Dark energy

In physical cosmology , dark energy is a form of matter or energy that would be present throughout space, producing a pressure that tends to accelerate the expansion of the Universe , resulting in a repulsive gravitational force . Considering the existence of dark energy is the most frequent way to explain recent observations that the Universe appears to be in accelerated expansion. In the standard model of cosmology, dark energy contributes almost three-quarters of the total mass-energy of the Universe.

Summary

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  • 1 Nature of Dark energy
  • 2 History
  • 3 Discovery of dark energy
  • 4 Effects of Dark energy on the universe
  • 5 Sources

Nature of Dark energy

The exact nature of dark energy is up for debate. It is known to be very homogeneous, not very dense, but its interaction with none of the fundamental forces other than with gravity is not known . Since it is not very dense, about 10−29 g / cm³, it is difficult to carry out experiments to detect it. Dark energy has a great influence on the Universe, since it is 70% of all energy and because it uniformly occupies interstellar space. The two main models are the quintessence and the cosmological constant.

History

The cosmological constant was first proposed by Albert Einstein as a means of obtaining a stable solution to Einstein’s field equation that would lead to a static Universe , using it to compensate for gravity . The mechanism was not only an inelegant example of “fine-tuning”, for it was soon shown that Einstein’s static Universe would be unstable because local heterogeneities would eventually lead to uncontrolled expansion or contraction of the Universe. The balance is unstable: if the Universe expands slightly, then the expansion releases the energy from the vacuum, which causes even more expansion. In the same way, a slightly contracting Universe will continue to contract.

These types of disturbances are inevitable, due to the irregular distribution of matter in the Universe. Edwin Hubble’s observations showed that the Universe is expanding and that it is not static at all. Einstein referred to his failure to predict a dynamic Universe, in contrast to a static Universe, as “his big mistake.” After this statement, the cosmological constant was long ignored as a historical curiosity.

Alan Guth proposed in the 1970s that a negative pressure field , similar in concept to dark energy, could lead to cosmic inflation in the pre-early Universe. Inflation postulates that some repulsive forces , qualitatively similar to dark energy, result in a huge and exponential expansion of the Universe shortly after the Big Bang . Such expansion is an essential feature of many current Big Bang models. However, inflation must have occurred at a much higher energy than the dark energy we observe today and it is thought to have ended completely when the Universe was only a fraction of a second away.

It is unclear what relationship, if any, exists between dark energy and inflation. Even after inflationary models have been accepted, the cosmological constant is thought to be irrelevant in the current Universe.

The term “dark energy” was coined by Michael Turner in 1998 . At that time , the problem of the lost mass of primordial nucleosynthesis and the large-scale structure of the Universe was established and some cosmologists had begun to theorize that there was an additional component to our Universe. The first direct proof of dark energy came from observations of the acceleration of expansion of supernovae , by Adam Riess and later confirmed by Saul Perlmutter .

This resulted in the model Lambda-CDM , which until 2006 was consistent with a number of the latest rigorously increasing cosmological observations 2005 from the Supernova Legacy Survey . The first SNLS results revealed that the average behavior of dark energy behaves like Einstein’s cosmological constant with an accuracy of 10%. The results from the Hubble Space Telescope Higher-Z Team indicate that dark energy has been present for at least 9 billion years and during the period preceding cosmic acceleration.

Discovery of dark energy

In 1998 observations of very distant Type 1 supernovae, made by the Supernova Cosmology Project at the Lawrence Berkeley National Laboratory and the High-z Supernova Search Team , suggested that the expansion of the Universe was accelerating. Since then, this acceleration has been confirmed by several independent sources: measurements of microwave background radiation , gravitational lenses, primitive nucleosynthesis of light elements, and the large-scale structure of the Universe, as well as an improvement in the measurements of supernovae. have been consistent with the Lambda-CDM model.

Type 1a supernovae provide the main direct proof of the existence of dark energy. According to Hubble’s Law , all distant galaxies apparently move away from the Milky Way , showing a redshift in the light spectrum due to the Doppler effect. Measurement of the scale factor at the time light was emitted from an object is easily obtained by measuring the redshift of the object in recession. This offset indicates the age of a distant object proportionally, but not absolutely.

For example, studying the spectrum of a quasar you can know if it was formed when the Universe was 20% or 30% of the current age, but you cannot know the absolute age of the Universe. For this it is necessary to accurately measure the cosmological expansion. The value that this expansion represents today is called the Hubble Constant. To calculate this constant, standard candles are used in cosmology , which are certain astronomical objects with the same absolute magnitude, which is known, in such a way that it is possible to relate the observed brightness, or apparent magnitude, to the distance.

Without the standard candles, it is impossible to measure the redshift-distance relationship of Hubble’s law. Type 1a supernovae are one of those standard candles, due to their large absolute magnitude, making it possible to see them even in the most distant galaxies. In 1998 several observations of these supernovae in very distant (and therefore young) galaxies demonstrated that the Hubble constant is not such, but that its value varies with time. Until that time it was thought that the expansion of the Universe was slowing down due to the gravitational force ; however, it was discovered that it was accelerating, so there must be some kind of force to accelerate the Universe.

Consistency in absolute magnitude for Type 1a supernovae is favored by the model of an old white dwarf star that gains mass from a companion star and grows to the precisely defined Chandrasekhar limit. With this mass, the white dwarf is unstable under thermonuclear leakage and explodes as a Type 1a supernova with a characteristic brightness. The observed brightness of the supernova is painted against its redshift and this is used to measure the history of the expansion of the Universe.

These observations indicate that the expansion of the Universe is not slowing down, as would be expected for a matter-dominated Universe, but rather accelerating. These observations are explained by assuming that there is a new type of energy with negative pressure. The existence of dark energy, however, is necessary to reconcile the measured geometry of space with the sum total of matter in the Universe. The latest microwave background radiation measurements from the WMAP satellite indicate that the Universe is very close to being flat.

For the shape of the Universe to be flat, the mass / energy density of the Universe has to be equal to a certain critical density. Subsequent observations of the microwave background radiation and the proportion of elements formed in the Big Bang have placed a limit on the amount of baryonic and dark matter that can exist in the Universe, which only accounts for 30% of the critical density.

This implies the existence of an additional form of energy that accounts for 70% of the remaining energy mass. These studies indicate that 73% of the Universe’s mass is made up of dark energy, 23% is dark matter (cold dark matter and hot dark matter) and 4% baryonic matter. The theory of large-scale structure of the Universe, which determines the formation of structures in the Universe (stars, quasars, galaxies, and galactic clusters), also suggests that the density of matter in the Universe is only 30% of the critical density .

Effects of Dark energy on the universe

The most direct consequence of the existence of dark energy and the acceleration of the Universe is that it is older than previously thought. If the age of the Universe is calculated based on the current data of the Hubble constant (71 ± 4 (km / s) / Mp), we obtain an age of 10 billion years, less than the age of the oldest stars that it is possible to observe in globular clusters, which creates an insurmountable paradox. Cosmologists estimate that the acceleration started about 9 billion years ago.

Before that, expansion was thought to be slowing down, due to the attractive influence of dark matter and baryons. The density of dark matter in an expanding Universe disappears faster than dark energy and finally dominates dark energy. Specifically, when the volume of the Universe doubles, the density of dark matter is halved but the density of dark energy remains almost unchanged (exactly constant in the case of a cosmological constant). Taking dark energy into account, the age of the Universe is about 13.7 billion years old (according to data from the WMAP satellite in 2003 ), which solves the age paradox of the oldest stars.

If the acceleration continues indefinitely, the end result will be that the galaxies outside the Virgo Supercluster will move beyond the event horizon: they will not be visible again, because their radial velocity will be greater than the speed of light. This is not a violation of special relativity and the effect cannot be used to send a signal between them. There really is no way to define “relative speed” in a curved space-time. Relative velocity and velocity can only be defined with full meaning in flat spacetime or in sufficiently small (infinitesimal) regions of curved spacetime.

In turn, it prevents any communication between them and the object passes without contacting. The Earth , the Milky Way and the Virgo supercluster, however, would remain virtually undisturbed while the rest of the universe recedes. In this scenario, the local supercluster would eventually undergo hot death, just as it was intended for a flat, matter-dominated Universe, before measurements of cosmic acceleration.

The microwave background indicates that the geometry of the Universe is flat, that is, the Universe has just enough mass for the expansion to continue indefinitely. If the Universe, instead of plane were closed, it would mean that the gravitational attraction of the mass that forms the Universe is greater than the expansion of the Universe, reason why this one would contract again ( Big Crunch ). However, when studying the mass of the Universe, it was soon detected that there was a lack of material for the Universe to be flat.

This “lost matter” was called dark matter. With the discovery of dark energy, it is now known that the fate of the Universe no longer depends on its geometry, that is, on the amount of mass in it. At first the expansion of the Universe was slowed down by gravity, but about 4 billion years ago the dark energy surpassed the effect of the gravitational force of matter and the acceleration of the expansion began.

The ultimate future of the Universe depends on the exact nature of dark energy. If this is a cosmological constant, the future of the Universe will be very similar to that of a flat Universe. However, in some quintessential models, called phantom energy, the density of dark energy increases over time, causing exponential acceleration. In some extreme models the acceleration would be so fast that it would overcome nuclear attractive forces and destroy the Universe in about 20,000 million years, in the so-called Big Rip.

There are some very speculative ideas about the future of the Universe. One suggests that phantom energy causes a divergent expansion, which would imply that the effective force of dark energy continues to grow until it dominates the rest of the forces of the Universe. Under this scenario, dark energy would eventually shatter all gravitationally bounded structures, including galaxies and solar systems, and eventually overtake the nuclear and electrical forces to smash the atoms themselves , ending the Universe in a Big Rip.

On the other hand, dark energy can dissipate over time or even become attractive. Such uncertainties open up the possibility that gravity could still lead to the Universe contracting itself in a “Big Crunch”. Some scenarios, such as the cyclical model, suggest that this may be the case. While these ideas are not supported by observations, they cannot be excluded. Acceleration measurements are crucial to determine the final destination of the Universe in the Big Bang Theory.

 

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