Cosmic Background Temperature Enters the Liquid Water Range

Approximately 10 million years after the Big Bang, the cosmic microwave background cooled to 373 K (100°C), entering the temperature range where liquid water can exist at standard pressure. For the following 7 million years, the entire universe remained between 273 K and 373 K. Astrophysicist Abraham Loeb identified this window in 2014 as a theoretical habitable epoch, though the universe then consisted almost entirely of hydrogen and helium with trace lithium. Standard models indicate no stars or heavy elements yet existed to form rocky substrates. The timing derives from the well-established CMB cooling relation T = T₀(1+z).