Question

What is redshift theory?

Answer 1

The evidence that the universe is expanding comes with something called the red shift of light. Light travels to Earth from other galaxies. As the light from that galaxy gets closer to Earth, the distance between Earth and the galaxy increases, which causes the wavelength of that light to get longer.

You can find more on:-

https://study.com/.../evidence-for-the-big-bang-theory-background-radiation-red-shift-a...