Evidence for Cosmological Distances

The first strong indication that gamma ray bursts are at cosmological distance was their distribution in space. Detailed analysis indicated that they were distributed isotropically (no preferred direction from Earth), but that they were not distributed uniformly through the volume of space (not homogenously distributed). This favors the cosmological interpretation. More recently, Doppler shift measurements discussed elsewhere in this module have given more direct proof that they are at cosmological distances.

Local or Cosmological?

One very important question is whether gamma ray bursts are local (in the halo of our galaxy, say) or cosmological (at such large distances that the Hubble Law is relevant). There was considerable debate on this issue in the early study of gamma ray bursts, but more recent observations make it highly likely that gamma ray bursts are occurring at cosmological distances (see the adjacent box).

This makes them extremely interesting, because for them to be seen at very large distances they must correspond to events in which as much as 100 times the energy of a supernova is being liberated in a short period in the form of gamma rays. Furthermore, the mechanism producing the gamma rays must be such as to allow the gamma rays to escape without too much interaction with surrounding matter, because that interaction would convert the gamma rays to light of longer wavelength and we would see the energy at other wavelengths instead of in gamma rays.