y building bigger telescopes, going to space, and looking from ultraviolet to visible to infrared wavelengths, we can view stars and galaxies as far back as stars and galaxies go. But for millions of years in the Universe, there were no stars, no galaxies, nor anything that emitted visible light. Prior to that, the only light that existed was the leftover glow from the Big Bang, along with the neutral atoms created during the first few hundred thousand years. For those millions of years, there's simply never been a way to gather information from the electromagnetic part of the spectrum. But a combination of advances in computing and the new construction of an array of thousands of large-scale radio telescopes across twelve countries opens up an incredible possibility like never before: the ability to map the neutral atoms themselves.
Distant sources of light – even from the cosmic microwave background – must pass through clouds of gas. If there's neutral hydrogen present, it can absorb that light, or, if it's excited in some way, it can emit light of its own.
Distant sources of light – even from the cosmic microwave background – must pass through clouds of gas. If there's neutral hydrogen present, it can absorb that light, or, if it's excited in some way, it can emit light of its own.
How can you see neutral atoms? After all, unless you're dealing in either reflected light or with atoms that are themselves in an excited state, neutral atoms are some of the most optically boring materials that there are. Atoms are made of negatively charged electrons surrounding a positively charged nucleus, capable of occupying a variety of quantum states. But early on, for millions of years after the Big Bang, 92% of the atoms are the most boring type that exists: hydrogen, with a single proton and electron. While many different energy states exist, without any external source to excite it, hydrogen atoms are doomed to live in the lowest-energy (ground) state.
More:
https://www.forbes.com/sites/startswithabang/2017/06/14/the-future-of-astronomy-thousands-of-radio-telescopes-that-can-see-beyond-the-stars/#72448c864c46
