Dark energy - Revision history

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	Instead of introducing a new energy component, other physicists argue that the accelerating cosmic expansion signals a breakdown of general relativity on cosmological scales, which would require a modified theory of gravity.		Instead of introducing a new energy component, other physicists argue that the accelerating cosmic expansion signals a breakdown of general relativity on cosmological scales, which would require a modified theory of gravity.

			== Current and future probes ==

	[[File:DESI on the Mayall 4-meter Telescope.jpg\|thumb\|The Dark Energy Spectroscopic Instrument (DESI) installed on the Nicholas U. Mayall 4-metre Telescope at Kitt Peak National Observatory. Credit: NOIRLab/KPNO/NSF/AURA/P. Marenfeld (CC BY 4.0).]]		[[File:DESI on the Mayall 4-meter Telescope.jpg\|thumb\|The Dark Energy Spectroscopic Instrument (DESI) installed on the Nicholas U. Mayall 4-metre Telescope at Kitt Peak National Observatory. Credit: NOIRLab/KPNO/NSF/AURA/P. Marenfeld (CC BY 4.0).]]

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Add DESI and Euclid probe images to Current and future probes — AI-assisted (RonzzWikiCowriter) (via update-page on MediaWiki MCP Server)

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	Instead of introducing a new energy component, other physicists argue that the accelerating cosmic expansion signals a breakdown of general relativity on cosmological scales, which would require a modified theory of gravity.		Instead of introducing a new energy component, other physicists argue that the accelerating cosmic expansion signals a breakdown of general relativity on cosmological scales, which would require a modified theory of gravity.

	~~== Current and future probes ==~~		[[File:DESI on the Mayall 4-meter Telescope.jpg\|thumb\|The Dark Energy Spectroscopic Instrument (DESI) installed on the Nicholas U. Mayall 4-metre Telescope at Kitt Peak National Observatory. Credit: NOIRLab/KPNO/NSF/AURA/P. Marenfeld (CC BY 4.0).]]

			[[File:Euclid spacecraft (ESA artist impression).jpg\|thumb\|left\|Artist's impression of ESA's Euclid space telescope in orbit. Credit: ESA (CC BY-SA 3.0 IGO).]]

	As of September 2026, the nature of dark energy remains a core objective of observational cosmology. The Dark Energy Spectroscopic Instrument (DESI) and the European Space Agency's Euclid space telescope are measuring the expansion history of the universe and the growth of cosmic structure over most of cosmic history, and the Nancy Grace Roman Space Telescope, launched in August 2026, is extending the supernova and weak-lensing measurements to much larger samples. The key observable is the equation-of-state parameter ''w'' and its possible variation with time: any deviation from −1 would rule out a pure cosmological constant and point to one of the dynamical alternatives.		As of September 2026, the nature of dark energy remains a core objective of observational cosmology. The Dark Energy Spectroscopic Instrument (DESI) and the European Space Agency's Euclid space telescope are measuring the expansion history of the universe and the growth of cosmic structure over most of cosmic history, and the Nancy Grace Roman Space Telescope, launched in August 2026, is extending the supernova and weak-lensing measurements to much larger samples. The key observable is the equation-of-state parameter ''w'' and its possible variation with time: any deviation from −1 would rule out a pure cosmological constant and point to one of the dynamical alternatives.

Rongzhou: /* Additional observational evidence */

2026-09-03T07:45:03Z

Additional observational evidence

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	[[File:Universe-timeline.jpg\|thumb\|upright=1.8\|The history of the universe, from the Big Bang to the present day. The expansion first slowed under the pull of matter and radiation, then accelerated as dark energy came to dominate. Credit: NASA/WMAP Science Team (public domain).]]		[[File:Universe-timeline.jpg\|thumb\|upright=1.8\|The history of the universe, from the Big Bang to the present day. The expansion first slowed under the pull of matter and radiation, then accelerated as dark energy came to dominate. Credit: NASA/WMAP Science Team (public domain).]]

			== The physics of dark energy ==

			Under the assumptions of homogeneity and isotropy on large scales, the expansion of the universe is described by the Friedmann equations, which follow from general relativity:

			{{#content:Q1543}}

			{{#content:Q1544}}

			Here ''a'' is the scale factor (the Hubble parameter is H = ȧ/''a''), ''k'' the spatial curvature, ρ the total mass density, ''p'' the pressure, G the gravitational constant, ''c'' the speed of light, and Λ the cosmological constant.

			The second equation demonstrates that any fluid with a non-negative pressure, such as ordinary matter (''p'' = 0) or radiation (''p'' = ρ''c''²/3), results in a more negative right-hand side, decelerating the expansion of the universe. The observed acceleration in the rate of expansion of the universe can only be explained by a dominant component with negative pressure satisfying ρ + 3''p''/''c''² < 0.

			Cosmologists characterise each component by its dimensionless equation-of-state parameter

			{{#content:Q1545}}

			with ''w'' = 0 for non-relativistic matter, ''w'' = 1/3 for radiation, and ''w'' = −1 for a cosmological constant.

			Mathematically, any component with ''w'' < −1/3 contributes to the acceleration of the rate of expansion of the universe. For this component to dominate the energy budget, it must also dilute more slowly than matter.

			The simplest component satisfying ''w'' < −1/3 is the cosmological constant itself, which behaves exactly as a fluid of constant energy density

			{{#content:Q1546}}

			with a constant negative pressure

			{{#content:Q1547}}

			so that its equation-of-state parameter is exactly

			{{#content:Q1548}}

			As the universe expands and matter is diluted, such a constant-density component becomes steadily more dominant, explaining why the acceleration of the cosmic expansion began only when the universe was already several billion years old, as dark energy had overtaken matter.<ref>{{#cite:Q1542}}</ref>

	== Additional observational evidence ==		== Additional observational evidence ==

Rongzhou: /* The physics of dark energy */

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The physics of dark energy

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 == Multiple hypotheses on the nature of dark energy ==

Rongzhou: /* History */

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History

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 Dark energy, unlike ordinary matter and energy, is not gathered into galaxies and clusters by gravity but spread almost uniformly throughout space. Furthermore, while ordinary matter and energy are diluted by the expansion of the universe, the density of dark energy in space changes very slowly as the universe expands.<ref>{{#cite:Q1542}}</ref> As such, the fraction of dark energy in the content of the universe has grown over time, causing it to increasingly dominate the dynamics of the cosmos.
 === Type Ia supernovae and the return of the cosmological constant ===
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 One year later, in 1999, the Supernova Cosmology Project published the analysis of 42 type Ia supernovae at redshifts between 0.18 and 0.83, and the data similarly favoured a positive cosmological constant, with 99% confidence.<ref>{{#cite:Q1541}}</ref>
 == Additional observational evidence ==

Rongzhou: /* Type Ia supernovae and the return of the cosmological constant */

2026-09-03T07:31:17Z

Type Ia supernovae and the return of the cosmological constant

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	=== Type Ia supernovae and the return of the cosmological constant ===		=== Type Ia supernovae and the return of the cosmological constant ===

	~~With~~ the cosmological constant set to zero, the standard Big Bang models predict that the expansion of the universe must decelerate: gravity pulls every galaxy towards every other, slowing down the expansion initiated by the Big Bang. How strongly the expansion decelerates depends on how much matter the universe contains, ~~so by the 1990s cosmologists were attempting to measure the deceleration~~, ~~expecting to find that the expansion~~ was ~~slowing down.~~		The story of the cosmological constant, however, was not over:

	~~Measuring~~ the expansion history ~~requires~~ objects whose intrinsic brightness is known well enough that their apparent brightness reveals their distance, nicknamed ~~'''~~standard candles~~''' by cosmologists~~. ~~During the 1990s~~, ~~type~~ Ia supernovae ~~became the standard candles of choice: they are~~ the explosions of white dwarfs that have accreted matter beyond a critical mass, ~~which makes their~~ peak brightness ~~remarkably uniform~~. ~~The small~~ remaining differences ~~are~~ corrected by the width–luminosity relation, which links how fast the supernova fades to how bright it is at its peak~~, turning type Ia supernovae into "standardisable" candles usable out to redshifts of order one~~.		With the cosmological constant set to zero, the standard Big Bang models predict that the expansion of the universe must decelerate: gravity pulls every galaxy towards every other, slowing down the expansion initiated by the Big Bang.

			For decades, scientists were however unable to measure experimentally the rate of cosmic expansion. This changed in the 1990s, when scientists sufficiently mastered the so-called "'''standard candles'''" method, which measures the expansion history of the universe via objects whose intrinsic brightness is known well enough that their apparent brightness reveals their distance, hence nicknamed "standard candles". A reliable "standard candle" class, it turned out, was Ia supernovae, the explosions of white dwarfs that have accreted matter beyond a critical mass, emitting a remarkably uniform peak brightness. Small remaining differences can be corrected by the width–luminosity relation, which links how fast the supernova fades to how bright it is at its peak.

	[[File:SN1994D type Ia supernova.jpg\|thumb\|The type Ia supernova SN 1994D (bright spot, lower left) in the galaxy NGC 4526, imaged by the Hubble Space Telescope. Credit: NASA/ESA, the Hubble Key Project Team and the High-Z Supernova Search Team (public domain).]]		[[File:SN1994D type Ia supernova.jpg\|thumb\|The type Ia supernova SN 1994D (bright spot, lower left) in the galaxy NGC 4526, imaged by the Hubble Space Telescope. Credit: NASA/ESA, the Hubble Key Project Team and the High-Z Supernova Search Team (public domain).]]

	Two ~~rival~~ teams set out to ~~apply them:~~ the ~~Supernova Cosmology Project, led by [[Person~~:~~Saul Perlmutter\|Saul Perlmutter]], and~~ the High-Z Supernova Search Team, led by [[Person:Adam Riess\|Adam Riess]] and [[Person:Brian Schmidt\|Brian Schmidt]]~~. Both expected to observe the deceleration caused by gravity. Instead, both found the opposite.~~		Two independent teams set out to measure the rate of cosmic expansion in the 1990s with Ia supernovae as standard candles: the High-Z Supernova Search Team, led by [[Person:Adam Riess\|Adam Riess]] and [[Person:Brian Schmidt\|Brian Schmidt]], and the Supernova Cosmology Project, led by [[Person:Saul Perlmutter\|Saul Perlmutter]]. Both expected to observe a deceleration caused by gravity as predicted by standard Big Bang models.

	In 1998, the High-Z team reported the analysis of 16 high-redshift type Ia supernovae together with 34 nearby ones. The high-redshift supernovae were on average 10–15% farther away than expected in a low-density universe without a cosmological constant, and ~~the data unanimously favoured eternally expanding models with a positive cosmological constant, ΩΛ > 0, and a currently accelerating expansion.<ref>{{#cite:Q1540}}</ref> One year later,~~ the Supernova Cosmology Project ~~published the analysis of 42 type Ia supernovae at redshifts between 0.18 and 0.83. The data indicated that the cosmological constant is non-zero and positive~~, ~~with 99% confidence.<ref>{{#cite:Q1541}}</ref>~~

	Both teams had therefore measured not a deceleration but an acceleration of the cosmic expansion. Within general relativity, such an acceleration requires a dominant component with negative pressure, precisely what a cosmological constant, or something like it, would provide. The constant had returned, this time under the name of dark energy.

	[[Person:Saul Perlmutter\|Saul Perlmutter]], [[Person:Brian Schmidt\|Brian Schmidt]] and [[Person:Adam Riess\|Adam Riess]] shared the 2011 Nobel Prize in Physics for the discovery of the accelerating expansion of the universe through observations of distant supernovae.

	~~[[File:Universe~~-~~timeline.jpg\|thumb\|upright=1~~.8\|The ~~history of the~~ universe, ~~from~~ the ~~Big Bang to the present day. The~~ expansion ~~first slowed under the pull of matter and radiation~~, ~~then accelerated as dark energy came to dominate~~. ~~Credit~~: ~~NASA~~/~~WMAP Science Team (public domain).]]~~		In 1998, the High-Z team reported the analysis of 16 high-redshift type Ia supernovae together with 34 nearby ones. The high-redshift supernovae were on average 10–15% farther away than expected in a low-density universe without a cosmological constant, and the data unanimously favoured eternally expanding models with a currently accelerating cosmic expansion, consistent with a positive cosmological constant (ΩΛ > 0).<ref>{{#cite:Q1540}}</ref>

	~~The term dark energy was coined~~ in ~~analogy with dark matter: like that component it is invisible~~, and the ~~name is intended to leave open what the new ingredient actually is~~.		One year later, in 1999, the Supernova Cosmology Project published the analysis of 42 type Ia supernovae at redshifts between 0.18 and 0.83, and the data similarly favoured a positive cosmological constant, with 99% confidence.<ref>{{#cite:Q1541}}</ref>

	== Additional observational evidence ==		== Additional observational evidence ==

Rongzhou: /* The cosmological constant and the expanding universe */

2026-09-03T07:08:41Z

The cosmological constant and the expanding universe

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	The first theoretical ancestor of dark energy was introduced by Albert Einstein in 1917, who called it the '''cosmological constant''', usually written Λ. It featured in his field equations of general relativity as an energy uniform in space and constant in time, necessary for the static universe assumed at the time.		The first theoretical ancestor of dark energy was introduced by Albert Einstein in 1917, who called it the '''cosmological constant''', usually written Λ. It featured in his field equations of general relativity as an energy uniform in space and constant in time, necessary for the static universe assumed at the time.

	~~The~~ discovery that the universe is expanding ~~would soon make~~ the constant unnecessary. Following the mathematical work of Alexander Friedmann, Georges Lemaître derived in 1927 an expanding-universe solution of general relativity and connected the expansion to the redshift of distant galaxies, obtaining ~~the~~ linear distance–radial-velocity relation established observationally two years later by Edwin Hubble.<ref>{{#cite:Q1526}}</ref><ref>{{#cite:Q1527}}</ref> With the universe shown to be expanding rather than static, Einstein's original ~~reason~~ for introducing the constant disappeared~~; Einstein~~ is said to have later called it his "biggest blunder", and it was dropped from the standard Big Bang models that followed~~. Its story, however, was not over: the term remained available in the equations, and observations at the end of the 20th century would bring it back~~.		10 years later, the discovery that the universe is expanding rendered the constant unnecessary. Following the mathematical work of Alexander Friedmann, Georges Lemaître derived in 1927 an expanding-universe solution of general relativity and connected the expansion to the redshift of distant galaxies, obtaining a linear distance–radial-velocity relation. This relation was further established observationally two years later by Edwin Hubble.<ref>{{#cite:Q1526}}</ref><ref>{{#cite:Q1527}}</ref> With the universe shown to be expanding rather than static, Einstein's original justification for introducing the constant disappeared: He is said to have later called it his "biggest blunder", and it was dropped from the standard Big Bang models that followed.

	=== Type Ia supernovae and the return of the cosmological constant ===		=== Type Ia supernovae and the return of the cosmological constant ===

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Rongzhou: /* The physics of dark energy */

2026-09-03T06:52:38Z

The physics of dark energy

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	{{#content:Q1546}}		{{#content:Q1546}}

	~~with~~ pressure ''p'' = −ρ''c''²~~, i.e., with an equation~~-of-~~state parameter~~ ''w'' = −1. As the universe expands and matter is diluted, such a constant-density component becomes steadily more dominant, explaining why the acceleration of the cosmic expansion began only when the universe was already several billion years old, as dark energy had overtaken matter.<ref>{{#cite:Q1542}}</ref>		Since its pressure is

			''p'' = −ρ''c''² <!-- change to {{#content:xxx}} -->

			We have

			''w'' = −1 <!-- change to {{#content:xxx}} -->

			As the universe expands and matter is diluted, such a constant-density component becomes steadily more dominant, explaining why the acceleration of the cosmic expansion began only when the universe was already several billion years old, as dark energy had overtaken matter.<ref>{{#cite:Q1542}}</ref>

	== Multiple hypothesis on the nature of the dark energy ==		== Multiple hypothesis on the nature of the dark energy ==

Rongzhou: /* Multiple hypothesis on the nature of the dark energy */

2026-09-03T06:45:13Z

Multiple hypothesis on the nature of the dark energy

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	@startuml		@startuml
	!theme bluegray		!theme bluegray
	title ~~What could~~ dark energy ~~be?~~		title Three hypothesis on the nature of dark energy

	rectangle "Dark energy" as DE		rectangle "Dark energy" as DE