What Was It Like When Starlight First Broke Through The Universe’s Neutral Atoms?
“The light created in the earliest era of stars and galaxies all plays a role. The ultraviolet light works to ionize the matter around it, enabling visible light to progressively farther and farther as the ionization fraction increases. The visible light gets scattered in all directions until reionization has gotten far enough to enable our best telescopes today to see it. But the infrared light, also created by the stars, passes through even the neutral matter, giving our 2020s-era telescopes a chance to find them.
When starlight breaks through the sea of neutral atoms, even before reionization completes, it gives us a chance to detect the earliest objects we’ll ever have seen. When the James Webb Space Telescope launches, that will be the first thing we look for. The most distant reaches of the Universe are within our view. We just have to look and find out what’s truly out there.”
Something existing in our Universe is not quite the same as something being detectable in our Universe. We know that, at some point in our past, we created the first generation of stars, the second generation of stars, and the very first galaxies to exist in our Universe. But in order to detect them, there has to be some way for that light to travel through the Universe to our observatories and telescopes monitoring the skies today. There’s an obstacle standing in the way of that, though: the neutral atoms formed just hundreds of thousands of years after the Big Bang. When the first hints of starlight begin permeating through space, they encounter these neutral atoms, which largely thwarts them. It takes hundreds of millions of years for starlight to win.