The high speed of stars and apparent presence of ‘dark matter’
in the satellite galaxies that orbit our Milky Way Galaxy
presents a direct challenge to Newton’s theory of gravitation,
according to physicists from Germany, Austria and Australia.
An image of the Draco satellite dwarf galaxy. (Credit:
Mischa Schirmer, University of Bonn)
Professor Pavel Kroupa of the University of Bonn’s
Argelander-Institut fuer Astronomie (AlfA) will discuss the
results of the team’s two studies in a presentation on Wednesday
22nd April at the European Week of Astronomy and Space Science
conference at the University of Hertfordshire.
Together with scientists at the University of Vienna and the
Australian National University in Canberra, the AlfA team looked
at the small dwarf galaxies that orbit the Milky Way. Some of
these contain only a few thousand stars and so are relatively
faint and difficult to find. Standard cosmological models
predict the presence of hundreds of these companions around most
of the larger galaxies, but up to now only 30 have been observed
around the Milky Way.
The team of scientists looked at the distribution of these
satellite dwarf galaxies and discovered they were not where they
should be. “There is something odd about their distribution”,
explains Professor Kroupa. “They should be uniformly arranged
around the Milky Way, but this is not what we found.” The
astronomers discovered that the eleven brightest of the dwarf
galaxies lie more or less in the same plane - in a kind of disk
shape - and that they revolve in the same direction around the
Milky Way (in the same way as planets in the Solar System
revolve around the Sun).
Professor Kroupa and the other physicists believe that this can
only be explained if today’s satellite galaxies were created by
ancient collisions between young galaxies. Team member and
former colleague Dr Manuel Metz, now at the Deutsches Zentrum
fuer Luft- and Raumfahrt, also worked on the study. “Fragments
from early collisions can form the revolving dwarf galaxies we
see today” comments Dr Metz. But he adds that this introduces a
paradox. “Calculations suggest that the dwarf satellites cannot
contain any dark matter if they were created in this way. But
this directly contradicts other evidence. Unless the dark matter
is present, the stars in the galaxies are moving around much
faster than predicted by Newton’s standard theory of
gravitation.”
Dr Metz continues, “The only solution is to reject Newton’s
theory. If we live in a Universe where a modified law of
gravitation applies, then our observations would be explainable
without dark matter.”
With this evidence, the team share the convictions of a number
of groups around the world who believe that some of the
fundamental principles of physics have been incorrectly
understood. If their ideas are correct, it will not be the first
time that Newton’s theory of gravitation has been modified. In
the 20th century it happened when Einstein introduced his
Special and General Theories of Relativity and again when
quantum mechanics was developed to explain physics on sub-atomic
scales. The anomalies detected by Dr. Metz and Professor Kroupa
and their collaborators imply that where weak accelerations
predominate, a ‘modified Newtonian dynamic’ may have to be used.
If the scientists are right then this has far-reaching
consequences for our understanding of the Universe we live in.