Question about the Big Bang and CMB

  • #1
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This may seem like a naive or obvious question, which is why I posted it here.

How do we know the cosmic microwave background came from the Big Bang? How can we tell how old radiation is?
  • #2
How do we know the cosmic microwave background came from the Big Bang?

It didn't. Not directly at least. It is from an event known as recombination. The CMB is the final bit of thermal radiation from the hot plasma that filled the early universe. It was simply the radiation that was released just prior to, and during, the transition from a plasma that was opaque to EM radiation to a gas of hydrogen and helium that was transparent to EM radiation. Basically, as the universe aged and expanded, the plasma cooled and became less dense. Eventually it reached a point where it had cooled sufficiently for protons to recombine with electrons to form atoms.

Prior to recombination this thermal radiation was simply reabsorbed by the plasma. However, once the universe became transparent to EM radiation that final bit that was emitted right at the transition was able to travel freely. This is what we see when we look at the CMB except that it's been redshifted substantially by expansion.

How can we tell how old radiation is?

It's quite complicated, as we can't directly measure the age of the universe or of the light. Instead we have to infer the ages based on other things, such as the proportions of different types of stars in star clusters, the temperature of white dwarfs, and the Hubble constant. I wish I had a good article to send you to, but I do not. Perhaps someone else here can give you a good reference.
  • #3
Thank you so much! This is of great help!
  • #4
There could be a psychological reason why your question has not been very much asked. If CMB had just been discovered and someone afterwards had said hey, I think I've got a fantastic explanation, people might have been persuaded round to the idea more gradually and grudgingly. But when it was predicted I think people said, oh interesting maybe, h'mm, but no one can actually see this radiation - until then! , suddenly one day 15 years after it had been predicted it showed up, found by people some of whom didn't even know it was supposed to exist, and had properties pretty much as predicted, that discovery sequence somehow had more impact.

I think epistemologists have debated whether something being predicted gives more weight as evidence than the same explanation of the same phenomena given after their discovery. I don't know whether they reached an agreed conclusion. It seems to me that logically the status is the same in the two cases. But psychologically prediction convinces much more than postdiction.

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