I may be giving a public lecture on this topic in the near future, and this will be the first such lecture I would have given. Therefore I shall practice on my few readers, and try and present my research in laymen terms. Please feel free to question me and highlight any areas that I have not been clear in or issues that need further elaboration.
I shall start this series with an overview of the origins and content of the Big bang theory.
The Big Bang Theory
Einsteins theory of gravitation
Gravitation is the force that attracts masses (bodies) to each other. The Newtonian view (i.e. according to Isaac Newton) was that gravitation was like an invisible force field connecting bodies to each other, the strength of this force increased in proportion to mass and decreased in proportion to the distance between the bodies. This idea was revolutionized by Albert Einstein. He made the connection between the mass of the body and the geometry of space. Basically a mass would distort space, effectively creating a dent in space, so when another body passed by, its path would be affected by this distortion.
To get a better idea of this phenomenon, imagine a rubber sheet nailed at its corners to a square frame, now run a small ball across this sheet and it will follow a straight path (ignore friction). Now, in the middle of the sheet place a large heavy ball, you will note that the sheet dents with the centre of the dent where the large ball is. Flick a smaller ball tangentially to the dent and it will spiral around the ball, flick it towards the large ball and it will fall towards the larger ball following a curved path. In this analogy the rubber sheet is space, and it is in this manner that gravitation works.
Point 1: masses distort the geometry of space.
Einsteins original theory predicted that space should be expanding, but this was in contradiction to the theory at the time that the universe was static. As a result of this Einstein added a term to his equation to counteract their prediction of an expanding space.
Point 2: Einsteins theory predicted an expanding universe, but he modified them so that they predicted a static universe.
Hubbles paradigm changing discovery:
Not long after Einsteins inception of his theory of gravity, the astronomer Edwin Hubble made a paradigm changing discovery. He rather accurately measured the doppler shift of neighboring galaxies.
Doppler shift: when a car is moving towards you the sound of the engine appears to be increasing in frequency, and when it is moving away from you the sound appears to be decreasing in frequency. This phenomenom also applies to light. Light can be viewed as a wave, and when the source of the light of moving towards you the frequency appears to increase and vice versa. A lower frequency corresponds to the infrared end of the light spectrum (see figure) and a higher frequency corresponds to the ultraviolet (or blue) end.
Anyway, Hubble discovered that on average, the galaxies exhibited a red shift and that the further a galaxy was away from us the faster it was moving away. In other words he discovered that the universe was expanding. Well, Einstein was left a bit red faced following this discovery. Friedmann and Lemaitre then independently derived the Friedman Lemaitre equations from Einsteins equations showing that the Universe is expanding.
Point 3: the universe is expanding.
Point 4: Einstein was embarrassed, and reverted to his original equations.
The Cosmological Principle and the Inception of the Big Bang theory
The Cosmological Principle states that on large scales the universe is both homogeneous and istoropic. In simple terms that means that in general, the properties of the universe are the same regardless of the direction of observation. This principle are apparent in Lemaitres equations, which showed that the universe was also expanding.
Lemaitre took this property of a future expanding universe one step further, and predicted that the universe was smaller in the past. This can be extrapolated to a point in the past when the volume of the universe was zero, this is known as the big bang singularity.
So what does the big bang theory tell us?
Well, there is a relationship between the size and temperature of the universe, namely as the universe expands it also cools. So the early universe was very hot, so hot in fact that matter would have not existed. Instead the universe was radiation dominated, it was a gloop of photons (particles of light) and highly energised subatomic particles. In this gloop particles and their nemesis the anti-particles would collide with such energy that they gave off radiation in the form of gamma rays which in turn decayed to form the particle-anti particle pairs. This state of affairs could not be supported as the universe cools, and eventually the annihilation of particles with their anti particles ceased to occur.
At this stage higher forms of matter could be sustained, i.e. electrons, protons, neutrons. The density was still quite high though, and photons were often deflected off electrons (known as Thompson scattering) and therefore a photon could not travel very far before it bounced off an electron. The free path of the photon was short, and the universe was effectivley opaque to light.
The universe was adament in its expansion though, and eventually the density of matter dropped so that light could travel freely without hindrance. The electrons had anyway combined with the protons and neutrons to form neutral hydrogen rendering them practically harmless to photons. This time, when the photons are released from their Thompson prison is known as matter radiation equality.
The freed light, this relic of the early history of the universe, is known as the Cosmic Background Radiation, and permeates the universe till today. The properties of this radiation/light can be accuratley estimated using the black body analogy and it was calculated that the Cosmic Background Radiation would have a temperature today of 2.74 Kelvin (about -270.3 Celcius) , this was first measured by Penzias and Wilson in the 1960s.
The theory also offers a neat chronology of events to how stars and elementary chemicals are produced, but this part can be neglected for the purpose of this presentation, since we are primarily concerned with the very early history of the universe.
Next: what the big bang theory does not predict