COSMIC DAWN by James Geach The iRst Galaxies Astronomers are traveling backwards in time to observe our universe's early history. T here's a certain exhilaration in hunting for the most distant galaxies in the universe. It's that feeling of pushing back the boundaries. Seeing a bit farther than anyone before you. But I bet if you ask most astrophysicists who study the formation and evolution of galaxies, it's not the distance in space that excites them, but the distance in time. Electromagnetic radiation travels at a finite speed, and the light emitted by distant galaxies can take billions of years - large fractions of the age of the universe - to reach us. We see those distant galaxies as they were in the past, at the moment the light left them. Our ultimate goal is to detect the light emitted by the first galaxies: the cosmic dawn. Remarkably, we're getting close. Any long-exposure image will reveal a universe teeming with galaxies; current estimates put the total number of galaxies around the trillion 14 A PR I L 2 018 * SK Y & TELESCOPE mark. As the sensitivity of CCD cameras improves and the size of telescopes increases, it has become routine for deep astronomical surveys to detect galaxies seen at a time when the universe was a small fraction of its current age. These infant galaxies are much too far away for light-years to be a useful metric. Rather than thinking in distance units, we use the galaxies' cosmological redshift (see box on page 17) as a proxy for their distance. The universe - space itself - has been expanding ever since it came into existence in the Big Bang. The wavelength of an electromagnetic wave traveling through the expanding universe will stretch so that blue light emitted by a distant galaxy will have become red light by the time we detect it. The amount the wavelength has stretched tells us how much the universe has expanded since the photon began its journey. If we know the history of the expansion (since space