In physical cosmology and
astronomy dark energy is a hypothetical form of energy that permeates all of space and tends to increase
the rate of expansion of the universe. Dark energy
is the most popular way to explain recent observations that the universe appears
to be expanding at an accelerating rate. In the standard model of cosmology, dark energy
currently accounts for 74% of the total mass energy of the universe.
Two proposed forms for dark energy are the cosmological
constant, a constant
energy density filling space homogeneously, and scalar fields such as quintessence
dynamic quantities whose energy density can vary in time and space.
Contributions from scalar fields that are constant in space
are usually also included in the cosmological constant. The cosmological constant
is physically equivalent to vacuum energy. Scalar fields which do change in
space can be difficult to distinguish from a cosmological constant because the
change may be extremely slow.
High-precision measurements of the expansion
of the universe are
required to understand how the expansion rate changes over time. In general
relativity, the evolution of the expansion rate is parameterized by the
cosmological equation of state. Measuring the
equation of state of 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 Lambatta - CDM model,
which has been referred to as the "standard model" of cosmology because of its
precise agreement with observations. Dark energy has been used as a crucial
ingredient in a recent attempt to
formulate a cyclic model for the universe.
is the evidence underpinning the existence of Dark Energy?
In 1998, published observations of Type
la supernovae("one-A") by the High-z supernova
search team followed in
1999 by the Supernva cosmology project suggested
that the expansion of the universe is accelerating. Since then, these
observations have been corroborated by several independent sources. Measurements
of the cosmic microwave background, gravitational lensing and the large scale
the cosmos as well as improved measurements of supernovae have been consistent
with the Lambda-CDM model.
Supernovae are useful for cosmology because they are excellent standard
candles across cosmological distances. They allow the expansion history of
the Universe to be measured by looking at the relationship between the distance
to an object and its redshift, which
gives how fast it is receding from us. The relationship is roughly linear,
according to Hubbles law. It is relatively easy to measure redshift, but finding the distance to
an object is more difficult. Usually, astronomers use standard candles:
objects for which the intrinsic brightness, the absolute magnitude, is known. This allows
the object's distance to be measured from its actually observed brightness, or
apparent magnitude. Type Ia supernovae are the best-known standard candles across
cosmological distances because of their extreme, and extremely consistent,
Cosmic Microwave Background
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, most recently by the WMAP satellite, indicate
that the universe is very close to flat. For the shape of the universe to be flat, the mass/energy density of the universe
must be equal to a certain critical density. The total amount of matter
in the universe (including baryons and dark matter), as measured by the CMB, 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%. The most recent
WMAP observations are consistent with a universe made up of 74% dark energy, 22%
dark matter, and 4% ordinary matter.
The exact nature of this dark energy is a matter of speculation. It is known
to be very homoheneous, not very dense and is not known to interact
through any of the fundamental forces other than gravity. Since it is
not very dense—roughly 10−29 grams per cubic centimeter—it is hard to
imagine experiments to detect it in the laboratory. Dark energy can only have
such a profound impact on the universe, making up 74% of all energy, because it
uniformly fills otherwise empty space. The two leading models are quintessence and the cosmological
constant. Both models include the common characteristic that dark energy must
have negative pressure.
The simplest explanation for dark energy is that it is simply the "cost of
having space": that is, a volume of space has some intrinsic, fundamental
energy. This is the cosmological constant, sometimes called Lambda (hence Lambda
after the Greek letter Λ, the symbol used to mathematically represent this
quantity. Since energy and mass are related by E =
mc2, Einstein's theory of general
relativity predicts that it will have a gravitational effect. It is
sometimes called a vacuum energy because it is the energy density of empty vacuum. In fact, most theories of particle
physics 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). The
cosmological constant is estimated by cosmologists to be on the order of
10−29g/cm³, or about 10−120 in reduced
Planck units. However, particle physics predicts a natural value of 1 in reduced
Planck units, a large discrepancy which is still lacking in explanation.
The cosmological constant has negative pressure equal to its energy density
and so causes the expansion of the universe to accelerate. The reason why a
cosmological constant has negative pressure can be seen from classical
thermodynamics; Energy must be lost from inside a container to do work on the
container. 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 box of vacuum energy actually increases when the volume increases (dV
is positive), because the energy is equal to ρV, where ρ (rho) is
the energy density of the cosmological constant. Therefore, p is negative
and, in fact, p = −ρ.
A major outstanding problem is that most quantam
field theories predict a huge cosmological constant from the energy of the quantum
vacuum, more than 100 orders of magnitude too large. This would
need to be cancelled almost, but not exactly, by an equally large term of the
opposite sign. Some supersymmetric theories require a cosmological
constant that is exactly zero, which does not help. The present scientific
consensus amounts to extrapolating the empirical evidence where it is relevant to
predictions, and fine tuning
theories until a more elegant solution is found. Philosophically, our most
elegant solution may be to say that if things were different, we would not be
here to observe anything — the anthropic principle. Technically, this amounts to checking theories against macroscopic observations.
Unfortunately, as the known error-margin in the constant predicts the fate of
the universe more than its present
state, many such "deeper" questions remain unknown.
Another problem arises with inclusion of the cosmic constant in the standard
model: i.e., the appearance of solutions with regions of discontinuities at low matter density.
Discontinuity also affects the past sign of the pressure assigned to the cosmic
constant, changing from the current negative pressure to attractive, as one
looks back towards the early Universe. A systematic, model-independent
evaluation of the supernovae data supporting inclusion of the cosmic constant in
the standard model indicates these data suffer systematic error. The supernovae
data are not overwhelming evidence for an accelerating Universe expansion which
may be simply gliding. A
numerical evaluation of WMAP and supernovae data for evidence that our local
group exists in a local void with poor matter density compared to other
locations, uncovered possible conflict in the analysis used to support the
cosmic constant. These
findings should be considered shortcomings of the standard model, but only when
a term for vacuum energy is included.
In spite of its problems, the cosmological constant is in many respects the
most economical solution to the problem of cosmic acceleration. One number
successfully explains a multitude of observations. Thus, the current standard
model of cosmology, the Lambda-CDM model, includes the cosmological constant as
an essential feature.
Future of the Universe
Cosmologists estimate that the acceleration began roughly 5 billion
years ago. Before that, it is thought that the expansion was decelerating, due
to the attractive influence of dark matter and baryons. The density of dark matter in an expanding
universe decreases more quickly than dark energy, and eventually the dark energy
dominates. Specifically, when the volume of the universe doubles, the density of
dark matter is halved but
the density of dark energy is nearly unchanged (it is exactly constant in the
case of a cosmological constant).
If the acceleration continues indefinitely, the ultimate result will be that
galaxies outside the local supercluster will move beyond the cosmic horizon: they will
no longer be visible, because their line off site velocity becomes greater than the
speed of light. This is not a
violation of special relativity, and the effect cannot be
used to send a signal between them. (Actually there is no way to even define
"relative speed" in a curved spacetime. Relative speed and velocity can only be
meaningfully defined in flat spacetime or in sufficiently small (infinitesimal)
regions of curved spacetime). Rather, it prevents any communication between them
as the objects pass out of contact. The earth, the milky way and the virgo supercluster, however, would remain virtually undisturbed while the rest of
the universe recedes. In this scenario, the local supercluster would ultimately
suffer heat death, just as was thought for
the flat, matter-dominated universe, before measurements of cosmic acceleration.
There are some very speculative ideas about the future of the universe. One
suggests that phantom energy causes divergent expansion, which would
imply that the effective force of dark energy continues growing until it
dominates all other forces in the universe. Under this scenario, dark energy
would ultimately tear apart all gravitationally bound structures, including
galaxies and solar systems, and eventually overcome the electrical and nuclear
forces to tear
apart atoms themselves, ending the universe in a "Big Rip". On the other hand, dark energy might
dissipate with time, or even become attractive. Such uncertainties leave open
the possibility that gravity might yet rule the day and lead to a universe that
contracts in on itself in a "Big Crunch". Some scenarios, such as the cyclic
model suggest this could be the case. While
these ideas are not supported by observations, they are not ruled out.
Measurements of acceleration are crucial to determining the ultimate fate of the
universe in big bang theory.
The cosmological constant was first proposed by Einstein as a mechanism to obtain a stable
solution of the gravitational field equation that
would lead to a static universe, effectively using dark energy to balance
gravity. Not only was the mechanism an inelegant example of fine tuning, it was soon realized that Einstein's
static universe would actually be unstable because local inhomogeneities would
ultimately lead to either the runaway expansion or contraction of the universe.
The equilibrium is unstable: if the universe
expands slightly, then the expansion releases vacuum energy, which causes yet
more expansion. Likewise, a universe which contracts slightly will continue
contracting. These sorts of disturbances are inevitable, due to the uneven
distribution of matter throughout the universe. More importantly, observations
made by Edwin Hubble showed that the universe appears to be expanding and not static at all. Einstein
famously referred to his failure to predict the idea of a dynamic universe, in
contrast to a static universe, as his greatest blunder. Following this
realization, the cosmological constant was largely ignored as a historical
Alan Guth proposed in the
1970s that a negative pressure field, similar in concept to dark energy, could
drive cosmic inflation in the very early universe. 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. Such expansion is an essential feature of
most current models of the Big Bang. 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.
It is unclear what relation, if any, exists between dark energy and inflation.
Even after inflationary models became accepted, the cosmological constant was
thought to be irrelevant to the current universe.
The term "dark energy" was coined by Michael
Turnerin 1998. By that time, the
missing mass problem of big bang nucleosynthesis and large scale strucuture
established, and some cosmologists had started to theorize that there was an
additional component to our universe. The first direct evidence for dark energy
came from supernova observations of accelerated expansion, and later
confirmed in Perlmutter et al..
resulted in the Lambda CDM mode, which as of 2006 is
consistent with a series of increasingly rigorous cosmological observations, the
latest being the 2005 Supernova Legacy Survey. 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 per cent.
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