<p>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.</p>
<p>The value of <span class="math-container">$\Omega_b h^2$</span>, where <span class="math-container">$\Omega_b$</span> is the ratio of the baryon density to the critical density for a flat universe and <span class="math-container">$h$</span> 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.</p>
<p><a href="https://i.sstatic.net/vFID6.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/vFID6.jpg" alt="CMB variations" /></a></p>
<p><em>The dependence of CMB angular variations on the baryon density (from Dodelson &amp; Schmidt 2021 Modern Cosmology)</em></p>
<p>Primordial nucleosynthesis calculations also show that the primordial abundances of helium and deuterium depend on <span class="math-container">$\Omega_b h^2$</span>.</p>
<p><a href="https://i.sstatic.net/KlRUT.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/KlRUT.jpg" alt="Primordial abundances" /></a></p>
<p><em>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 <a href="http://www.astro.ucla.edu/%7Ewright/BBNS.html" rel="nofollow noreferrer">http://www.astro.ucla.edu/~wright/BBNS.html</a>).</em></p>
<p>Combining these measurements can give the value of <span class="math-container">$\Omega_b$</span>, but combining either with a value for the Hubble parameter <span class="math-container">$(h \simeq 0.7)$</span> would also give a value for <span class="math-container">$\Omega_b$</span>.</p>
<p>There are of course (small) uncertainties in all of these numbers, at the level of 10 per cent or so.</p>

CC BY-SA 4.0
2021-09-19T14:32:35.217Z
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?
Alex
CC BY-SA 4.0
2021-09-19T15:03:55.617Z
@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.
ProfRob
CC BY-SA 4.0
2021-09-19T15:48:13.080Z
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?"
Alex
CC BY-SA 4.0
2021-09-19T16:21:01.217Z
I'll stick to answering your original question. If you have others they should be posted as new questions. @Alex
ProfRob
CC BY-SA 4.0
2021-09-19T12:53:57.857Z
2021-09-19T13:04:23.590Z
2021-09-19T13:04:23.590Z
ProfRob
<p>In <a href="https://phys.org/news/2021-09-universe.html" rel="nofollow noreferrer">https://phys.org/news/2021-09-universe.html</a> it says:&quot;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.&quot;</p>
<p>What is the basis for the estimate of the total amount of the Universe's &quot;normal&quot; 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?</p>

CC BY-SA 4.0
2021-09-19T18:50:42.037Z
I've rolled back to avoid closing as "needs more focus"
James K
CC BY-SA 4.0
2021-09-19T19:45:49.390Z
@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...
Alex
CC BY-SA 4.0
2021-09-19T19:54:06.717Z
This seems to be a feature of your questions, which I was hesitant to answer in the first place.
ProfRob
CC BY-SA 4.0
2021-09-19T19:58:59.403Z
see [ask] and [answer]
James K
CC BY-SA 4.0
2021-10-03T12:11:34.807Z
https://scitechdaily.com/part-of-the-universes-missing-matter-found-thanks-to-very-large-telescope/
Alex
<universe><matter>
Question re distribution of the baryonic matter in the Universe
Alex
# Question re distribution of the baryonic matter in the Universe

(Asked by: username_0 on 2021-09-18)

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?

**Tags:** universe, matter

> **Comments:**
> **username_1**: I've rolled back to avoid closing as "needs more focus"  
> **username_0**: @username_1 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...  
> **username_2**: This seems to be a feature of your questions, which I was hesitant to answer in the first place.  
> **username_1**: see [ask] and [answer]  
> **username_0**: https://scitechdaily.com/part-of-the-universes-missing-matter-found-thanks-to-very-large-telescope/  

---

## 1 answers

### Answer by username_2 on 2021-09-19. Score: 3

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](https://i.sstatic.net/vFID6.jpg)](https://i.sstatic.net/vFID6.jpg)

*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](https://i.sstatic.net/KlRUT.jpg)](https://i.sstatic.net/KlRUT.jpg)

*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](http://www.astro.ucla.edu/%7Ewright/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.

> **Comments:**
> **username_0**: 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?  
> **username_2**: @username_0 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.  
> **username_0**: 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?"  
> **username_2**: I'll stick to answering your original question. If you have others they should be posted as new questions. @username_0  

---

