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.[1][2]
Contents 1 History of discovery and previous speculation 1.1 Einstein's cosmological constant 1.2 Inflationary dark energy 1.3 Change in expansion over time 2 Nature 2.1 Technical definition 3 Evidence of existence 3.1 Supernovae
3.2 Cosmic microwave background
3.3 Large-scale structure
3.4 Late-time integrated Sachs-Wolfe effect
3.5 Observational
4 Theories of dark energy 4.1 Cosmological constant 4.2 Modified gravity 4.2.1 Quintessence 4.2.2 Interacting dark energy 4.3 Variable dark energy models 4.4 Observational skepticism 5 Implications for the fate of the universe 6 In philosophy of science 7 See also 8 Notes 9 References 10 External links History of discovery and previous speculation[edit]
Einstein's cosmological constant[edit]
The "cosmological constant" is a constant term that can be added to
Diagram representing the accelerated expansion of the universe due to dark energy. High-precision measurements of the expansion of the universe are
required to understand how the expansion rate changes over time and
space. 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). Measuring the equation of state for dark energy is one of the
biggest efforts in observational cosmology today. Adding the
cosmological constant to cosmology's standard FLRW metric leads to the
Lambda-CDM model, which has been referred to as the "standard model of
cosmology" because of its precise agreement with observations.
As of 2013, the
Nature timeline view • discuss • edit -13 — – -12 — – -11 — – -10 — – -9 — – -8 — – -7 — – -6 — – -5 — – -4 — – -3 — – -2 — – -1 — – 0 — cosmic expansion Earliest light cosmic speed-up Solar System water Single-celled life photosynthesis Multicellular life Land life Earliest gravity Dark energy Dark matter ← Earliest universe (−13.80) ← Earliest stars ← Earliest galaxy ← Earliest quasar/sbh ←
←
←
←
← Earliest Earth (−4.54) ← Earliest life ← Earliest oxygen ← Atmospheric oxygen ← Earliest sexual reproduction ← Cambrian explosion ← Earliest humans L i f e P r i m o r d i a l Axis scale: billion years
Also see:
The nature of dark energy is more hypothetical than that of dark
matter, and many things about the nature of dark energy remain matters
of speculation.[19]
Distance measurements and their relation to redshift, which suggest the universe has expanded more in the last half of its life.[22] The theoretical need for a type of additional energy that is not matter or dark matter to form the observationally flat universe (absence of any detectable global curvature). It can be inferred from measures of large scale wave-patterns of mass density in the universe. Supernovae[edit] A
In 1998, the High-Z
Estimated division of total energy in the universe into matter, dark
matter and dark energy based on five years of
The existence of dark energy, in whatever form, is needed to reconcile
the measured geometry of space with the total amount of matter in the
universe. Measurements of cosmic microwave background (CMB)
anisotropies indicate that the universe is close to flat. For the
shape of the universe to be flat, the mass/energy density of the
universe must be equal to the critical density. The total amount of
matter in the universe (including baryons and dark matter), as
measured from the CMB spectrum, accounts for only about 30% of the
critical density. This implies the existence of an additional form of
energy to account for the remaining 70%.[26] The Wilkinson Microwave
H ( z ) = − 1 1 + z d z d t ≈ − 1 1 + z Δ z Δ t . displaystyle H(z)=- frac 1 1+z frac dz dt approx - frac 1 1+z frac Delta z Delta t . The reliance on a differential quantity, Δz/Δt, can minimize many common issues and systematic effects; and as a direct measurement of the Hubble parameter instead of its integral, like supernovae and baryon acoustic oscillations (BAO), it brings more information and is appealing in computation. For these reasons, it has been widely used to examine the accelerated cosmic expansion and study properties of dark energy. Theories of dark energy[edit] Dark energy's status as a hypothetical force with unknown properties makes it a very active target of research. The problem is attacked from a great variety of angles, such as modifying the prevailing theory of gravity (general relativity), attempting to pin down the properties of dark energy, and finding alternative ways to explain the observational data. The equation of state of Dark
Cosmological constant[edit]
Main article: Cosmological constant
Further information:
Estimated distribution of matter and energy in the universe[42] The simplest explanation for dark energy is that it is an intrinsic,
fundamental energy of space. This is the cosmological constant,
usually represented by the Greek letter Λ (Lambda, hence Lambda-CDM
model). Since energy and mass are related according to the equation E
= mc2, Einstein's theory of general relativity predicts that this
energy will have a gravitational effect. It is sometimes called a
vacuum energy because it is the energy density of empty vacuum.
The cosmological constant has negative pressure equal to its energy
density and so causes the expansion of the universe to accelerate. The
reason a cosmological constant has negative pressure can be seen from
classical thermodynamics. In general, energy must be lost from inside
a container (the container must do work on its environment) in order
for the volume to increase. Specifically, a change in volume dV
requires work done equal to a change of energy −P dV, where P
is the pressure. But the amount of energy in a container full of
vacuum actually increases when the volume increases, because the
energy is equal to ρV, where ρ is the energy density of the
cosmological constant. Therefore, P is negative and, in fact,
P = −ρ.
There are two major advantages for the cosmological constant. The
first is that it is simple. Einstein had in fact introduced this term
in his original formulation of general relativity such as to get a
static universe. Although he later discarded the term after Hubble
found that the universe is expanding, a nonzero cosmological constant
can act as dark energy, without otherwise changing the Einstein field
equations. The other advantage is that there is a natural explanation
for its origin. Most quantum field theories predict vacuum
fluctuations that would give the vacuum this sort of energy. This is
related to the Casimir effect, in which there is a small suction into
regions where virtual particles are geometrically inhibited from
forming (e.g. between plates with tiny separation).
A major outstanding problem is that the same quantum field theories
predict a huge cosmological constant, more than 100 orders of
magnitude too large.[11] This would need to be almost, but not
exactly, cancelled by an equally large term of the opposite sign. Some
supersymmetric theories require a cosmological constant that is
exactly zero,[43] which does not help because supersymmetry must be
broken.
Nonetheless, the cosmological constant is the most economical solution
to the problem of cosmic acceleration. Thus, the current standard
model of cosmology, the Lambda-CDM model, includes the cosmological
constant as an essential feature.
Modified gravity[edit]
The evidence for dark energy is heavily dependent on the theory of
general relativity. Therefore, it is conceivable that a modification
to general relativity also eliminates the need for dark energy. There
are very many such theories, and research is ongoing.[44][45] The
measurement of the speed of gravity with the gravitational wave event
Conformal gravity
Dark
Notes[edit] ^ [72] Frieman, Turner & Huterer (2008) p. 6: "The
References[edit] ^ Overbye, Dennis (20 February 2017). "Cosmos Controversy: The
External links[edit] Dark
v t e Dark matter Forms of dark matter Baryonic dark matter
Cold dark matter
Hot dark matter
Light dark matter
Mixed dark matter
Warm dark matter
Self-interacting dark matter
Hypothetical particles Axino Axion Dark photon Holeum LSP Minicharged particle Neutralino Sterile neutrino SIMP WIMP Theories and objects Cuspy halo problem
Dark fluid
Dark galaxy
Dark globular cluster
Search experiments Direct detection ANAIS
ArDM
ADMX
CDEX (1, 10, 1T)
CDMS (I, II, SuperCDMS, GEODM)
CoGeNT
COSINE (100)
COUPP (4, 3600)
CRESST (I, II)
Indirect detection AMS-02 ATIC CALET CAST DAMPE IceCube MOA OGLE PAMELA Other projects MultiDark PVLAS SNOLAB Potential dark galaxies HE0450-2958 HVC 127-41-330 Smith's Cloud VIRGOHI21 Related Dark energy
Exotic matter
v t e Breakthrough of the Year Science journal 1996:
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