A new paper suggests that planets from the remnants of the universe's earliest stars could have supported life on dim, warm planets. Credit: NASA/WMAP Science Team
By: Ker Than, Inside Science
(Inside Science) -- Life in the universe could be much older than previously thought, forming as early as fifteen million years after the Big Bang, according to a provocative new idea proposed by a Harvard astrophysicist.
In this scenario for the early universe, rocky planets born from the dregs of massive, primordial stars would have been warmed by the heat of a radiation that permeated all of space, which was much hotter back then than it is now. One of these ancient worlds could have supported liquid water on its surface irrespective of its distance to a star, and thus been habitable to primitive forms of Earth-like organisms, said Avi Loeb, who chairs the Harvard astronomy department.
With the discovery of exoplanets, Loeb said, scientists are beginning to seriously consider that life-as-we-know-it exists in other places.
"What I’m saying here is that it can also be extended to other, earlier, times," he said.
Loeb said that if proven, his idea would weaken the case for the anthropic principle. This theory, popular among many scientists, posits that the values of key laws and constants of the universe such as the electromagnetic force, the mass of a neutron, and perhaps most importantly, the energy density of empty space itself, known as the cosmological constant, appear to be fine-tuned to sustain life as we know it. Otherwise we would not exist. But, if life could develop under conditions as extreme and alien as those of the early universe, then that suggests that scientists revisit the idea that the conditions in our own mature universe are uniquely suited to harbor life.
“There are cosmologists who argue that the values of the cosmological constant that we observe today must have been this way so that we would exist on the surface of a planet like the Earth, around a [star] like the sun, in a galaxy as massive as the Milky Way,” Loeb said.
But that might not be the case. Life could have started much earlier, Loeb said, when the cosmological constant was many orders of magnitude larger than it is today, during a time that, according to the anthropic principle, life could not survive.
“If that’s the case, then the universe wasn’t designed for us to exist in it. We might be latecomers and not at the center of the biological universe. Life existed earlier. It’s not a big deal,” he added.
Loeb said the idea that life might exist in earlier times occurred to him in the shower a few years ago.
“That’s usually the place where I get peace and quiet,” he said. “Nobody bothers me. I don’t get any emails. I can think about things.”
What Loeb realized that day was that the temperature of the cosmic microwave background (CMB), the relic radiation left over from the Big Bang that permeates the entire universe, has varied widely throughout time. Today, it is close to absolute zero; 400,000 years after the Big Bang, during an era known as recombination, when hydrogen atoms first formed, the CMB was nearly as hot as the surface of the sun. But during a very brief time window, between 10 to 17 million years after the Big Bang, the temperature of the CMB was about 80 degrees Fahrenheit, or close to room temperature.
This period also happens to coincide roughly with when some theoretical calculations suggest that the first stars in the universe could have formed. When those ancient stars -– which were more massive than our sun and had far shorter lifespans -- died, they would have exploded as supernovae and seeded the space around them with the heavy elements necessary for the creation of rocky planets.
Once formed, these early worlds would not have needed the heat of stars to warm them, because they would have been enveloped by space that was warmed by the CMB itself.
“Thus, the earliest time that life could have started was about 15 million years after the Big Bang. The current age of the universe is about 13.8 billion years old, so we’re talking about a time when the universe was just a tenth of a percent of its present age,” Loeb said.
Greg Laughlin, an astronomer at the University of California, Santa Cruz, said Loeb’s idea was a “very much out-of-the-box” idea that was almost certainly wrong -- if only because most scientific hypotheses turn out to be incorrect.
“I would not bet money that life existed at that early of a time,” said Laughlin, who was not involved in the study.
Nevertheless, ideas like the one Loeb proposes are useful, Laughlin said, because they help push the envelope of what is considered possible.
“It shows that we need to consider environments that are very different from what we have here on Earth,” he said. “I think the idea that there are habitable planets that look just like the Earth, orbiting sun-like stars that pepper the galaxy, is naïve. These kinds of far-reaching speculations do a very good job of broadening the discussion in ways that are useful.”
Ker Than is a freelance writer living in California's Bay Area. He tweets at @kerthan. Reprinted with permission from Inside Science, an editorially independent news product of the American Institute of Physics, a nonprofit organization dedicated to advancing, promoting and serving the physical sciences.