Do dwarf galaxies favour MOND over Dark Matter? (Forwarded)

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CONTACTS

Garry W. Angus
School of Physics and Astronomy
SUPA, University of St Andrews
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KY16 9SS
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Website: http://star-www.st-and.ac.uk/~gwa2/

EMBARGOED UNTIL 0001 BST, 2 April 2008

Ref.: PN 08/18 (NAM 09)

Do dwarf galaxies favour MOND over Dark Matter?

A detailed analysis of eight dwarf galaxies that orbit the Milky Way
indicates that their orbital behaviour can be explained more accurately
with Modified Newtonian Dynamics (MOND) than by the rival, but more widely
accepted, theory of dark matter. The results will be presented by Garry
Angus, of the University of St Andrews, at the RAS National Astronomy
Meeting in Belfast on Wednesday 2nd April.

"MOND was first suggested to account for things that we see in the distant
universe. This is the first detailed study in which we've been able to
test out the theory on something close to home. The MOND calculations and
the observations appear to agree amazingly well. We've also found some
exciting tidal effects predicted by MOND that we should be able to test
through future observations and simulations," said Angus.

Essentially, MOND adds a new constant of nature (a0) to physics, besides
the speed of light, Planck's constant among 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 is so small that it goes 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 we
observe in galaxies or clusters of galaxies, then the constant makes a
significant difference to the resulting gravitational forces.

When MOND is applied to the population of dwarf galaxies, one effect is
that tidal forces from the Milky Way, which have a negligible effect in
classical Newtonian Mechanics, can actually make a big difference. This is
particularly significant for the dwarfs orbiting our Galaxy most closely.

"In these dwarf galaxies, the internal gravity is very weak compared to
the gravity of the Milky Way. MOND suggests that the Milky Way is a bit
like a bank that loans out gravity to nearby dwarf galaxies to make them
more stable. However, there are conditions on the loan: if the dwarf
galaxies start to approach the bank, the loan is gradually reduced or even
cancelled and the dwarfs must pay it back. In two galaxies, we've seen
what could be signs that they've come too close too quickly and are unable
to repay the loan fast enough. This appears to have caused disruption to
their equilibrium," said Angus.

Angus used MOND to calculate the ratio of mass to amount of light emitted
by the stars in the dwarf galaxies from the observed random velocities of
the stars collected independently. He also calculated the orbital paths of
the stars in the dwarf galaxies. In all eight cases, the MOND calculations
for the orbits were within predictions. For six of the eight galaxies, the
calculations were also a good match to expected values for mass-to-light
ratios; however for two galaxies, Sextans and Draco, the ratios were very
high, which could well suggest tidal effects. The value for Sextans could
also be due to poor quality measurements of the galaxy's luminosity, which
are improving all the time for these ultra dim objects.

"These tidal effects can be tested by updating the 13 year old luminosity
of Sextans and making accurate observations of the orbits of Draco and
Sextans around the Milky Way. We also need to carry out some detailed
simulations to understand the exact mechanisms of the tidal heating," said
Angus.

If Newton's gravity holds true, the dark matter needed in the dwarfs has
constant density in the centre which is contrary to theoretical
predictions, which suggest density should rise to the centre.

"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," said Angus.

IMAGES
[http://www.ras.org.uk/images/stories...satellites.jpg
(424KB)]
Satellite galaxies of the Milky Way.

NOTES FOR EDITORS

RAS NATIONAL ASTRONOMY MEETING

The RAS National Astronomy Meeting (NAM 2008) is hosted by Queen's
University Belfast. It is principally sponsored by the RAS and the STFC.
NAM 2008 is being held together with the UK Solar Physics (UKSP) and
Magnetosphere, Ionosphere and Solar-Terrestrial (MIST) spring meetings.

MODIFIED NEWTONIAN DYNAMICS (MOND)

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.

DWARF GALAXIES

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.

Andrew Yee wrote:
Do dwarf galaxies favour MOND over Dark Matter?

A detailed analysis of eight dwarf galaxies that orbit the Milky Way
indicates that their orbital behaviour can be explained more accurately
with Modified Newtonian Dynamics (MOND) than by the rival, but more widely
accepted, theory of dark matter. The results will be presented by Garry
Angus, of the University of St Andrews, at the RAS National Astronomy
Meeting in Belfast on Wednesday 2nd April.

It seems to me that the University of St. Andrews are a bunch of rabble
rousers and malcontents for Dark Matter. :-)

First, one of their staff, Dr. HongSheng Zhao, throws an alternative
theory to Dark Matter and Dark Energy called Dark Fluid, which
completely explains away the behaviour we see out of galaxies and
galactic clusters without using Dark Matter.

Dark Fluid: Dark Matter And Dark Energy May Be Two Faces Of Same Coin
http://www.sciencedaily.com/releases...0131094056.htm

And now showing MOND to be a stronger theory than Dark Matter as well?
Will their heresy never end? :-)

Yousuf Khan