The hunt for signs of intelligent life elsewhere in the cosmos has been frustratingly quiet. But maybe the reason aliens aren't talking is because they had to contend with brutally high doses of radiation. If anyone is out there, they may be living deep below vast oceans, making it unlikely they would be seeking to communicate with surface dwellers.
A new analysis of cosmic evolution suggests that planets in the early universe were slammed with bursts of radiation thousands to millions of times higher than Earth has ever faced. That's because black holes and star formation were more vigorous during these epochs, and everything in the universe was also much closer together, allowing for denser doses of radiation than planets face today.
"We live in a calm time in the universe," says Paul Mason of New Mexico State University. "The past has been much more violent, especially on the short term."
Mason worked with Peter Biermann of the Max Planck Institute of Radio Astronomy in Germany to understand how radiation from both inside and outside of galaxies could affect the evolution of life. They found that life on the surfaces of planets would have had a difficult time taking hold in the first half of the 13.8-billion-year life of the universe.
To reach their conclusion, the pair rewound the expanding universe to better understand the impact that denser galactic neighborhoods of the past could have had on one another. They also examined the role the Milky Way's magnetic field may have played on life in our home galaxy. Mason presented the results earlier this month at the American Astronomical Society's 227th meeting in Kissimmee, Florida.
Some of the most dangerous regions for life in all epochs are those with frequent star formation, like the center of a galaxy. That's because where stars are born, they also die. When those deaths come as violent supernovae, nearby planets can be doused with radiation or stripped of their protective atmospheres, exposing surface life to even more radiation from stars and other cosmic sources.
Star formation is an ongoing issue in galaxies, but according to Mason, both the births of stars and their explosive deaths occurred more rapidly in the early years of the Milky Way.
"Throughout the history of the galaxy, we see that a lot of star formation occurred, mostly in the past," Mason says.
Galactic centers also make bad neighbors because most of them contain supermassive black holes. These black holes are often actively feeding, which hurls damaging radiation toward any nearby planets. While the Milky Way's central black hole isn't active today, Mason says there is a good chance that it was in the past.
Even then, the outskirts of galaxies, where star formation is calm and no supermassive black holes reside, may not have been as safe as once thought. The Milky Way and other galaxies have weak magnetic fields of their own. And according to physicist Glennys Farrar of New York University, while the primary source of the Milky Way's magnetic field remains a mystery, its effects can be both helpful and harmful for evolving life.
For instance, charged particles from supernovae and supermassive black holes can interact with the galactic magnetic field, which would then distribute the damaging rays. Cosmic rays can survive in the field for 10 million years, Mason adds, giving them plenty of time to percolate to a galaxy's outer edges.
"You could be far away from the center and still be affected by what goes on at the center," Mason says. Overall, the radiation levels in the first half of the life of the universe could be a thousand times higher in its galaxies, but spikes from the galactic centers as the central black holes fed could reach as much as 10 million times higher, providing a dramatic increase that could be bad for surface-based life.
"For any particular galaxy in the universe, the outbursts of its own galactic center would probably be the most damaging sources of cosmic rays," says Mason.
If life evolved beneath an ocean or underground, it could be shielded from some or all of the radiation. However, Mason points out that the path toward complex societies on Earth required life moving from the seas to land. It's possible that alien societies could exist beneath the oceans of other planets, although finding signs of them with today's technology would be extremely difficult.
A hint of good news comes from globular clusters, groups of gravitationally bound stars that orbit galaxies. The Milky Way has more than 150 of these satellites, while larger galaxies can contain hundreds or even thousands.
Stars in these clusters tend to form at about the same time, within only a handful of generations. Those that explode in supernovae die fairly rapidly, leaving behind long-lived siblings that have plenty of time to build planets that would be free from constant radiation baths.
Several pieces of research have looked at globular clusters as potential neighborhoods for life. While some scientists suggest that stars in these clusters would lack the necessary material to build planets, other researchers point to some of the diverse planets found so far by NASA's Kepler space telescope, which formed despite a dearth of these materials in their host stars.
Aside from reduced supernovae radiation, the high stellar density in globular clusters means that most stars have neighbors lying far closer than our relatively isolated sun, allowing for greater chances of interstellar travel and communication.
Based on the rate of cosmic expansion, Mason suggests that the universe would have reached a state most favorable for life no more than 7 to 9 billion years after the Big Bang. From that point on, there might be "pockets of habitability"—life-friendly zones that could avoid local sources of cosmic radiation.
In search of those pockets, globular clusters may be even better places to scan than galaxies, Mason says: "Globular clusters have an advantage, with some caveats."
However, even these clusters may not completely escape the radiation risk. As they orbit their parent galaxies, they may pass close by or even through the galactic plane. Even this brief encounter could expose planets in the clusters to periodic spikes in cosmic rays. They would also interact, at least briefly, with the magnetic field of their parent galaxy, which means they could be exposed to any radiation trapped inside.
High-energy cosmic rays from the centers of other galaxies, as well as enigmatic gamma-ray bursts, could singe planets inside globular clusters as well. This would have been a more significant problem in the past, because galaxies once lay much closer together than they do today, making encounters with other galaxies even more frequent.
These extragalactic radiation events would be rarer but far more powerful. According to Jeremy Webb, a postdoctoral fellow at Indiana University, globular clusters lack magnetic fields of their own. This means they have no shield from even the less dangerous cosmic rays cast by their neighbors. And while the magnetic field of the cluster's partner galaxy could help deflect some of the weaker rays, Mason says that the strongest of them would still manage to penetrate.
"There's no place to hide," Mason says. "Even in a globular cluster, you can't hide from those."