The next lecture meeting of the Society will be on Thursday May 19th and is entitled Space Geodesy – Getting the Measure of the Earth and it will be given by Professor Marek Ziebart from the department of Civil, Environmental & Geomatic Engineering at UCL.
The Annual General Meeting will take place on Thursday June 18th and as usual will be preceded by wine & cheese (£2.00) and if time allows, followed by a scientific entertainment.
The agenda will include the annual reports from Officers and Sections and the election of five Ordinary Members of Council for the coming year. Council proposes the following:
|Secretary||Dr. Julie Atkinson|
|Treasurer & Membership Secretary||John Tennant|
|Programme Secretary||Jim Brightwell|
|Ordinary Members (max 5)||Martin Williams, Roger O'Brien, Peter Stern, Julia Daniels|
Dr. Leo McLaughlin has resigned from the Council due to reasons of health, leaving a vacancy. Council proposes Dr. Kevin Devine to fill the vacancy but invites further nominations to the above posts; such nominees shall be duly proposed and seconded and have agreed to serve if elected.
Peter R Wallis
One of the most intriguing problems today in cosmology and physical science is that of Dark Matter. It seems to outweigh the ordinary matter seen in the universe by some 5.5 to 1, as evidenced by its gravitational effect upon stars and galaxies. But so far we have not been able to detect it by any other measurements. What is it?
Most of the attempts to detect it are based on a speculation that it consists of Weakly Interacting Massive Particles left over in the Big Bang in which our universe started 13.7 billion years ago. They are called WIMPs, but this is not an explanation, merely a description. Several searchers for WIMPs base their plans on the still controversial theory of 'super-symmetry', in which all the fundamental particles in the 'standard model' of physics have heavier, but so far unobserved, partners. One of these is the neutralino which seems to have appropriate properties to explain dark matter; it would have feeble interactions with conventional matter but have a mass in the range of 50 to a few thousand times that of a proton.
So several teams are building equipment to search for the neutralino. The philosophy is that one needs a very large detector, in view of the very weak interaction, and that it is composed itself of particles generally similar in mass to that postulated for the hypothetical neutralino, in order that a collision should have a sufficient recoil effect to be observable. And the detector should be placed deep underground to minimise cosmic and other radiation.
Several of the teams use tanks of liquid xenon, which has an atomic mass of 131, suitable for detecting neutralinos at the lighter end of the spectrum. One team is located at the Sasso National Laboratory, Aquila, Italy. It began operating in 2006 with 15 kg of xenon, finding nothing, but was upgraded in 2009 to 161 kg, called 'XENON 100'. So far an eleven-day run has found nothing. This suggests that neutralinos, if they exist, have a mass greater than 100 Giga-electron volts (Gev). Results from a recent 100-day run are, so far, obscure due to trace contamination of the xenon giving increased background noise. Two other larger xenon detectors in S Dakota and Kamioka, Japan, have not yet started operating. Other teams use germanium and silicon crystals, which are the only solid elements that can be made pure enough. CoGeNT, in Souda, Minnesota, has provisionally claimed detections of particles in the range of 7 to 11 Gev, but consider it necessary to delay publication till more results have been obtained, partly as the result conflicts with that from XENON 100.
Currently a debate has broken out  whether a different approach could be used. Apparently dark matter particles could have their own anti-particles. Put enough together in one place and some could annihilate each other, generating gamma radiation. Such a place could be the centre of our galaxy. Dan Hooper, an astronomer at the Fermi National Accelerator Laboratory at Batavia, Illinois, claims he has found such evidence from NASA's Fermi Gamma-ray Space Telescope, indicative of particles in the 7.3 to 9.3 Gev range, similar to the germanium results. Others are sceptical, as the centre of the galaxy is so complex and we know little about it.
The XENON 100 and CoGeNT teams are expected to release their full year's results this year and two other teams with larger xenon detectors are expected to start operating soon. One is the Large Underground Detector LUX, with 350 kgs of xenon, and the other is the Japanese XMASS, with 1000 kgs, but Japan's recent problems may delay it.
 Adam Mann, "Hunting of the Dark", Nature, 24 March 2011 Back
Way back in the 1930's it was discovered that galaxies in clusters were moving too rapidly to be explained by the gravitational interaction of their individual components. This led astronomers to conclude that there was a huge 'Missing Mass', something unseen that was causing the discrepancy. The problem was compounded when in the 1980's the rotation of galaxies also showed that the stars in the outer regions appeared to be moving at speeds similar to the stars nearer the centre. It was as if the stars were 'fixed' to some gigantic wheel. In order to explain these anomalies using our present understanding of gravity, it was necessary to invent something that has a mass in the order of 5 times that of the calculated mass of the entire galaxy yet was totally invisible.
It was his work on the motions of galaxies in the Coma cluster in 1933 that led the Swiss astronomer and flawed genius Fritz Zwicky to coin the term 'Dark Matter' to account for the 'Missing Mass'. Working at Mt. Palomar, Zwicky employed the Virial theorem * which allows the total kinetic energy in complex systems to be calculated. His results indicated that the mass of the cluster based on the speed of its galaxies was at least ten times greater than the mass calculated from its light output. The search was then on to identify the origin and nature of 'Dark Matter' but so far, we have been unable to prove conclusively that it exists at all, let alone provide a convincing description of its composition. Terms like WIMPS, MACHOS and now NEUTRALINOS are bandied about in a vain attempt to describe its nature. Today according to some authorities 'Dark Matter' represents 80% of the matter in the entire Universe while ordinary baryonic matter (the parts we can actually see) represents just 20%.
But could there possibly be another explanation to the 'Missing Mass' problem that confounds cosmologists? Recently published work demonstrates a radical alternative approach to this long standing problem.
Consider this proposition: "What if gravity behaves differently when applied to the masses and comparatively low accelerations of stars in galaxies?" "What if Newton's second law (f = ma) is not sacrosanct and does not hold in certain special circumstances." After all, Einstein showed that Newton's laws do not hold in regions of intense gravitational fields and our understanding of gravity can hardly be described as complete. Is it remotely possible that Newton's laws also do not hold when considering the very low accelerations of stars in galaxies, where for a given force less mass is required? Newtonian laws of gravity work demonstrably in the Solar System but in the Solar System accelerations are in the order of a million times greater than the very weak accelerations acting on stars in galaxies. This is the basis for a study called MOND – Modified Newtonian Dynamics carried out by Mordehai Milgrom  of the Weizmann Institute.
In order to test this theory it is necessary to determine the mass and velocities of stars in a large sample of galaxies. Work on this has recently been published by Stacy McGaugh  of the University of Maryland. McGaugh studied gas-rich galaxies because determining gas mass is more reliable than determining stellar mass. The study has shown that the relationship between mass and velocity was indeed that predicted by MOND, and MOND has made some interesting predictions in other areas. However, all is not plain sailing as the theory has run into considerable trouble in other applications. For example, it underestimates the mass in galactic clusters by as much as 50%, it cannot account for bending light in examples of gravitational lensing and more seriously, it violates the law of conservation of momentum – although there is a relativistic version Tensor-Vector-Scalar theory (TeVeS) developed by Jacob Bekenstein  that does not violate this law and embraces the phenomenon of gravitational lensing as well.
All this has the potential for further study because although we have lived with the theoretical existence of vast quantities of 'Dark Matter' for eight decades, we are still searching and we still can't find any!
If we apply the principle of 'Occam's Razor' we might conclude that the reason that we can't find any is simply because it does not exist. Maybe there is after all another explanation for the 'Missing Mass' that does not rely on the 'invention' of ever more exotic and elusive fundamental particles.
1 Milgrom, Mordehai: "Does Dark Matter really exist?" Scientific American Aug 2002 42-50-52 Back
2 Sanders, Robert; McGaugh, Stacy S, "Modified Newtonian Dynamics as an alternative to Dark Matter" Annual Review of Astronomy and Astrophysics 40, 263 - 317 Back
3 Bekenstein, Jacob D (2004) "Relativistic gravitation theory for the modified Newtonian dynamics paradigm" Phys,Rev. D70 (8): 083509 Back
Further reading: "Can tweaking gravity make Dark Matter go away?" Longstaff. Alan: Astronomy Now April 2011
* The Virial theorem:
M = (V2.R)/G where:
M = Mass
V = Velocity
R = distance from galactic centre
G = Gravitational Constant = 6.67 x 1011
Last updated 27-Jan-2018