Annalee Newitz in Slate:
These days when astronomers discover a planet, the news is usually accompanied by the disappointing report that it’s not in a “habitable zone,” which is to say the exact orbit required to keep water in a liquid state. If the planet is too close to its star, all the water has boiled away; if the planet is too distant, the water is frozen solid. Given that life as we know it requires water, most astronomers assume that life could only develop on a planet in its solar system’s habitable zone.
But in the early universe, as Loeb speculates in a paper published in Astrobiologylate 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.
More here.