When an intense jet of gamma rays( a gamma ray burst) from a newly formed black hole strikes a planet, its strips the ozone shield, killing complex life. This occurs frequently enough to explain why we haven't seen signals of extraterrestrial civilizations, especially from galaxies low in heavy elements.
A new study confirms the potential hazard of nearby gamma-ray bursts (GRBs), and quantifies the probability of an event on Earth and more generally in the Milky Way and other galaxies. The authors find a 50% chance that a nearby GRB powerful enough to cause a major life extinction on the planet took place during the past 500 million years (Myr). They further estimate that GRBs prevent complex life like that on Earth in 90% of the galaxies.
The luminosity of long GRBs – the most powerful ones – is so intense that they are observed throughout the universe (CERN Courier April 2009 p12). If one were to happen nearby, the intense flash of gamma rays illuminating the Earth for tens of seconds could severely damage the thin ozone layer that absorbs ultraviolet radiation from the Sun. Calculations suggest that a fluence of 100 kJ/m2would create a depletion of 91% of this life-protecting layer on a timescale of a month, via a chain of chemical reactions in the atmosphere. This would be enough to cause a massive life-extinction event. Some scientists have proposed that a GRB could have been at the origin of the Ordovician extinction some 450 Myr ago, which wiped out 80% of the species on Earth.
GRBs are more frequent in low-mass galaxies ...The Milky Way would therefore be among only 10% of all galaxies in the universe – the larger ones – that can sustain complex life in the long-term.
The two theoretical astrophysicists also claim that GRBs prevent evolved life as it exists on Earth in almost every galaxy that formed earlier than about five-thousand-million years after the Big Bang...
This could help explain Enrico Fermi’s paradox on the absence of evidence for an extraterrestrial civilization. [emphasis mine]
Assuming extraterrestrial civilizations might be hazardous to us, as Stephen Hawking suggests, this might be a good thing. As long as our luck holds.