1. Before Hydrogen formation : ProtonElectronHydrogen10 9 Photons Photons scatter off the free electrons, so the universe is “ opaque ” Lots of photons have E > 13.6eV  Hydrogen can ’ t form… but the Universe is expanding, making photons “ stretch ” … Temp  3000 Kelvin t  380,000 yr.

2. During Hydrogen formation : ProtonElectronHydrogen10 9 Photons Hydrogen starts to form, so the Universe suddenly gets “ clearer ” … Temp ≈ 3000 Kelvin t ≈ 380,000 yr. During this time, the photons will have their “ last interaction ” with the electrons, and carry away information about their environment…

3. After Hydrogen formation : ProtonElectronHydrogen10 9 Photons Very few free electrons  Universe is “ transparent ” Temp  3000 Kelvin t  380,000 yr. The photons travel freely, only being affected by gravity… and the universe continues to expand.

4. t = today (13.7 gigayears)  matter = 10 9 times less Temp ≈ 3 Kelvin Cosmic Microwave Background (CMB) photons: today ~ 1 mm to 10 cm

10. 13.7 Giga light-years

11. Gravity Wave 13.7 Giga light-years

12. Scattering of light by electrons 1. The electric field of a light wave shakes an electron along the direction of polarization.

13. Scattering of light by electrons 2. Light is not emitted in the direction of shaking! Green = probability of emitting in that direction…

14. Environment around electrons at t=380,000 years leads to polarization Uniform “ glow ” around electron  “ shaking ” in all directions  all polarizations emitted equally.

15. Environment around electrons at t=380,000 years leads to polarization Non-uniform “ glow ” around electron  preferential “ shaking ”  polarized emission.

16. Gravity waves stretch space…

17. … and create variations

18. Gravity Wave