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The Big Bang Theory
Introduction
The Big Bang Model is a broadly accepted theory for the origin and evolution of our universe. It states that some 13.7 billion years ago, the portion of the universe we can see today was compressed into an infinitely dense singularity. Since that time the universe has expanded until it is now the much cooler and vast cosmos we observe today.
The theory was first proposed by Georges Lemaître in 1931. In 1927 Lemaître proposed that the recession of a distant galaxy, as measured by Edwin Hubble, was due to expansion of the universe. He arrived at this using a particular solution to Einstein's equations for General Relativity* and the assumption of the Cosmological Principle
By 1929 Hubble had, through a series of detailed measurements using the Mount Wilson Observatory, arrived at the realisation that the further away a galaxy was, the faster it was receding from earth (this is now called Hubble's Law). This provided the first support for Lemaître's new theory of expansion. In 1931 Lemaître suggested that, if one were to trace time backwards, the universe would contract until it could contract no further, bringing the whole universe together in a 'primeval atom'.
By the end of World War II two hypotheses had emerged regarding the cosmos. One was the 'steady state' theory, championed by Fred Hoyle. The other was Lemaître's theory*. Ironically it was Hoyle who actually coined the phrase 'Big Bang' during a BBC Radio broadcast in March 1949 - intending to mock the theory.
The final nail in the coffin of the alternate 'steady state' theory came, in the view of most physicists/cosmologists, in 1964 when Arno Penzias and Robert Wilson discovered the Cosmic Microwave Background.
*The particular solution had already been calculated by Alexander Friedmann in 1922 and is known as the Friedmann equations. It seems certain that Lemaitre arrived at the equations independently.
If we extrapolate the state of the universe backwards in time, using General Relativity, we arrive at an infinitely dense and 'hot' state at a finite time in the past. This is generally called the 'singularity' and marks the point at which the theory of General Relativity breaks down. This hot, dense state is known as the Big Bang and is regarded as the 'birth' of the Universe (or at least that part of the Universe which is visible to us).
In order to arrive at a precise time for this event, measurements of Type Ia supernovae, fluctuations in the Cosmic Microwave Background radiation, and the correlation function of galaxies are used. When the calculations are performed, using these metrics, the age of the Universe comes out to 13.73 ± 0.12 billion years.
Support for the model
The first major experimental support for the Big Bang model came in 1964 with the discovery of the Cosmic Microwave Background radiation predicted by the model. Most physicists saw this as the clinching evidence that the other main theory - steady-state theory - was wrong and that the Big Bang model was correct.
Other evidence:
- The relative abundance of the lighter elements. The model predicts that there should be a large abundance of helium nuclei (He-3 and He-4), with a dash of deuterium (a form of hydrogen with a proton-neutron nucleus), lithium and beryllium. All the heavier elements are made in the core of Stars. This prediction has been tested experimentally by examining the amount of deuterium in the spectrum of stars and the observations agree with the model. Also, as we look further into space (back in time) the lighter elements become even more abundant relatively - exactly what we would predict.
- The age of stars. The model predicts that stars would start to form around 200 million years after the initial Big Bang. it follows that we should not observe stars that are older than this - and we don't.
- Evolution of galaxies. The model predicts that galaxies will evolve as gravity dominates and galaxies collide and interact with each other. Thus as we look back in time we should be able to see different types of galaxies corresponding to the different ages. This is exactly what we do observe.
- Time dilation in supernova brightness curves. Light travelling through the expanding universe should undergo red-shift as a result (the wavelength is stretched as the universe expands). Analysis of the spectra of different supernovae show this effect quite clearly - the spectra of older (more distant) supernovae are indeed red-shifted.
- Tolman tests. The model also predicts that the brightness of light sources should decrease as the fourth power of the redshift. The early universe was full of plasma, and photons were constantly scattering off of all of the ionized matter. This means that the universe a perfect absorber; no photons could leave the universe. It follows that the whole universe (or at least that part that was causally connected) is in thermal equilibrium and can be described as having a unique temperature. In classical thermodynamics, photons emitted by a black-body, at a given temperature, have a very specific distribution of energies and, as Tolman showed in 1934, this is what is observed.
In fact much more evidence for the model exists - for more details go here
