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The Question

(Submitted April 04, 2000)

I am an adult who recently visited the Hayden Planetarium at the Museum of Natural History and was totally blown away by what I learned. In trying to learn even more about the universe, I've picked up several books but am stuck on the theory of recombination and how it relates to transparency. I just don't get what it means that the universe became transparent to background radiation at the time of recombination (300,000 years into the universes lifespan). I think I understand the idea of recombination well enough, but what does it mean that at that point the universe became transparent to the radiation? Does it mean that the universe separated from the radiation and began to form the beginning of galaxies? Or is the word transparency used in the more generally understood meaning; that at the point of recombination the matter could seen through.

Your time on this is much appreciated. I am simply fascinated by this and want to gain a better understanding of what's 'out there' and why it's there.

The Answer

You're asking a very good question. What "transparent" means is that after the recombination period the photons can actually "escape" because they're not constantly colliding and being re-emitted. "Recombination" is a sort of absolute limit for us to see.

I assume you know that looking away from the Earth is the same as looking back in time. For example, the Sun is at 8 light minutes from us, so when we look at the Sun, we are seeing it the way it was 8 minutes ago. Now suppose that we could built an incredible powerful telescope to peer at objects very, very, very far. The "recombination" limit tells us that even if we could do such thing, we will never see beyond that time (because before no photon could "escape").

I hope this answers your question.

Ilana Harrus for

the "Ask an Astrophysicist" team.

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