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In cosmology, dark energy is a hypothetical form of energy which permeates all of space and has negative pressure resulting in an effective "repulsive gravitational force". Dark energy may account for the accelerating universe as well as a significant portion of the mass in the universe. Two proposed forms of dark energy are the cosmological constant and quintessence, where the former is static and the latter is dynamic. Distinguishing between the two requires high precision measurements of the expansion of the universe to see how the speed of the expansion changes over time. Making such measurements is a topic of current research.

Proposal

The cosmological constant was first proposed by Albert Einstein as a mechanism to balance gravitation and lead to a static universe. However, this mechanism had two problems. The first is that it would not lead to a stable static universe. If a static universe expanded slightly, it would cause a release of energy which would produce more expansion. Conversely if the universe contracted slightly, it would absorb energy leading to further contraction. The second problem were observations by Hubble that seemed to indicate that the universe was expanding and not static. For a long time, the cosmological constant was largely ignored as an historical curiosity. However in the 1970's Alan Guth proposed that the cosmological constant could drive cosmic inflation in the early universe. Still the cosmological constant was believed to be irrelevant to the current universe until the late 1990's, when due to satellites and the golden age of telescopes, high precision measurements were made of distant supernovae and the cosmic microwave background, which are most easily explained if some form of dark energy does exist.

Nature of phenomena

Because of its repulsive nature, dark energy tends to cause the expansion of the universe to accelerate, rather than slow down as would be expected in a purely matter dominated universe. An accelerating universe is exactly what was observed by looking at the most distant supernova.

Another argument comes from studies of the total energy density of the universe. It has long been known from theoretical and observational arguments that the total energy density of the universe is very near the critical density needed to make the universe "flat" (i.e. the curvature of space-time, defined in general relativity, goes to zero on large scales. See: shape of the universe). Since energy is equivalent to mass (special relativity: E = mc), this is usually expressed in terms of a critical mass density needed to make the universe flat. Observations of the luminous matter only account for 2-5% of the necessary mass density. Dark matter, i.e. matter which doesn't emit enough light to be seen, has long been hypothesized to make up this missing mass, but observations of galaxies and clusters made during the 1990s, strongly argued that dark matter couldn't account for more than ~25% of the critical mass density. Remarkably, the supernova observations predict that dark energy should make up ~70% of the critical energy density, thus when added to the mass-energy of matter, the total energy density comes out exactly as needed to make the universe "flat".

Speculation

The exact nature of this dark energy is largely a matter of speculation. Some believe that dark energy might be "vacuum energy", represented by the "cosmological constant" (λ) in general relativity. The simplest explanation is to posit a "cosmological constant", meaning a constant uniform density of dark energy throughout all of space that is independent of time or the universe's expansion. This is the form of dark energy introduced by Einstein, and is consistent with our limited observations to date. If dark energy takes this form, it suggests that it is a fundamental property of the universe. Alternatively, dark energy might arise out of some type of particle, referred to as quintessence. Some theories suggest such particles could have been created during the big bang in sufficient abundances to permeate all of space. However, if this were the case they might be expected to clump and vary in density as a function of time. No evidence of this is yet available, but neither can it be ruled out.

Inflation

It should also be noted that some form of dark energy is the most likely explanation of cosmic inflation during the big bang. Such inflation is an essential feature of most current theories of cosmology and structure formation. It is unclear whether the dark energy present today is related to the dark energy that could have caused inflation.

February 2004 Hubble Space Telescope evidence

According to a group led by Adam Riess at the Space Telescope Science Institute, Baltimore, the universe's dark energy probably won't destroy the universe any sooner than about 30 billion years from now.

They used the Hubble Space Telescope to find very distant supernovae that exploded when the universe was less than half its current age. The apparent brightness of a certain type of supernova gives cosmologists a way to measure the expansion rate of the universe at different times in the past.

Cosmologists understand almost nothing about dark energy even though it appears to comprise about 70 percent of the universe. They are seeking to uncover its two most fundamental properties: its strength and its permanence.

In a paper to be published in the Astrophysical Journal, Riess and his collaborators have made the first meaningful measurement of the second property, its permanence.

Currently, there are two leading interpretations for the dark energy as well as many more exotic possibilities. It could be an energy percolating from empty space as Einstein's theorized "cosmological constant," an interpretation which predicts that dark energy is unchanging and of a prescribed strength.

An alternative possibility is that dark energy is associated with a changing energy field called "quintessence."

This field would be causing the current acceleration — a milder version of the inflationary episode from which the early universe emerged.

When astronomers first realized the universe was accelerating, the conventional wisdom was that it would expand forever. However, until we better understand the nature of dark energy—its properties—other scenarios for the fate of the universe are possible.

If the repulsion from dark energy is or becomes stronger than Einstein's prediction, the universe may be torn apart by a future "Big Rip," during which the universe expands so violently that first the galaxies, then the stars, then planets, and finally atoms come unglued in a catastrophic end of time. Currently this idea is very speculative, but being pursued by theorists.

At the other extreme, a variable dark energy might fade away and then flip in force such that it pulls the universe together rather than pushing it apart.

This would lead to a "big crunch" where the universe ultimately implodes. According to Riess, this appears to be the least-likely scenario.

External links

• HubbleSite press release: New Clues About the Nature of Dark Energy: Einstein May Have Been Right After All