(1) Two photons collide w/ total energy twice mass energy of an electron: create one electron and one positron
(2) when electron & anti electron collided, annihilate and leave behind two photons - transform all their mass energy back into photon energy
Combined density of matter + energy value at which geometry is flat.
> CD = overall geometry spherical (closed) < CD = overall geometry saddle shaped (open)
If the universe were infinite, unchanging, and everywhere the same, the entire night sky would be light (not dark!)
Finite number of stars or u changes w time? —> While universe contains infinite number of time, we see only those that lie within observable universe
Why Believe the Big Bang?
(1) Expansion of the U (2) Ages of stars in MW (3) Cosmic Blackbody radiation - uniform & BB (4) Helium abundance explained
Big Bang Proof (1) Expanding Universe
Universe cannot stand still because attractive gravity of mass causes curvature trying to bring mass back together — U always has to be shrinking or expanding. Space between galaxies is expanding! According to GR, in past (approx. 14Gyrs ago),U was small point - before this, time did not exist, and relativity equations cannot see beyond point.
Big Bang Proof (2): Explains ages of stars in MW
Speed of light is finite (3e8 m/s) so when we see smth very far away, we see it was in past. I.e lookback time Redshift z of galaxy describes how much U & wavelengths have grown since light emitted 1 + z = today/ emitted = size of U today/size emitted Z = 0 (here & now)