What if We’ve Misunderstood Our Place in the Universe? a Harvard Astronomer Thinks We Have.
But in the early universe, as Loeb speculates in a paper published in Astrobiology late last year, everything would have been a habitable zone. 10 to 20 million years after the Big Bang, the universe was still bathed in that warm gas we saw in the CMB, but it had cooled down to a temperature that would keep water liquid no matter where it was relative to its star. The ambient temperature of the universe would provide enough heat to turn an ice giant like Neptune into a water giant. That’s why Loeb has dubbed this era the “habitable epoch.”
It would have been a weird time for life to evolve, though. Many of the building blocks of life on Earth, like carbon and metals, exist only because of the massive stellar explosions called supernovas which signal the deaths of stars. In a universe where so few stars had been born, even fewer would have died. This was a period when solid matter was an anomaly, before most of the elements on the periodic table existed.
Stars would have been few and far between. “Life might have been more isolated than it is today,” Loeb said. “Now we are members of a galaxy, with tens of billions of stars not far away.” Still, Loeb said, the rare stars and planets would form hotter, more energetic regions in the sea of warm gas. There would be energy to kick-start life forms and liquid water would slosh across the surface of planets with atmosphere. Also, the relative isolation of these worlds would have protected them from threats like cosmic radiation and asteroid bombardment—two dangers that have nearly extinguished life on Earth more than once.
Would this life have been intelligent? “No,” Loeb said. “I’m talking about very simple organisms like algae.” Because the universe was changing so quickly, species would only have about a million years to evolve on a planet before the warm gas clouds around it cooled enough to change the environment radically. Still, a million years is enough time for a single-celled creature to evolve. And another simple species, more adapted to the colder world, could evolve to take its place. But could a humanlike civilization arise in one of those evolutionary windows? The odds are slim—consider that it took roughly 65 million years for the small, fluffy mammals of the Tertiary period to evolve into modern humans.