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Saturday, December 09, 2006

THROUGH A UNIVERSE DARKLY

BY STACY MCGAUGH
Special to makkah998.com

People have always asked and sought answers to the big questions. How big is the Universe? How old? What is our place in it? And for as long as we could speak, we've been making up the answers.

We may be a privileged generation: the first in history to know the right answers to the big questions. According to Einstein's theory of General Relativity, the universe can’t be static and eternal - it must either expand or contract. This was such a remarkable prediction that Einstein himself tried to deny it, inserting a fudge factor into his theory - the "cosmological constant" - to hold the universe steady. A decade later, Hubble discovered that the universe was in fact expanding. Einstein repudiated the cosmological constant as his "greatest blunder" and Big Bang cosmology was born.

Big Bang cosmology provided the modern framework for answering the big questions. But some basic questions remained rather open. How old is the universe? It depends on the expansion rate and the density of gravitating mass, which acts to slow the expansion. For many years, cosmology was defined by the search for these two numbers. Measuring either to accuracies better than a factor of two has been a generations-long struggle.

In the past decade, this long struggle has paid off. The Hubble Space Telescope measured the expansion rate to unprecedented accuracy. A host of experiments has constrained the mass density, culminating most recently with NASA's WMAP satellite, widely credited for heralding in the era of "precision cosmology." Cosmic parameters are now constrained to an accuracy of typically ten percent. We no longer say that the age of the universe is ten to twenty billion years. It is blithely stated to be 13.7 billion years. Just a decade ago, to quote a number past the decimal point would have drawn hoots of derision from one's cosmologist colleagues.

There is a dark side to this universe. The total amount of mass in the universe greatly exceeds that which we can see. Vera Rubin and her collaborators at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington provided convincing proof for the existence of dark matter a quarter century ago with observations of the rotation curves of spiral galaxies. Galactic rotation curves plot the variations in orbital velocities of stars and gas within the galaxy as a function of their distance from the galactic center.

These rotation curves remain flat (or almost constant) when, if all the mass in these galaxies was accounted for by ordinary matter, they should drop. That they don't requires the invocation of an unseen dark matter --- a revelation that eventually won Rubin the National Medal of Science.

Modern measurements require that dark matter exceed ordinary matter by a factor of six. This dark matter must be in some novel form, widely believed to be some as yet undiscovered particle from the menagerie of particle physics. The existence of this dark matter is an absolute necessity to our modern cosmology, but has yet to be detected in the laboratory. If found it would imply a new physics beyond the standard model of particle physics.

As if a universe in which most of the mass is invisible weren't strange enough, a variety of cosmic measures, most notably the brightness of supernovae seen to explode billions of light years distant, imply that the cosmic expansion rate is accelerating. This remarkable observation flies in the face of Einstein's pure gravity theory. Gravity between masses is strictly attractive; the cumulative density of mass in the universe, both luminous and dark, should act to slow the cosmic expansion. Instead, it seems to be speeding up!

Einstein himself provided the solution to this problem, in the form of his repudiated cosmological constant. Invented to hold the universe steady, such a fudge to his theory in effect plays the role of anti-gravity, pushing instead of pulling. This is precisely what we need to accelerate the expansion rate. Moreover, the fact that the push exceeds the pull means that there is even more "dark energy" (the modern generalization of Einstein's cosmological constant) than there is dark matter.

But wait. The anti-gravity that is dark energy means that General Relativity as Einstein originally formulated it - the version without a cosmological constant that successfully predicted the expansion of the universe - is not adequate to describe the universe. We have to modify it by adding dark energy. If we must generalize General Relativity, why stop there? Perhaps the need for dark matter is actually an indication of the need to modify gravity.


A few years after Rubin's pioneering observations, physicist Mordehai Milgrom of Israel’s Weizmann Institute suggested as much. Indeed, many versions of modified gravity theories were tried and rejected. Usually, these supposed a change to the force law on some large-scale size, a scale appropriate to galaxies. This approach does not work. The version suggested by Milgrom, MOND (for Modified Newtonian Dynamics), envisioned a modification of Newton's Second Law at a different kind of scale: at very low acceleration. (The centripetal acceleration experienced by a star in a galactic orbit is roughly one part in 100,000,000,000 of that we feel on the surface of the earth!) Of all the proposed modified gravity theories, this seemed theoretically to be the most outrageous. Yet it worked like magic in fitting the rotation curves of spiral galaxies.

An image of a putative dark matter envelope surrounding the Elliptical galaxy NGC 4555 taken by the NASA Chandra X-ray Observatory.


I have spent most of my career studying a class of objects known as Low Surface Brightness Galaxies. These diffuse galaxies rotate like spirals, but have a much larger mean separation between their stars. Like most astronomers, I had dismissed and largely ignored Milgrom's idea. When I learned he had made a number of very specific predictions for this class of objects, I assumed that my data would falsify his theory. Instead, I found it impossible to make sense of the data in terms of dark matter. In these ghostly galaxies, where dark matter must outweigh luminous stars and gas, it was ordinary matter that seemed to be calling the shots. To my growing incredulity, each observation that was puzzling in the context of dark matter theory turned out to be confirmation of one of Milgrom's longstanding predictions.

Still, Milgrom's idea seems to many too crazy to be possible. Crazier than dark matter. And dark energy. While it works uncannily well at describing the rotation curves of galaxies, it is less clear whether it works in other systems. Worse, it seemed utterly at odds with General Relativity.


This has changed recently with the advent of TeVeS (Tensor-Vector-Scalar) gravity theory. TeVeS was introduced in 2004 by Jacob Bekenstein, a prize-winning physicist at Israel’s Hebrew University renowned for his work on the thermodynamics of black holes. Bekenstein showed for the first time that it is indeed possible to write a theory encompassing both General Relativity and MOND. This can be achieved by incorporating two additional theoretical elements (scalar and vector fields) into General Relativity’s standard theory which already requires a tensor field. The existence of such an all-encompassing theory removes the principal theoretical objection to modified gravity as a real physical alternative to dark matter.

It is early days for testing TeVeS. Can it provide a satisfactory description of cosmology? One test will be to see if it can alleviate the apparent theoretical need for dark energy as well as that for dark matter. If such a sweeping explanation is possible, it might well be preferable to the patchwork cosmology espoused at present. More locally, Bekenstein has suggested that TeVeS could be tested here in the solar system, as MOND-like effects might manifest themselves in small regions in interplanetary space where the gravitational effects of the sun and planets balance. There is a rich array of experiments to pursue which have the potential to generate a true revolution in physics.

I remain perplexed by the stubborn success of MOND in individual galaxies. If dark matter theory is correct, this should not happen. So far, laboratory searches for dark matter have turned up empty. One begins to wonder - does the dark matter required by standard theory actually exist?


Throughout the history of the human endeavor to understand the cosmos, the limits on our observational horizon have flavored the answers to the big questions offered by our imaginations. We have made tremendous observational progress and genuinely know more now than ever. But have we finally found the right answers to the big questions? Or do these new answers simply open the door to a new set of questions?

Images: NASA

Stacy McGaugh, who wrote his doctoral thesis on "The Physical Properties of Low Surface Brightness Galaxies" at the University of Michigan, is an associate professor of astronomy at the University of Maryland in College Park.


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