest and physicist, Georges Lemaitre, who is sometimes referred to as the father of the Big Bang theory, showed that the equations of general relativity predicted an unstable universe that contracted or expanded. With our current knowledge we know that a static universe is impossible. Our observations that the sky is dark at night are very important. If the Universe was indeed static and unchanging, the whole sky would have been as bright as the Sun itself with starlight filling the whole Universe with energy. Every line of sight would be filled with starlight as bright as the Sun.

Lemaitre used Einstein's equations to prove that the Universe had a beginning where all matter and energy were compressed into a single compact particle which he called a "primeval atom". He claimed that it was the disintegration of this primeval atom that gave birth to the Universe. It was therefore not surprising that he was referred to as the father of the Big Bang theory. Later Stephen Hawking and Roger Penrose used Einstein's equations to prove that the Universe was born from a singularity, a point in space-time where everything became infinite. This was probably the most significant development in the mathematical proof of the origin of the Universe.

The expanding Universe

The astronomer usually credited for the discovery that the Universe is expanding, Edwin Hubble, was not in fact the first astronomer to provide observational evidence of an expansion. The American astronomer, V M. Slipher, used the spectra of stars to measure the velocity of nearby galaxies. Using the Doppler effect of light, Slipher could determine that distant galaxies are moving away from us. From the results of his observations Slipher concluded that the Universe is expanding.

It was however Edwin Hubble who determined not just that the Universe is expanding and that the galaxies are receding. Hubble concluded that the external galaxies were like our own Milky Way. He was able to measure the distances to some of the receding galaxies and discovered a relation between the distances to and their velocities. The velocity of a receding galaxy was directly proportional to its distance from us. Therefore a galaxy twice as far as another was moving away from us at twice the speed. Making use of the constant of proportionality, now called the Hubble Constant, he could also measure the rate of expansion of the Universe, i.e., the average value of velocity of recession divided by distance, now estimated as being about 70 km/s/megaparsec.

His result led to the belief that if the Universe is expanding then everything must have been closer in the past. Going back far enough the Universe must have been very, very small, as small as a quantum object from which our Universe was born. This is the Big Bang theory.

The Cosmic Microwave Background Radiation (CMBR)

George Gamow realised that if there had been a Big Bang then there must still be residual radiation left from the enormous heat when the Universe was born. The Cosmic Microwave Background Radiation was discovered by accident in 1965 by two Bell Laboratory scientists, Arno Penzias and Robert Wilson, as they tracked down sources of radio interference. Almost immediately after its discovery the CMBR was recognized as the relic of the early stages of the expansion of the Universe. About three hundred thousand years after the Big Bang the expanding Universe had cooled down enough to allow photons to travel freely without clashing with other particles such as electrons. The photons (radiation) from this epoch in the evolution of our Universe can today be observed as a faint glow of microwaves (photons in the microwave wavelength). The discovery of the CMBR was the final evidence that scientists needed to establish the Big Bang theory as a scientific fact.

Concluding comments

To talk of the Big Bang theory would be incomplete if the role of quantum mechanics in the understanding of the very early moments of the quantum sized universe is not mentioned. The uncertainty principle, the most basic tenet of quantum mechanics, enabled scientists to understand that a runaway chance quantum fluctuation in the primordial quantum vacuum could have set in motion the birth of the Universe. Subsequent events such as the exponential inflation could only be explained by quantum principles. The continuous fluctuation of energy in the quantum field which are believed to be responsible for tiny variations of temperature in the CMBR provided the 'seeds' from which stars and galaxies were formed. Not all scientists accept the Big Bang theory to explain the birth of the Universe. Nor do we fully understand the incredible temperature and densities that supposedly existed in the first few microseconds after the Big Bang.

To learn more about these fascinating ideas and others, please contact the author.

Frikkie de Bruyn

Director: Cosmology Section of ASSA
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Title Annotation:cosmology corner
Author:de Bruyn, Frikkie
Publication:Monthly Notes of the Astronomical Society of Southern Africa
Geographic Code:6SOUT
Date:Dec 1, 2009
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