When was cosmic dawn? Some of the most distant galaxies known hold a clue

Astronomy and astrophysics

29 June 2021

Light from the early days of the Universe helps to pin-point when the stars switched on after the Big Bang.

If you know someone’s age, it’s easy to work out when they were born. Now astronomers have performed that same calculation for some of the Universe’s most ancient galaxies. That has allowed them to estimate the timing of cosmic dawn — the period when the very first stars appeared.

Nicolas Laporte at the University of Cambridge, UK, and his colleagues examined six galaxies that had previously been identified as potentially ancient. To estimate the galaxies’ age, the team studied the starlight coming from them, looking for a hydrogen signature that becomes more pronounced as the stars in a galaxy age. The group also determined that light from some of the galaxies had travelled for more than 13 billion years before it reached Earth, which means we are seeing them as they were less than 550 million years after the Big Bang.

Taken together, the two measurements suggest that the first stars in these galaxies lit up when the Universe was between 250 million and 350 million years old.

The authors expect NASA’s James Webb Space Telescope, which is due to launch late this year, to be able to see galaxies such as these in their earliest stages of formation.