Cold dark matter

Cold dark matter

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Cold dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

(or CDM) is the improvement of the big bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 theory that contains the additional assumption that most of the matter in the Universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 consists of material that cannot be observed by its electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 (dark) and whose constituent particles move slowly (cold). , most cosmologists favor the cold dark matter theory as a description of how the universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

), to the lumpy distribution of galaxies and their clusters
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

 we see today — the large-scale structure of the universe. The theory was originally published in 1984 by United States physicists Joel R. Primack
Joel Primack
Joel R. Primack is a professor of physics and astrophysics at the University of California, Santa Cruz and is a member of the Santa Cruz Institute for Particle Physics. Dr. Primack received his A.B. from Princeton University in 1966 and his Ph.D. from Stanford University in 1970. According to...

, George Blumenthal
George Blumenthal
George R. Blumenthal is an American astrophysicist, astronomer, professor, and academic administrator. He is the tenth chancellor of the University of California, Santa Cruz.-Biography:...

, and Sandra Moore Faber
Sandra M. Faber
Sandra Moore Faber is a University Professor of Astronomy and Astrophysics at the University of California, Santa Cruz, and works at the Lick Observatory.In 1966, she obtained a B.A., with high honors, in physics from Swarthmore College....

.

Primordial
Primordial black hole
A primordial black hole is a hypothetical type of black hole that is formed not by the gravitational collapse of a large star but by the extreme density of matter present during the universe's early expansion....

 intermediate mass black holes (created in the big bang instead of by mass accretion) between 30 and 300,000 solar masses in galactic halos are consistent with observations of wide binaries
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

 as well as microlensing and galactic disk stability.

In the cold dark matter theory, structure grows hierarchically, with small objects collapsing first and merging in a continuous hierarchy to form more and more massive objects. In the hot dark matter
Hot dark matter
Hot dark matter is a hypothetical form of dark matter which consists of particles that travel with ultrarelativistic velocities. The best candidate for the identity of hot dark matter is the neutrino. Neutrinos have very small masses, and do not take part in two of the four fundamental forces, the...

 paradigm, popular in the early eighties, structure does not form hierarchically (bottom-up), but rather forms by fragmentation (top-down), with the largest supercluster
Supercluster
Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.-Existence:...

s forming first in flat pancake-like sheets and subsequently fragmenting into smaller pieces like our galaxy the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

. The predictions of hot dark matter strongly disagree with observations of large-scale structure, whereas the cold dark matter paradigm is in general agreement with the observations.

Three important discrepancies between the predictions of the cold dark matter paradigm and observations of galaxies and their clustering in space have arisen, however, creating a potential crisis for the whole picture.
  • The cuspy halo problem
    Cuspy halo problem
    The cuspy halo problem arises from cosmological simulations that seem to indicate cold dark matter would form cuspy distributions — that is, increasing sharply to a high value at a central point — in the most dense areas of the universe. This would imply that the center of our galaxy,...

     is that cold dark matter predicts that the density distribution of DM halos be much more peaked than what is observed in galaxies by investigating their rotation curve.
  • The missing satellites problem
    Dwarf galaxy problem
    The dwarf galaxy problem is one that arises from numerical cosmological simulations that predict the evolution of the distribution of matter in the universe. Dark matter seems to cluster hierarchically and in ever increasing number counts for smaller and smaller sized halos...

     is that cold dark matter predicts large numbers of small dwarf galaxies about one thousandth the mass of the Milky Way
    Milky Way
    The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

    . These are not observed.
  • The angular momentum problem is that cold dark matter predicts significant amounts of low angular momentum material, which is not present in most disk galaxies.


All of these problems have a number of proposed solutions. However, it remains unclear whether they represent a real crisis for the CDM paradigm, or an indication that the model needs further development.

The CDM theory makes no predictions about exactly what the cold dark matter particles are, and one large weakness in the cold dark matter theory is that it is unclear what the dark matter consists of. The candidates fall into three categories which are "humorously" named, as is common in physics.
  • WIMPs or Weakly Interacting Massive Particles assume that the dark matter is some sort of heavy unknown particle. Unfortunately, there is no known particle with the required properties. The search for these involves attempts at direct detection by highly sensitive detectors and attempts at production by particle accelerator
    Particle accelerator
    A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...

    s.
  • Axion
    Axion
    The axion is a hypothetical elementary particle postulated by the Peccei-Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics...

    s have actually been the first cold dark matter candidates. Their cosmology is very different from the other particles of the Standard Model
    Standard Model
    The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

     (& beyond), being potentially CDM even with a very small mass. Being a consequence of the solution to the Strong CP problem in QCD
    QCD
    The initialism QCD may refer to:* Quantum chromodynamics, the theory describing the Strong Interaction* Quantum circuit diagram* Quality, Cost, Delivery, a three-letter acronym used in lean manufacturing...

    , they are also still hypothetical and actively sought.
  • MACHOs or Massive Compact Halo Object
    Massive compact halo object
    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 matter in galaxy halos. A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar...

    s
    assume that the dark matter consists of condensed objects such as black hole
    Black hole
    A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

    s, neutron star
    Neutron star
    A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...

    s, white dwarf
    White dwarf
    A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

    s, very faint star
    Star
    A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

    s, or non-luminous objects like planet
    Planet
    A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

    s. The search for these consists of using gravitational lensing to see the effect of these objects on background galaxies.


It is also thought that CDM may be made of purely gravitating inflationary relics - so-called dark matter "X-particles" such as Holeum
Holeum
Holeums are stable, quantized gravitational bound states of primordial or micro black holes.-Introduction:Holeums were proposed by L.K. Chavda and Abhijit Chavda in 2002. They have all the properties associated with cold dark matter...

s., or simply of surface-like singularities in the metric field whose Einstein–Hilbert action coincides with that of closed bosonic strings
.

See also

  • Dark matter
    Dark matter
    In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

    • Hot dark matter
      Hot dark matter
      Hot dark matter is a hypothetical form of dark matter which consists of particles that travel with ultrarelativistic velocities. The best candidate for the identity of hot dark matter is the neutrino. Neutrinos have very small masses, and do not take part in two of the four fundamental forces, the...

       (HDM)
    • Warm dark matter
      Warm dark matter
      Warm dark matter is a hypothesized form of dark matter that has properties intermediate between those of hot dark matter and cold dark matter, causing structure formation to occur bottom-up from above their free-streaming scale, and top-down below their free streaming scale. The most common WDM...

       (WDM)
  • Lambda-CDM model
    Lambda-CDM model
    ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...

  • Modified Newtonian dynamics
    Modified Newtonian dynamics
    In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...