Astronomers often use the term ‘redshift’ when describing how far away a distant object is. When an object like stars or galaxies move away from us, its light waves are stretched into longer wavelengths or lower frequencies and we say that the light is redshift.
Astronomers use redshifts to track the rotation of our galaxy, to measure approximate distances to very distant galaxies and also measures the speeds of galaxies. The more distant an object, the more it will be redshifted. This means that the further away the galaxies are, the faster they are moving. As the space between the galaxies expands, the space inside the universe is also expanding. The space is expanding everywhere, so the more distant an object is, the more rapidly it appears to be moving away. This is similar to an explosion, where the bits moving fastest travel furthest from the explosion.
Redshift data provides evidence that the universe, including space itself, is expanding. Astronomers have discovered that all distant galaxies are moving away from us and that the farther away they are, the faster they are moving; which causes the light from the galaxies to be redshifted. As a result of the Big Bang, he universe is expanding as the most of the galaxies within it are moving away from each other.
Hubble made two important observations that led him from the straight line he saw in his diagram to the picture of the expanding universe.
- First, the line in his distance-redshift diagram does not depend on direction in the sky - in one direction we see redshifts, as if galaxies are moving away from us, and in the opposite direction we also see redshifts, not blueshifts.
- From the second observation, we know that our region of space isn't special in any way - we don’t see an edge in any direction. This means that while all galaxies appear to be moving away from us, we are not at the center of the universe - the universe has no center.
An observer in any other galaxy would see the same line in a Hubble diagram. The expanding universe model would not have worked if Hubble had found anything except a straight line in his distance-redshift diagram.
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