The Big Bang Theory
The Big Bang theory suggests that the universe started at one point 13.8 billion years ago.
There are a number of pieces of evidence for this:
- Redshift (Hubble's Law)
- Cosmic Microwave Background Radiation (CMBR)
- Abundance of Hydrogen and Helium
- The darkness of the night sky (Olbers' Paradox)
Cosmic Microwave Background Radiation
During the Big Bang there was a lot of heat, detectable as infrared radiation. This has been redshifted to a microwave signal which is detectable by television and radio receivers, and is the reason for the static noise when you are not tuned into a station.
Black Body Radiation
Hotter objects emit more radiation than colder objects. Objects want to cool down by releasing electromagnetic radiation. This is mainly infrared, however if an object is hot enough, it will emit visible light or even higher frequency radiation.
A perfect black body is an object which absorbs and emits all parts of the electromatic spectrum equally. In a black body, the wavelengths of light released depend on on the temperature of the body.
To summarise, the hotter the object, the shorter the peak wavelength, and the higher the intensity.
According to Hubble's Law,
We can determine information about stars and galaxies in space by looking at the electromagnetic radiation emitted - but this means that if you have matter which does not emit any form of EM radiation, it is impossible for us to observe. We call this matter dark matter.
The stars far away from the centre of the Milky Way are orbiting the centre faster than they should based on calculations using the visible mass. The only way this is possible is if a lot more hidden mass exists on the edge of the galaxy, proving the abundant existence of dark matter.
Another proof of its existence is gravitational lensing - the bending of light around heavy objects. Light from some sources are bending a lot more than they should based on the visible mass around it, showing that there must be a lot more invisible mass.