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Dark matter page 2 (continued from page 1)

Composition

An important property of all dark matter is that it behaves like and is modeled like a perfect fluid, meaning that it does not have any internal resistance or viscosity.[65] This means that dark matter particles should not interact with each other (except through gravity), i.e. they move past each other without ever bumping or colliding. Also as discussed above, "cold" theories, as opposed to the "warm" or "hot" perspectives on the composition of dark matter, gained favor at better explaining observable phenomena.

[edit] Detection

If the dark matter within our galaxy is made up of Weakly Interacting Massive Particles (WIMPs), then a large number must pass through the Earth each second. There are many experiments currently running, or planned, aiming to test this hypothesis by searching for WIMPs. Although WIMPs are a more popular dark matter candidate,[7] there are also experiments searching for other particle candidates such as axions. It is also possible that dark matter consists of very heavy hidden sector particles which only interact with ordinary matter via gravity.

These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of WIMP annihilations.[66]

An alternative approach to the detection of WIMPs in nature is to produce them in the laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect WIMPs produced in collisions of the LHC proton beams; because a WIMP has negligible interactions with matter, it may be detected indirectly as (large amounts of) missing energy and momentum which escape the LHC detectors, provided all the other (non-negligible) collision products are detected.[67] These experiments could show that WIMPs can be created, but it would still require a direct detection experiment to show that they exist in sufficient numbers in the galaxy, to account for dark matter.[68]

[edit] Direct detection experiments

Direct detection experiments typically operate in deep underground laboratories to reduce the background from cosmic rays. These include: the Soudan mine; the SNOLAB underground laboratory at Sudbury, Ontario (Canada); the Gran Sasso National Laboratory (Italy); the Boulby Underground Laboratory (UK); and the Deep Underground Science and Engineering Laboratory, South Dakota (US).

The majority of present experiments use one of two detector technologies: cryogenic detectors, operating at temperatures below 100mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect the flash of scintillation light produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include: CDMS, CRESST, EDELWEISS, EURECA. Noble liquid experiments include ZEPLIN, XENON, DEAP, ArDM, WARP and LUX. Both of these detector techniques are capable of distinguishing background particles which scatter off electrons, from dark matter particles which scatter off nuclei. Other experiments include SIMPLE and PICASSO.

The DAMA/NaI, DAMA/LIBRA experiments have detected an annual modulation in the event rate,[69] which they claim is due to dark matter particles. (As the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount depending on the time of year). This claim is so far unconfirmed and difficult to reconcile with the negative results of other experiments assuming that the WIMP scenario is correct.[70]

Directional detection of dark matter is a search strategy based on the motion of the Solar System around the galactic center. By using a low pressure TPC, it is possible to access information on recoiling tracks (3D reconstruction if possible) and to constrain the WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun is travelling (roughly in the direction of the Cygnus constellation) may then be separated from background noise, which should be isotropic. Directional dark matter experiments include DMTPC, DRIFT, Newage and MIMAC.

On 17 December 2009 CDMS researchers reported two possible WIMP candidate events. They estimate that the probability that these events are due to a known background (neutrons or misidentified beta or gamma events) is 23%, and conclude "this analysis cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject either event as signal."[71]

More recently, on 4 September 2011, researchers using the CRESST detectors presented evidence[72] of 67 collisions occuring in detector crystals from sub-atomic particles, calculating there is a less than 1 in 10,000 chance that all were caused by known sources of interference or contamination. It is quite possible then that many of these collisions were caused by WIMPs, and/or other unknown particles.

[edit] Indirect detection experiments

Indirect detection experiments search for the products of WIMP annihilation. If WIMPs are Majorana particles (the particle and antiparticle are the same) then two WIMPs colliding would annihilate to produce gamma rays, and particle-antiparticle pairs. This could produce a significant number of gamma rays, antiprotons or positrons in the galactic halo. The detection of such a signal is not conclusive evidence for dark matter, as the backgrounds from other sources are not fully understood.[7][66]

The EGRET gamma ray telescope observed an excess of gamma rays, but scientists concluded that this was most likely a systematic effect.[73] The Fermi Gamma-ray Space Telescope, launched June 11, 2008, is searching for gamma ray events from dark matter annihilation.[74] At higher energies, the ground-based MAGIC gamma-ray telescope has set limits to the existence of dark matter in dwarf spheroidal galaxies [75] and clusters of galaxies.[76]

The PAMELA payload (launched 2006) has detected an excess of positrons, which could be produced by dark matter annihilation, but may also come from pulsars. No excess of anti-protons has been observed.[77]

WIMPs passing through the Sun or Earth are likely to scatter off atoms and lose energy. This way a large population of WIMPs may accumulate at the center of these bodies, increasing the chance that two will collide and annihilate. This could produce a distinctive signal in the form of high energy neutrinos originating from the center of the Sun or Earth.[78] It is generally considered that the detection of such a signal would be the strongest indirect proof of WIMP dark matter.[7] High energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this.

WIMP annihilation from the Milky Way Galaxy as a whole may also be detected in the form of various annihilation products.[79] The Galactic Center is a particularly good place to look as it contains the largest dark matter abundance.[80]

[edit] Alternative theories

Although dark matter is the most popular theory to explain the various astronomical observations of galaxies and galaxy clusters, there has been no direct observational evidence of dark matter. Some alternative theories have been proposed to explain these observations without the need for a vast amount of undetected matter. They broadly fall into the categories of modified gravity laws, and quantum gravity laws. The difference between modified gravity laws and quantum gravity laws is that modified gravity laws simply propose alternative behaviour of gravity at astrophysical and cosmological scales, without any regard to the quantum scale. Both posit that gravity behaves differently at different scales of the universe, making the laws established by Newton and Einstein insufficient.

[edit] Modified gravity laws

One group of alternative theories to dark matter assume that the observed inconsistencies are due to an incomplete understanding of gravitation rather than invisible matter. These theories propose to modify the laws of gravity instead.

The earliest modified gravity model to emerge was Mordehai Milgrom's Modified Newtonian Dynamics (MOND) in 1983, which adjusts Newton's laws to create a stronger gravitational field when gravitational acceleration levels become tiny (such as near the rim of a galaxy). It had some success in predicting galactic-scale features, such as rotational curves of elliptical galaxies, and dwarf elliptical galaxies, etc. It fell short in predicting galaxy cluster lensing. However, MOND was not relativistic, since it was just a straight adjustment of the older Newtonian account of gravitation, not of the newer account in Einstein's general relativity. Work began soon after to make MOND conform to General Relativity. It's an ongoing process, and many competing theories have emerged based around the original MOND theory, such as TeVeS, and MOG or STV gravity, phenomenological covariant approach,[81] etc.

In 2007, John W. Moffat proposed a modified gravity theory based on the Nonsymmetric Gravitational Theory (NGT) that claims to account for the behavior of colliding galaxies.[82] This theory still requires the presence of non-relativistic neutrinos, or other candidates for (cold) dark matter, to work.

Another proposal utilizes a gravitational backreaction in an emerging theoretical field that seeks to explain gravity between objects as an action, a reaction, and then a back-reaction. Simply, an object A affects an object B, and the object B then re-affects object A, and so on: creating somewhat of a feedback loop that strengthens gravity.[83]

Recently, another group has proposed a modification of large scale gravity in a theory named "dark fluid". In this formulation, the attractive gravitational effects attributed to dark matter are instead a side-effect of dark energy. Dark fluid combines dark matter and dark energy in a single energy field that produces different effects at different scales. This treatment is a simplified approach to a previous fluid-like model called the Generalized Chaplygin gas model where the whole of spacetime is a compressible gas.[84] Dark fluid can be compared to an atmospheric system. Atmospheric pressure causes air to expand, but part of the air can collapse to form clouds. In the same way, the dark fluid might generally expand, but it also could collect around galaxies to help hold them together.[84]

Another set of proposals is based on the possibility of a double metric tensor for space-time.[85] It has been argued that time reversed solutions in general relativity require such double metric for consistency, and that both Dark Matter and Dark Energy can be understood in terms of time reversed solutions of general relativity.[86]

[edit] Quantum Gravity

Main article: Quantum gravity

Quantum Gravity is an active wide-ranging theoretical physics field that encompasses many different competing theories, and even many different competing families of theories. It is also sometimes known as the Theory of Everything or TOE. Basically, it is a class of theories that attempts to reconcile the two great not-yet-reconciled laws of physics, gravitation with quantum mechanics, and obtain corrections to the current gravitational laws. Examples of quantum gravity theories are Superstring theory, its successor M-Theory, as well as the competing Loop Quantum Gravity.

In a sense, quantum gravity is a much more ambitious field of study than dark matter, since quantum gravity is an all-encompassing attempt to reconcile gravity with the other fundamental forces of nature, whereas dark matter is simply a classical physics solution for a classical gravity problem. It is hoped that once a testable quantum gravity theory emerges, that one of its side benefits will be to explain these various gravitational mysteries from first principles rather than through empirical methods alone.

Some Superstring/M-Theory cosmologists 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. M-Theory envisions that the universe is made up of more than the observable 3 spatial and 1 time dimensions, and that there are up to 11 dimensions altogether. The remaining dimensions are hidden from our full view and only show up at the quantum levels. However, if there are particles or energy that exist only within these alternate dimensions, then they might account for the gravitational effects currently attributed to dark matter.

Loop quantum gravity and its subset Loop quantum cosmology envisions spacetime itself as being made up of elementally small particles, or quanta. This is quite different from how we usually envision empty space, as being simply empty, i.e. full of nothing: LQG and LQC says even empty space is actually made of something. Each particle of spacetime in various ways loops up (combines and twists) with adjacent particles of spacetime to create all of the matter and energy we see in the universe today. In this sense, if matter is just crumpled up spacetime, then even the empty untwisted space near a large body of matter would be put under more tension than empty untwisted space far away from matter; think of a long chain that you crumple up in the middle, the uncrumpled chainlinks near the crumpled up portion would still feel a large tension. This can be thought of as the same effect as dark matter. Chain links far away from the twists would feel little or no tension and would be in a state of relaxation, this can be analogous to dark energy.

In a 2004 study at the University of Mainz in Germany,[87] it has been found that if one applies just a standard quantum mechanical approach to Newton's Gravitational constant at various scales within the astrophysical realm (i.e. scales from solar systems up to galaxies), it can be shown that the Gravitational constant is not so constant anymore and actually starts to grow. The implication of this is that if the Gravitational constant grows at different scales, then dark matter is not needed to explain galactic rotational curves.

[edit] Popular culture

Main article: Dark matter in fiction

Mentions of dark matter occur in some 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.

[edit] See also

Stylised Lithium Atom.svg Physics portal

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[edit] Further reading

Author:Bling King
Published:Sep 25th 2011
Modified:Dec 31st 2011
3

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