Editing Zero-point energy (section)

=== Dark energy ===
{{Main|Dark energy}}
{{unsolved|physics|Why does the large zero-point energy of the [[vacuum]] not cause a large cosmological constant? What cancels it out?{{sfnp|Rugh|Zinkernagel|2002}}<ref name=scientificamerican0588-106 />{{sfnp|Battersby|2016}}}}
In the late 1990s it was discovered that very distant [[supernova]]e were dimmer than expected suggesting that the universe's expansion was accelerating rather than slowing down.{{sfnp|Riess et al.|1998}}{{sfnp|Perlmutter et al.|1998}} This revived discussion that Einstein's cosmological constant, long disregarded by physicists as being equal to zero, was in fact some small positive value. This would indicate empty space exerted some form of [[Negative energy|negative pressure or energy]].

There is no natural candidate for what might cause what has been called dark energy but the current best guess is that it is the zero-point energy of the vacuum, but this guess is known to be off by 120 [[order of magnitude|orders of magnitude]].<ref>{{cite magazine|last1=Clark|first1=Stuart|title=The Universe is Flat as a Pancake|magazine=New Scientist|date=2016|volume=232|issue=3097|page=35|url=https://archive.org/details/NewScientistOctober29/page/n35/mode/2up}}</ref>

The [[European Space Agency|European Space Agency's]] [[Euclid (spacecraft)|Euclid telescope]], launched on 1 July 2023, will map galaxies up to 10 billion light years away.<ref>{{Cite news |date=2023-07-01 |title=The Dark Universe Is Waiting. What Will the Euclid Telescope Reveal? |work=The New York Times |language=en |url=https://www.nytimes.com/2023/07/01/science/spacex-euclid-launch.html |access-date=2023-08-23 |last1=Miller |first1=Katrina }}</ref> By seeing how dark energy influences their arrangement and shape, the mission will allow scientists to see if the strength of dark energy has changed. If dark energy is found to vary throughout time it would indicate it is due to [[Quintessence (physics)|quintessence]], where observed acceleration is due to the energy of a [[scalar field]], rather than the cosmological constant. No evidence of quintessence is yet available, but it has not been ruled out either. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some scientists think that the best evidence for quintessence would come from violations of Einstein's [[equivalence principle]] and [[equivalence principle#Tests of the Einstein equivalence principle|variation of the fundamental constants]] in space or time.<ref name="Carroll1998">{{cite journal|last1=Carroll|first1=Sean M.|title=Quintessence and the Rest of the World: Suppressing Long-Range Interactions|journal=Physical Review Letters|volume=81|issue=15|year=1998|pages=3067–3070|issn=0031-9007|doi=10.1103/PhysRevLett.81.3067|arxiv = astro-ph/9806099 |bibcode = 1998PhRvL..81.3067C |s2cid=14539052|url=https://cds.cern.ch/record/356711/files/9806099.pdf}}</ref> Scalar fields are predicted by the ''[[Standard Model]] of particle physics'' and [[string theory]], but an analogous problem to the cosmological constant problem (or the problem of constructing models of [[cosmological inflation]]) occurs: [[renormalization]] theory predicts that scalar fields should acquire large masses again due to zero-point energy.