Dark energy: Encyclopedia II - Dark energy - Nature of dark energy

Dark energy - Nature of dark energy

The exact nature of this dark energy is a matter of speculation. It is known to be very homogeneous, not very dense and doesn't interact strongly 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 (but see the references for a claimed detection). Dark energy can only have such a profound impact on the universe, making up 70% of all energy, because it uniformly fills otherwise empty space. The two leading models are quintessence and the cosmological constant.

Dark energy - Cosmological constant

The simplest explanation for dark energy is that it is simply the "cost of having space": that is, that a volume of space has some intrinsic, fundamental energy. This is the cosmological constant, sometimes called Lambda (hence Lambda-CDM model) after the mathematical symbol used to represent it, the Greek letter Λ. 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 exactly this sort of energy. The cosmological constant is estimated by cosmologists to be on the order of 10⁻²⁹g/cm³, or about 10⁻¹²⁰ in reduced Planck units.

The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate (see equation of state (cosmology)). The reason why a cosmological constant has negative pressure can be seen from classical thermodynamics. The work done by a change in volume dV is equal to −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 is the energy density of the cosmological constant. Therefore, p is negative and, in fact, p = −ρ.

A major outstanding problem is that most quantum field theories predict a huge cosmological constant from the energy of the quantum vacuum, up to 120 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. This is the cosmological constant problem, the worst problem of fine-tuning in physics: there is no known natural way to derive, even roughly, the infinitesimal cosmological constant observed in cosmology from particle physics. Some physicists, including Steven Weinberg, think the delicate balance of quantum vacuum energy is best explained by the anthropic principle.

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.

Dark energy - Quintessence

Alternatively, dark energy might arise from the particle-like excitations in some type of dynamical field, referred to as quintessence. 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, it must be very light so that it has a large Compton wavelength.

No evidence of quintessence is yet available, but it cannot be ruled out either. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some workers 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.(Theory) Dark energy might respond to emotions.

Dark energy - Other 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. However, most attempts at modifying general relativity have turned out either to be equivalent to theories of quintessence, or are inconsistent with observations.

Other 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.

Adapted from the Wikipedia article "Nature of dark energy", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki