Where Have All the Gamma Ray Bursts Gone?

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Astronomers are confused.

As if gamma-ray bursts (GRBs) weren’t mysterious enough, there’s something else to add to the bag of confusion. GRB events are missing from the furthermost reaches of the Universe. Right around the time when there should be a lot of GRBs, during the “star forming epoch” (when stars were just beginning to evolve after the Big Bang), there appears to be none. Zero. There’s no ancient flashes of massive star death to be found. What’s more, there doesn’t appear to be any afterglow from previous gamma-ray bursts either.

So what’s going on? Were there no GRB events before 12.8 billion years ago? Possibly… although there might be another answer. They are out there but we just can’t see them.

Gamma-ray bursts are the biggest and brightest explosions in our Universe since the Big Bang. When a GRB detonates, it can easily outshine its host galaxy containing billions of stars. These energetic events have been observed since the 1960’s and only until recently have astronomers found an explanation as to what GRBs are. A GRB occurs when a young metal-poor massive star has used up all its fuel and, like a supernova, collapses under its own gravitational field. The rapid-spinning star then funnels intense beams of radiation from its poles in the form of gamma-rays. Should one of these beams be directed toward Earth, we see a disproportionately bright explosion (as a vast amount of energy is channelled through the poles). Until the “collapsar model” was devised, astronomers were at a loss to explain these energetic events.

The collapsar model appears to explain GRBs lasting for two seconds or more. However, there is another class of GRB, of much shorter timescales, that does not fit in with the collapsar model. Short-period GRBs may be the result of violent interactions between black holes and a neutron stars.

So, does this mean GRBs are becoming less mysterious? Actually, GRB theory has just become a little more complicated. It would appear that no GRBs occurred before 12.8 billion years ago. Last month, the most distant (and therefore oldest) GRB was detected 12.8 billion light years away, but that in itself is strange.

During the time when the first stars started to form (around 13.4 billion years ago), they were by definition “metal-poor” stars (heavier elements, such as metals, were only possible after several generations of stellar evolution), so this should be a period of time when GRBs were regularly lighting up the night sky. However, according to observations of the most distant galaxies containing the youngest stars, GRB events seem to be non-existent.

One explanation put forward is the effect of red shift. As the Universe expands, space-time stretches. As light travels from the most distant reaches of the Universe, perhaps the light itself from GRBs has been so stretched (red-shifted) that the electromagnetic emissions simply cannot be detected by our instrumentation. These huge explosions could be happening, but as the emitted light has been so red-shifted, by the time the light reaches us, perhaps the emission does not resemble a GRB. Even the afterglow of one of these massive explosions would be unrecognisable in this case, the light observed would be shifted all the way into the infrared.

So will any GRBs be discovered further away than 12.8 billion light years? I think we’ll have to wait until we build some improved infrared observatories or recognise what a distant, ancient GRB looks like…

Source: NASA