After the Big Bang and before the first stars ignited, the universe was a very dark and cold place. There were no galaxies, no supernovae, and no quasars. The universe primarily consisted of neutral hydrogen gas floating in an omnipresent sea of background radiation leftover from the Big Bang. Over time, gravity slowly shepherded the densest regions of hydrogen gas into compact clouds, which ultimately collapsed to form the first stars.When these primordial stars first began shining within the pitch-black void, they blasted the surrounding hydrogen gas with ultraviolet radiation. This excited the hydrogen atoms within the gas, causing them to absorb energy from the background radiation at one particular frequency — 1.4 gigahertz. Theoretically, astronomers knew that they should be able to detect the absorption or corresponding emission from this process, but until this study, they have been unable to do so.“The problem is, due to the expanding universe, this absorption would be observed at some [unknown] lower frequency,” said Peter Kurczynski, a program officer with the National Science Foundation who supported the study, in a video by NSF , “Finding that frequency, finding the absorption that comes when the first stars turn on, would be like listening to every station on your car stereo at once, and being able to tell that your favorite is missing.”To find this unknown signal, the team of researchers used an Earth-based instrument called a radio spectrometer, located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia. As part of the Experiment to Detect the Global Epoch of reionization Signature (EDGES), the team measured the vast majority of the southern sky. After collecting the average radio spectrum for all astronomical signals, the team combed over the data searching for minute fluctuations in the signal’s power as a function of frequency.Initially, the team was searching for frequencies that corresponded to later points in cosmic time, but in 2015, they extended their search to lower frequencies, which would have come from even earlier. “As soon as we switched our system to this lower range, we started seeing things that we felt might be a real signature,” said Alan Rogers, a scientist at MIT’s Haystack Observatory and co-author of the study, in a press release “We see this dip most strongly at about 78 megahertz," he said, "and that frequency corresponds to roughly 180 million years after the Big Bang. In terms of a direct detection of a signal from the hydrogen gas itself, this has got to be the earliest.”