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?
The truth is we need them both, as they make the Universe transparent to different types of light. Come get the full story today.
What Was It Like When The Universe First Made Atoms?
“The rest is history. Sure, it takes more than 100,000 years for the process to complete, but this is how the Universe does it. This two-photon transition, rare though it is, is the process by which neutral atoms first form. It takes us from a hot, plasma-filled Universe to an almost-equally-hot Universe filled with 100% neutral atoms. Although we say that the Universe formed these atoms 380,000 years after the Big Bang, this was actually a slow, gradual process that took about 100,000 years on either side of that figure to complete. Once the atoms are neutral, there is nothing left for the Big Bang’s light to scatter off of. This is the origin of the CMB: the Cosmic Microwave Background.”
The Universe got off to a start in quite a hurry. By time just four minutes had passed since the Big Bang, we had already created more matter than antimatter, gotten rid of all the unstable, fundamental particles, given mass to the Universe, and fused the first elements of the periodic table. The next step would be to make neutral atoms: the building blocks of all the complex structure we know of in the cosmos. Yet it didn’t take minutes, hours, days, or even months. It took hundreds of thousands of years to get there; an eternity compared to all that came before.
Yet, after enough time went by, we made it. Here’s the cosmic story of what the Universe was like, and how we got there.