An overwhelming weight of evidence has convinced cosmologists that the universe came into existence at a definite moment in time, some 14 billion years ago, in the form of a superhot, superdense fireball of energetic radiation. This is known as the Big Bang theory. Until the arrival of the Big Bang theory the universe was believed to be essentially eternal and unchanging, represented by the Steady State model. The first clear hint that the universe might change as time passes came in 1917 when Albert Einstein developed his General Theory of Relativity. Einstein realised that his equations said that the universe must be either expanding or contracting, but it could not be standing still, because if it were then gravity would attract all the galaxies towards one another. This was, at the time, a revolutionary concept, so revolutionary that Einstein refused to believe it and introduced his infamous 'cosmological constant' into the equations so that the sums agreed that the universe could be static. He later claimed it was the biggest blunder of his career. It was in 1920 that Edwin Hubble discovered that the universe was expanding by measuring the light from distant galaxies. This discovery was followed in 1927 by Georges Lemaitre, a Belgian astronomer, who was the first person to produce a version of what is now known as the Big Bang model.
It is necessary to understand that the Big Bang did not begin as a huge explosion within the universe, the Big Bang created the universe. A popular misconception is that it happened within the universe and that it is expanding through it. This causes people to wonder where in the universe it started, as if by running the clock backwards we would reach the point where all the galaxies come together in the centre of the universe. The universe does not have a centre, any more than the surface of a sphere has a centre, there is no preferred place that could be termed the centre. I know this sounds odd, it must have a centre, mustn't it? The problem we have here is we are trying to visualise the universe in the standard 3 dimensions that we are familiar with and therefore expect to find a centre to an expanding sphere. The universe, however, is not an expanding 3 dimensional sphere, it contains also the dimension of time and many other dimensions as well. By way of an illustration imagine a balloon with dots painted on the surface to represent the galaxies. If the balloon is now inflated we can see that all the dots are moving away from one another, just as the galaxies are in the real universe, and we can also see that on the surface of the balloon there is no centre point from which all the galaxies are moving away from. I am not suggesting that we are existing on the 'outside' of an expanding bubble, only that we cannot visualise the entire expanding universe.
Let's begin with a brief look at how the Big Bang describes the creation and evolution of the universe before moving on to some of the evidence to support the theory and the problems associated with the theory.
The Big Bang theory
The standard model of the Big Bang theory proposes that the universe emerged from a singularity, at time zero, and describes all that has happened since 0.0001 (10-4) of a second after this moment of creation. The temperature of the universe at that time was 1,000 billion degrees Kelvin (1012) and had a density that of nuclear matter, 1014 grams per cubic centimetre (the density of water is 1 gram per cubic centimetre). Under these extreme conditions, the photons of the 'background' radiation carry so much energy that they are interchangeable with particles. Photons create pairs of particles and antiparticles which annihilate one another to make energetic photons in a constant interchange of energy in line with Einstein's equation E = mc2. Because of a small asymmetry in the way the fundamental interactions work, slightly more particles were produced than antiparticles - about one in a billion more particles than antiparticles.
When the universe had cooled to the point that photons no longer had the energy required to make particles, all the paired particles and antiparticles annihilated, and the one in a billion particles left over settled down to become stable matter.
One-hundredth of a second after time zero the temperature had fallen 90% to 100 billion K. By one-tenth of a second after time zero the temperature was down to 30 billion K. The temperature after 13.8 seconds was down to 3 billion K, and by three minutes and two seconds had cooled to 1 billion K, only 70 times hotter than the centre of the Sun today. At this temperature nuclei of deuterium and helium could be formed and stick together despite collisions with other particles.
During the fourth minute after time zero reactions took place that locked up the remaining neutrons in helium nuclei, as described by Gammow et al in 1940 and Fred Hoyle and others in the 1960's. This epoch ended with just under 25% of the nuclear material converted into helium, and the rest left behind as lone protons - hydrogen nuclei.
By just over 30 minutes after time zero, all of the positrons had annihilated with almost all of the electrons - with again one in a billion left over - to produce the background radiation proper, and the temperature had dropped to 300 million K, and the density was only 10% of that of water. At this temperature stable atoms were still not able to form.
The interactions between electrons and photons continued for 300,000 years, until the universe had cooled to 6000 K, roughly the temperature of the surface of the Sun, and the photons were becoming too weak to knock electrons off atoms.
Over the next 500,000 years the background radiation decoupled, and had no more significant interaction with matter. The Big Bang was in effect over, and the universe left to expand and cool. About 1 million years after time zero, stars and galaxies could begin to form. Nucleosynthesis inside stars convert hydrogen and helium to make heavier elements, eventually giving rise to our Sun, the Earth and ourselves.
heres the link to the site where i got this from, also has evidence
http://www.thekeyboard.org.uk/The%20Big%20Bang%20Theory.htm
