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2 years ago in Cosmology , Physics By Aliya Tazeen

Was there light before stars formed, and how did it originate?

In my cosmology lectures, we talk about the CMB flash, then a dark period, then the "first light" of stars. This seems too neat. Were there really no photons for hundreds of millions of years? I'm trying to build an accurate timeline of the universe's radiative history and want to understand if any processes generated faint light before stellar ignition.

 

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By Ruhi Malhotra Answered 1 year ago

It wasn't utterly dark, but it was profoundly dim. The dominant light was indeed the fading afterglow of the CMB, shifting into the infrared. However, there was another, fainter kind of "light": the 21-centimeter hyperfine line emission from neutral hydrogen. This radio wavelength radiation is not light we can see, but it's a crucial electromagnetic signature. So, the universe wasn't a perfect void of photons; it was filled with a uniform, cold bath of CMB photons and a feeble whisper of 21-cm radiation. The dramatic change with the first stars was the introduction of point sources of intense, high-energy light that began to reheat and reionize the fog.

 

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