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.

Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate.

Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe.

The mass-energy of dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount.

The density of dark energy is very low (~ 7 × 10−30 g/cm3) much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the mass-energy of the universe because it is uniform across space.

Two proposed forms for dark energy are the cosmological constant, representing 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 can be formulated to be equivalent to the zero-point radiation of space i.e. the vacuum energy.

Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.

Einstein’s cosmological constant

The “cosmological constant” is a constant term that can be added to Einstein’s field equation of general relativity.

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“.

The cosmological constant was first proposed by Einstein as a mechanism to obtain a solution of the gravitational field equation that would lead to a static universe, effectively using dark energy to balance gravity.

Einstein gave the cosmological constant the symbol Λ (capital lambda). Einstein stated that the cosmological constant required that ’empty space takes the role of gravitating negative masses which are distributed all over the interstellar space’.

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.

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.

Further, observations made by Edwin Hubble in 1929 showed that the universe appears to be expanding and not static at all. Einstein reportedly referred to his failure to predict the idea of a dynamic universe, in contrast to a static universe, as his greatest blunder.

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.

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.

Nearly all inflation models predict that the total (matter+energy) density of the universe should be very close to the critical density.

During the 1980s, most cosmological research focused on models with critical density in the matter only, usually 95% cold dark matter and 5% ordinary matter.

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 value for the Hubble constant lower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering.

These difficulties became stronger after the discovery of anisotropy in the cosmic microwave background by the COBE spacecraft in 1992, and several modified CDM models came under active study through the mid-1990s: these included the Lambda-CDM model and a mixed cold/hot dark matter model.

The first direct evidence for dark energy came from supernova observations in 1998 of accelerated expansion in Riess et al. and in Perlmutter et al., and the Lambda-CDM model then became the leading model.

Soon after, dark energy was supported by independent observations: in 2000, the BOOMERanG and Maxima cosmic microwave background experiments observed the first acoustic peak in the CMB, showing that the total (matter+energy) density is close to 100% of the critical density.

Then in 2001, the 2dF Galaxy Redshift Survey gave strong evidence that the matter density is around 30% of critical. The large difference between these two supports a smooth component of dark energy making up the difference.

Much more precise measurements from WMAP in 2003–2010 have continued to support the standard model and give more accurate measurements of the key parameters.

The term “dark energy“, echoing Fritz Zwicky’s “dark matter” from the 1930s, was coined by Michael Turner in 1998.

Nature

The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation.

Dark energy is thought to be very homogeneous and not very dense and is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied and un-massive — roughly 10−27 kg/m3 — it is unlikely to be detectable in laboratory experiments.

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 empty space.

Independently of its actual nature, dark energy would need to have a strong negative pressure (repulsive action), like radiation pressure in a metamaterial, to explain the observed acceleration of the expansion of the universe.

According to general relativity, the pressure within a substance contributes to its gravitational attraction for other objects just as its mass density does.

This happens because the physical quantity that causes matter to generate gravitational effects is the stress-energy tensor, which contains both the energy (or matter) density of a substance and its pressure and viscosity.

In the Friedmann–Lemaître–Robertson–Walker metric, it can be shown that a strong constant negative pressure in all the universe causes an acceleration in the expansion if the universe is already expanding, or deceleration in contraction if the universe is already contracting. This accelerating expansion effect is sometimes labeled “gravitational repulsion“.

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 last half of its life.
  • The theoretical need for a type of additional energy that does not matter or dark matter to form the observationally flat universe (absence of any detectable global curvature).
  • Measures of large-scale wave-patterns of mass density in the universe.

Implications for the fate 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 matter.

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).

Projections into the future can differ radically for different models of dark energy. 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.

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.

Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.

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” towards us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Uses of the proper distance).

Assuming the dark energy is constant (a cosmological constant), the current distance to this cosmological event horizon is about 16 billion light-years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event were less than 16 billion light years away, but the signal would never reach us if the event were more than 16 billion light-years away.

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.

Planet Earth, the Milky Way, and the Local Group of which the Milky Way is a part would all remain virtually undisturbed as the rest of the universe recedes and disappears from view.

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.

There are other, more speculative ideas about the future of 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.

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“, 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. While none of these are supported by observations, they are not ruled out.

In the philosophy of science

In the philosophy of science, dark energy is an example of an “auxiliary hypothesis“, an ad hoc postulate that is added to a theory in response to observations that falsify it.

It has been argued that the dark energy hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper.

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*This article was originally published at en.wikipedia.org.