When Did The Universe Become Transparent To Light?
“The Universe became transparent to the light left over from the Big Bang when it was roughly 380,000 years old, and remained transparent to long-wavelength light thereafter. But it was only when the Universe reached about half a billion years of age that it became fully transparent to starlight, with some locations experiencing transparency earlier and others experiencing it later.
To probe beyond these limits requires a telescope that goes to longer and longer wavelengths. With any luck, the James Webb Space Telescope will finally open our eyes to the Universe as it was during this in-between era, where it’s transparent to the Big Bang’s glow but not to starlight. When it opens its eyes on the Universe, we may finally learn just how the Universe grew up during these poorly-understood dark ages.”
There are two ways that astrophysicists talk about the Universe becoming transparent. The first is when the particles from the Big Bang finally form neutral atoms, becoming “transparent” to the leftover photons from that era. The second is hundreds of millions of years later, when those same neutral atoms are reionized, and starlight can travel freely through intergalactic space. Which one is right? When did the Universe become transparent to light?