The phrase "cosmic dawn" refers to the very beginning of the universe, when the first stars and galaxies started to form. It's like the sunrise of our universe. Full definition
This could mean that there was more radiation than predicted at cosmic dawn, or that the neutral hydrogen was being cooled by something. (science.howstuffworks.com)
That's valuable information because astronomers are intensely interested in this first generation of stars, in an era often called cosmic dawn. (sciencemag.org)
In effect, this research has opened a window into cosmic dawn, one that started 180 million years after our universe was born and ended 70 million years later — a period that represents the short time span of the first stars. (science.howstuffworks.com)