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Asunto:[debunker] prueba
Fecha:Viernes, 23 de Noviembre, 2001  20:53:14 (-0500)
Autor:illu minati <illu03 @.......com>

ASTRONOMY

Do Magnetic Fields Explain the Enigma
Of Rotation in Spiral Galaxies?
by Eduardo Battaner
EDITOR’S NOTE
Astronomers were surprised in the
1970s when they discovered that the
rotation of galaxies seems not to obey
Newton’s laws. Instead of dropping off
with increasing distance from the nucle-
us, as Newton’s laws predict, rotational
velocity remains roughly constant. The
change in rotational velocity with in-
creasing distance from the nucleus is
expressed in a galaxy’s rotation curve.
Beyond the inner portion of the galaxy,
rotation curves are observed to be a!-
ways more or less flat (velocity is rough-
ly constant in kilometers per second).
Astronomers assumed that changes in
brightness along a galactic radial line
were an adequate measure of differ-
ences in mass concentration, and on
that basis (with the help of Newton’s
laws) expected a drop-off of velocity.
(Astronomers refer to ‘Keplerian rota-
tion,” but it is Newton, who introduced
the factor of mass, on whom they rely.)
As a result of this enigma, hypotheses
have been put forward arguing that
there is matter in galaxies that is invisi-
ble because it does not radiate. Such
“dark matter,” according to these hy-
potheses, is gravitationally significant,
sufficing to make the rotation curves
flat. There is, after all, no known law of
nature by which mass is obliged to an-
nounce itself by emitting electromag-
netic radiation at intensities and wave-
lengths that can be detected. The
question then arises, Why is there al-
ways just enough dark matter to keep
the rotation rate constant, regardless of
large differences in the sizes and masses
of the galaxies?
Another approach, pursued here by
Spanish physicist Eduardo Battaner, is
to ask whether magnetic fields in galac-
tic discs might create sufficient added
centripetal force to allow higher veloci-
ties than otherwise possible.
A
large variety of observational, theo-
retical and even metaphysical rea-
sons are now invoked to justify the belief
that the visible universe is but a small
fraction of the whole. The dark matter
hypothesis was at first brilliant and risky,
but now has a large acceptance in the
astrophysical and particle physics com-
munities. It provides a cumbersome
view of the universe and of our under-
standing of it. The unobservable matter
may be 100 times more massive than the
observable.
Suppose that the elementary particle
constituting dark matter is the axion.*
There would then be 10^9 axions/cm3,
which means 1016 axions per proton!
Our familiar galaxies would move in a
sea of unknown nature, and even the
mass of a galaxy becomes practically an
unknown quantity. However, all the as-
trophysical difficulties introduced by this
hypothesis are “philosophically” com-
pensated because dark matter could
make possible a flat universe, one that
is mathematically simple.
The observational bases for the exis-
tence of unobservable mass are, howev-
er, not so firm. Some of the most reliable
presumed evidence is provided by the
observation of large rotation velocities of
the gas on the peripheries of spiral galax-
ies. A large mass must be present to pre-
vent the large centrifugal force from dis-
persing this gas (Figure 1).
The problem can be introduced sim-
ply. At very large distances r from the
nucleus, the mass density should have
decreased so much that the gravitational
force would just be that created by a
point-like mass at the center of the
galaxy, having the mass of the whole
galaxy M; that is, GMm/r2 (G being the
gravitational constant and m the mass of
Note
Terms defined in the glossary are Indicated in
the text by an asterisk at their first appearance.
a particle at distance r). The outward
centrifugal force is mv2/r, v being the
circular velocity.
In the absence of other forces, both
gravitational and centrifugal forces must
be in balance, therefore:
A study of the Andromeda galaxy by
Vera Rubin in about 1970 was the first
to disclose that the rotational motion of
a galaxy was not what was expected.
Andromeda (M3 1) is shown here.


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