Astronomers now know that there is much more to the universe than meets the eye. The luminous and non-luminous normal matter makes up about 4 percent of the total mass and energy density of the universe. Dark matter, which emits no light and cannot be directly observed, comprises another 24 percent of the total, while dark energy dominates with about 72 percent.
Most of the universe is unknown and only indirectly detected. We can see its effects on galaxies and the expansion of the universe, but we have yet to identify the underlying source. That may seem unsettling, but to a scientist, it is exciting. There are more great mysteries to explore and solve! Dark Energy View All Articles. Accelerating the Expansion of the Universe.
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What is Dark Energy? The universe is expanding, and that expansion stretches light traveling through space in a phenomenon known as cosmological redshift. The greater the redshift, the greater the distance the light has traveled. Hustak STScI In the late s, astronomers found evidence that the expansion of the universe was not slowing down due to gravity as expected. Within the Hubble Deep Field-North region, astronomers pinpointed a blaze of light from one of the farthest supernova ever seen.
In a close-up view of that region left a white arrow points to a faint elliptical, the home of the exploding SN ff. The supernova itself right is distinguished by the white dot in the center.
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 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 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. Alan Guth and Alexei Starobinsky proposed in that a negative pressure field, similar in concept to dark energy, could drive cosmic inflation in the very early universe.
Unlocking the Mystery
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. These models were found to be successful at forming realistic galaxies and clusters, but some problems appeared in the late s: in particular, the model required a value for the Hubble constant lower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering.
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The first direct evidence for dark energy came from supernova observations in of accelerated expansion in Riess et al. The large difference between these two supports a smooth component of dark energy making up the difference. Much more precise measurements from WMAP in — 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 s, was coined by Michael Turner in 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 estimated from the curvature of the universe and 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. As of , the Lambda-CDM model is consistent with a series of increasingly rigorous cosmological observations, including the Planck spacecraft and the Supernova Legacy Survey.
First results from the SNLS reveal that the average behavior i. The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation. 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 [ dubious — discuss ]. This accelerating expansion effect is sometimes labeled "gravitational repulsion". In standard cosmology, there are three components of the universe: matter, radiation, and dark energy. Matter is anything whose energy density scales with the inverse cube of the scale factor, i. This can be understood intuitively: for an ordinary particle in a square box, doubling the length of a side of the box decreases the density and hence energy density by a factor of eight 2 3.
For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift. Thus, unlike ordinary matter, it does not get diluted with the expansion of space.
In , the Supernova Cosmology Project  followed by suggesting that the expansion of the universe is accelerating. Schmidt , and Adam G. Riess for their leadership in the discovery. Since then, these observations have been corroborated by several independent sources. Measurements of the cosmic microwave background , gravitational lensing , and the large-scale structure of the cosmos , as well as improved measurements of supernovae, have been consistent with the Lambda-CDM model.
Supernovae are useful for cosmology because they are excellent standard candles across cosmological distances. They allow researchers to measure the expansion history of the universe by looking at the relationship between the distance to an object and its redshift , which gives how fast it is receding from us.
The relationship is roughly linear, according to Hubble's law. It is relatively easy to measure redshift, but finding the distance to an object is more difficult. Usually, astronomers use standard candles: objects for which the intrinsic brightness, or absolute magnitude , is known. This allows the object's distance to be measured from its actual observed brightness, or apparent magnitude. Type Ia supernovae are the best-known standard candles across cosmological distances because of their extreme and consistent luminosity.
Recent observations of supernovae are consistent with a universe made up The existence of dark energy, in whatever form, is needed to reconcile the measured geometry of space with the total amount of matter in the universe. Measurements of cosmic microwave background CMB anisotropies indicate that the universe is close to flat. For the shape of the universe to be flat, the mass-energy density of the universe must be equal to the critical density.
A survey, the WiggleZ galaxy survey of more than , galaxies, provided further evidence towards the existence of dark energy, although the exact physics behind it remains unknown. The data confirmed cosmic acceleration up to half of the age of the universe 7 billion years and constrain its inhomogeneity to 1 part in Accelerated cosmic expansion causes gravitational potential wells and hills to flatten as photons pass through them, producing cold spots and hot spots on the CMB aligned with vast supervoids and superclusters. A new approach to test evidence of dark energy through observational Hubble constant data OHD has gained significant attention in recent years.
The core of this idea is the measurement of the differential age evolution as a function of redshift of these cosmic chronometers. Thus, it provides a direct estimate of the Hubble parameter. For these reasons, it has been widely used to examine the accelerated cosmic expansion and study properties of dark energy. An attempt to directly observe dark energy in a laboratory failed to detect a new force.
Dark energy's status as a hypothetical force with unknown properties makes it a very active target of research.parderesi.tk
Dr. Adam Riess - Dark Energy
The problem is attacked from a great variety of angles, such as modifying the prevailing theory of gravity general relativity , attempting to pin down the properties of dark energy, and finding alternative ways to explain the observational data. The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space. It is sometimes called a vacuum energy because it is the energy density of empty vacuum. The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate.
The reason a cosmological constant has negative pressure can be seen from classical thermodynamics. In general, energy must be lost from inside a container the container must do work on its environment in order for the volume to increase. There are two major advantages for the cosmological constant. The first is that it is simple. Einstein had in fact introduced this term in his original formulation of general relativity such as to get a static universe. Although he later discarded the term after Hubble found that the universe is expanding, a nonzero cosmological constant can act as dark energy, without otherwise changing the Einstein field equations.
The other advantage is that there is a natural explanation for its origin. Most quantum field theories predict vacuum fluctuations that would give the vacuum this sort of energy. This is related to the Casimir effect , in which there is a small suction into regions where virtual particles are geometrically inhibited from forming e. A major outstanding problem is that the same quantum field theories predict a huge cosmological constant , more than orders of magnitude too large. Some supersymmetric theories require a cosmological constant that is exactly zero,  which does not help because supersymmetry must be broken.
Nonetheless, the cosmological constant is the most economical solution to the problem of cosmic acceleration. Thus, the current standard model of cosmology, the Lambda-CDM model, includes the cosmological constant as an essential feature. In quintessence models of dark energy, the observed acceleration of the scale factor is caused by the potential energy of a dynamical field , referred to as quintessence field.
Quintessence differs from the cosmological constant in that it can vary in space and time. In order for it not to clump and form structure like matter, the field must be very light so that it has a large Compton wavelength. No evidence of quintessence is yet available, but it has not been ruled out either. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some scientists think that the best evidence for quintessence would come from violations of Einstein's equivalence principle and variation of the fundamental constants in space or time.