Wednesday, April 16, 2008

Divisive idea 'explains galaxies'

By Paul Rincon
Science reporter, BBC News, Belfast

Dwarf galaxies (Image: Nasa)
Dwarf galaxies are relatively small galaxies containing a few billion stars

A controversial theory of physics may explain some aspects of galaxy behaviour better than rival, but more widely accepted, ideas.

That is the claim of an astronomer who studied eight so-called dwarf galaxies.

Modified Newtonian Dynamics (Mond) is proposed as an alternative to the widely accepted theory of dark matter to explain the dynamics of galaxies. (Wikipedia)

Garry Angus, from St Andrews University, said Mond effects could be very important for small galaxies.

Details of the study have been presented at the UK National Astronomy Meeting in Belfast and a press release.


Mond would add a new constant of nature - called a0 - to physics, besides the speed of light, Planck's constant, and many others.

Above it, accelerations are exactly as predicted by Newton's second law, which says that a force equals an object's mass times its acceleration.

Below it, gravity decays with distance from a mass, rather than distance squared.
This constant would be so small that it would go unnoticed with the large accelerations that we experience in day-to-day life.

For instance, when we drop a ball the gravity is 100 billion times stronger than a0 and the accelerated motion of the Earth round the Sun is 50 million times stronger.

However, when objects are accelerating extremely slowly, as is seen in galaxies or clusters of galaxies, the constant makes a significant difference to the resulting gravitational forces.

Problem cases

Mr Angus took two key parameters of the eight Milky Way dwarf galaxies and tried to fit them to predictions made by the Mond theory.

These two parameters were the ratio of mass to the amount of light emitted by the stars in the dwarf galaxies (the mass-to-light ratio) and the orbital paths of stars in the galaxies.

"Six out of the eight definitely fit the available data very, very well," the University of St Andrews researcher told BBC News.

"There are two problem cases, and we're going to run simulations to check them."

The most troublesome of these problem cases is a dwarf galaxy called Draco.

But Mr Angus said this could be because this galaxy was falling towards the Milky Way at a speed of 300km/s.

Our galaxy's gravity could be acting upon it, such that tidal forces were beginning to tear Draco apart.

The value for Sextans could also be due to tidal effects but also due to using old measurements of the galaxy's luminosity. Mr Angus said the data were improving all the time for these ultra dim objects.

But he explained: "The larger the systems you go to, the worse Mond fits the data. Presumably there is something we're not understanding at the scales of clusters of thousands of galaxies and upwards."

At these scales, dark matter seems to be the only realistic proposal. This invisible form of matter is thought to make up some 22% of the Universe. The matter we can see makes up a paltry 4%.

The origins of the dark matter theory date back to 1933, when the Swiss astronomer Fritz Zwicky found evidence for unseen mass within a galaxy cluster. This became known as the "missing mass problem".

Observational evidence for dark matter has since been obtained from the study of the motions of galaxies. And this continues to be the dominant theory to explain observations of galaxies.


The MOND theory was first put forward in 1983 by Mordehai Milgrom, now at the Weizmann Institute of Science in Rehovot, Israel. The constant, a0 is equal to 1.2 x 10-10 m/s2. Milgrom points out that this value is also the acceleration that you get by dividing the speed of light by the lifetime of the universe.


There are fourteen dwarf galaxies known to orbit the Milky Way, including the Large and Small Magellanic Clouds. The eight selected for this study are all spherical in shape and lie around 240-750 million trillion (x10^16) kilometres outside the Milky Way.

"Even without direct detection, the dark matter theory is difficult to prove or refute and although we may not be able to prove whether MOND is correct, by carrying out these kind of tests we can see if it continues to hold up or if it is definitely ruled out,"

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