In https://phys.org/news/2021-09-universe.html it says:"Scientists thus discovered some of the universe's missing baryons, thereby confirming that 80–90% of normal matter is located outside of galaxies, an observation that will help expand models for the evolution of galaxies."

What is the basis for the estimate of the total amount of the Universe's "normal" baryonic matter (as referred in the above article's quote), which gave the authors of the article the grounds to claim that 80–90% of it is located outside of galaxies?

  • $\begingroup$ I've rolled back to avoid closing as "needs more focus" $\endgroup$
    – James K
    Sep 19, 2021 at 18:50
  • $\begingroup$ @James K Aren't my 3 additional follow up questions complement the original one? In my understanding only providing answers for all 4 of them would provide comprehensive coverage of the subject matter. When asking my question I didn't anticipate receiving narrow focused answer... $\endgroup$
    – Alex
    Sep 19, 2021 at 19:45
  • $\begingroup$ This seems to be a feature of your questions, which I was hesitant to answer in the first place. $\endgroup$
    – ProfRob
    Sep 19, 2021 at 19:54
  • $\begingroup$ see How to Ask and How to Answer $\endgroup$
    – James K
    Sep 19, 2021 at 19:58
  • $\begingroup$ scitechdaily.com/… $\endgroup$
    – Alex
    Oct 3, 2021 at 12:11

1 Answer 1


The total baryonic mass density can be estimated by combining analysis of the cosmic microwave background, estimates of the primordial abundances of helium and deuterium and measurements of the Hubble parameter.

The value of $\Omega_b h^2$, where $\Omega_b$ is the ratio of the baryon density to the critical density for a flat universe and $h$ is the value of the present-day Hubble parameter divided by 100 km/s/Mpc, is directly constrained by modelling the small scale anisotropies in the CMB.

CMB variations

The dependence of CMB angular variations on the baryon density (from Dodelson & Schmidt 2021 Modern Cosmology)

Primordial nucleosynthesis calculations also show that the primordial abundances of helium and deuterium depend on $\Omega_b h^2$.

Primordial abundances

Primordial abundances versus baryon density. The horizontal bands are measurements of the primordial helium and deuterium abundances. The curves are nucleosynthesis predictions. The vertical grey band is the constraint from the CMB. Note the excellent agreement. (From http://www.astro.ucla.edu/~wright/BBNS.html).

Combining these measurements can give the value of $\Omega_b$, but combining either with a value for the Hubble parameter $(h \simeq 0.7)$ would also give a value for $\Omega_b$.

There are of course (small) uncertainties in all of these numbers, at the level of 10 per cent or so.

  • $\begingroup$ Thanks, as I understood your answer, you have provided the estimate of the AVERAGE baryonic matter density across the entire Universe (yes?). Now, from the referred (via link) in my question work (as I understood it), it follows that the actual distribution of the baryonic matter in the Universe greatly varies (comparing such within and outside galaxies), so there is a considerable difference between the average baryonic matter density within galaxies and the average baryonic matter density outside of galaxies. How the cited work arrived to the evaluation of the each of 2 average densities? $\endgroup$
    – Alex
    Sep 19, 2021 at 14:32
  • 1
    $\begingroup$ @Alex so your question is then "how do we calculate the amount of baryonic matter in galaxies?" and not the question you have written above which unambiguously asks how the total baryonic mass density is estimated. $\endgroup$
    – ProfRob
    Sep 19, 2021 at 15:03
  • $\begingroup$ This is the chain of questions, which logically follow each other - starting from the first one, which you have answered - the 3 next questions (they seems have to be tied together) are: 1) "how do we calculate the average density of baryonic matter, contained inside galaxies?" 2) "how do we calculate the average density of baryonic matter, contained outside of galaxies?" 3) "how average densities obtained in 1) and 2) combined together give the estimation of the overall density of baryonic matter in the Universe provided in your original answer?" $\endgroup$
    – Alex
    Sep 19, 2021 at 15:48
  • 1
    $\begingroup$ I'll stick to answering your original question. If you have others they should be posted as new questions. @Alex $\endgroup$
    – ProfRob
    Sep 19, 2021 at 16:21

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