Cosmology

What galactic redshift tells us

• Two key observations from looking at galaxies all across the sky:

1. Every distant galaxy is redshifted; they are all moving away from the Earth

2. The further away a galaxy is, the greater the redshift, so the faster it is moving away

• This relationship, distance proportional to recession speed, is known as Hubble's law
• It tells us that the Universe is expanding uniformly. Each galaxy sees every other galaxy receding, just like dots on the surface of an inflating balloon

Why this is evidence for the Big Bang

The logic chain runs as follows:

1. Distant galaxies show redshift → they are moving away from us
2. The further away a galaxy, the bigger its redshift → distant galaxies are moving away faster
3. If you rewind time, all galaxies must once have been very close together
4. Rewind enough, and the entire Universe was concentrated in a single small region
5. That fits the Big Bang picture: a hot dense beginning, followed by 14 billion years of expansion

The galactic redshift observations were one of the first major pieces of evidence for the Big Bang, first measured by Edwin Hubble in the 1920s, and refined ever since with better telescopes and supernova observations. The discovery of the CMB in 1964 then provided the second, independent piece of evidence.

What galactic redshift does NOT mean

• The galaxies are not moving through space away from a centre. Instead, space itself is expanding between them, carrying them apart (the balloon analogy)
• There is no single point in space that the Universe is expanding from; every point is moving away from every other point
• The Earth is not at the centre of the Universe. Any observer in any galaxy would see all the other galaxies redshifted by the same amount, just as every dot on the inflating balloon sees all the other dots receding