what is red shift and how do we know about it?
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I see alot of ***how*** explanations but no ***why***
If you detonate a nuclear bomb in space, you would see the explosion expand outwards equally in all directions. Just like throwing a rock in the water, you’ll see the explosion wave large at first then dissipate the farther and farther away it goes from the source. Like sound the closer/farther you get from the source, the louder/quieter it is.
**But Why**
The reason for this is the nature of energy waves in our universe. In space as your explosion expands from its origin the shell of the explosion sphere keeps increasing, so the energy of the explosion must keep covering a larger and larger area over time. Like those expanding ball toys. To compensate for this and obey conservation of energy, the intensity/amplitude of the energy wave decreases as the area it covers increases.
**Le Shift But How**
Now a star is that explosion, but its moving and constantly exploding every second. So as the distance between you changes you will be getting either bigger or smaller waves over time, if we see the wave energy dissipate with distance we call it red shift or increase as the source gets closer blue shift. We also call this the doppler effect. As others have said you can experience this with sound by standing next to a busy road.
This also means a star with no frequency shift is moving in parallel with us, for a moment at least
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