Editing Cosmological constant (section)

=== Quantum field theory <span id="Cosmological constant problem"></span> ===
{{see also|Cosmological constant problem}}
{{unsolved|physics|Why does the [[zero-point energy]] of the quantum vacuum not cause a large cosmological constant? What cancels it out?}}
A major outstanding [[Unsolved problems in physics|problem]] is that most [[quantum field theory|quantum field theories]] predict a huge value for the [[quantum fluctuation|quantum vacuum]]. A common assumption is that the [[quantum fluctuation|quantum vacuum]] is equivalent to the cosmological constant. Although no theory exists that supports this assumption, arguments can be made in its favor.<ref>{{harvp|Rugh|Zinkernagel|2001|p=?}}</ref>

Such arguments are usually based on [[dimensional analysis]] and [[effective field theory]]. If the universe is described by an effective local quantum field theory down to the [[Planck scale]], then we would expect a cosmological constant of the order of <math display=inline>M_{\rm pl}^2</math> (<math display=inline>1</math> in reduced Planck units). As noted above, the measured cosmological constant is smaller than this by a factor of ~10<sup>120</sup>. This discrepancy has been called "the worst theoretical prediction in the history of physics".<ref name="CC Problem"/>

Some [[supersymmetry|supersymmetric]] theories require a cosmological constant that is exactly zero, which further complicates things. This is the cosmological constant problem, the worst problem of [[Fine-tuning (physics)|fine-tuning]] in [[physics]]: there is no known natural way to derive the tiny cosmological constant used in [[physical cosmology|cosmology]] from [[particle physics]].

No vacuum in the [[string theory landscape]] is known to support a metastable, positive cosmological constant, and in 2018 a group of four physicists advanced a controversial conjecture which would imply that [[Swampland (physics)|no such universe exists]].<ref>{{cite web |last1=Wolchover |first1=Natalie |title=Dark Energy May Be Incompatible With String Theory |url=https://www.quantamagazine.org/dark-energy-may-be-incompatible-with-string-theory-20180809/ |website=[[Quanta Magazine]] |publisher=Simons Foundation |access-date=2 April 2020 |date=9 August 2018}}</ref>