Why do galaxies appear to move away from each other?

Galaxies appear to move away from each other due to the ongoing expansion of the universe.

The phenomenon of galaxies moving away from each other is a direct consequence of the Big Bang theory. According to this theory, the universe began as a hot, dense point nearly 13.8 billion years ago. Since then, it has been expanding. This expansion is not just the galaxies moving through space, but the actual stretching of space itself. This means that every point in the universe is moving away from every other point.

This movement is observed as a redshift in the light from distant galaxies. When we look at the light from these galaxies, we find that the spectral lines have been shifted towards the red end of the spectrum. This is known as the Doppler Effect, which is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. In the context of galaxies, this redshift is interpreted as a sign that the light source is moving away from us.

The rate at which galaxies are moving away from us is given by Hubble's Law. Named after the American astronomer Edwin Hubble, this law states that the speed at which a galaxy is moving away from us is directly proportional to its distance from us. In other words, the further away a galaxy is, the faster it is moving away from us. This observation supports the idea that the universe is expanding uniformly.

However, it's important to note that not all galaxies are moving away from us. Some nearby galaxies, like the Andromeda galaxy, are actually moving towards the Milky Way due to gravitational attraction. But on a large scale, most galaxies appear to be moving away from each other, indicating the overall expansion of the universe.