Are Gamma-Ray Bursts Keeping Life from Developing in the Universe?

​The universe might be a radiation-scorched, lifeless place after all. Just as soon as a planet, save for a relative handful of well-sheltered rocks, becomes life-harboring and friendly, it gets nuked back to a barren wasteland.

This is one conclusion of a​ new paper examining the likely prevalence of gamma-ray burst (GRB) events throughout the Milky Way and universe at-large, particularly of the sort—long gamma-ray bursts or LGRBs—that could strip away a planet's protective ozone layer and blast its inhabitants with very high-energy photons.

The conclusion sounds wingnutty, but comes via the peer-reviewed Physical Review Letters and the astrophysicist Tsvi Piran, the paper's lead author and one of the leading theorists of cosmic gamma-ray origins. While Piran doesn't say that life is impossible elsewhere, he does conclude that it's unlikely in most of the universe save for a few outlying galactic neighborhoods, like our own.

Even this neighborhood, at the outer edge of the Milky Way, wasn't always so protected. The general idea is that as galaxies become more dense and less metallic, the likelihood of neutron star mergers producing gamma-ray bursts goes up. Piran offers a 50 percent likelihood that a devastating GRB blasted Earth within the past half-billion years, perhaps precipitating a mass-extinction event.

Gamma-ray doom ​would look like this: the incoming rays would collide with nitrogen molecules in the atmosphere, which would then break apart into nitric oxide compounds. Nitric oxide is smog, basically, and it would tear the ozone layer up, ​leaving life exposed to the Sun's harmful UV rays. Plants would die off, and skin cancer would become the new common cold. The nitric haze, while allowing in UV rays, would block out visible light, or some large part of it, leaving humans to starve in something like a nuclear winter. Acid rain would become the norm.

Fortunately, the probability of a lethal GRB goes down quickly the further we are from the galactic center. This means that Earth is relatively protected, yet most other habitable exoplanets would be found closer in. Near the center of a galaxy, lethal GRBs are almost guaranteed—periodic biological cleanses.

What's more, the universe only becomes more compact as we go back in time, and for much of history, life as we know it would be just plain impossible

"It seems the survival of life, as we know it on Earth, was only a recent phenomenon in the history of the Universe caused by the growth of large galaxies," Piran et al write. "Life forms that might have existed earlier or that exist today in other regions of the Universe that are much more susceptible to significant GRB bombardment must have been much more resilient to radiation than life on Earth."

The authors note that regular gamma-ray wipes might be a good thing for the overall development of higher-order life, but if that life happened to already exist at the time, it would likely die a miserable death. In the interim, we should probably worry about other sorts of ​radiation dooms.

A pre-print open-access version of this paper is ​hosted at arXiv.