A gamma ray burst produces somewhere around 1044 J of energy, according to Wikipedia. Only a small portion of that energy will be absorbed by other objects.

The rest of that energy will go on to travel the universe indefinitely I presume?

How do we know how much of the universes total mass-energy is in "in transit" in the form of gamma rays and other forms of radiation?


2 Answers 2


The energy density of the universe is quite well known. Most seems in the form of what is called "dark energy", which accelerates cosmic expansion. Next comes dark matter, accounting for some 20%, then baryonic matter, about 4%. Electromagnetic radiation (photons) contributes so little that I forgot how much. Most of the energy density in photons is in the cosmic background (CMB), the remnant of an ealier epoch when electromagnetic radiation dominated the energy budget of the universe.

The cosmic expansion makes the energy density of photons decay faster than that of matter, because (in addition to the decrease in spatial density $\propto(1+z)^{-3}$) photons also get red-shifted, adding another factor $(1+z)^{-1}$.

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    $\begingroup$ "Most of the energy density in photons is in the cosmic background (CMB)" - do you have an argument or a reference for this? $\endgroup$ Jan 19, 2014 at 17:07

No energy is in transit as GRBs, just like Gamma rays that started in a GRB.

If you remember that all that energy is in the form of radiation, it obeys the same laws as ordinary light: the farther, the dimmer (inverse of square of the distance law).

So if you see stars dimmer as they are farther from you, same happens to the RGBs. Radiation from the stars expands infinitely (becoming dimmer and dimmer) until absorbed by clouds or just becoming dimmer than the background.

Same happens to gamma rays from GRBs. It is just that since rays are more energetic, they are less easily absorbed, and since GRBs themselves are more energetic, the becoming dimmer than background distance is bigger.

  • $\begingroup$ I guess I should clarify the question. A burst is of course a burst. But if energy leaves a star in the form of gamma rays as a result of a GRB, and it has the potential to sterilize Earth a million years later, then during that million years, its energy must be in transit, unobservable I presume. $\endgroup$
    – frodeborli
    Jan 16, 2014 at 22:25
  • $\begingroup$ The energy in gamma-ray bursts is deposited in an ultrarelativistic stream of light particles, which only later due to shocks from collision with ISM gives rise to observed gamma-rays and other stuff like afterglows. Also, GRBs, not RGBs. $\endgroup$ Jan 16, 2014 at 22:56
  • $\begingroup$ @AlexeyBobrick I think the precise formation mechanism of GRBs is still under debate, so your statement can only reflect a possible model, but is no definite widely accepted paradigm. $\endgroup$
    – Walter
    Jan 18, 2014 at 10:20
  • $\begingroup$ @Walter: a reference? $\endgroup$ Jan 18, 2014 at 15:50
  • $\begingroup$ What is under debate is the central engine: what happens in the inner 100 km of a core collapsing star. There is some debate about how exactly the spectrum is produced. There is no debate, however, about where the gamma-rays are generated or what does the jet mainly consist of. $\endgroup$ Jan 18, 2014 at 15:52

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