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Posted 27 April 2008

Pillars of the Big Bang Theory

by Frikkie de Bruyn


When you drink water, remember the hydrogen in
the water was forged in the furnace of  the big bang.
We are truly part of the universe.  

1. Introduction

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 thought of 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 spacetime 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 changed 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 it changed cosmology so dramatically.


2. Einstein’s general theory of relativity


The first pillar on which our understanding of the big bang is based is Einstein’s general theory of relativity. The period preceding general relativity was dominated by Newton’s theory of universal gravitation. The effects of gravity have been described by general relativity as the geometry of spacetime. Space and time have been united in a new dimension, spacetime, which was no longer a passive background as described by Newton, but it became an active player in shaping the universe. General relativity improved Newton’s theory, which could only predict motions up to galactic scale, to predictions of collections of matter on a cosmic scale. But Einstein’s equations described an unstable universe which is expanding or contracting. Einstein did not like this and he introduced a cosmological constant to his equations describing a static, unchanging universe in line with his own believes.

Shortly after Einstein published his general relativity theory, the Dutch physicist, Willem de Sitter, solved the general relativity equations using 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 spacetime where everything became infinite. This was probably the most significant development in the mathematical prove of the origin of the universe.


3. The expanding universe

The astronomer usually credited for the discovery that the universe is expanding, Edwin Hubble, was not the first astronomer to provide observational evidence of an expanding universe. 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, the American astronomer, Edwin Hubble, who determined not just that the universe is expanding and the velocities of recession of the galaxies, but he also measured the distances to the receding galaxies. From his measurements Hubble concluded that the galaxies were so far away from us that they were galaxies like our own. He further discovered a relation between the distance to these galaxies 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 of the nearer galaxy. Making use of the Hubble constant he could also measured the rate of expansion of the universe; the average value of velocity of recession divided by distance which is about 70 km/s/megaparsec. The conclusion to which Hubble and thousands of other scientists came was 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.


4. The Cosmic Microwave Background Radiation (CMBR)

It was rumoured that the maverick scientist, George Gamow, said that if there was a big bang then there must still be a residual radiation left from the enormous heat when the universe was born. The third pillar of the big bang theory, the CMBR, was discovered by accident in 1965 by the Bell Laboratory scientists 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 CBR was the final evidence that scientists needed to establish the big bang theory as a scientific fact.


5. Concluding comments

A discussion of the pillars 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.

Frikkie de Bruyn

Those who would like to read more about the big bang in non-technical terms, the following book might be useful:

Karen C. Fox, The Big Bang Theory. What it is, where it came from, and why it works. 


Return to Cosmology Corner main page.

 

 

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