Editing Astronomy (section)

== Unsolved problems in astronomy ==
{{Main|List of unsolved problems in astronomy}}
In the 21st century there remain important unanswered questions in astronomy. Some are cosmic in scope: for example, what are [[dark matter]] and [[dark energy]]? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown.<ref name="physics questions">{{cite web|url=http://www.pnl.gov/energyscience/01-02/11-questions/11questions.htm
 |title = 11 Physics Questions for the New Century
|publisher = Pacific Northwest National Laboratory
|access-date =12 August 2006 |archive-url = https://web.archive.org/web/20060203152634/http://www.pnl.gov/energyscience/01-02/11-questions/11questions.htm |archive-date = 3 February 2006}}</ref> What will be the [[ultimate fate of the universe]]?<ref>{{cite web
|last = Hinshaw|first = Gary|date = 15 December 2005
|url = http://map.gsfc.nasa.gov/m_uni/uni_101fate.html
|title = What is the Ultimate Fate of the Universe?
|publisher = NASA WMAP|access-date =28 May 2007| archive-url= https://web.archive.org/web/20070529145436/http://map.gsfc.nasa.gov/m_uni/uni_101fate.html| archive-date= 29 May 2007 | url-status= live}}</ref> Why is the abundance of [[lithium]] in the cosmos four times lower than predicted by the standard [[Big Bang]] model?<ref>{{Cite journal|last1=Howk|first1=J. Christopher|last2=Lehner|first2=Nicolas|last3=Fields|first3=Brian D.|last4=Mathews|first4=Grant J.|date=6 September 2012|title=Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud|journal=Nature|language=en|volume=489|issue=7414|pages=121–23|doi=10.1038/nature11407|pmid=22955622|arxiv = 1207.3081 |bibcode = 2012Natur.489..121H |s2cid=205230254}}</ref> Others pertain to more specific classes of phenomena. For example, is the [[Solar System]] normal or atypical?<ref>{{cite journal | title=How special is the Solar system? | last1=Beer | first1=M. E. | last2=King | first2=A. R. | last3=Livio | first3=M. | last4=Pringle | first4=J. E. | journal=Monthly Notices of the Royal Astronomical Society | volume=354 | issue=3 | pages=763–768 | date=November 2004 | doi=10.1111/j.1365-2966.2004.08237.x | doi-access=free | arxiv=astro-ph/0407476 | bibcode=2004MNRAS.354..763B | s2cid=119552423 }}</ref> What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses—the [[initial mass function]]—apparently regardless of the initial conditions?<ref>{{cite journal
|last = Kroupa|first = Pavel
|title=The Initial Mass Function of Stars: Evidence for Uniformity in Variable Systems
|journal=Science|date=2002|volume=295|issue=5552
|pages=82–91
|doi=10.1126/science.1067524
|pmid=11778039
|arxiv = astro-ph/0201098 |bibcode = 2002Sci...295...82K
|s2cid = 14084249
}}</ref> Likewise, questions remain about the formation of the [[protogalaxy|first galaxies]],<ref>{{cite web|title=FAQ – How did galaxies form?|url=http://origins.stsci.edu/faq/galaxies.html|publisher=NASA|access-date=28 July 2015|archive-url=https://web.archive.org/web/20150628054952/http://origins.stsci.edu/faq/galaxies.html|archive-date=28 June 2015}}</ref>  the origin of [[supermassive black hole]]s,<ref>{{cite web|title=Supermassive Black Hole|url=http://astronomy.swin.edu.au/cosmos/S/Supermassive+Black+Hole|publisher=Swinburne University|access-date=28 July 2015|archive-date=14 August 2020|archive-url=https://web.archive.org/web/20200814110807/https://astronomy.swin.edu.au/cosmos/S/Supermassive+Black+Hole|url-status=live}}</ref> the source of [[ultra-high-energy cosmic ray]]s,<ref>{{cite journal|journal=Annual Review of Astronomy and Astrophysics|title=The Origin of Ultra-High-Energy Cosmic Rays|last=Hillas|first=A.M.|volume=22|date=September 1984|doi=10.1146/annurev.aa.22.090184.002233|pages=425–44|quote=This poses a challenge to these models, because [...]|bibcode = 1984ARA&A..22..425H }}</ref> and more.

Is there other [[Extraterrestrial life|life in the Universe]]? Especially, is there [[Extraterrestrial intelligence|other intelligent life]]? If so, what is the explanation for the [[Fermi paradox]]? The existence of life elsewhere has important scientific and philosophical implications.<ref>{{cite web
|url=http://www.astrobio.net/debate/236/complex-life-elsewhere-in-the-universe
|archive-url=https://web.archive.org/web/20110628214416/http://www.astrobio.net/debate/236/complex-life-elsewhere-in-the-universe
|archive-date=28 June 2011
|url-status=dead
|title = Rare Earth: Complex Life Elsewhere in the Universe?
|work = Astrobiology Magazine|access-date =12 August 2006|date=15 July 2002
}}</ref><ref>{{cite web|url=http://www.bigear.org/vol1no2/sagan.htm|title=The Quest for Extraterrestrial Intelligence|last=Sagan|first=Carl|work=Cosmic Search Magazine|access-date=12 August 2006|archive-url=https://web.archive.org/web/20060818144558/http://www.bigear.org/vol1no2/sagan.htm|archive-date=18 August 2006 |url-status=live}}</ref>