  As a copious source of gamma-rays, a nearby Galactic Gamma-Ray Burst (GRB)
can be a threat to life. Using recent determinations of the rate of GRBs, their
luminosity function and properties of their host galaxies, we estimate the
probability that a life-threatening (lethal) GRB would take place. Amongst the
different kinds of GRBs, long ones are most dangerous. There is a very good
chance (but no certainty) that at least one lethal GRB took place during the
past 5 Gyr close enough to Earth as to significantly damage life. There is a
50% chance that such a lethal GRB took place during the last 500 Myr causing
one of the major mass extinction events. Assuming that a similar level of
radiation would be lethal to life on other exoplanets hosting life, we explore
the potential effects of GRBs to life elsewhere in the Galaxy and the Universe.
We find that the probability of a lethal GRB is much larger in the inner Milky
Way (95% within a radius of 4 kpc from the galactic center), making it
inhospitable to life. Only at the outskirts of the Milky Way, at more than 10
kpc from the galactic center, this probability drops below 50%. When
considering the Universe as a whole, the safest environments for life (similar
to the one on Earth) are the lowest density regions in the outskirts of large
galaxies and life can exist in only ~ 10% of galaxies. Remarkably, a
cosmological constant is essential for such systems to exist. Furthermore,
because of both the higher GRB rate and galaxies being smaller, life as it
exists on Earth could not take place at $z > 0.5$. Early life forms must have
been much more resilient to radiation.
