Editing Observable universe (section)

== Matter and mass ==
=== Number of galaxies and stars ===
The observable universe contains as many as an estimated 2 trillion galaxies<ref name="BBC-20231129">{{cite news |last=Gunn |first=Alistair |date=29 November 2023 |title=How many galaxies are there in the universe? – Do astronomers know how many galaxies exist? How many can we see in the observable Universe? |url=https://www.skyatnightmagazine.com/space-science/how-many-galaxies-in-universe |url-status=live |archiveurl=https://archive.today/20231203021645/https://www.skyatnightmagazine.com/space-science/how-many-galaxies-in-universe |archivedate=3 December 2023 |accessdate=2 December 2023 |work=[[BBC Sky at Night]]}}</ref><ref>{{cite journal |title=New Horizons spacecraft answers the question: How dark is space? |website=phys.org |url=https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |access-date=January 15, 2021 |language=en |archive-date=January 15, 2021 |archive-url=https://web.archive.org/web/20210115110710/https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |url-status=live }}</ref><ref>{{cite news |last1=Howell |first1=Elizabeth |title=How Many Galaxies Are There? |url=https://www.space.com/25303-how-many-galaxies-are-in-the-universe.html |website=Space.com |access-date=March 5, 2021 |date=March 20, 2018 |archive-date=February 28, 2021 |archive-url=https://web.archive.org/web/20210228013433/https://www.space.com/25303-how-many-galaxies-are-in-the-universe.html |url-status=live }}</ref> and, overall, as many as an estimated 10<sup>24</sup> stars<ref name="ESA-2019">{{cite web |author=Staff |title=How Many Stars Are There In The Universe? |url=https://www.esa.int/Our_Activities/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe |date=2019 |work=[[European Space Agency]] |access-date=September 21, 2019 |archive-date=September 23, 2019 |archive-url=https://web.archive.org/web/20190923134902/http://www.esa.int/Our_Activities/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe |url-status=live }}</ref><ref>{{Cite book|chapter=The Structure of the Universe|doi=10.1007/978-1-4614-8730-2_10|title=The Fundamentals of Modern Astrophysics|pages=279–294|year=2015|last1=Marov|first1=Mikhail Ya.|isbn=978-1-4614-8729-6}}</ref> &ndash; more stars (and, potentially, Earth-like planets) than all the [[Sand|grains of beach sand]] on planet [[Earth]].<ref name="SU-20020201">{{cite web |last=Mackie |first=Glen |title=To see the Universe in a Grain of Taranaki Sand |url=http://astronomy.swin.edu.au/~gmackie/billions.html |date=February 1, 2002 |work=[[Centre for Astrophysics and Supercomputing]] |access-date=January 28, 2017 |archive-date=June 30, 2012 |archive-url=https://archive.today/20120630205715/http://astronomy.swin.edu.au/~gmackie/billions.html |url-status=live }}</ref><ref name="CNET-20150319">{{cite news |last=Mack |first=Eric |date=19 March 2015 |title=There may be more Earth-like planets than grains of sand on all our beaches – New research contends that the Milky Way alone is flush with billions of potentially habitable planets – and that's just one sliver of the universe. |url=https://www.cnet.com/science/the-milky-way-is-flush-with-habitable-planets-study-says/ |url-status=live |archiveurl=https://archive.today/20231201144523/https://www.cnet.com/science/the-milky-way-is-flush-with-habitable-planets-study-says/ |archivedate=1 December 2023 |accessdate=1 December 2023 |work=[[CNET]]}}</ref><ref name="MNRAS-20150313">{{cite journal |last1=Bovaird |first1=T. T. |last2=Lineweaver |first2=C. H. |last3=Jacobsen |first3=S. K. |date=13 March 2015 |title=Using the inclinations of Kepler systems to prioritize new Titius–Bode-based exoplanet predictions |url=https://academic.oup.com/mnras/article/448/4/3608/970734 |url-status=live |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=448 |issue=4 |pages=3608–3627 |arxiv=1412.6230 |doi=10.1093/mnras/stv221 |doi-access=free |archiveurl=https://archive.today/20231201151205/https://academic.oup.com/mnras/article/448/4/3608/970734 |archivedate=1 December 2023 |accessdate=1 December 2023}}</ref> Other estimates are in the hundreds of billions rather than trillions.<ref name=":0">{{cite journal |last1=Lauer |first1=T. R. |last2=Postman |first2=M. |last3=Spencer |first3=J. R. |last4=Weaver |first4=H. A. |last5=Stern |first5=S. A. |last6=Gladstone |first6=G. R. |last7=Binzel |first7=R. P. |last8=Britt |first8=D. T. |last9=Buie |first9=M. W. |last10=Buratti |first10=B. J. |last11=Cheng |first11=A. F. |last12=Grundy |first12=W. M. |last13=Horányi |first13=M. |last14=Kavelaars |first14=J. J. |last15=Linscott |first15=I. R. |last16=Lisse |first16=C. M. |last17=McKinnon |first17=W. B. |last18=McNutt |first18=R. L. |last19=Moore |first19=J. M. |last20=Núñez |first20=J. I. |last21=Olkin |first21=C. B. |last22=Parker |first22=J. W. |last23=Porter |first23=S. B. |last24=Reuter |first24=D. C. |last25=Robbins |first25=S. J. |last26=Schenk |first26=P. M. |last27=Showalter |first27=M. R. |last28=Singer |first28=K. N. |last29=Verbiscer |first29=A. J. |last30=Young |first30=L. A. |date=2022 |title=Anomalous Flux in the Cosmic Optical Background Detected with New Horizons Observations |journal=The Astrophysical Journal Letters |volume=927 |issue=1 |pages=l8 | doi=10.3847/2041-8213/ac573d|arxiv=2202.04273 |bibcode=2022ApJ...927L...8L | doi-access=free}}</ref><ref name="ann21001">{{cite news |last=Lauer |first=Todd |title= NOIRLab Scientist Finds the Universe to be Brighter than Expected |url= https://noirlab.edu/public/announcements/ann21001/ |date=12 January 2021 |work=[[NOIRLab]] |access-date=12 January 2021 }}</ref><ref name="arxiv:2011.03052">{{cite journal |last1=Lauer |first1=Tod R. |last2=Postman |first2=Marc |last3=Weaver |first3=Harold A. |last4=Spencer |first4=John R. |last5=Stern |first5=S. Alan |last6=Buie |first6=Marc W. |last7=Durda |first7=Daniel D. |last8=Lisse |first8=Carey M. |last9=Poppe |first9=A. R. |last10=Binzel |first10=Richard P. |last11=Britt |first11=Daniel T. |last12=Buratti |first12=Bonnie J. |last13=Cheng |first13=Andrew F. |last14=Grundy |first14=W. M. |last15=Horányi |first15=Mihaly |last16=Kavelaars |first16=J. J. |last17=Linscott |first17=Ivan R. |last18=McKinnon |first18=William B. |last19=Moore |first19=Jeffrey M. |last20=Núñez |first20=J. I. |last21=Olkin |first21=Catherine B. |last22=Parker |first22=Joel W. |last23=Porter |first23=Simon B. |last24=Reuter |first24=Dennis C. |last25=Robbins |first25=Stuart J. |last26=Schenk |first26=Paul |last27=Showalter |first27=Mark R. |last28=Singer |first28=Kelsi N. |last29=Verbiscer |first29=Anne J. |last30=Young |first30=Leslie A. |title=New Horizons Observations of the Cosmic Optical Background |journal=The Astrophysical Journal |date=11 January 2021 |volume=906 |issue=2 |pages=77 |doi=10.3847/1538-4357/abc881 | arxiv= 2011.03052 |bibcode=2021ApJ...906...77L |hdl=1721.1/133770 |s2cid=226277978 |doi-access=free }}</ref> The estimated total number of stars in an [[Cosmic inflation|inflationary universe]] (observed and unobserved) is 10<sup>100</sup>.<ref name="SR-20200203">{{cite journal |last=Totani |first=Tomonori |title=Emergence of life in an inflationary universe |date=3 February 2020 |journal=[[Scientific Reports]] |volume=10 |number=1671 |page=1671 |doi=10.1038/s41598-020-58060-0 |pmid=32015390 |arxiv=1911.08092 |bibcode=2020NatSR..10.1671T |doi-access=free |pmc=6997386 }}</ref>

=== <span class="anchor" id="Matter content"></span> Matter content—number of atoms ===
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{{Main|Abundance of the chemical elements}}
Assuming the mass of ordinary matter is about {{val|1.45|e=53|u=kg}} as discussed above, and assuming all atoms are [[hydrogen atom]]s (which are about 74% of all atoms in the Milky Way by mass), the estimated total number of atoms in the observable universe is obtained by dividing the mass of ordinary matter by the mass of a hydrogen atom. The result is approximately 10<sup>80</sup> hydrogen atoms, also known as the [[Eddington number]].

=== Mass of ordinary matter ===
The mass of the observable universe is often quoted as 10<sup>53</sup>&nbsp;kg.<ref name="Paul Davies 2006 43">{{cite book |author=Davies |first=Paul |url=https://archive.org/details/cosmicjackpotwhy0000davi/page/43 |title=The Goldilocks Enigma |date=2006 |publisher=First Mariner Books |isbn=978-0618592265 |page=[https://archive.org/details/cosmicjackpotwhy0000davi/page/43 43–]}}</ref> In this context, mass refers to ordinary (baryonic) matter and includes the [[interstellar medium]] (ISM) and the [[intergalactic medium]] (IGM). However, it excludes [[dark matter]] and [[dark energy]]. This quoted value for the mass of ordinary matter in the universe can be estimated based on critical density. The calculations are for the observable universe only as the volume of the whole is unknown and may be infinite.

=== Estimates based on critical density ===
Critical density is the energy density for which the universe is flat.<ref>See [[Friedmann equations#Density parameter]].</ref> If there is no dark energy, it is also the [[density]] for which the expansion of the universe is poised between continued expansion and collapse.<ref>{{cite book |author=Kaku |first=Michio |url=https://books.google.com/books?id=cKULZJpcJBwC |title=Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos |publisher=Knopf Doubleday |year=2006 |isbn=978-0307276988 |page=385 |language=en-us}}</ref> From the [[Friedmann equations]], the value for <math>\rho_\text{c}</math> critical density, is:<ref>{{cite book |author=Schutz |first=Bernard F. |url=https://books.google.com/books?id=iEZNXvYwyNwC&pg=PA361 |title=Gravity from the ground up |date=2003 |publisher=Cambridge University Press |isbn=978-0521455060 |pages=361– |language=en-uk}}</ref>
: <math>\rho_\text{c} = \frac{3 H^2}{8 \pi G},</math>
where ''G'' is the [[gravitational constant]] and {{nowrap|1=''H'' = ''H''<sub>0</sub>}} is the present value of the [[Hubble constant]]. The value for ''H''<sub>0</sub>, as given by the European Space Agency's Planck Telescope, is ''H''<sub>0</sub> = 67.15 kilometres per second per megaparsec. This gives a critical density of {{val|0.85|e=-26|u=kg/m3}}, or about 5 hydrogen atoms per cubic metre. This density includes four significant types of energy/mass: ordinary matter (4.8%), neutrinos (0.1%), [[cold dark matter]] (26.8%), and [[dark energy]] (68.3%).<ref name="planck_cosmological_parameters">{{cite journal | arxiv=1303.5076 | title=Planck 2013 results. XVI. Cosmological parameters | author=Planck collaboration | journal=Astronomy & Astrophysics | date=2013|bibcode = 2014A&A...571A..16P | doi=10.1051/0004-6361/201321591 | volume=571 | pages=A16| s2cid=118349591 }}</ref> 

Although neutrinos are [[Standard Model]] particles, they are listed separately because they are [[Scale factor (cosmology)#Radiation-dominated era|ultra-relativistic]] and hence [[Equation of state (cosmology)#Ultra-relativistic particles|behave]] like radiation rather than like matter. The density of ordinary matter, as measured by Planck, is 4.8% of the total critical density or {{val|4.08|e=-28|u=kg/m3}}. To convert this density to mass we must multiply by volume, a value based on the radius of the "observable universe". Since the universe has been expanding for 13.8 billion years, the [[Comoving and proper distances|comoving distance]] (radius) is now about 46.6 billion light-years. Thus, volume ({{sfrac|4|3}}''πr''<sup>3</sup>) equals {{val|3.58|e=80|u=m3}} and the mass of ordinary matter equals density ({{val|4.08|e=-28|u=kg/m3}}) times volume ({{val|3.58|e=80|u=m3}}) or {{val|1.46|e=53|u=kg}}.