Written by Nancy
Atkinson
Draco satellite dwarf galaxy. Credit: Mischa Schirmer,
University of Bonn
A group of physicists say that the distribution of satellite
galaxies that orbit the Milky
Way, as well as the apparent dark matter within them,
presents a direct challenge to Newton’s theory of gravitation,
as the galaxies are not where they should be. “There is
something odd about their distribution,” said Professor Pavel
Kroupa from the University of Bonn in Germany. “They should be
uniformly arranged around the Milky Way, but this is not what we
found.” 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 physicists say that Newton’s theory of gravitation should be
modified.
The astronomers from Germany, Austria and Australia looked at
the small dwarf galaxies that orbit the Milky Way and 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). Some of these contain only a few thousandstars and
so are relatively faint and difficult to find.
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 Dr.
Manuel Metz said, “Fragments from early collisions can form the
revolving dwarf galaxies we see today, but 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.”
Metz added, “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.
The two studies will appear in papers in Monthly Notices of the
Royal Astronomical Society and the Astrophysical Journal.