Tuesday 23 July 2013

A team of scientists at the University of Salford has developed an extension of Einstein’s theory of general relativity.  This update of Einstein's equations, to appear online in the July edition of the research journal Physical Review , may have major implications for our understanding of galaxies and the universe.  It could also have ramifications in distinct areas of theoretical physics that rely on general relativity.

Astronomers have been searching since the 1930s for a mysterious substance called ‘dark matter’.  The consensus view of the scientific community has been that such dark matter had to exist - to explain observations of anomalous speeds of galaxies.  Yet, to date, this search has remained fruitless.  This new version of general relativity, developed at the University of Salford, explains the observed speeds without the need to assume that dark matter exists. 

The work was carried out by Edmund Chadwick, Tim Hodgkinson and Graham McDonald of the University’s Materials & Physics Research Centre

Tim Hodgkinson explained: "Some people might argue that Einstein's theory, in which there is curvature of space and time due to mass, already includes a form of space-time expansion.  We introduced an explicit expansion factor in general relativity to see what would result.  Initially, we had only the slightest suspicion that this would lead us to consideration of whether dark matter existed.  So, it was a true revelation to us when we realised that this could explain the anomalous observations that have puzzled the astrophysics community for over 80 years."

An earlier theory called MOND (MOdified Newtonian Dynamics) attempted to explain the observed speed anomaly by a modification of Newton's Law of Gravity. Application of the MOND law has indeed had great success in fitting astronomical observations – without a need to assume the existence of dark matter.  However, this Newtonian modification is somewhat ad-hoc, in the sense that it was designed specifically to fit the anomalous observational data.

Edmund Chadwick said that: “Since MOND is not really based on a physical or dynamical principle, it has been a central goal of recent theoretical physics research to reproduce a MOND-scaling effect through a satisfactory update of Einstein’s general relativity. After working through some rather complex mathematics, a remarkably simple result finally emerged. We found a simple way to link gravitational theory with the characteristics of MOND, and there is also a hint at a possible physical origin of this linkage.”

On the connection of this result to an underlying physical phenomenon, Graham McDonald added: "During the calculations an odd, but beautifully simple, expression emerged for how this new expansion could vary in space.  If the theory was correct, then this mathematical expression had to have some kind of physical interpretation.  We eventually found that the speed of spherical shock waves (just like those in acoustic explosions) has exactly this type of dependence.

“This seems like a pretty bizarre connection, until one notes the prevalence of shock waves in current theories of galaxies and the universe.  In particular, many scientists believe that the Big Bang itself was just like an acoustic explosion.

“Ideally, the application of this updated general relativity theory to galactic-scale problems was going to have some connection to the dynamical character of an underlying physical phenomenon.  This new work has thus taken steps towards both a gravitational theory that supports MOND-type scaling and providing possible physical reasons for this scaling. These developments now pose many further questions regarding the origin and evolution of galaxies and the universe."

The full paper can be downloaded here: http://arxiv.org/abs/1307.3937

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