Big Bang Evidence, the CMB and Redshift Calculations (Paper 2)

Explained

Two pieces of evidence, and the calculation that goes with one

The Big Bang theory says the universe began extremely small, hot and dense, and has been expanding and cooling ever since. Two observations support it, and questions here almost always ask for one or both.

Redshift

Light from distant galaxies arrives with a longer wavelength than it left with, shifted towards the red end of the spectrum. Longer wavelength means lower frequency, and it happens because the source is moving away from us.

Two features of the observation matter. Almost every galaxy shows a redshift, so almost everything is receding. And the further away a galaxy is, the greater its redshift, so the more distant galaxies are receding faster.

That pattern is not evidence that we are at the centre of anything. Space itself is expanding, so every observer anywhere sees the same thing, with everything moving away from them. Winding the expansion backwards gives a universe that was once much smaller, hotter and denser.

The cosmic microwave background

Microwave radiation arrives from every direction in space, at almost exactly the same intensity whichever way you look.

It is the radiation left over from the hot early universe. It was originally much shorter in wavelength, and it has been stretched into the microwave region by the expansion of space over billions of years.

Its uniformity is the point. A local source would come from one direction; this comes from all of them, which is what you would expect from something that filled the whole universe.

The calculation

The change in wavelength divided by the original wavelength equals the speed of the galaxy divided by the speed of light.

Two habits protect the marks. The change in wavelength means the observed value minus the original, and it is divided by the original rather than the observed. And the speed of light is 3.0 times ten to the eight metres per second, so the wavelengths must be in metres too, which usually means converting from nanometres first.

Check the answer is a small fraction of the speed of light. A galaxy receding faster than light would be a sign that a power of ten has gone astray.

What the theory does not say

The Big Bang was not an explosion into empty space, and there is no point you could travel to and call the centre. Space itself expanded, and it is still expanding, which is why the galaxies are separating without moving through space in the ordinary sense.

Nor does the theory say what happened before, or what caused it. It describes the universe from an extremely early moment onwards, and questions asking what caused the Big Bang are outside what the evidence supports.