A brief explanation on The Doppler Effect and how it effects sound, light and our understanding of the Universe.
The Doppler Effect
This is when electron-magnetic waves are squished as the source comes towards you or is stretched as it travels away. When it is squished the frequency of the waves become higher. When it is stretched the frequency becomes lower.
This can be applied to sound; for example if a police car coming towards you, the siren is high pitch. But as it then drives away it becomes lower.
Light (Red and Blueshift)
It can also be applied to light, for example if an object is travelling towards you at high speed, the light being emitted is squished, therefore moving all light towards the blue part of the spectrum. This is known as Blueshift. The opposite is also true, if an object is travelling away from you, the light being emitted is stretched therefore moving all light towards the red part of the spectrum. This is called Redshift.
Big Bang Theory
The Doppler effect is used by astronomers to help support the big bang theory. This is because most galaxies are Redshifting, therefore travelling away from us. This means they are must have come from a center point (where the explosion happened) and are still expanding.
I hoped this helps, Thanks