Date: January 19, 2012

Title: Encore:Does Dark Matter Really Exist?

Podcaster: Jim Stratigos

Organization: The Atlanta Astronomy Club

Links: http://www.atlantaastronomy.org/

This podcast originally aired on June 12, 2009:
http://365daysofastronomy.org/2009/06/12/june-12th-does-dark-matter-really-exist/

Description: Is most of the matter in the universe a mysterious and invisible substance known as ‘dark matter’? Observations of the rotational curves of galaxies and certain properties of the cosmic microwave background can not be explained with current theories unless the universe contains many times the mass that composes the stars and gas that we can observe directly. While the search for dark matter goes on, some scientists are not convinced it exists and have proposed an alternative explanation knows as Modified Gravity Theory or MOG. They believe that Einstein’s theory of gravity is incomplete and that a new gravitational theory can explain these anomalous observations.

Bio: Jim Stratigos is an engineer, serial entrepreneur, part-time astronomy student and dedicated amateur astronomer. He is currently an industry consultant advising companies on telecommunications technologies. Following a 35 year career in satellite, wireless and semiconductor technology he has finally found time to pursue his true passions which include visual and radio astronomy and astrophotography. You can read more about Jim at www.dogstar-observatory.com.(web site will be working soon)

Sponsor: This episode of the “365 Days of Astronomy” podcast is sponsored by — NO ONE. Please consider sponsoring a day two so we can continue to bring me daily ‘infotainment’.

Transcript:

Welcome to another 365 Days of Astronomy podcast. My name is Jim Stratigos and I am an avid amateur astronomer with an interest in astrophysics and cosmology. When you look up into the nigh sky and see how star formation and galaxy interactions are still shaping the structure of the universe you can’t help but to wonder what makes it all tick. My desire to learn more about the inner workings of things has lead me to on-line study programs at Swinburne University and to eagerly devour every popular book on the subjects of cosmology and gravity. While I am just an engineer and not an astrophysicist, the concepts of the big bang, nucleosynthesis, supernova, stellar evolution and the cosmic microwave background at least seem plausible. But there is one concept that I just can’t seem to come to grips with and thats the existence of dark matter. Could nature have been so devious as to make ordinary matter – the stuff you and I are made of – a minor component of all mass in the universe?

If you listened to all the pronouncements from the astronomical community about dark matter ‘discoveries’ you might assume that the case for its existence was a done deal – you would be wrong. Despite some pretty strong evidence that something unseen is causing some very strange phenomenon, we have not actually detected dark matter and there are some who doubt it even exists.

First, a brief review on why we believe dark matter exists. Way back in 1933 astronomers Fritz Zwicky and Sinclair Smith first became aware that galaxy clusters behaved as if their total masses were many times that attributable to the stars and gas that they could see. Galaxies within these clusters were rotating so fast that they should have been flung out into the void if standard gravitational theory was correct. Something was amiss!

Another set of observations made in the 1970’s gave impetus to the theory that much of the mass in the universe was unseen. Vera Rubin noted that the rotational curve of spiral galaxies did not follow Newton’s law of gravitation. The orbital speed of stars should drop as their distance from the center of galaxies increased but they didn’t – they stayed constant resulting in a flat rotational curve. Either Newton was wrong or something unseen was adding significant mass to galaxies.

Yet another anomaly from observational astronomy was noted while measuring gravitational lensing. Lensing occurs when light from a distant galaxy is bent by the combined gravity of a galaxy cluster between the Earth and the distant galaxy. Predicted by Einstein decades before they were observed, lensing galaxies also acted as if the amount of matter in the cluster was many times what should be there from stars and gas.

Finally, observations of the cosmic microwave background radiation made by the WMAP satellite show minute temperature oscillations that could be explained if most of the matter in the universe was composed of something other than normal protons, neutrons and electrons.

Dark matter has been postulated as an explanation for these observations and for the past two decades scientists have been trying to figure out exactly what it is. Initial theories that dark matter consists of dead stars and other unseen conventional matter have been discarded in favor of the current ‘cold dark matter’ theory that postulates the existence of massive particles that do not interact with ordinary matter other than via the force of gravity. Called “weakly interacting massive particles” or WIMPS for short, these theoretical particles should be detectable by their rare collisions with ordinary matter and possibly produced at very high energies once the Large Hadron Collider comes on line. While the theory predicts that WIMP particles should outnumber ordinary matter by about 6:1, and that trillions should be passing through the earth each second, no one has actually been able to detect them despite dozens of experiments operating for many years. This lack of experimental evidence of dark matter particles has lead some to consider other explanations for these observational anomalies.

An alternative explanation is to look at gravity itself. Perhaps the existing theories of gravity are wrong on the scales of galaxies and clusters despite overwhelming experimental evidence they are accurate on the scale of the solar system. Could there be an unknown aspect of gravity that causes its effects to deviate from Newton’s law at large distances?

The first serious attempt at a modified theory of gravity was put forth in 1981 by Mordehai Milgrom in his Modified Newtonian Dynamics or MOND theory. MOND modifies Newton’s 2nd law of motion for very small accelerations (for example the environment found near the edges of galaxies). Although MOND has been able to accurately predict effects such as galaxy rotation curves it has not been able to explain other gravitational anomalies without requiring dark matter.

Another explanation for the supposed effects of dark matter has been proposed by Dr. John Moffat, professor emeritus of physics at the University of Waterloo. Dr. Moffat is also a resident affiliate member of the Perimeter Institute for Theoretical Physics in Ontario, Canada. His recent book, Reinventing Gravity, provides an in-depth and readable summary of the dark matter problem and various alternative explanations including his own modified gravity theory called MOG. MOG proposes modifications to Einstein’s theory of gravitation to account for a fifth force of nature in addition to the forces of gravity, electromagnetism, and the strong and weak forces. This fifth force is repulsive and weakens gravity inside the deep gravitational fields of galaxies and clusters. At extreme distances its effects diminish leaving a stronger gravitational field in such a way as to extend the apparent gravitational reach of massive objects. Beyond this critical distance (which is predicted by MOG) gravity decreases as expected according to Newton’s law.

Along with the fifth force, MOG also contains a gravitational constant that varies with space and time unlike Newton’s fixed constant. As a result, MOG also neatly provides alternative explanations of the big bang and black holes but these are topics for another podcast.

MOG has done an excellent job explaining galactic rotational curves and gravitational lensing without needing dark matter. It also accurately predicts the stability of galaxy clusters without requiring any additional mass.

Moffat has also shown that MOG can account for the acoustical wave peaks observed in the cosmic microwave background that have been attributed to the effects of dark matter in the early universe.

The most recent test of Moffat’s theory was provided by observations of the Bullet Cluster – two galaxy clusters that have collided and supposedly separated from their dark matter. While the stars and gasses of the two clusters were not effected by the collision, gravitational lensing was still observed from a region outside of the visible matter thought to contain the dark matter. Since dark matter only interacts with ordinary matter via gravitation it was left behind. Headlines and paper titles proclaimed ‘dark matter observed’ and ‘a direct empirical proof of the existence of dark matter’. But is it really? Can the lensing effects seen in the Bullet cluster only be explained if over 75% of the matter in the universe is unseen? According to a paper published by Moffat and his colleague Joel Brownstein, all of the effects observed in the Bullet Cluster can be explained by MOG. While the math is complicated, the bottom line is that the additional force in MOG produces a mass distribution that accounts for the lensing effects without requiring dark matter.

Any good theory of how the universe works should make testable predictions. For objects on the order of the size of the sun and earth, MOG predicts the same orbital precession as does Einstein’s general relativity whether its in the solar system or within near relativistic orbiting systems such as binary pulsars. MOG also accurately predicts the orbital speeds of stars within globular clusters which ought to contain dark matter but for some reason act as if they don’t. One of the more interesting predictions to come from MOG is that the inertial and gravitational masses of a small object will be different at very small accelerations, such as in a near weightless environment. While such tiny accelerations and masses are very difficult to accurately measure, if such difference are discovered then in addition to doing away with the need for dark matter, MOG may also begin to shed light on some of the fundamental mysteries of mass, inertia and gravity.

So what’s next in the battle between dark matter and alternative theories of gravity? Experiments to measure gravity and acceleration over very small distances may lead to results that support MOG. If, on the other hand, one of the many experiments designed to detect dark matter particles succeeds and these results can be verified, then MOG could fade away as have other alternatives to Newton’s and Einstein’s theories of gravity. Stay tuned!

To learn more about John Moffat and his theory visit his Wikipedia entry. You can also purchase his very readable book, Reinventing Gravity at Amazon.com.

Thanks for listening and clear skies.

End of podcast:

365 Days of Astronomy
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