Physics Colloquium: Gamma Ray Bursts and Extinctions Events in the Universe

Abstract:

Gamma-Ray Bursts (GRBs) are short and intense bursts of soft gamma-rays.  A local burst in our own galaxy, the Milky Way, could have devastating effects on life on Earth. I describe the possible harmful effects of a nearby GRB and compare them to other astronomical phenomena that are a threat to life. Using recent estimates of the rate of GRBs and its dependence on properties of the host galaxies I estimate the frequency of life threatening GRBs on Earth and elsewhere in the Milky Way. A potentially lethal GRB is likely to occur at our neighborhood once per billion years.  Indeed, the Ordovician extinction, which occurred around 440 million years ago, had certain signatures of a GRB-induced event. Turning to the Galaxy I show that GRBs are much more frequent in the inner region of the Milky Way and that they were much more common at earlier epochs.  This suggests that GRBs would have hampered the development of advanced life forms at this region and throughout the Milky Way at earlier times. Finally turning to the whole Universe I show that GRBs pose a threat to life in most small galaxies. The outskirts of large galaxies, like our Milky Way, are the most hospitable regions for life.