Since space is mostly empty, one might first think that the majority of the photons, e.g. emissions from the stars since the beginning of the universe, are still in flight and not absorbed by anything.

Is this true?

Is there any estimates that the proportion is relative to the total amount of photons ever created?

(Let's consider the photons that have made it into space first, e.g. exclude the photons that gets absorbed and re-emitted inside stars for now).

  • $\begingroup$ @RoryAlsop Maybe it's easier if we narrow the scope a little? E.g. exclude the infancy of universe and consider a relatively steady-state like now. Sort of like Olbers' paradox (I've heard about it but forgot the name). Has there been any estimate of how much photons that have been emitted but not absorbed (now sure in what kind of units). $\endgroup$ Dec 6, 2019 at 19:42
  • 3
    $\begingroup$ Both number density and energy density is dominated not by astrophysical processes (such as stars and quasars), but by the cosmic microwave background photons, emitted long before the existence of stars. 5.3% of those photons have been absorbed, but 94.7% are "still in flight". There are, on average, 411 CMB photons per cm$^3$. See this answer on physics.SE. $\endgroup$
    – pela
    Dec 6, 2019 at 21:02

1 Answer 1


Peebles and Fukugita's excellent "The cosmic energy inventory" estimates that about $10^{-4.3}$ of the total mass-energy is in the form of cosmic background radiation, and $10^{-5.7}$ in the form of radiation from stars.

The estimate that the perturbation due to plasma and other interactions of the background radiation is around $10^{-8.5}$, so clearly most photons have never been captured by anything and are still in flight.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .