The Big Bang Theory states that other galaxies are moving away from our own at great speed, in all directions, as if they have all been propelled by an ancient explosive force. Before this occurred it is believed that the entire universe was compressed into a mass that was only a few millimetres across called the primordial atom or the singularity. This mass contained all of the universe's matter and radiation therefore it was extremely dense. It took a mere three minutes for the atoms that make up the universe as we know it to be created. From there the universe expanded and became what it is today. Even as you are reading this now our universe is still expanding.
Nobody really knows what existed before the Big Bang or how it occurred; however there are some theories involving the possibility that the singularity came out of a black hole from another universe and the big bang occurred from there. How our universe came to be is one of the greatest mysteries of all time.
The Big Bang Theory was proposed by a Belgian priest and cosmologist named George Lemaître in the 1927. George was born in 1894 in Belgium. However the term 'Big Bang' was in fact coined by Fred Hoyle; the creator of the Steady State Theory. George's theory was supported by four different people and their three theories. These theories are:
CMBR (Cosmic Microwave Background Radiation) - Arno Penzias and Robert Wilson discovered cosmic microwave radiation. They heard a strange buzzing sound coming thorugh their satellite; at first it was thought to be pigeon poo affecting the signal. However it was later discovered that it is the electromagnetic radiation left over from the occurrence of the big bang, therefore it is further proof that the Big Bang occurred.
REDSHIFT - Edwin Hubble's redshift theory supports the Big Bang. A redshift is when the light from a galaxy is moving away from you and the observed wavelength of the light appears longer. As it moves away it moves towards the red end of the spectrum and if it is moving towards you it appears blue. The fact that redshift is occurring is evidence that galaxies are moving away just as the Big Bang theory states. There are three different types of redshift and they are all caused differently. The three types are the doppler redshift, cosmological redshift and gravitational redshift.
Doppler Effect - The doppler effect was proposed in 1842 by Christian Doppler. It is the changes in sound as something moves towards you or away from you. Like with the redshift theory when the source of the sound moves away it changes to the red end of the spectrum and as it comes closer it goes to the blue end. A common example is the siren of an ambulance. As it drives towards you the sirens seem close together and very loud, however when it drives away the siren seems to be distant and the sounds are further apart.
RELATIVITY - In 1905 Albert Einstein theorized that the universe has four dimensions, the first three are space, and the fourth is called spacetime. This is a dimension in which time and space are linked. Einstein's theory of relativity states that two people observing the same event in the same way could see the singular event occurring at two different times, depending upon their distance from the event. Basically distance and time are not absolute parameters and depend on the observer. Einstein's theories of relativity resulted in the equation E = mc2 (energy = mass x speed of light squared), this equation means that matter and energy are interchangeable and connects the mass of an object with its potential energy. This relates back to the Big Bang theory as many of George Lemaître's discoveries provided solutions to the equations of the theory of relativity.
Hubble Constant - In the 1920's Edwin Hubble made the discovery that our universe is in fact expanding. As the objects move further and further away they begin receding from us at greater speeds. The velocity can be given by an expression known as Hubble's Law: V = H x R. These discoveries are proof for the Big Bang theory and they can help to estimate the length of time it had been expanding for since the Big Bang. This information can lead us to an estimate for the age of the Universe.
The Steady State Theory
The Steady State Theory was proposed by Hermann Bondi, Thomas Gold, and Sir Fred Hoyle in 1948.The theorystarted to gain a large following in the 1950's and 1960's. However the popularity of the theory died off after the discovery of the cosmic microwave radiation in the late 1960's. The steady state theory faced major criticism and debate for political and religious reasons around the world. In the 1950's Pope Pious XII stated that the big bang theory affirmed the idea of a creator and was in line with Christian beliefs. Because of that people started to associate the steady-state theory with atheism.
The theory states that the universe looks the same from every spot in the universe and at every time. A universe has no beginning and no end according to the theory and that even though the universe is expanding, it does not change it's look over time. For this to be true, new matter must be formed to keep the density mostly equal over time; this matter condenses into new galaxies that replace receded galaxies. Whereas with the big bang theory the density of gravity falls as the universe expands in size. The idea of the steady state theory was discredited due to the fact that there is now evidence to prove that the universe is changing and does not look the same or have a constant density.
REDSHIFT - The Redshift theory provides evidence for the steady state theory because it proves that our galaxy is in fact expanding as the steady state theory suggests. According to the redshift theory when something moves away it shifts towards the red end of the spectrum. As our galaxy expands the stars appear on the red end of the spectrum therefore proving some elements of the steady state theory.
HUBBLE CONSTANT - Edwin Hubble made a ground breaking discovery in the 1920's that provided evidence for the steady state theory. He discovered that it is true that our universe was expanding. This theory is however also proof for the Big Bang so there is still no concrete evidence to show that the steady state theory is correct.
Evidence for the Steady State theory was also found when it was discovered that there were some miscalculations in the Big Bang theory. Below is an excerpt from the book, 'Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos' by physicist Michio Kaku. 'One defect in the [big bang] theory was that Hubble, because of errors in measuring light from distant galaxies, had miscalculated the age of the universe to be 1.8 billion years. Geologists claimed that Earth and the solar system were probably many billions of years old. How could the universe be younger than its planets?' This is also the reason that Hoyle was so opposed to the big bang theory. This miscalculation of course does not prove either theory but did bring some favor towards the steady state because it was thought that the big bang could have other faults in it as well as this one.