Photons in the Expanding Universe

Earlier on this site I saw part of an answer in a question that confused me. It stated: "...Light and anti-light are the same thing. So if two beams of light collide and destroy each other (a very improbable process under current conditions, but possible) the output is a balanced mixture of particle/antiparticle pairs. Back in the day, when photons were a lot denser than now, those types of collisions were common." Why are photons less dense than before?
- Ameerah (age 14)
Philadelphia, PA, USA

It's just that our space has been expanding. That dilutes the photons, because the same number are now spread over a bigger volume. It also stretches out their wavelengths, making each one lower energy. A pair of low-energy photons doesn't have enough energy to make an electron-positron pair, even if they collide.

Mike W.

(published on 01/03/2013)