Gamma ray bursts are not that rare

Yesterday it was reported that scientists have suggested that an anomalous peak in radioactive materials discovered in antarctic ice sediments and in ancient Japanese cedar trees could be explained by a gamma ray burst hitting the Earth in the 8th century CE.

The BBC radio report I head described gamma ray bursts as “extremely rare” and the website article – and much other coverage – repeats the idea that they are rare events.

But they are not.

There are an estimated $1.25\times10^{11}$ galaxies in the universe. It is estimated that a gamma ray burst happens at least once every million years in any galaxy – or approximately every $3\times10^8$ days. That means that today there will be approximately 1000 gamma ray bursts. Now, let’s assume that due to relativistic effects  we can at most only observe one-tenth of the universe, that still means 100 events in the observable part of the universe (how big the observable universe is in comparison to the universe is another matter however).

Of gamma ray bursts are narrowly beamed so even with this high rate of production not many get seen on Earth (probably a good thing given the energies involved), but they are far from rare.

Of course, events in our galaxy are rare (the fact that we are here at all is testament to that), but on the universal scale that is drawing a very tight boundary on the region being tested.