Is it a map of the observable universe? Is it the big bang? Very unclear
In: Planetary Science
To elaborate (slightly) on the fantastically-named u/BoobsBlissfulBabe’s response, you can view the CMB as a photograph of the universe as it was at roughly 300,000 years old.
It hinges on a tight coupling between photons (“particles” of light) and electrons in hydrogen atoms. Quantum mechanics tells us that there’s a minimum energy a photon needs to knock an electron out of the atom. After the Big Bang, every photon in the universe was energetic enough to do this — so the universe was a massive, cosmic game of billiards. A photon would slam into a hydrogen atom, dislodging the electron and leaving the nucleus (a proton). That electron would then slam into a proton, forming a hydrogen atom… and releasing a photon or photons with the excess energy. Rinse and repeat.
The universe, though, was expanding — and as you’ll know if you’ve ever pumped up tyres by hand, compressing gases makes them hot, and conversely expanding them makes them cooler. Similarly with the universe as a whole. At roughly 300,000 years old, it grew cold enough that there were fewer and fewer photons that would knock electrons out of hydrogen, and soon there were none.
Suddenly photons could “free-stream” — just carry on their merry way, through the ages, until eventually they hit our televisions and radio telescopes, including one run by Penzias and Wilson for Bell Labs who picked up a consistent, irritating static. After cleaning out all the pigeon poo from their radio telescope they still had this annoying static. It was quickly realised that this was actually static made by photons from the CMB.
The net result is that Penzias and Wilson got a Nobel prize for, essentially, cleaning their telescope of pigeon poo; and that we have in the sky a photo of the universe as it was a few hundred thousand years after the Big Bang. Of course, that photo’s screened: predominantly, the galaxy gets in the way, and so does any amount of intervening matter. Even so, it’s an extraordinarily rich source of information on cosmology.
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