Editing Hierarchy problem (section)

=== Cosmological constant ===
{{main article|Cosmological constant problem}}
In [[physical cosmology]], current observations in favor of an [[accelerating universe]] imply the existence of a tiny, but nonzero [[cosmological constant]]. This problem, called the '''cosmological constant problem''', is a hierarchy problem very similar to that of the Higgs boson mass problem, since the cosmological constant is also very sensitive to quantum corrections, but its calculation is complicated by the necessary involvement of [[general relativity]] in the problem. Proposed solutions to the cosmological constant problem include modifying and/or extending gravity,<ref name="dark universe">Bull, Philip, Yashar Akrami, Julian Adamek, Tessa Baker, Emilio Bellini, Jose Beltrán Jiménez, Eloisa Bentivegna et al. "Beyond ΛCDM: Problems, solutions, and the road ahead." Physics of the Dark Universe 12 (2016): 56-99.</ref><ref>{{cite journal|last=Ellis |first=George F. R. |authorlink=George F. R. Ellis |title=The trace-free Einstein equations and inflation |journal=[[General Relativity and Gravitation]] |year=2014 |volume=46 |issue=1 |pages=1619 |doi=10.1007/s10714-013-1619-5 |arxiv=1306.3021|bibcode=2014GReGr..46.1619E |s2cid=119000135 }}</ref><ref>{{cite journal|last=Percacci |first=R. |title=Unimodular quantum gravity and the cosmological constant |doi=10.1007/s10701-018-0189-5 |arxiv=1712.09903 |year=2018 |journal=[[Foundations of Physics]] |volume=48 |number=10 |pages=1364–1379|bibcode=2018FoPh...48.1364P |s2cid=118934871 }}</ref> adding matter with unvanishing pressure,<ref name="Luongo Muccino pp. 2-3">{{cite journal |last1=Luongo |first1=Orlando |last2=Muccino |first2=Marco |title=Speeding up the Universe using dust with pressure |journal=Physical Review D |volume=98 |issue=10 |date=2018-11-21 |issn=2470-0010 |doi=10.1103/physrevd.98.103520 |pages=2–3|arxiv=1807.00180 |bibcode=2018PhRvD..98j3520L |s2cid=119346601 }}</ref> and UV/IR mixing in the Standard Model and gravity.<ref>{{cite journal|title=Effective Field Theory, Black Holes, and the Cosmological Constant|last1=Cohen|first1=Andrew|last2=Kaplan|first2=David B.|last3=Nelson|first3=Ann|journal=Physical Review Letters|volume=82|issue=25|date=21 June 1999|pages=4971–4974|doi=10.1103/PhysRevLett.82.4971|arxiv=hep-th/9803132|bibcode=1999PhRvL..82.4971C|s2cid=17203575}}</ref><ref>{{cite arXiv|title=Densities of States and the CKN Bound|author1=Nikita Blinov|author2=Patrick Draper|eprint=2107.03530|date=7 July 2021|class=hep-ph}}</ref> 

Some physicists have resorted to [[anthropic reasoning]] to solve the cosmological constant problem,<ref>{{cite journal|last1=Martel|first1=Hugo|author2-link=Paul R. Shapiro|last2=Shapiro|first2=Paul R.|last3=Weinberg|first3=Steven|title=Likely Values of the Cosmological Constant|journal=The Astrophysical Journal|date=January 1998|volume=492|issue=1|pages=29–40|doi=10.1086/305016|arxiv=astro-ph/9701099|bibcode=1998ApJ...492...29M|s2cid=119064782}}</ref> but it is disputed whether such anthropic reasoning is scientific.<ref>{{cite book | author = Penrose, R. |author-link = Roger Penrose | title = The Emperor's New Mind | url = https://archive.org/details/emperorsnewmindc00penr | url-access = registration | publisher = Oxford University Press | isbn = 978-0-19-851973-7 | date =1989}} Chapter 10.</ref><ref>{{cite journal | author = Starkman, G. D. | author2 = Trotta, R. | title = Why Anthropic Reasoning Cannot Predict Λ | journal = Physical Review Letters | volume = 97 |page = 201301 | date = 2006 | doi = 10.1103/PhysRevLett.97.201301 | pmid = 17155671 | issue = 20 | bibcode=2006PhRvL..97t1301S|arxiv = astro-ph/0607227 | s2cid = 27409290 }} See also this [http://www.physorg.com/news83924839.html news story.]</ref>