A new paper published in Nature this week reports on the measurements of thermal radiation from such events.
A long-standing theory that still awaits testing predicts that absorption of UV radiation from early stars by nearby clouds of hydrogen could have driven TS back down to TG, but not lower. In other words, the cosmic dawn would make the gas seem colder when observed at radio frequencies. This would create an absorption feature in the spectrum of the background radiation left over from the Big Bang.
Bowman et al. now report the possible detection of just such an absorption signal. The authors measured TS , averaged over much of the sky and over a contiguous range of radio frequencies; each frequency provides a window on a different time in the Universe’s past. The measurement is very difficult because it must be performed using an extremely well-calibrated VHF radio antenna and receiver, to enable the weak cosmological signal to be separated from much stronger celestial signals and from those within the electronics systems of the apparatus used.
For those of you who are not familiar with science, when you read the link, please read how the experimenters made the effort to ensure that their results are not due to their experimental technique or instrumentation.