Satellite Galaxies Challenge Newtonian Model

April 22, 2009

The high speed of stars and apparent presence of “Ëœdark 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. 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 “Ëœmodified 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.

Image Caption: An image of the Draco satellite dwarf galaxy. Credit: Mischa Schirmer, University of Bonn

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