Due to the toy model nature of concordance cosmology, some experts believe that a more accurate general relativistic treatment of the structures on all scales in the
real universe may do away with the need to invoke dark energy.
Independently of its actual nature, dark energy would need to have a strong negative pressure to explain the observed acceleration of the expansion of the universe.
Before these observations, scientists thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time.
Evidence of existence The evidence for dark energy is indirect but comes from three independent sources: • Distance measurements and their relation to redshift, which suggest
the universe has expanded more in the latter half of its life.
 Theories of dark energy Dark energy’s status as a hypothetical force with unknown properties makes it a very active target of research.
 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.
Inhomogeneous cosmologies, which attempt to account for the back-reaction of structure formation on the metric, generally do not acknowledge any dark energy contribution to
the universe’s energy density.
For these reasons, this method has been widely used to examine the accelerated cosmic expansion and study properties of dark energy.
 • 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).
 Assuming that the lambda-CDM model of cosmology is correct, as of 2013, the best current measurements indicate that dark energy contributes 68% of the total energy
in the present-day observable universe.
 Variable dark energy models The density of dark energy might have varied in time during the history of the universe.
These models were found to be successful at forming realistic galaxies and clusters, but some problems appeared in the late 1980s: in particular, the model required a value
for the Hubble constant lower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering.
If considered as a “source term” in the field equation, it can be viewed as equivalent to the mass of empty space (which conceptually could be either positive or negative),
or “vacuum energy”.
 However, because of the accelerating expansion, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they
emit past that point will never be able to reach us at any time in the infinite future because the light never reaches a point where its “peculiar velocity” toward us exceeds the expansion velocity away from us (these two notions of velocity
are also discussed in Uses of the proper distance).
However, the cosmological constant problem asserts that there is a huge disagreement between the observed values of vacuum energy density and the theoretical large value of
zero-point energy obtained by quantum field theory.
 In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological
constant boundary (w = −1) from above to below.
The cosmological constant was first proposed by Einstein as a mechanism to obtain a solution to the gravitational field equation that would lead to a static universe, effectively
using dark energy to balance gravity.
First results from the SNLS reveal that the average behavior (i.e., equation of state) of dark energy behaves like Einstein’s cosmological constant to a precision of 10%.
Technical definition See also: Friedmann equations In standard cosmology, there are three components of the universe: matter, radiation, and dark energy.
 Yet other possibilities are that the accelerated expansion of the universe is an illusion caused by the relative motion of us to the rest of the universe,
or that the statistical methods employed were flawed.
In this scenario, the Local Group would ultimately suffer heat death, just as was hypothesized for the flat, matter-dominated universe before measurements of cosmic acceleration.
For a cosmological constant, or any other model that predicts that the acceleration will continue indefinitely, the ultimate result will be that galaxies outside the Local
Group will have a line-of-sight velocity that continually increases with time, eventually far exceeding the speed of light.
Measuring the equation of state for dark energy is one of the biggest efforts in observational cosmology today.
 The mechanism was an example of fine-tuning, and it was later realized that Einstein’s static universe would not be stable: local inhomogeneities would ultimately
lead to either the runaway expansion or contraction of the universe.
Using baryon acoustic oscillations, it is possible to investigate the effect of dark energy in the history of the Universe, and constrain parameters of the equation of state
of dark energy.
 A different approach uses a cosmological extension of the equivalence principle to show how space might appear to be expanding more rapidly in the voids surrounding
our local cluster.
Other mechanism driving acceleration Modified gravity See also: Massive gravity The evidence for dark energy is heavily dependent on the theory of general relativity.
Soon after, dark energy was supported by independent observations: in 2000, the BOOMERanG and Maxima CMB experiments observed the first acoustic peak in the CMB, showing that
the total (matter+energy) density is close to 100% of critical density.
In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until matter–radiation equality, which triggers quintessence
to start behaving as dark energy, eventually dominating the universe.
The phantom energy model of dark energy results in divergent expansion, which would imply that the effective force of dark energy continues growing until it dominates all
other forces in the universe.
 This is not a violation of special relativity because the notion of “velocity” used here is different from that of velocity in a local inertial frame of reference, which
is still constrained to be less than the speed of light for any massive object (see Uses of the proper distance for a discussion of the subtleties of defining any notion of relative velocity in cosmology).
However, inflation must have occurred at a much higher energy density than the dark energy we observe today and is thought to have completely ended when the universe was just
a fraction of a second old.
High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time and space.
 Interacting dark energy This class of theories attempts to come up with an all-encompassing theory of both dark matter and dark energy as a single phenomenon that
modifies the laws of gravity at various scales.
The vacuum energy, that is, the particle-antiparticle pairs generated and mutually annihilated within a time frame in accord with Heisenberg’s uncertainty principle in the
energy-time formulation, has been often invoked as the main contribution to dark energy.
Without introducing a new form of energy, there was no way to explain how scientists could measure an accelerating universe.
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.
 However, scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be prolonged.
 Recent results from the Hubble Space Telescope Higher-Z Team indicate that dark energy has been present for at least 9 billion years and during the period preceding cosmic
 Cosmic microwave background Estimated division of total energy in the universe into matter, dark matter and dark energy based on five years of WMAP data.
 Quintessence Main article: Quintessence (physics) In quintessence models of dark energy, the observed acceleration of the scale factor is caused by the potential
energy of a dynamical field, referred to as quintessence field.
 Astrophysicist Ethan Siegel states that, while such alternatives gain a lot of mainstream press coverage, almost all professional astrophysicists are confident
that dark energy exists, and that none of the competing theories successfully explain observations to the same level of precision as standard dark energy.
The first observational evidence for its existence came from measurements of supernovas, which showed that the universe does not expand at a constant rate; rather, the universe’s
expansion is accelerating.
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).
 Inflationary dark energy Alan Guth and Alexei Starobinsky proposed in 1980 that a negative pressure field, similar in concept to dark energy, could drive cosmic
inflation in the very early universe.
 Work done in 2013 based on the Planck spacecraft observations of the CMB gave a more accurate estimate of 68.3% dark energy, 26.8% dark matter, and 4.9% ordinary matter.
 This conjecture would not rule out other models of dark energy, such as quintessence, that could be compatible with string theory.
 Observational Hubble constant data A new approach to test evidence of dark energy through observational Hubble constant data (OHD), also known as cosmic chronometers,
has gained significant attention in recent years.
 Change in expansion over time Diagram representing the accelerated expansion of the universe due to dark energy.
 The measurement of the speed of gravity in the first gravitational wave measured by non-gravitational means (GW170817) ruled out many modified gravity theories as
explanations to dark energy.
 A: CPL Model, B: Jassal Model, C: Barboza & Alcaniz Model, D: Wetterich Model Cosmological constant Main article: Cosmological constant Further information: Equation
of state (cosmology) Estimated distribution of matter and energy in the universe The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space.
They allow researchers to measure the expansion history of the universe by looking at the relationship between the distance to an object and its redshift, which gives how
fast it is receding from us.
 The final component is dark energy: it is an intrinsic property of space and has a constant energy density, regardless of the dimensions of the volume under consideration
(ρ ∝ a0).
Such uncertainties leave open the possibility of gravity eventually prevailing and lead to a universe that contracts in on itself in a “Big Crunch”, or that there may
even be a dark energy cycle, which implies a cyclic model of the universe in which every iteration (Big Bang then eventually a Big Crunch) takes about a trillion (1012) years.
During the 1980s, most cosmological research focused on models with critical density in matter only, usually 95% cold dark matter (CDM) and 5% ordinary matter (baryons).
 A recent proposal speculates that the currently unexplained excess observed in the XENON1T detector in Italy may have been caused by a chameleon model of dark energy.
 As galaxies approach the point of crossing this cosmological event horizon, the light from them will become more and more redshifted, to the point where the wavelength
becomes too large to detect in practice and the galaxies appear to vanish completely (see Future of an expanding universe).
This so-called late-time Integrated Sachs–Wolfe effect (ISW) is a direct signal of dark energy in a flat universe.
The reason dark energy can have such a profound effect on the universe, making up 68% of universal density in spite of being so dilute, is that it uniformly fills otherwise
 Some people argue that the only indications for the existence of dark energy are observations of distance measurements and their associated redshifts.
However, it dominates the universe’s mass–energy content because it is uniform across space.
In this scenario, dark energy doesn’t actually exist, and is merely a measurement artifact.
As more space comes into existence, more of this energy-of-space would appear.
 Dark energy could in principle interact not only with the rest of the dark sector, but also with ordinary matter.
For the shape of the universe to be flat, the mass–energy density of the universe must be equal to the critical density.
Some scientists think that the best evidence for quintessence would come from violations of Einstein’s equivalence principle and variation of the fundamental constants in
space or time.
For example, if we are located in an emptier-than-average region of space, the observed cosmic expansion rate could be mistaken for a variation in time, or acceleration.
In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales.
Modern observational data allows us to estimate the present density of dark energy.
[‘Taken from Frieman, Turner, & Huterer (2008):: 6, 44
“The Universe has gone through three distinct eras:
Radiation-dominated, z ≳ 3000 ;
Matter-dominated, 3000 ≳ z ≳ 0.5 ; and
Dark-energy-dominated, 0.5 ≳ z .
The evolution of
the scale factor is controlled by the dominant energy form:
(for constant w ). During the radiation-dominated era,
during the matter-dominated era,
and for the dark energy-dominated era, assuming w ≃ −1 asymptotically
together, all the current data provide strong evidence for the existence of dark energy; they constrain the fraction of critical density contributed by dark energy, 0.76 ± 0.02 , and the equation-of-state parameter:
w ≈ −1 ± 0.1 [stat.] ± 0.1 [sys.]
assuming that w is constant. This implies that the Universe began accelerating at redshift z ~ 0.4 and age t ~ 10 Ga . These results are robust – data from any one method can be removed without compromising the constraints – and they are not substantially
weakened by dropping the assumption of spatial flatness.”: 44
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Photo credit: https://www.flickr.com/photos/chorip/495094547/’]