Cosmic Microwave Background Radiation refers to the uniform spread of microwave radiation found throughout the universe. It is an integral piece of evidence supporting the Big Bang Theory. In 1948, CMB was predicted by Ralph Alpher and other scientists but it wasn’t until 1965 when Arno Penzias and Robert Wilson’s radio receiver detected physical evidence of this strange phenomena through abnormal temperatures.

Initially, the universe was a hot, opaque soup of plasma containing free electrons. These free electrons would scatter light and prevent it from travelling unhindered. Overtime, as space cooled in temperature, free electrons fused with protons and neutrons to make neutral atoms. The absence of these free electrons resulted in a transparent universe in which light could now traverse. This very light emitted by photons 380,000 years after the Big Bang, when the universe became transparent, is known as Cosmic Background Radiation. The radiation we observe today from Earth used to be extremely powerful and had a high frequency - however, after billions of years this light has been red shifted and regressed to radio waves. By observing the properties of light released billions of years ago, scientists realised that the Big Bang Theory predicted the same characteristics and concluded it as substantial evidence.  

Hubble’s law is a formula which depicts the recession velocity of a galaxy based on its position: the farther away the galaxy, the faster it recedes. In short, the law also helped discover that light emitted from nearby galaxies such as Andromeda was being redshifted and therefore the universe was expanding. In 1998, the cause of this accelerating expansion was confirmed to be dark energy, a mysterious force which scientists are still studying today. If the universe is expanding, it means that at some point in time space was compacted into a singularity, an event which the Big Bang Theory is founded on.         

*Redshift is a physical phenomenon when electromagnetic radiation is “stretched” into red wavelengths because it is moving away from the observer.  

During the cooling period of the universe, the earliest elements/isotopes such as hydrogen, deuterium, helium-3, helium-4 and lithium-7 were created. The Big Bang theory predicts a specific ratio/amount of these elements made in the aftermath of the “explosion.” These calculations are supported by the composition of distant galaxies and ancient stars which contain this ratio of elements.    

*New stars like the Sun cannot be counted as evidence because they have been made from older generations of stars and as a result, their composition will largely differ from stars formed shortly after the Big Bang.