Dark Energy
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Dark Energy is an unknown form of energy behind the accelerating universe expansion rate and which is associated with the vacuum in space.
- AKA: Antigravity.
- Counter-Example(s):
- See: Hubble's Law, Cosmological Constant, Standard Model, Friedmann Equations, Einstein Field Equations.
References
2016
- (Wikipedia, 2015) ⇒ http://en.wikipedia.org/wiki/Dark_energy Retrieved: 2016-27-02
- QUOTE: 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 68.3% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, respectively, and other components such as neutrinos and photons contribute a very small amount. Again on a mass–energy equivalence basis, the density of dark energy (~ 7 × 10−30 g/cm3) is very low, much less than the density of ordinary matter or dark matter within galaxies. However, it comes to dominate the mass–energy of the universe because it is uniform across space. 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 can be formulated to be equivalent to vacuum energy. Scalar fields that 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 and space. In general relativity, the evolution of the expansion rate is parameterized by the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring the equation of state for 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.
2016
- (Physics of the Universe Website,2016) ⇒ http://www.physicsoftheuniverse.com/topics_bigbang_accelerating.html , Retrieved:2016-27-02
- QUOTE: If 74% of the total mass of the universe consists of dark energy, and about 85% of the remaining actual matter (representing about 22% of the total) is dark matter (see the section on Dark Matter for more discussion of this), then this suggests that only around 4% of the universe consists of what we think of as "normal", everyday, atom-based matter such as stars, intergalactic gas, etc. As of 2013, based on cosmic microwave background radiation data from the Planck satellite, the latest figures are closer to 68%, 27% and 5% respectively. Nowadays, this is generally accepted as the "standard model" of the make-up of the universe. So, for all our advances in physics and astronomy, it appears that we can still only see, account for and explain a small proportion of the totality of the universe, a sobering thought indeed.
- Incorporating dark energy into our model of the universe would neatly account for the "missing" three-quarters of the universe required to cause the observed acceleration in the revised Big Bang theory. It also makes the map of the early universe produced by the WMAP probe fit well with the currently observed universe. Carlos Frenk's beautiful 3D computer models of the universe resemble remarkably closely the actual observed forms in the real universe (taking dark matter and dark energy into account), even if not all scientists are convinced by them. Alternative theories, such as Mordehai Milgrom's idea of "variable gravity", are as yet poorly developed and would have the effect of radically modifying all of physics from Newton onwards. So dark energy remains the most widely accepted option.
- Further corroboration of some kind of energy operating in the apparent vacuum of space comes from the Casimir effect, named after the 1948 experiments of Dutch physicists Hendrik Casimir and Dirk Polder. This shows how smooth uncharged metallic plates can move due to energy fluctuations in the vacuum of empty space, and it is hypothesized that dark energy, generated somehow by space itself, may be a similar kind of vacuum fluctuation.
2005
- (Feng et al., 2005) ⇒ Bo Feng, Xiulian Wang, and Xinmin Zhang. "Dark energy constraints from the cosmic age and supernova." Physics Letters B 607.1 (2005): 35-41. ⇒ http://www.sciencedirect.com/science/article/pii/S0370269304017447
2003
- (Peebles et al., 2003) ⇒ P. Peebles, E. James, and Bharat Ratra. "The cosmological constant and dark energy." Reviews of Modern Physics 75.2 (2003): 559. ⇒ http://arxiv.org/pdf/astro-ph/0207347.pdf