What is Dark Energy and Dark Matter
Based on Big Bang cosmology, dark energy is a hypothetical form of energy that is around the space and has a negative pressure. The introduction of dark energy is today the most popular among cosmologists to explain the observations of an accelerating universe, as well as to fill a significant portion of the missing mass of the universe (about 90%).
Two proposed forms of dark energy are the cosmological constant, a constant energy density filling space homogeneously, and quintessence, a dynamic field whose energy density varies in space and time. The cosmological constant is physically equivalent to the energy of the vacuum. Differing possibility requires accurate measurements of the universe to understand how the rate of expansion changes over time. The coefficient of expansion is parameterized by the equation of state. Calculate the equation of state of dark energy is one of the biggest efforts in observational cosmology.
The addition of a cosmological constant in the basic theory of cosmology (see Friedmann-Lemaitre-Robertson-Walker) led to the adoption of a model called Lambda-CDM model. This model is consistent with the cosmological observations confirmed.
The dark energy term was coined by Michael Turner.
The acceleration of the universe was confirmed in the nineties. To explain this phenomenon, we hypothesized an anti-gravity force that permeates the entire universe. The announcement that the data confirm an acceleration of the expansion was given by Saul Perlmutter of Berkeley Lab on 8 January 1998.
Albert Einstein, who lived in an era dominated by the steady state theory, to achieve this stasis inserted in the field equations of general relativity theory of the famous cosmological constant, to counteract the effects of gravity. When Edwin Hubble discovered that the universe was expanding, Einstein retracted his idea, calling it "my biggest mistake." According to the theory of relativity, the effect of such a negative pressure is similar, qualitatively, to a large-scale anti-gravity force.
When Richard Feynman and others developed the quantum theory of matter, they realized that the vacuum had a well-defined energy, given by the virtual particles that are formed in pairs, which annihilate each other. Through accurate measurements, it was realized that the error was not that of Einstein: an undetectable form of energy permeating the vacuum, and its action is anti-gravity accelerating the expansion of the universe. As a side result, the age of the universe is less than that estimated on the basis of a constant rate of expansion. We still do not understand or what is this force should be, nor how it is linked to the inflationary theory. In 2004, Christian Beck of Queen Mary University of London and Michael Mackey of McGill University in Montreal have developed a theory that connects these fluctuations to dark energy, and suggested the experimental measurement of dark energy through the Josephson junction.
However, some models of quantum gravity, including loop quantum gravity, can explain cosmological properties in other ways, without recourse to dark energy.
Evidence Of Acceleration Of The Universe
In the late nineties, observations of Type Ia supernovae suggested that the universe's expansion was accelerating. These observations were confirmed by many other independent sources: the cosmic microwave background radiation, the age of the universe, the abundances of the elements due to primordial nucleosynthesis, the large scale structure of the universe and the measurements of the Hubble parameter, as well as accurate analysis of the supernovae. All these factors confirm the Lambda-CDM model.
Type Ia supernovae provide the best evidence for the existence of dark energy. The measurement of the speed of removal of objects is simply obtained by measuring the red shift (redshift) of the object. Finding the distance of the object instead is a more complex problem. To do this you need to find standard candles: objects whose absolute magnitude is known, in order to relate the apparent magnitude at a distance. Without standard candles, it is impossible to measure the relationship between the Hubble law distance and redshift. Type Ia supernovae are the best standard candles for cosmological observation, since they are very bright and burn only when the mass of an old white dwarf reaches the Chandrasekhar limit. The distances of the supernovae are measured on the basis of their speed, and this method is also used to determine the history of the universe. These findings indicate that the universe is slowing down, what you would expect in a universe dominated by matter, but is mysteriously accelerated. The observations are then explained by postulating a kind of energy with negative pressure (see the equation of state in cosmology for a mathematical explanation): dark energy.
The existence of dark energy, in whatever form, also solves the problem of "missing mass". The theory of primordial nucleosynthesis regulates the formation of light elements in the early universe, such as helium, deuterium and lithium. The theory of large-scale structure of the Universe regulates the formation of the structure of the universe, stars, quasars, galaxies and galaxy clusters and groups. Both these theories suggest that the density of baryons and cold dark matter in the universe is about 30% of the critical density for closure of the universe. This is the density needed to make the curvature of the universe nothing. The measurements of the cosmic microwave background, recently performed by the WMAP satellite, indicate that the universe is very close to a zero curvature. So, we know that some forms of energy must be the remaining 70%.
Nature Of Dark Energy
The exact nature of dark energy is the subject of research. It is known to be homogeneous, not very dense and does not interact strongly through any of the fundamental forces except gravity. Since it is not very thick, about 10-29 grams per cubic centimeter, it is hard to find in laboratory experiments. Dark energy can only have an impact on the universe, which would constitute 70% of all energy as it uniformly fills all the gaps. The two most important are the cosmological constant and quintessence.
The simplest explanation for dark energy is the "price to have room", a volume of space has inherent and fundamental energy. This is the cosmological constant, sometimes called Lambda (hence the Lambda-CDM model) from the mathematical symbol used to represent: the Greek letter Λ. Since energy and mass are linked by the formula E = mc ², Einstein's general theory of relativity predicts that there will be gravitational effects. Sometimes it is called vacuum energy because it is the energy density of a void. In fact, most theories of particle physics predicts that vacuum fluctuations would give exactly that kind of energy. The cosmological constant is estimated to be of the order of about 10-29 or 10-123 g / cm 3 in Planck units.
The cosmological constant has negative pressure equal to the density of its energy and therefore causes the accelerating expansion of the universe. The reason was that the constant pressure value can be found in classical thermodynamics. The work produced by a change in volume dV is equal to-p dV, where p is the pressure. But the amount of energy in an empty container of energy actually increases as the volume increases (dV is positive and then), because the energy is equal to ρV, where ρ is the energy density of the cosmological constant. Then p is negative and in fact p =-ρ.
One of the greatest unsolved problems of physics is that most quantum field theories provide a huge value for the constant energy of the quantum vacuum, up to 123 orders of magnitude more. This would mean that should be avoided by a force equal and opposite sign. Some supersymmetric theories require that the cosmological constant is exactly zero. This is the problem of the cosmological constant, the worst problem of precision in physics: there is a natural (known) to obtain, even approximately, the infinitesimal cosmological constant observed in cosmology from particle physics. Some physicists, among them Steven Weinberg, think that the delicate balance of quantum vacuum energy is best explained by the anthropic principle.
Despite these problems, the cosmological constant is in many respects the most economical solution to the problem of cosmic acceleration. The standard model of these days includes the cosmological constant as an essential characteristic.
Alternatively, dark energy could arise by excitation of particles in some types of dynamic fields, and called quintessence. This differs from the cosmological constant because it can vary in space and time. So that this does not form structures such as subject, must be very light so as to have a Compton wavelength is very large.
There is no evidence of the quintessence now, but can not be discarded a priori. Generally expected to accelerate expansion of the universe less than the cosmological constant. Some scientists believe that the best evidence of the quintessence resulting from violation of the equivalence principle of Einstein and variations of fundamental constants in space and time. The scalar fields are expected by the standard model and string theory, but a problem similar to that of the cosmological constant (or the problem of constructing models of cosmic inflation) shows: the renormalization theory has it that the scalar fields acquire a large mass .
The problem of cosmic coincidence wonders why the cosmic acceleration begins when it is estimated to have begun. If you had started sooner, structures such as galaxies in the universe would not have had time to train, live and, at least we know to exist. Those who argue the anthropic principle sees this as a strong proof of their thesis. However, many models of quintessence behave capable of resolving the problem. In these models, the quintessence field has a density that track the radiation density until matter and radiation are equal. This means that the quintessence starts to behave as dark energy, and ultimately to dominate the universe. This sets the low energy scale of dark energy.
Some special cases of quintessence are phantom energy, where the energy density increases with time of the essence, and essence-k (kinetic quintessence) which has a non-standard form of kinetic energy. These can have unusual properties: phantom energy for example can cause the Big Rip.
Some theorists think that dark energy and cosmic acceleration are the evidence of a failure of general relativity on scales larger than those of superclusters of galaxies. One of the alternative models are the theories of MOND (Modified Newtonian Dynamics, modified Newtonian dynamics). However, most attempts to modify general relativity proved or equivalent to theories of quintessence, or inconsistent with the observations.
Other theorists think instead of dark energy and cosmic acceleration are the evidence of a failure of the standard Big Bang model, since it forces us to admit the presence of something not (yet) experiments.
Other ideas come from dark energy, string theory, brane cosmology (en: Brane cosmology) and the holographic principle, but have not yet proved as effective as those of the quintessence and the cosmological constant. In analogy with phenomena observed in liquids, it should be assumed that the almost infinite number of infinitesimal collisions between an almost infinite number of particles can produce a kind of pressure that widens the ball of our universe.
Implications For The Fate Of The Universe
Cosmologists speculate that the acceleration is started about 5 billion years ago. Before that, it is believed that the expansion was decelerating due to the influence of the attraction of dark matter and baryons. The density of dark matter in an expanding universe disappears more quickly than dark energy, and finally this domain. Specifically, when the volume of the universe doubles, the density of dark matter is halved, but the dark energy remains almost unchanged (exactly the same in the case of the cosmological constant).
If the acceleration continues indefinitely, the ultimate result will be that galaxies outside the local supercluster will move beyond the cosmic horizon: they will not be more visible, because their relative speed becomes greater than the speed of light. This is not a violation of special relativity, and the effect can not be used to send signals between the galaxies. In reality there is no way to define "relative speed" in a curved spacetime. The relative speed and speed can only be defined in a flat spacetime or in the sections sufficiently small (infinitesimal) curved spacetime. Rather, it prevents any communication between the galaxies and objects lose contact. The Earth, the Milky Way and the Local Supercluster, however, would remain virtually undisturbed while the rest of the universe moves away. In this scenario, the local supercluster would suffer heat death just as you thought and material for the flat universe, before measurements of cosmic acceleration.
There are different ideas about the future of the universe. One of these suggests that the phantom energy will cause an expansion divergent, implying that the effective force of dark energy continues to grow to dominate all other forces of the universe. In this scenario the dark energy would destroy all structures linked by gravity, including galaxies and solar systems, and finally overcome the forces and nuclear power by destroying the atoms themselves, making the universe end with a Big Rip (large tear). On the other hand, dark energy might disappear with time, or even become attractive. These uncertainties leave open the possibility that gravity can dominate the universe leading to a contraction called "Big Crunch". Some models, such as cyclic say that this is the end. Although these ideas are not supported by observations, are not even discarded. The acceleration measurements are important in determining the ultimate fate of the universe in the Big Bang theory.
In Science Fiction And Gaming
Dark energy is often mentioned (and even manipulated) in the famous video game series Half-Life 2. As a science fiction series, the vision goes beyond a bit of dark energy 'from those descriptions in physical theories concerning this phenomenon: it appears to have been manipulated by technological devices ranging from guns to teleporters to fusion reactors. Dark energy, in fact, is here presented as the primary energy source of the overwhelming power called Combine. The Combine uses this energy to power their fortresses (the Citadels, huge buildings like skyscrapers tall colossal whole kilometers), its own weapons (assault rifles combine shoot real bullets dark energy) and its transporter system .
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