Dark energy

[[File:Dark Energy.jpg|165px|right|thumb|Acceleration of the Universe]]
{{Astronomy}}
'''Dark energy''' is 
the final chapter of ''Half-Life 2''
 the term for a proposed form of energy that is thought to permeate all space.  Dark energy has a repulsive effect, much weaker than [[gravity]], but permeating all of space means that falling off is irrelevant. The true nature of dark energy is one of the currently unsolved problems in [[physics]].

The abundance of [[dark matter]] is 26.8% of the total mass of the [[Universe]], which is far larger than the 4.9% of mass represented by ordinary (or baryonic) matter, such as interstellar gas. But 68.3% of the mass of the Universe is bound up in the mass-equivalence of dark energy.

== Evidence ==
Not only is the [[Expanding universe|Universe expanding]], its rate of expansion is increasing with [[time]]. Dark energy is the name given to the mysterious "something" that is fueling this universal inflation. Such energy is necessary only if the universe is flat; an "open" universe (an "anti-de Sitter spacetime", one with hyperbolic geometry--negative scalar curvature universally) experiences an accelerating metric expansion anyway. However, our observations suggest the universe is flat (a "minkowski" spacetime, euclidean geometry, zero scalar curvature universally), and an anti-de Sitter spacetime is unstable and thus cannot be a description of our universe anyway.<ref>Moschidis, Georgios (2018-12-11). "A proof of the instability of AdS for the Einstein--massless Vlasov system". arXiv:[https://arxiv.org/abs/1812.04268 1812.04268]</ref>

Dark energy is the successor to such past scientific gap-fillers as caloric, phlogiston, [[Luminiferous aether|aether]] and [[Einstein]]'s "biggest blunder" &mdash; the cosmological constant (it's more of a reinterpretation than a successor; dark energy appears in the {{wpl|Einstein Field Equations}} as a cosmological constant, using the same symbol Einstein did). Whether it stays as it is, becomes modified into a more concrete form, or is replaced by an even stranger space filler remains to be seen.

The {{wpl|European Space Agency|}} (ESA) has launched in July 2023 the mission ''{{wpl|Euclid (spacecraft)|Euclid}}'', which will hopefully improve our knowledge of it.

==The darkest side of dark energy==
Some theorized forms of dark energy as {{wpl|Phantom energy}}, if they existed or were dark energy to become them, could bring [[Ultimate fate of the Universe#Ripping everything apart: Big Rip|the end of the Universe at least as we know it]].

==See also==
*[[Dark matter]]
*[[Dark flow]]

==External links==
*[http://www.popsci.com/science/article/2011-05/new-study-confirms-dark-energy-tearing-universe-apart Largest-Ever Survey of Cosmic History Confirms That Dark Energy Exists, Is Tearing the Universe Apart]
*[http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html Dark Energy]  from [[NASA|NASA's]] Imagine the Universe
*[http://news.nationalgeographic.com/news/2008/08/080811-dark-energy.html Most Direct Evidence of Dark Energy Detected]

==References==
{{reflist|2|80%}}

[[Category:physics]]
[[Category:Astronomy]]
[[Category:Cosmology]]
{{stub|astronomy}}

Categories: physics, Astronomy, Cosmology