What is the "surface of last scattering"?

Answer 1

Hi Paul

The last scattering surface is the name given by cosmologists to the time period in the universe's history when the universe became transparent to radiation.

About 300 000 years after the big bang the temperature and energy density had dropped sufficiently for neutral atoms to form. A lot of free electrons were captured by hydrogen and helium nuclei. When this happens there are far fewer charged particles or "ions" about, and so the universe ceases to behave like an ionised gas. High temperature ionised gases (or plasmas) are largely opaque to radiation (ie photons). The mean free path time for a photon is shorter as it interacts with the ions. Once neutral atoms form the photons "fly free".

The significance is that these photons we're discussing will become the cmbr (cosmic microwave background) photons of today. Since the last interaction most of the cmbr photons had was at the last scattering surface, a sample of the cmbr sea provides a map of the universe's density (etc) at that period. This gives a tremendous amount of informaiton about what the (relatively) early universe was like.

The final thing to note is that the last scattering surface isn't a surface like you'd be used to (eg the top of a table or the surface of a pool). It's a space-time surface, made by taking a 3-D "hyperslice" through space-time at a given time coordinate. It's called the "last scattering" surface because it is the last time the cmbr photons were scattered by interactions (in bulk).


Hope this helps!
The Chicken