Everything about Dark Energy totally explained
In
physical cosmology,
dark energy is a hypothetical form of
energy that permeates all of space and tends to increase the
rate of expansion of the
universe. Assuming the existence of 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 almost three-quarters 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 or
moduli,
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
Lambda-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.
Negative pressure
Dark energy causes expansion because it has strong
negative pressure.
A substance has positive pressure when it pushes outward on its surroundings. This is the usual situation for fluids. Negative pressure (tension) exists when the substance instead pulls on its surroundings. A common example of negative pressure occurs when a solid is stretched to support a hanging weight.
According to the
Friedmann-Lemaître-Robertson-Walker metric, which is an application of
General Relativity to cosmology, the pressure within a substance contributes to its gravitational attraction for other things just as its mass density does. Negative pressure causes a gravitational repulsion.
The gravitational repulsive effect of dark energy's negative pressure is greater than the gravitational attraction caused by the energy itself (for example
Mass-energy equivalence). At the cosmological scale, it also overwhelms all other forms of gravitational attraction, resulting in the accelerating expansion of the universe.
The solution to the apparent contradiction of "pushing causing attraction" and "pulling causing repulsion" is given by:
- The pushing of positive pressure (pulling of negative pressure) is a non-gravitational force which just moves substances around within space without changing space itself. However, the gravitational attraction (repulsion) it causes operates on space itself, decreasing (increasing) the amount of space between things. This determines the size of the universe.
- It isn't necessary for these two effects to act in the same direction. In fact, they act in opposite directions (similar to Newton's Third Law in classical mechanics).
Evidence for dark energy
In 1998, observations of
type Ia supernovae ("one-A") by the
Supernova Cosmology Project at the
Lawrence Berkeley National Laboratory and the
High-z Supernova Search Team 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 structure of the cosmos as well as improved measurements of supernovae have been consistent with the
Lambda-CDM model.
The type Ia supernovae provide the most direct evidence for dark energy. Measuring the scale factor at the time that light was emitted from an object is accomplished easily by measuring the
redshift of the receding object. Finding the distance to an object is a more difficult problem, however. It is necessary to find
standard candles: objects for which the actual brightness, what astronomers call the
absolute magnitude, is known, so that it's possible to relate the observed brightness, or
apparent magnitude, to the distance. Without standard candles, it's impossible to measure the redshift-distance relation of
Hubble's law. Type Ia supernovae are the best known standard candles for cosmological observation because they're very bright and thus visible across billions of light years. The consistency in absolute magnitude for type Ia supernovae is explained by the favored model of an old
white dwarf star which gains mass from a companion star and grows until it reaches the precisely defined
Chandrasekhar limit. At this mass, the
white dwarf is unstable to thermonuclear runaway and explodes as a type Ia supernova with a characteristic brightness. The observed brightness of the supernovae are plotted against their redshifts, and this is used to measure the expansion history of the universe. These observations indicate that the expansion of the universe isn't
decelerating, which would be expected for a matter-dominated universe, but rather is mysteriously accelerating. These observations are explained by postulating a kind of energy with negative pressure (see
equation of state (cosmology) for a mathematical explanation):
dark energy.
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%. 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, which is appearance of solutions with regions of discontinuities (see
classification of discontinuities for three examples) at low matter density. The 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.
Quintessence
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.
Quintessence differs from the cosmological constant in that it can vary in space and time. In order for it not to clump and form
structure like matter, the field must be very light so that it has a large
Compton wavelength.
No evidence of quintessence is yet available, but it hasn't been ruled out either. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. 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.
Scalar fields are predicted by the
standard model and
string theory, but an analogous problem to the cosmological constant problem (or the problem of constructing models of
cosmic inflation) occurs:
renormalization theory predicts that scalar fields should acquire large masses.
The
cosmic coincidence problem asks why the
cosmic acceleration began when it did. If
cosmic acceleration began earlier in the universe, structures such as
galaxies would never have had time to form and life, at least as we know it, would never have had a chance to exist. Proponents of the
anthropic principle view this as support for their arguments. However, many models of quintessence have a so-called
tracker behavior, which solves this problem. 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. This naturally sets the low
energy scale of the dark energy.
Some special cases of quintessence are
phantom energy, in which the energy density of quintessence actually increases with time, and k-essence (short for kinetic quintessence) which has a non-standard form of
kinetic energy. They can have unusual properties: phantom energy, for example, can cause a
Big Rip.
Alternative ideas
Some theorists think that dark energy and
cosmic acceleration are a failure of
general relativity on very large scales, larger than
superclusters. It is a tremendous extrapolation to think that our law of gravity, which works so well in the
solar system, should work without correction on the scale of the universe. Most attempts at modifying general relativity, however, have turned out to be either equivalent to theories of
quintessence, or inconsistent with observations. It is of interest to note that if the equation for gravity were to approach r instead of r^2 at large, intergalactic distances, then the acceleration of the expansion of the universe becomes a mathematical artifact, negating the need for the existence of Dark Energy.
Alternative ideas for dark energy have come from
string theory,
brane cosmology and the
holographic principle, but have not yet proved as compelling as quintessence and the cosmological constant. On string theory, an article in the journal
Nature described:
String theories, popular with many particle physicists, make it possible, even desirable, to think that the observable universe is just one of 10500 universes in a grander multiverse, says [LeonardSusskind, a cosmologist at Stanford University in California]. The vacuum energy will have different values in different universes, and in many or most it might indeed be vast. But it must be small in ours because it's only in such a universe that observers such as ourselves can evolve.
Yet another, "radically conservative" class of proposals aims to explain the observational data by a more refined use of established theories rather than through the introduction of dark energy, focusing, for example, on the gravitational effects of density inhomogeneities or on consequences of electroweak symmetry breaking in the early universe.
Implications for the fate of the universe
Cosmologists estimate that the acceleration began roughly 5 billion years ago. Before that, it's 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: that'll no longer be visible, because their line-of-sight velocity becomes greater than the speed of light. This isn't a violation of special relativity, and the effect can't be used to send a signal between them. (Actually there's 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 and 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're not ruled out. Measurements of acceleration are crucial to determining the ultimate fate of the universe in big bang theory.
History
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 curiosity.
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 Turner in 1998. By that time, the missing mass problem of big bang nucleosynthesis and large scale structure was 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, in Riess et al 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 acceleration.
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