when a car comes towards you it "pushes" the sound ahead and makes the wave length shorter, so you get a higher sound.
When the car moves away from you it "drags" the sound behind it and stretching the wavelength to a lower tone..
This is a short animation that may explain it more visual:
The same Doppler shift works with light, of course you cannot hear that but you can see it when you use a prism to see what colors the light is made off:
You might notice that "white" light is made up of several colors, they are the colors of the rainbow.
The light spectrum is shown like this:
On the diagram you can see the left, the highest wavelength of light : Ultraviolet, the human eye cannot see this, the for humans visible part starts with red and ends with blue , The longer wave next to red also cannot be seen by the human eye and is called Infrared.
This diagram shows the spectrum of a stationary light source, the wavelength of the light is shown on top.
Now we get back to Mr Hubble in 1929.
While working at the Carnegie Observatories in Pasadena, California, he was photographing the light spectra of a number of distant galaxies.
There he noticed that compared to the stationary light, the red in this spectrum was shifted..
And by calculating using the Doppler measurements he discovered that the universe was expanding , the galaxies were moving away from us, his measurements were even so precise that he discovered they are moving away from us and are gaining speed, they are accelerating ..
Other astronomers have already discovered that the universe is not just a piece of empty space filled with stars but that our region is actually part of the (virtual) surface of an expanding sphere, like a ball that is constantly being inflated..
This has been confirmed by the data collected with various astronomical research space probes
On a crime scene when the police finds a bullet in a wall they can calculate the trajectory where the bullet came from.
From the position of shrapnel from a bomb, they can calculate where the bomb was placed when it exploded.
Here is a nice video with explosions, but than reversed , the film running backwards:
When you apply the same to the universe, virtually play the film backwards you will finally and up in a single dot where everything originated..
This has been proven Mathematically, by advanced computer models and by experiments in particle accelerators like the large Hadron Collider in France and Switzerland where they hoop to get a glimp from before the Big Bang.
And finally all is confirmed by the background radiation data gathered from measurements of the NASA's Wilkinson Microwave Anisotropy Probe.
The Big Bang is a proven fact
Strictly spoken "The Big Bang" is the wrong term, it was not an explosion, it was an expansion, and we are still experiencing the effects of it
What happened before the Big Bang ? we STILL don't know
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