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Dark Matter

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Vanessa Korias
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« on: April 10, 2007, 12:44:19 am »

Alternative explanations

A proposed alternative to physical dark matter particles has been to suppose that the observed inconsistencies are due to an incomplete understanding of gravitation. 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 corrects Newton's laws at small acceleration. 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. 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. A theory of modified gravity (MOG) proposed by John W. Moffatt, based upon the Nonsymmetric Gravitational Theory (NGT), is also an alternative to dark matter.

In August 2006, a study of colliding galaxy clusters claimed to show that even in a modified gravity hypothesis, the majority of the mass must be some form of dark matter by demonstrating that when regular matter is "swept away" from a cluster, the gravitational effects of dark matter (which is thought to be non-interacting aside from its gravitational effect) remain.[17] A study claims that TeVeS may be able to produce the observed effect, but this still requires the majority of the mass to be in the form of dark matter, possibly in the form of ordinary neutrinos.[18] Also Nonsymmetric Gravitational Theory has been claimed to qualitatively fit the observations without needing exotic dark matter.[19]

In another class of theories one attempts to reconcile gravitation with quantum mechanics and obtains corrections to the conventional gravitational interaction. In scalar-tensor theories, scalar fields like the Higgs field couples to the curvature given through the Riemann tensor or its traces. 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. Using an approach based on the exact renormalization group, M. Reuter and H. Weyer have shown[20] that Newton's constant and the cosmological constant can be scalar functions on spacetime if one associates renormalization scales to the points of spacetime.

http://en.wikipedia.org/wiki/Dark_matter
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