Editing Cosmological constant (section)

== Predictions ==

=== 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>

=== Anthropic principle ===
One possible explanation for the small but non-zero value was noted by [[Steven Weinberg]] in 1987 following the [[anthropic principle]].<ref>{{harvp|Weinberg|1987}}.</ref> Weinberg explains that if the vacuum energy took different values in different domains of the universe, then observers would necessarily measure values similar to that which is observed: the formation of life-supporting structures would be suppressed in domains where the vacuum energy is much larger. Specifically, if the vacuum energy is negative and its absolute value is substantially larger than it appears to be in the observed universe (say, a factor of 10 larger), holding all other variables (e.g. matter density) constant, that would mean that the universe is closed; furthermore, its lifetime would be shorter than the age of our universe, possibly too short for intelligent life to form. On the other hand, a universe with a large positive cosmological constant would expand too fast, preventing galaxy formation. According to Weinberg, domains where the vacuum energy is compatible with life would be comparatively rare. Using this argument, Weinberg predicted that the cosmological constant would have a value of less than a hundred times the currently accepted value.<ref>{{harvp|Vilenkin|2006|pages=138–139}}.</ref> In 1992, Weinberg refined this prediction of the cosmological constant to 5 to 10 times the matter density.<ref>{{harvp|Weinberg|1992|p=182}}.</ref>

This argument depends on the vacuum energy density being constant throughout spacetime, as would be expected if dark energy were the cosmological constant. There is no evidence that the vacuum energy does vary, but it may be the case if, for example, the vacuum energy is (even in part) the potential of a scalar field such as the residual [[inflaton]] (also see ''[[Quintessence (physics)|Quintessence]]''). Another theoretical approach that deals with the issue is that of [[multiverse]] theories, which predict a large number of "parallel" universes with different laws of physics and/or values of fundamental constants. Again, the anthropic principle states that we can only live in one of the universes that is compatible with some form of intelligent life. Critics claim that these theories, when used as an explanation for fine-tuning, commit the [[inverse gambler's fallacy]].

In 1995, Weinberg's argument was refined by [[Alexander Vilenkin]] to predict a value for the cosmological constant that was only ten times the matter density,<ref>{{harvp|Vilenkin|2006|page=146}}.</ref> i.e. about three times the current value since determined.

=== Failure to detect dark energy ===
An attempt to directly observe and relate quanta or fields like the [[chameleon particle]] or the [[symmetron]] theory to dark energy, in a laboratory setting, failed to detect a new force.<ref>{{cite journal |last1=Sabulsky |first1=D. O. |last2=Dutta |first2=I. |last3=Hinds |first3=E. A. |last4=Elder |first4=B. |last5=Burrage |first5=C/ |last6=Copeland |first6=E. J. |year=2019 |title=Experiment to Detect Dark Energy Forces Using Atom Interferometry |journal=Physical Review Letters |volume=123 |issue=6 |pages=061102 |arxiv=1812.08244 |bibcode=2019PhRvL.123f1102S |doi=10.1103/PhysRevLett.123.061102 |pmid=31491160 |s2cid=118935116}}</ref> Inferring the presence of dark energy through its interaction with baryons in the [[cosmic microwave background]] has also led to a negative result,<ref>{{cite journal |last1=Vagnozzi |first1=S. |last2=Visinelli |first2=L. |last3=Mena |first3=O. |last4=Mota |first4=D. |year=2020 |title=Do we have any hope of detecting scattering between dark energy and baryons through cosmology? |journal=Mon. Not. R. Astron. Soc. |volume=493 |issue=1 |pages=1139 |arxiv=1911.12374 |bibcode=2020MNRAS.493.1139V |doi=10.1093/mnras/staa311 |doi-access=free}}</ref> although the current analyses have been derived only at the linear perturbation regime. It is also possible that the difficulty in detecting dark energy is due to the fact that the cosmological constant describes an existing, known interaction (e.g. electromagnetic field).<ref>{{Cite journal |last=Ogonowski |first=Piotr |date=2023-01-09 |title=Proposed method of combining continuum mechanics with Einstein Field Equations |url=https://www.worldscientific.com/doi/10.1142/S0218271823500104 |journal=International Journal of Modern Physics D |volume=32 |issue=3 |language=en |pages=2350010–2350024 |doi=10.1142/S0218271823500104 |issn=0218-2718|arxiv=2212.13113 |bibcode=2023IJMPD..3250010O |s2cid=254778036 }}</ref>