Our universe is about 13 billion years old, and for roughly 3.5 billion of those years, life has been wriggling all over our planet. But what was going on in the universe before that time? It’s possible that there was a period shortly after the Big Bang when the entire universe was teeming with life. Harvard astronomer Avi Loeb calls this period the “habitable epoch,” and he believes that its existence changes how humans should understand our place in the cosmos.
We have one snapshot of life in the early universe, taken about 400,000 years after the Big Bang. This image is known as the Cosmic Microwave Background (CMB), and it’s what astronomers see when they aim their telescopes at the farthest edges of space, capturing light that has been traveling through the universe for billions of years—and from billions of years ago. Remember, light takes a while to reach Earth (it travels at only 186,000 miles per second), so the starlight you see in the sky at night is often thousands of years old. The CMB is a lot older than that. It’s from the time when the universe hadn’t yet developed stars.
Photo courtesy NASA/WMAP Science Team
If the CMB looks to you like a lot of glowing blobs, that’s because it is. Radio astronomers Arno Penzias and Robert Wilson won a Nobel Prize for discovering that these blobs were actually relics of warm gas spreading outward shortly after the Big Bang. Some regions of the gas appear denser than others, and these areas eventually formed stars and galaxies as the universe aged and cooled. But for millions of years, the universe was in a kind of interim state between lumpy gas and the cool, galaxy-studded darkness of today. That’s where Loeb’s habitable epoch comes in.
It was a time when the very first solid objects were forming in the universe, about 10 to 20 million years after the Big Bang. “The first objects were very small,” Loeb told me by phone from his office at Harvard. By small, he meant they didn’t approach the mass of even a moderately sized galaxy like our own Milky Way. In fact, there were no galaxies at that time—only large stars, probably embedded in dark matter. “We can do simulations of these early stars, and what people find is that they were tens to hundreds of times more massive than the Sun.” These giant stars, floating alone, easily could have had rocky worlds like Earth in orbit around them.
And that’s when things get interesting. 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 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.
The habitable epoch might have been a lonely, strange time to be alive. But if Loeb is right—and other physicists, such as Princeton’s Freeman Dyson, believe he is—then life may be a lot less rare than we ever imagined. “It’s almost like a Copernican Revolution in our thinking about life,” Loeb said. “Once we believed Earth was the center of the universe. Then Copernicus and others said, hey, it’s actually the Earth that’s moving around the Sun.” Suddenly, Earth wasn’t so special; we weren’t at the center of all things. Loeb is suggesting that maybe life on Earth isn’t so special, either.
“For a long time, we’ve had this preconception that life is here on Earth, but the universe is dead,” Loeb said. “But maybe we should be thinking of this as a living universe. We may be relative latecomers to the game.” If life becomes an important ingredient in the development of the cosmos, it unseats humans as the all-important observers of our universe. It suggests that many other eyes watched the skies before our sun was even lit.
For Loeb, the habitable epoch is part of a fuller understanding of our universe as a place where life might well be common. The problem is that even if life were common, it would be very hard to detect on other planets. “Suppose there’s a nuclear war elsewhere in our galaxy,” Loeb suggested. “How do you detect that with telescopes? The energy released is so small we wouldn’t even be able to see it if it happened on the nearest star.” Currently astronomers are trying to design instruments that would be able to find life on other worlds, perhaps by looking for telltale signs of molecular oxygen, which is almost always created by life forms.
But in the meantime, Loeb has one piece of advice for cosmologists. “Until proven wrong, we should assume we are not special.”
