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2Physics Quote:
"Lasers are light sources with well-defined and well-manageable properties, making them an ideal tool for scientific research. Nevertheless, at some points the inherent (quasi-) monochromaticity of lasers is a drawback. Using a convenient converting phosphor can produce a broad spectrum but also results in a loss of the desired laser properties, in particular the high degree of directionality. To generate true white light while retaining this directionality, one can resort to nonlinear effects like soliton formation."
-- Nils W. Rosemann, Jens P. Eußner, Andreas Beyer, Stephan W. Koch, Kerstin Volz, Stefanie Dehnen, Sangam Chatterjee
(Read Full Article: "Nonlinear Medium for Efficient Steady-State Directional White-Light Generation"
)

Sunday, March 04, 2007

Limit on Size of Dark Matter Clumps

Joseph Silk (photo courtesy: Oxford University)

Considering that the theory of gravitation is correct, when cosmologists analyze various observed data of our universe, they arrive at an intriguing observation -- that there's not nearly enough visible matter to hold the universe together. In fact, up to 95% appears to be missing.
The idea of the existence of dark matter originates from this. 'Dark matter' is supposed to be that illusive mass that remains invisible to modern day telescopes because it does not interact strongly with electromagnetic waves.

According to the model agreed upon by most physicists so far, dark matter could exist either as an accumulation of as-yet unseen 'weakly interacting massive particles' (WIMPs), or large clumps of 'massive compact objects' (MCOs) that do not emit any observable amount of radiation – or even as a mixture of both types.

Now Benton Metcalf from the Max Planck Institute for Astrophysics in Germany and Joseph Silk from the University of Oxford in the UK have attempted to see just how large these MCOs can be. They analysed the light from a supernovae five billion light years away. If an MCO had been there near the path of one of these light beams, the light would be undergo a measurable amount of dispersion by the MCO's gravitational field in an effect known as "gravitational lensing".

Because of the long path the light took to arrive the earth, the chances of a large MCO straying through would have been fairly high. But even after ploughing through data collected from almost 300 supernovae, the scientists could not find any dispersion caused by possible MCOs larger than one-hundredth the mass of the Sun. This implies that there is an 89% certainty they do not exist at all. Moreover, the physicists claim that MCOs larger than one-tenth the mass of the Earth can be confidently "eliminated" as the sole constituent of dark matter.

Until now many cosmologists believed in the existence of faint stars, neutron stars and black holes as significant constituents of dark matter. This result comes as a shock to those ideas. According to the recent Physical Review Letters paper by Metcalf and Silk, dark matter is more likely made of WIMPs.

Reference:
R. Benton Metcalf and Joseph Silk, "New Constraints on Macroscopic Compact Objects as Dark Matter Candidates from Gravitational Lensing of Type Ia Supernovae", Phys. Rev. Lett. 98, 099903 (E) (2007). Link to Abstract

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