Physical cosmology

Universe · Big Bang Age of the universe Timeline of the Big Bang Ultimate fate of the universe Components Lambda-CDM model Dark energy · Dark matter

This box: view • talk • edit

In astrophysics and cosmology, dark matter is a hypothetical form of matter that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. The age of the Universe is the time elapsed between the theory of the Big Bang and the present day This timeline of the Big Bang describes the events according to the Scientific theory of the Big Bang, using the cosmological time parameter of Comoving coordinates The ultimate fate of the universe is a topic in Physical cosmology. &LambdaCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter. In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion of the universe Astrophysics is the branch of Astronomy that deals with the Physics of the Universe, including the physical properties ( Luminosity, Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its A hypothesis (from Greek) consists either of a suggested explanation for a phenomenon (an event that is observable or of a reasoned proposal suggesting a possible Matter is commonly defined as being anything that has mass and that takes up space. In Physics, emission is the process by which the Energy of a Photon is released by another entity for example by an Atom whose Electrons Reflection is the change in direction of a Wave front at an interface between two different media so that the wave front returns into the medium from which Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Gravitation is a natural Phenomenon by which objects with Mass attract one another According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter accounts for the vast majority of mass in the observable universe. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. In Big Bang Cosmology, the observable universe is the region of space bounded by a Sphere, centered on the observer that is small enough that The observed phenomena consistent with dark matter observations include the rotational speeds of galaxies, orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of The Bullet cluster ( 1E 0657-56) consists of two colliding clusters of galaxies. Dark matter also plays a central role in structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background. Structure formation refers to a fundamental problem in Physical cosmology. The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning the formation of the first galaxies the way Anisotropy (pronounced with stress on the third syllable ˌænaɪˈsɒtrəpi is the property of being directionally dependent as opposed to Isotropy, which means homogeneity All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component. In Particle physics, a hadron ( from the ἁδρός hadrós, " stout, thick " ( "

The composition of dark matter is unknown but may include ordinary and heavy neutrinos, recently postulated elementary particles such as WIMPs and axions, astronomical bodies such as brown dwarfs and planets (collectively called MACHOs), primordial black holes and clouds of nonluminous gas. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost In Astrophysics, weakly interacting massive particles or WIMPs are hypothetical particles serving as one possible solution to the Dark matter problem The axion is a hypothetical Elementary particle postulated by Peccei-Quinn theory in 1977 to resolve the Strong-CP problem in Quantum chromodynamics Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of Dark A primordial black hole is a hypothetical type of Black hole that is formed not by the Gravitational collapse of a star but by the extreme density of matter present Also, matter that might exist in another universe but might affect ours via gravity would be consistent with some theories of brane cosmology. Brane cosmology refers to several theories in Particle physics and cosmology motivated by but not exclusively derived from Superstring theory and Current evidence favors models in which the primary component of dark matter is new elementary particles, collectively called nonbaryonic dark matter.

The dark matter component has vastly more mass than the "visible" component of the universe. The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy ^[1] At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic meter of space. Baryons are the family of Subatomic particles with a Baryon number of 1 Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion of the universe ^[2] Some hard-to-detect baryonic matter makes a contribution to dark matter but constitutes only a small portion. Baryonic dark matter is Dark matter (matter that doesn't emit light composed of Baryons i ^[3][4] Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much as the marking of early maps with "terra incognita. Terra incognita is the Latin term for "unknown land" used in Cartography for regions that have not been mapped or documented "^[2]

Observational evidence

The first to provide evidence and infer the existence of a phenomenon that has come to be called "dark matter" was Swiss astrophysicist Fritz Zwicky, of the California Institute of Technology (Caltech) in 1933. Fritz Zwicky ( February 14 1898 &ndash February 8 1974) was an American-based Swiss Astronomer of Bulgarian origin The California Institute of Technology (commonly referred to as Caltech) is a private, Coeducational research university located in Pasadena ^[5] He applied the virial theorem to the Coma cluster of galaxies and obtained evidence of unseen mass. In Mechanics, the virial theorem provides a general equation relating the average total Kinetic energy, \left\langle T \right\rangle of a stable system The Coma Cluster ( Abell 1656 is a large cluster of galaxies that contains over 1000 identified galaxies Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation Zwicky estimated the cluster's total mass based on the motions of galaxies near its edge. When he compared this mass estimate to one based on the number of galaxies and total brightness of the cluster, he found that there was about 400 times more mass than expected. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the "missing mass problem". Based on these conclusions, Zwicky inferred that there must be some non-visible form of matter which would provide enough of the mass and gravity to hold the cluster together.

Composite image of the Bullet cluster shows distribution of ordinary matter, inferred from X-ray emissions, in red and total mass, inferred from gravitational lensing, in blue

Much of the evidence for dark matter comes from the study of the motions of galaxies. The Bullet cluster ( 1E 0657-56) consists of two colliding clusters of galaxies. In Physics, emission is the process by which the Energy of a Photon is released by another entity for example by an Atom whose Electrons A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Many of these appear to be fairly uniform, so by the virial theorem the total kinetic energy should be half the total gravitational binding energy of the galaxies. The kinetic energy of an object is the extra Energy which it possesses due to its motion The Gravitational Binding energy of an object consisting of loose material held together by gravity alone is the amount of Energy required to pull all Experimentally, however, the total kinetic energy is found to be much greater: in particular, assuming the gravitational mass is due to only the visible matter of the galaxy, stars far from the center of galaxies have much higher velocities than predicted by the virial theorem. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, cannot be explained by only the visible matter. The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this. Galaxies show signs of being composed largely of a roughly spherically symmetric, centrally concentrated halo of dark matter with the visible matter concentrated in a disc at the center. Most of the mass of any galaxy is dominated by a component concentrated at the centre of the galaxy but dominating its dynamics throughout known as the Dark matter halo. Low surface brightness dwarf galaxies are important sources of information for studying dark matter, as they have an uncommonly low ratio of visible matter to dark matter, and have few bright stars at the center which impair observations of the rotation curve of outlying stars. A low surface brightness galaxy or LSB galaxy is a diffuse Galaxy with a Surface brightness that when viewed from Earth, is at least one

Gravitational lensing can be used to directly map the total distribution of mass, including both dark matter and visible material. A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of ^[6] In most regions of the universe, dark matter and visible material are found together,^[7] as expected because of their mutual gravitational attraction. However, the different forms of matter were separated by the violent collision of two clusters of galaxies about 150 million years ago, a system known as the Bullet Cluster. Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation The Bullet cluster ( 1E 0657-56) consists of two colliding clusters of galaxies. ^[8][9] Researchers mapped the distribution of mass using measurements of gravitational lensing, and compared it to X-ray maps showing hot gases, thought to constitute the large majority of ordinary matter in the clusters. The hot gases in the two clusters collided and slowed down: they now lie close to the point of impact. Conversely, the individual galaxies and the dark matter did not interact, so are now further apart. In several hundred more million years, gravitational attraction between the different concentrations of matter is expected to pull them back together, reproducing the usual configuration.

Galactic rotation curves

Main article: Galaxy rotation curve

Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy Dark matter can explain the velocity curve having a "flat" appearance out to a large radius

For 40 years after Zwicky's initial observations, no other corroborating observations indicated that the mass to light ratio was anything other than unity (a high mass-to-light ratio indicates the presence of dark matter). In Physical cosmology the mass to light ratio is the quotient between the total Mass of a Galaxy or a cluster and its Luminosity. Then, in the late 1960s and early 1970s, Vera Rubin, a young astronomer at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington presented findings based on a new sensitive spectrograph that could measure the velocity curve of edge-on spiral galaxies to a greater degree of accuracy than had ever before been achieved. Vera (Cooper Rubin (born 23 July 1928) is an Astronomer who has done pioneering work on Galaxy rotation rates The Carnegie Institution for Science (also called the Carnegie Institution of Washington (CIW) is a organization in the United States established to support Scientific A spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Together with fellow staff-member Kent Ford, Rubin announced at a 1975 meeting of the American Astronomical Society the astonishing discovery that most stars in spiral galaxies orbit at roughly the same speed, which implied that their mass densities were uniform well beyond the locations with most of the stars (the galactic bulge). The American Astronomical Society (AAS sometimes pronounced "double-A-S" is a US society of professional astronomers and other interested individuals A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth A spiral galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the In Astronomy, a bulge is a tightly packed group of Stars The term almost exclusively refers to the central group of stars found in most spiral galaxies. This result suggests that either Newtonian gravity does not apply universally or that, conservatively, upwards of 50% of the mass of galaxies was contained in the relatively dark galactic halo. Gravitation is a natural Phenomenon by which objects with Mass attract one another Met with skepticism, Rubin insisted that the observations were correct. Eventually other astronomers began to corroborate her work and it soon became well-established that most galaxies were in fact dominated by "dark matter"; exceptions appeared to be galaxies with mass-to-light ratios close to that of stars. Subsequent to this, numerous observations have been made that do indicate the presence of dark matter in various parts of the cosmos. Together with Rubin's findings for spiral galaxies and Zwicky's work on galaxy clusters, the observational evidence for dark matter has been collecting over the decades to the point that today most astrophysicists accept its existence. As a unifying concept, dark matter is one of the dominant features considered in the analysis of structures on the order of galactic scale and larger.

Velocity dispersions of galaxies

Rubin's pioneering work has stood the test of time. Measurements of velocity curves in spiral galaxies were soon followed up with velocity dispersions of elliptical galaxies. An elliptical galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated While sometimes appearing with lower mass-to-light ratios, measurements of ellipticals still indicate a relatively high dark matter content. Likewise, measurements of the diffuse interstellar gas found at the edge of galaxies indicate not only dark matter distributions that extend beyond the visible limit of the galaxies, but also that the galaxies are virialized up to ten times their visible radii. This has the effect of pushing up the dark matter as a fraction of the total amount of gravitating matter from 50% measured by Rubin to the now accepted value of nearly 95%.

There are places where dark matter seems to be a small component or totally absent. Globular clusters show no evidence that they contain dark matter, though their orbital interactions with galaxies do show evidence for galactic dark matter. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. For some time, measurements of the velocity profile of stars seemed to indicate concentration of dark matter in the disk of the Milky Way galaxy, however, now it seems that the high concentration of baryonic matter in the disk of the galaxy (especially in the interstellar medium) can account for this motion. A disc is a component of Disc galaxies, such as Spiral galaxies, or Lenticular galaxies. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply Galaxy mass profiles are thought to look very different from the light profiles. The typical model for dark matter galaxies is a smooth, spherical distribution in virialized halos. Such would have to be the case to avoid small-scale (stellar) dynamical effects. Recent research reported in January 2006 from the University of Massachusetts, Amherst would explain the previously mysterious warp in the disk of the Milky Way by the interaction of the Large and Small Magellanic Clouds and the predicted 20 fold increase in mass of the Milky Way taking into account dark matter. The University of Massachusetts Amherst (otherwise known as UMass Amherst, Massachusetts, or UMass) is a research and Land-grant university in The Large Magellanic Cloud (LMC is a nearby Satellite galaxy of our own galaxy the Milky Way. The Small Magellanic Cloud (SMC is a Dwarf galaxy. It contains several hundred million stars

In 2005, astronomers from Cardiff University claimed to discover a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21. Cardiff University (Prifysgol Caerdydd is a leading University located in the Cathays Park area of Cardiff, Wales, United Kingdom A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 Mly (18 VIRGOHI21 is a Dark matter halo in the Virgo cluster discovered in 2005 ^[10] Unusually, VIRGOHI21 does not appear to contain any visible stars: it was seen with radio frequency observations of hydrogen. Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times more dark matter than hydrogen and has a total mass of about 1/10th that of the Milky Way Galaxy we live in. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang and structure formation have suggested that such dark galaxies should be very common in the universe, but none had previously been detected. The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light If the existence of this dark galaxy is confirmed, it provides strong evidence for the theory of galaxy formation and poses problems for alternative explanations of dark matter.

Recently too there is evidence that there are 10 to 100 times fewer small galaxies than permitted by what the dark matter theory of galaxy formation predicts. There are also a small number of galaxies, like NGC 3379 whose measured orbital velocity of its gas clouds, show that it contains almost no dark matter at all. Messier 105 (also known as M105 and NGC 3379) is an Elliptical galaxy in the Constellation Leo. ^[11]

Missing matter in clusters of galaxies

Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter - Enlarge the image to see the lensing arcs. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into Abell 1689 is a stellar object described as being a Galaxy cluster and Gravitational lens in the constellation Virgo. Credits: NASA/ESA

Dark matter affects galaxy clusters as well. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation X-ray measurements of hot intracluster gas correspond closely to Zwicky's observations of mass-to-light ratios for large clusters of nearly 10 to 1. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Intergalactic space is the physical space between galaxies. Generally free of dust and debris intergalactic space is very close to a total Vacuum. Many of the experiments of the Chandra X-ray Observatory use this technique to independently determine the mass of clusters. The Chandra X-ray Observatory is a Satellite launched on STS-93 by NASA on July 23, 1999.

The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas, and an amount of dark matter equivalent to more than 10¹⁴ Suns. Abell 2029 is a large cluster of galaxies a billion light years away in the constellation Serpens, close to its boundary with Virgo. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies. ^[12] The measured orbital velocities of galaxies within galactic clusters have been found to be consistent with dark matter observations.

Another important tool for future dark matter observations is gravitational lensing. A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of Lensing relies on the effects of general relativity to predict masses without relying on dynamics, and so is a completely independent means of measuring the dark matter. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Strong lensing, the observed distortion of background galaxies into arcs when the light passes through a gravitational lens, has been observed around a few distant clusters including Abell 1689 (pictured right). Abell 1689 is a stellar object described as being a Galaxy cluster and Gravitational lens in the constellation Virgo. By measuring the distortion geometry, the mass of the cluster causing the phenomena can be obtained. In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters.

Perhaps more convincing, a technique has been developed over the last 10 years called weak gravitational lensing, which looks at minute distortions of galaxies observed in vast galaxy surveys due to foreground objects through statistical analyses. While the presence of any mass bends the path of light passing near it this effect rarely produces the giant arcs and multiple images associated with strong gravitational lensing. In Astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the Redshift of astronomical objects By examining the apparent shear deformation of the adjacent background galaxies, astrophysicists can characterize the mean distribution of dark matter by statistical means and have found mass-to-light ratios that correspond to dark matter densities predicted by other large-scale structure measurements. The correspondence of the two gravitational lens techniques to other dark matter measurements has convinced almost all astrophysicists that dark matter actually exists as a major component of the universe's composition.

Structure formation

Main article: structure formation

Dark matter is crucial to the Big Bang model of cosmology as a component which corresponds directly to measurements of the parameters associated with Friedmann cosmology solutions to general relativity. Structure formation refers to a fundamental problem in Physical cosmology. The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. &LambdaCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 In particular, measurements of the cosmic microwave background anisotropies correspond to a cosmology where much of the matter interacts with photons more weakly than the known forces that couple light interactions to baryonic matter. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena In Physics, a fundamental interaction or fundamental force is a mechanism by which particles interact with each other and which cannot be explained in terms Likewise, a significant amount of non-baryonic, cold matter is necessary to explain the large-scale structure of the universe. Baryons are the family of Subatomic particles with a Baryon number of 1 In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light

Observations suggest that structure formation in the universe proceeds hierarchically, with the smallest structures collapsing first and followed by galaxies and then clusters of galaxies. Structure formation refers to a fundamental problem in Physical cosmology. As the structures collapse in the evolving universe, they begin to "light up" as the baryonic matter heats up through gravitational contraction and the object approaches hydrostatic pressure balance. Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite Ordinary baryonic matter had too high a temperature, and too much pressure left over from the Big Bang to collapse and form smaller structures, such as stars, via the Jeans instability. Baryons are the family of Subatomic particles with a Baryon number of 1 The Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation Dark matter acts as a compactor of structure. This model not only corresponds with statistical surveying of the visible structure in the universe but also corresponds precisely to the dark matter predictions of the cosmic microwave background.

This bottom up model of structure formation requires something like cold dark matter to succeed. Large computer simulations of billions of dark matter particles have been used to confirm that the cold dark matter model of structure formation is consistent with the structures observed in the universe through galaxy surveys, such as the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest. The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic Redshift survey using a dedicated 2 In Astronomy, the 2dF Galaxy Redshift Survey (Two-degree-Field Galaxy Redshift Survey 2dF or 2dFGRS is a Redshift survey conducted by the In Astronomical spectroscopy, the Lyman alpha forest is the sum of Absorption lines arising from the Lyman alpha transition of the neutral Hydrogen These studies have been crucial in constructing the Lambda-CDM model which measures the cosmological parameters, including the fraction of the universe made up of baryons and dark matter. &LambdaCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter.

Dark matter composition

Unsolved problems in physics: What is dark matter? How is it generated? Is it related to supersymmetry?

Although dark matter was detected by its gravitational lensing in August 2006,^[13] many aspects of dark matter remain speculative. This is a list of some of the major unsolved problems in Physics. In Particle physics, supersymmetry (often abbreviated SUSY) is a Symmetry that relates elementary particles of one spin to another particle that A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of The DAMA/NaI experiment and its successor DAMA/LIBRA have claimed to directly detect dark matter passing through the Earth, though most scientists remain skeptical since negative results of other experiments are (almost) incompatible with the DAMA results if dark matter consists of neutralinos. The DAMA/NaI experiment was designed to detect Dark matter using the direct detection technique In Particle physics, the neutralino is a hypothetical particle part of the doubling of the menagerie of particles predicted by supersymmetric theories

Data from a number of lines of evidence, including galaxy rotation curves, gravitational lensing, structure formation, and the fraction of baryons in clusters and the cluster abundance combined with independent evidence for the baryon density, indicate that 85-90% of the mass in the universe does not interact with the electromagnetic force. The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy This "dark matter" is evident through its gravitational effect. Several categories of dark matter have been postulated.

• Baryonic dark matter - One candidate for missing dark baryonic matter is Rydberg matter, which has spectroscopic signatures in agreement with the unidentified infrared bands. Baryonic dark matter is Dark matter (matter that doesn't emit light composed of Baryons i Rydberg matter is a metastable state of highly excited atoms (see Rydberg atom) which are condensed in a solid- or liquid-like very low density matter ^[14]
• Nonbaryonic dark matter^[15] - which is divided into three different types:
  • Hot dark matter - nonbaryonic particles that move ultrarelativistically^[16]
  • Warm dark matter - nonbaryonic particles that move relativistically
  • Cold dark matter - nonbaryonic particles that move non-relativistically^[17]

Davis et al wrote in 1985:

Candidate particles can be grouped into three categories on the basis of their effect on the fluctuation spectrum (Bond et al 1983). Hot dark matter is a Hypothetical form of Dark matter which consists of particles that travel with Ultrarelativistic velocities In Physics, a particle is called ultrarelativistic when its speed is very close to the speed of light c such that its total energy E^2 = m^2 c^4 + p^2 c^2 Warm dark matter (WDM is theorized to have properties which are intermediate between those of Hot dark matter and Cold dark matter. Cold dark matter (or CDM) is a refinement of the Big bang theory that contains the additional assumption that most of the matter in the Universe consists If the dark matter is composed of abundant light particles which remain relativistic until shortly before recombination, then it may be termed "hot". The best candidate for hot dark matter is a neutrino [. . ] A second possibility is for the dark matter particles to interact more weakly than neutrinos, to be less abundant, and to have a mass of order 1eV. Such particles are termed "warm dark matter", because they have lower thermal velocities than massive neutrinos [. . ] there are at present few candidate particles which fit this description. Gravitinos and photinos have been suggested (Pagels and Primack 1982; Bond, Szalay and Turner 1982) [. . ] Any particles which became nonrelativistic very early, and so were able to diffuse a negligible distance, are termed "cold" dark matter (CDM). There are many candidates for CDM including supersymmetric particles^[18]

Hot dark matter consists of particles that travel with relativistic velocities. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial One kind of hot dark matter is known, the neutrino. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost Neutrinos have a very small mass, do not interact via either the electromagnetic or the strong nuclear force and are therefore very difficult to detect. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of In particle physics the strong interaction, or strong force, or color force, holds Quarks and Gluons together to form Protons and This is what makes them appealing as dark matter. However, bounds on neutrinos indicate that ordinary neutrinos make only a small contribution to the density of dark matter.

Hot dark matter cannot explain how individual galaxies formed from the Big Bang. The microwave background radiation as measured by the COBE and WMAP satellites, while incredibly smooth, indicates that matter has clumped on very small scales. Fast moving particles, however, cannot clump together on such small scales and, in fact, suppress the clumping of other matter. Hot dark matter, while it certainly exists in our universe in the form of neutrinos, is therefore only part of the story.

Estimated distribution of dark matter and dark energy in the universe

The Concordance Model requires that, to explain structure in the universe, it is necessary to invoke cold (non-relativistic) dark matter. Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo objects, or "MACHOs". Baryons are the family of Subatomic particles with a Baryon number of 1 Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of Dark However, studies of big bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter. In Physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than those of H-1 (i Baryons are the family of Subatomic particles with a Baryon number of 1

At present, the most common view is that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electrons, protons, neutrons, and known neutrinos. Baryons are the family of Subatomic particles with a Baryon number of 1 The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost The most commonly proposed particles are axions, sterile neutrinos, and WIMPs (Weakly Interacting Massive Particles, including neutralinos). The axion is a hypothetical Elementary particle postulated by Peccei-Quinn theory in 1977 to resolve the Strong-CP problem in Quantum chromodynamics A sterile neutrino is a hypothetical Neutrino that does not interact via any of the Fundamental interactions of the Standard Model except gravity In Particle physics, the neutralino is a hypothetical particle part of the doubling of the menagerie of particles predicted by supersymmetric theories None of these are part of the standard model of particle physics, but they can arise in extensions to the standard model. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them Many supersymmetric models naturally give rise to stable WIMPs in the form of neutralinos. In Particle physics, supersymmetry (often abbreviated SUSY) is a Symmetry that relates elementary particles of one spin to another particle that In Particle physics, the neutralino is a hypothetical particle part of the doubling of the menagerie of particles predicted by supersymmetric theories Heavy, sterile neutrinos exist in extensions to the standard model that explain the small neutrino mass through the seesaw mechanism. Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost In Theoretical physics, in the area of Quantum field theory, the seesaw mechanism is a mechanism to generate very small numbers from "reasonable numbers"

Detection of dark matter

These cosmological models predict that if WIMPs are what make up dark matter, trillions must pass through the Earth each second. Despite a number of attempts to find these WIMPs, none have yet been confirmedly found.

Experimental searches for these dark matter candidates have been conducted and are ongoing. These efforts can be divided into two broad classes: direct detection, in which the dark matter particles are observed in a detector; and indirect detection, which looks for the products of dark matter annihilations. Dark matter detection experiments have ruled out some WIMP and axion models. The axion is a hypothetical Elementary particle postulated by Peccei-Quinn theory in 1977 to resolve the Strong-CP problem in Quantum chromodynamics There are also several experiments claiming positive evidence for dark matter detection, such as DAMA/NaI, DAMA/LIBRA^[19] and EGRET, but these are so far unconfirmed and difficult to reconcile with the negative results of other experiments. The DAMA/NaI experiment was designed to detect Dark matter using the direct detection technique The Energetic Gamma Ray Experiment Telescope (EGRET was one of the four scientific instruments on NASA's Compton Gamma Ray Observatory satellite Several searches for dark matter are currently underway, including the Cryogenic Dark Matter Search in the Soudan mine, the XENON, DAMA/LIBRA and CRESST experiments at Gran Sasso and the ZEPLIN project at the Boulby Underground Laboratory (UK), and many new technologies are under development, such as the ArDM experiment. The Cryogenic Dark Matter Search ( CDMS) is a series of experiments designed to directly detect particle Dark matter in the form of WIMPs Using an array The Soudan Underground Mine State Park is a Minnesota state park at the site of the Soudan Underground Mine, on the south shore of Lake Vermilion The XENON Dark Matter Search Experiment aims to construct a next-generation Dark matter detector which will use liquid Xenon as the target material for finding The Cryogenic Rare Event Search with Superconducting Thermometers (CRESST is a collaboration of European experimental particle physics groups involved in the construction of cryogenic The Laboratori Nazionali del Gran Sasso is a Particle physics laboratory of the INFN, situated near the Gran Sasso Mountain in Italy ArDM (Argon Dark Matter is a Particle physics experiment based on a liquid Argon detector aiming at measuring signals from WIMPs (Weakly Interacting Massive

One possible alternative approach to the detection of WIMPs in nature is to produce them in the laboratory. Experiments with the Large Hadron Collider near Geneva may be able to detect WIMPs. Geneva (Genève is the second-most populous city in Switzerland (after Zürich) and is the most populous city of Romandy (the French -speaking Because a WIMP only has negligible interactions with matter, it can be detected as missing energy and momentum. It is also possible that dark matter consists of very heavy hidden sector particles which only interact with ordinary matter via gravity. In Particle physics, the term hidden sector refers to the collection of yet-unobserved Quantum fields and the corresponding hypothetical particles that do

The Cryogenic Dark Matter Search, in the Soudan Mine in Minnesota aims to detect the heat generated when ultracold germanium and silicon crystals are struck by a WIMP. Germanium (dʒɚˈmeɪniəm is a Chemical element with the symbol Ge and Atomic number 32 Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating The Gran Sasso National Laboratory at L'Aquila, in Italy, use xenon to measure the flash of light that occurs on those rare occasions when a WIMP strikes a xenon nucleus. Xenon (ˈzɛnɒn or) is a Chemical element represented by the symbol Xe. The results from April 2007, using 15 kg of liquid and gaseous xenon, failed to detect any, and in March 2008 the team started again using 150 kg of the material.

The GLAST space telescope, planned for launch in October 2008, searching gammawave events, may also detect WIMPs. WIMP supersymmetric particle and antiparticle collisions should release a pair of detectable gamma waves. The number of events detected will show to what extent WIMPs comprise dark matter.

With all these experiments together, scientists are becoming confident that WIMPs will be discovered in the near future. But some scientists are beginning to think that dark matter is composed of many different candidates. ^[20] WIMPs may thus only be a part of the solution.

Alternative explanations

Modifications of gravity

A proposed alternative to physical dark matter particles has been to suppose that the observed inconsistencies are due to an incomplete understanding of gravitation. Gravitation is a natural Phenomenon by which objects with Mass attract one another To explain the observations, the gravitational force has to become stronger than the Newtonian approximation at great distances or in weak fields. One of the proposed models is Modified Newtonian Dynamics (MOND), which adjusts Newton's laws at small acceleration. In Physics, Modified Newtonian dynamics ( MOND) is a theory that proposes a modification of Newton's Second Law of Dynamics (F = ma to explain Newton's laws of motion are three Physical laws which provide relationships between the Forces acting on a body and the motion of the However, constructing a relativistic MOND theory has been troublesome, and it is not clear how the theory can be reconciled with gravitational lensing measurements of the deflection of light around galaxies. This page is about the scientific concept of relativity for philosophical or sociological theories about relativity see Relativism. A gravitational lens is formed when the light from a very distant bright source (such as a Quasar) is "bent" around a massive object (such as a cluster of The leading relativistic MOND theory, proposed by Jacob Bekenstein in 2004 is called TeVeS for Tensor-Vector-Scalar and solves many of the problems of earlier attempts. Jacob David Bekenstein (born May 1, 1947) is a Physicist who has contributed to the foundation of Black hole thermodynamics and to other aspects Tensor-Vector-Scalar gravity (TeVeS is a proposed relativistic theory which purports to explain Galactic rotation curves without invoking Dark matter. However, a study in August 2006 reported an observation of a pair of colliding galaxy clusters whose behavior, it was claimed, was not compatible with any current modified gravity theories. ^[21]

In 2007, astronomer John W. Moffatt proposed a theory of modified gravity (MOG) based on the Nonsymmetric Gravitational Theory (NGT) that accounts for the behavior of colliding galaxies. John Moffat may refer to John Moffat (physicist (born 1932 professor at the University of Toronto known for his work suggesting the speed of light has varied In Theoretical physics, the nonsymmetric gravitational theory (NGT of John Moffat is a classical theory of Gravitation which tries to explain the mystery ^[22]

Quantum mechanical explanations

In another class of theories one attempts to reconcile gravitation with quantum mechanics and obtains corrections to the conventional gravitational interaction. Gravitation is a natural Phenomenon by which objects with Mass attract one another Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In scalar-tensor theories, scalar fields like the Higgs field couple to the curvature given through the Riemann tensor or its traces. Scalar-tensor theories are theories that include a scalar field as well as a Tensor field to represent an interaction especially the gravitational one In Physics, a scalar is a simple Physical quantity that is not changed by Coordinate system rotations or translations (in Newtonian mechanics or In Mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry In many of such theories, the scalar field equals the inflaton field, which is needed to explain the inflation of the universe after the Big Bang, as the dominating factor of the quintessence or Dark Energy. The inflaton is the generic name of the unidentified scalar field (and its associated particle that may be responsible for an episode of inflation in the very early In Physical cosmology, cosmic inflation is the idea that the nascent Universe passed through a phase of exponential expansion that The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. In Physics, quintessence is a hypothetical form of Dark energy postulated as an explanation of observations of an Accelerating universe. In Physical cosmology, dark energy is a hypothetical exotic form of Energy that permeates all of space and tends to increase the rate of expansion of the universe Using an approach based on the exact renormalization group, M. In Theoretical physics, renormalization group (RG refers to a mathematical apparatus that allows one to investigate the changes of a physical system as one views Reuter and H. Weyer have shown^[23] that Newton's constant and the cosmological constant can be scalar functions on spacetime if one associates renormalization scales to the points of spacetime. In Physical cosmology, the cosmological constant (usually denoted by the Greek capital letter Lambda: Λ was proposed by Albert Einstein as a modification Some M-Theory cosmologists also propose that multi-dimensional forces from outside the visible universe have gravitational effects on the visible universe meaning that dark matter is not necessary for a unified theory of cosmology. In Theoretical physics, M-theory is a new limit of String theory in which 11 dimensions of Spacetime may be identified

However it is important to remember that if dark matter passes through matter freely, a detector created from matter has little or no chance of either proving or disproving the existence of dark matter or energy.

Dark matter in popular culture

Main article: Dark matter in fiction

Mentions of dark matter occur in some video games and other works of fiction. Dark matter is occasionally described in Computer and video games and other works of Fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the properties of dark matter proposed in physics and cosmology.

See also

• Scalar Field Dark Matter
• SIMP
• Light Dark Matter
• Self-interacting dark matter
• Mirror matter
• Conformal gravity
• Robust Associations of Massive Baryonic Objects (RAMBOs)

References

External links

• Video on "The Mystery of Dark Matter" (Perimeter Institute for Theoretical Physics)
• Yale University Video Lecture on Dark Matter at Google Video
• Vacuum Modified Gravity as an explanation for flat galaxy rotation curves
• Another MOND is possible
• Video of Dark Matter
• Visibility/Detectability of Dark Matter (January 3, 2007)
• Found: Most of the Universe - by Phil Plait
• Hot Dark Matter - by Martin White
• Dark Matter Portal - by Frédéric Mayet
• "Most of our Universe is Missing" - Horizon video
• "Particle Dark Matter: Evidence, Candidates and Constraints" - by G. The Perimeter Institute for Theoretical Physics (shortened to Perimeter Institute or simply PI) is an independent resident-based research institute devoted to Bertone, D. Hooper and J. Silk
• "Experimental Searches for Dark Matter" - by Timothy J. Joseph Silk is the Savilian Chair of Astronomy at the University of Oxford. Sumner
• "Dark matter comes out of the cold" - BBC News 5 February 2005
• "Astronomers find Starless Galaxy" - BBC News 23 February 2005
• "In the Beginnings" - The Economist 13 January 2005
• Elliptical galaxies have dark matter halo as well
• Recent papers on dark matter on arXiv.org
• NASA Finds Direct Proof of Dark Matter (images), (text)
• Paper on Direct Proof of Dark Matter
• News and Video Release of 3D Map of Dark Matter's scaffolding at ESA/Hubble
• Dark matter exists CNN. Events 1576 - Henry of Navarre converts to Roman Catholicism in order to ensure his right to the throne of France. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Events 1455 - Traditional date for the publication of the Gutenberg Bible, the first Western Book printed from Movable Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. The Economist is an English-language weekly news and International affairs publication owned by The Economist Newspaper Ltd and edited in London Events 532 - Nika riots in Constantinople. 888 - Odo Count of Paris becomes King of the Franks Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. com
• Dark matter lacks extra gravitational force on Wikinews
• Dark matter introduction articles
• Avant News spoof on the true nature of dark matter
• "Team finds 'proof' of dark matter" Paul Rincon (2006) BBC online. Wikinews is a free-content News source Wiki and a project of the Wikimedia Foundation. First direct evidence for dark matter
• Dark Matter Exists by Sean M. Carroll. Sean M Carroll (born 1966 is a senior research associate in the Department of Physics at the California Institute of Technology.
• Cosmology's Missing Mass Problems by Robert Fritzius
• A big find in the hunt for elusive dark matter
• Seeing the unseeable (SeedMagazine. com)
• Dark Matter Halo From Dipole Gravity Model