I know that the CMB was created at the recombination, when photons could finally travel freely. But each of those photons was deflected one last time before this happened. Where's the matter that deflected the photons? Has it become the baryonic matter we see around us, or has it crossed the cosmic event horizon?


1 Answer 1


It is all around you. It was gravitationally attracted to clumps of dark matter where it reached sufficient density to begin to interact with itself (via the electromagnetic force), break up into smaller clumps which ultimately became stars, In the centre of these stars nuclear reactions converted a fraction of the hydrogen and helium nuclei into heavier isotopes, which (after several generations of stars) some of these heavier isotopes (and some of the hydrogen) eventually ended up in a planet orbiting a star, on which a series of chemical reactions between these isotopes ultimately resulted in me sitting here typing this.

Every baryon in the world was, at one time, a hydrogen or helium nucleus in the matter that filled the universe at the time time of the CMB.

  • $\begingroup$ In a general sense, the matter which emitted the CMB is the same matter which the stars and galaxies are made out of, but what about in the specific sense? That is, if I go out this evening and measure a particular photon of the CMB, where is the particle that last deflected that specific photon? About how far away from Earth is it likely to be? Could it be it in the Milky Way? Somewhere else in the visible universe? Is it beyond the cosmic event horizon? $\endgroup$
    – R.M.
    Commented Jul 9, 2022 at 0:57
  • $\begingroup$ Re your last sentence, doesn't Hawking radiation produce "new" baryons? $\endgroup$ Commented Jul 9, 2022 at 1:08
  • $\begingroup$ @R.M. since the CMB is basically the same age as the universe (to within 10^6 years), it's at least 13 billion ly away (but then, taking expansion into account, even further). And the CMB which bounced off your baryons is, conversely, shining on someone equally far away, if they are seeing it just now. $\endgroup$ Commented Jul 9, 2022 at 2:22
  • $\begingroup$ What made it "break up into smaller clumps" ? And what was the large clump that broke? $\endgroup$
    – Mazura
    Commented Jul 9, 2022 at 6:13
  • $\begingroup$ What made it "break up into smaller clumps" Anisotropy, amplified by gravity, followed by "Jens instability". The "large clump" was a mass of dark matter that now forms the halos of the galaxies in the virgo supercluster. $\endgroup$
    – James K
    Commented Jul 9, 2022 at 7:32

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