How does redshift provide evidence for galactic motion?

Redshift provides evidence for galactic motion by showing that galaxies are moving away from us, indicating an expanding universe.

Redshift is a phenomenon in physics where the wavelength of light or other electromagnetic radiation from an object is increased, or 'shifted' to the longer-wavelength (red) end of the spectrum. This is often observed in the light from distant galaxies. The redshift of a galaxy's light is directly proportional to its distance from us: the farther away a galaxy is, the more its light is redshifted.

This redshift is evidence for galactic motion because it's consistent with the Doppler Effect, a principle in physics that describes how the frequency of a wave changes when the source of the wave is moving relative to the observer. When a galaxy is moving away from us, the light waves it emits are stretched out, causing them to shift towards the red end of the spectrum. This is known as a 'redshift'. Conversely, if a galaxy were moving towards us, its light waves would be compressed, causing a 'blueshift'. However, when we observe distant galaxies, we almost always see a redshift, indicating that they are moving away from us.

This observation of redshift in distant galaxies is a key piece of evidence for the Big Bang theory, which posits that the universe began as a hot, dense point and has been expanding ever since. The fact that we see galaxies moving away from us in all directions suggests that this expansion is happening uniformly throughout the universe. Therefore, redshift provides crucial evidence for both the motion of galaxies and the larger-scale structure and history of the universe.