Redshift everywhere means the Universe is expanding
Definition
Big Bang Theory
The theory that the Universe began from a single point and has been expanding ever since.
- The light from distant galaxies is redshifted, which means those galaxies are receding from the Earth.
- The decisive detail is that this is true in every direction we look, so almost every distant galaxy is moving away from us.
- Redshift in the light from distant galaxies is therefore evidence that the Universe is expanding.
- That does not put the Earth at the centre of anything: space itself is stretching, so an observer in any galaxy would see the same thing.
- Imagine dots drawn on a balloon that is being blown up: every dot moves away from every other dot, and no dot is the centre.
- Run that expansion backwards and everything gets closer together, so the whole Universe must once have been packed into a tiny volume.
- This is why redshift supports the Big Bang Theory, the idea that the Universe started from a single point and has been expanding ever since.
Key Idea
The logic runs in one direction and it is worth writing it out in full: redshift, so galaxies are receding, so the Universe is expanding, so it was once much smaller.
Cosmic microwave background radiation: the afterglow that fills all of space
Definition
Cosmic microwave background radiation
Microwave radiation of a specific frequency that is observed at all points in space around us, left over from shortly after the Universe was formed.
- Point a detector anywhere in the sky and it picks up microwave radiation of a specific frequency, the same everywhere, known as cosmic microwave background radiation.
- It comes from no particular star or galaxy, which rules out any ordinary object as its source and points to something that happened everywhere at once.
- This radiation was produced shortly after the Universe was formed, when the whole Universe was extremely hot and dense.
- At that time it was emitted at much shorter wavelengths, as high-energy radiation rather than microwaves.
- As the Universe expanded, that radiation was stretched to longer and longer wavelengths along with the space it was travelling through.
- By now it has been stretched all the way into the microwave region of the electromagnetic spectrum, which is exactly where we detect it.
- It is the same stretching idea as redshift, applied not to one galaxy's light but to radiation filling the entire Universe.
Common Mistake
- Do not describe the Big Bang as an explosion into empty space, because space itself was expanding rather than matter flying outwards through a room that was already there.
- Do not claim the Earth is at the centre of the expansion just because every galaxy is receding from us, since every other galaxy sees exactly the same.
- Do not say the background radiation was emitted as microwaves, because it started at much shorter wavelengths and was stretched into the microwave region afterwards.
Exam technique
- Two different pieces of evidence are examined here, so read carefully whether the question wants redshift or the background radiation.
- Say in every direction when using redshift as evidence, because one receding galaxy on its own would prove nothing about the whole Universe.
- The mark most often lost on the background radiation is the stretching step, so state that expansion moved it into the microwave region rather than only that it exists.
- Write the word evidence into your answer, because these questions are about what supports the theory rather than about restating it.
Self review
- What does the redshift of light from distant galaxies tell us about the Universe?
- Why does the fact that redshift is seen in every direction matter?
- State the Big Bang Theory in one sentence.
- Where in the sky is cosmic microwave background radiation observed?
- Explain why that radiation is now found in the microwave region rather than at shorter wavelengths.
