Dark energy

energyvacuum energycauses an acceleration in the expansioncosmic accelerationcosmological dark energydark energy cosmologyexpansion of the universeexpansion of the Universe is acceleratingvacuum or dark energy
In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.wikipedia
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Physical cosmology

cosmologycosmologicalcosmologist
In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.
This model requires the universe to contain large amounts of dark matter and dark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations.

Lambda-CDM model

ΛCDMstandard cosmological modelLambda-CDM
Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe.
The letter \Lambda (lambda) represents the cosmological constant, which is currently associated with a vacuum energy or dark energy in empty space that is used to explain the contemporary accelerating expansion of space against the attractive effects of gravity.

Dark matter

dark matter detectiondark-mattermissing mass
The mass–energy of dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount.
In the standard Lambda-CDM model of cosmology, the total mass–energy of the universe contains 5% ordinary matter and energy, 27% dark matter and 68% of an unknown form of energy known as dark energy.

Expansion of the universe

expanding universeexpandingexpansion of space
Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate.
Physicists have postulated the existence of dark energy, appearing as a cosmological constant in the simplest gravitational models, as a way to explain the acceleration.

Cosmological constant

cosmological constant problemcosmological termexpansion
Two proposed forms of dark energy are the cosmological constant, representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. A major outstanding problem is that the same quantum field theories predict a huge cosmological constant, more than 100 orders of magnitude too large.
It is closely associated to the concepts of dark energy and quintessence.

Quintessence (physics)

quintessencedark energy
Two proposed forms of dark energy are the cosmological constant, representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space.
In physics, quintessence is a hypothetical form of dark energy, more precisely a scalar field, postulated as an explanation of the observation of an accelerating rate of expansion of the universe.

Zero-point energy

zero point energyzero-point fieldzero-point
The cosmological constant can be formulated to be equivalent to the zero-point radiation of space i.e. the vacuum energy.
Yet according to Einstein's theory of general relativity any such energy would gravitate and the experimental evidence from both the expansion of the universe, dark energy and the Casimir effect show any such energy to be exceptionally weak.

Negative mass

gravitationally repulsive negative massesnegative energiesnegative gravitational mass
Einstein stated that the cosmological constant required that 'empty space takes the role of gravitating negative masses which are distributed all over the interstellar space'.
In December 2018, the astrophysicist Jamie Farnes from the University of Oxford proposed a "dark fluid" theory, related, in part, to notions of gravitationally repulsive negative masses, presented earlier by Albert Einstein, that may help better understand, in a testable manner, the considerable amounts of unknown dark matter and dark energy in the cosmos.

Big Bang

Big Bang theoryThe Big Bangbig-bang
Inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the Big Bang.
More recently, measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy's existence.

Astronomy

astronomicalastronomerastronomers
In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.
Dark matter and dark energy are the current leading topics in astronomy, as their discovery and controversy originated during the study of the galaxies.

Matter

corporealsubstancematerial
The mass–energy of dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount.
This part of the universe does not include dark energy, dark matter, black holes or various forms of degenerate matter, such as compose white dwarf stars and neutron stars.

Michael Turner (cosmologist)

Michael TurnerMichael S. TurnerTurner
The term "dark energy", echoing Fritz Zwicky's "dark matter" from the 1930s, was coined by Michael Turner in 1998.
Michael S. Turner is a theoretical cosmologist, who coined the term dark energy in 1998.

Baryon acoustic oscillations

baryon acoustic oscillationBAOacoustic peaks
Soon after, dark energy was supported by independent observations: in 2000, the BOOMERanG and Maxima cosmic microwave background experiments observed the first acoustic peak in the CMB, showing that the total (matter+energy) density is close to 100% of critical density.
BAO measurements help cosmologists understand more about the nature of dark energy (which causes the accelerating expansion of the universe) by constraining cosmological parameters.

Deceleration parameter

acceleratingaccelerating ratedecelerate
Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate. The first direct evidence for dark energy came from supernova observations in 1998 of accelerated expansion in Riess et al. and in Perlmutter et al., and the Lambda-CDM model then became the leading model.
The value of w_i is 0 for non-relativistic matter (baryons and dark matter), 1/3 for radiation, and −1 for a cosmological constant; for more general dark energy it may differ from −1, in which case it is denoted w_{DE} or simply

Hubble's law

Hubble constantHubble parameterHubble flow
In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.
The cosmological constant has regained attention in recent decades as a hypothesis for dark energy.

Type Ia supernova

type IaType Ia supernovaetype 1a supernova
In 1998, the High-Z Supernova Search Team published observations of Type Ia ("one-A") supernovae.
Details of the pre-nova moments may help scientists better judge the quality of Type Ia supernovae as standard candles, which is an important link in the argument for dark energy.

Wilkinson Microwave Anisotropy Probe

WMAPWilkinson Microwave Anisotropy Probe (WMAP)WMAP satellite
Much more precise measurements from WMAP in 2003–2010 have continued to support the standard model and give more accurate measurements of the key parameters.
The WMAP data are very well fit by a universe that is dominated by dark energy in the form of a cosmological constant.

Gravitational lens

gravitational lensinggravitationally lenseddeflection of light
Measurements of the cosmic microwave background, gravitational lensing, and the large-scale structure of the cosmos, as well as improved measurements of supernovae, have been consistent with the Lambda-CDM model.
They may also provide an important future constraint on dark energy.

Vacuum

free spaceevacuatedhigh vacuum
It is sometimes called a vacuum energy because it is the energy density of empty vacuum.
According to modern understanding, even if all matter could be removed from a volume, it would still not be "empty" due to vacuum fluctuations, dark energy, transiting gamma rays, cosmic rays, neutrinos, and other phenomena in quantum physics.

Observable universe

large-scale structure of the universelarge-scale structurelarge-scale structure of the cosmos
Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. Measurements of the cosmic microwave background, gravitational lensing, and the large-scale structure of the cosmos, as well as improved measurements of supernovae, have been consistent with the Lambda-CDM model.
However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light (special relativity prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding; see uses of the proper distance for a discussion) and furthermore the expansion rate appears to be accelerating due to dark energy.

List of unsolved problems in physics

unsolved problems in physicsunsolved problem in physicsproblem
A major outstanding problem is that the same quantum field theories predict a huge cosmological constant, more than 100 orders of magnitude too large.
There are still some deficiencies in the Standard Model of physics, such as the origin of mass, the strong CP problem, neutrino mass, matter–antimatter asymmetry, and the nature of dark matter and dark energy.

General relativity

general theory of relativitygeneral relativity theoryrelativity
The "cosmological constant" is a constant term that can be added to Einstein's field equation of general relativity.
Observational evidence from redshift surveys of distant supernovae and measurements of the cosmic background radiation also show that the evolution of our universe is significantly influenced by a cosmological constant resulting in an acceleration of cosmic expansion or, equivalently, by a form of energy with an unusual equation of state, known as dark energy, the nature of which remains unclear.

Saul Perlmutter

Perlmutter
The first direct evidence for dark energy came from supernova observations in 1998 of accelerated expansion in Riess et al. and in Perlmutter et al., and the Lambda-CDM model then became the leading model. The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess for their leadership in the discovery.
These findings reinvigorated research into the nature of the universe, and especially into the role of dark energy.

Brian Schmidt

Brian P. SchmidtBrian Paul SchmidtProfessor Brian Schmidt
The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess for their leadership in the discovery.
The corroborating evidence between the two competing studies led to the acceptance of the accelerating universe theory and initiated new research to understand the nature of the universe, such as the existence of dark energy.

Shape of the universe

flatflat universeopen universe
In general relativity, the evolution of the expansion rate is estimated from the curvature of the universe and the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space).
Data from Wilkinson Microwave Anisotropy Probe (WMAP) as well as the Planck spacecraft give values for the three constituents of all the mass-energy in the universe – normal mass (baryonic matter and dark matter), relativistic particles (photons and neutrinos), and dark energy or the cosmological constant: