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Faced with flat rotation curves that seemed to flout Newton's laws, astronomers assumed the existence of a halo of dark matter around every spiral galaxy. Whatever the stuff was, it did not emit light, but it did exert a gravitational pull. The dark matter tugged on the stars, cranking up their speeds and creating the flat rotation curves. Milgrom decided to retrofit Newton…If you took high school physics, you may remember having Newton's most important equation pounded into your head:
F = ma. With this simple formula, known as Newton's second law, Newton forever linked forces (F) to their action on mass (m) in the form of acceleration (a). We all experience the relation between force and acceleration whenever we're in a car. As the car accelerates, we're forced back in our seats; when it decelerates, we're forced forward. Milgrom found that the best way to resolve the problem of the flat rotation curves was to modify this hallowed equation.
"I assumed that when the accelerations due to gravitational forces became very small, the formula changes to F = ma²/a0," Milgrom says. According to Milgrom, this change holds only when accelerations fall below one 10-billionth of a meter per second every second. Not only does this modification work best with the data, he adds, but the new constant, a0, may be of cosmological significance: Accelerating at this rate will take you from a resting state to the speed of light in the lifetime of the universe. Otherwise Newton's law operates as usual. So with MOND, stars in the outer reaches of galaxies move faster than expected, not because of the influence of some invisible matter but because Milgrom's amended version of Newton's second law increases the force acting on them.
To make MOND a serious alternative to dark matter, Milgrom's inspired guess needed to mature into a true theory, with a firm foundation in modern physics. And that meant confronting not just Newton but his wild-haired offspring, Einstein. It was Einstein who divined the interconnections between gravity, space, and time. For MOND to make headway in the field, someone was going to have to find a way to reconcile it with Einstein's masterpiece, the theory of general relativity.
On March 25, 2004, a paper from Physical Review Letters D appeared on the Los Alamos preprint server, a Web site where physicists post their newest articles. The paper, titled "Relativistic Gravitation Theory for the MOND Paradigm," was written by Jacob Bekenstein, Milgrom's collaborator since the 1980s. Building on earlier attempts, Bekenstein was finally able to generate a MOND theory that Einstein might have loved. The new theory was called TeVeS, an acronym for tensor, vector, and scalar—mathematical terms that describe how matter and energy interact with space and time in general relativity.
"TeVeS does everything," says Mario Livio with enthusiasm. A self-described agnostic in the MOND debate, but one with an obvious love for the underdog, Livio says that Bekenstein's work is "a phenomenal paper." In TeVeS, he adds, all the right things happen. Its results mesh with what physicists know about gravity from Einstein, and when gravity is very weak, it reduces to the behavior Milgrom envisioned in his first MOND papers. "With Bekenstein's theory we should now be able to explore all aspects of relativistic behavior," Milgrom says, unable to hide his pride. "This includes the bending of light by gravity, and in principle, the new theory should be OK for galaxy formation"… TeVeS has already passed a crucial first test, Milgrom says, because it can be used to explain the phenomenon of gravitational lensing, a cosmic optical illusion in which matter bends light. It's one of the stranger predictions of general relativity, and astronomers have confirmed it many times over.
Q: It's been said that dark matter theory is beginning to resemble the celestial mechanics of Ptolemy, who continually added epicycles to his models as the observations failed to conform to a geocentric universe. What would it take for scientists to abandon dark matter?
A: DM is encountering difficulties in the face of observations, and people keep inventing new DM with new properties to try and circumvent these difficulties. Over the years I have refrained from attacking DM, but others have started finding faults with it and that trend is increasing.
The theory has recently overcome some serious problems that had plagued it since its inception - such as how it fits with general relativity - and it is now able to make surprising predictions about the evolution of the universe.
What has made people sit up and take notice is a 2004 paper by Jacob Bekenstein of the Hebrew University of Jerusalem in Israel (New Scientist, 22 January 2005, p 10). His paper, the culmination of a 20-year quest to reconcile MOND with relativity, seems to have stood the test of time and has convinced several other groups to work on the theory and test its predictions. Their results suggest Bekenstein is on to... a theory of gravity that preserves the best aspects of relativity, but with modifications that could make dark matter obsolete.
Where do you go to look for modified Newtonian dynamics (MOND)? Try our own solar system. Since the early 1980s, NASA's Pioneer 10 and 11 space probes have been coasting away from the sun in the outer solar system. Puzzlingly, they have been decelerating more quickly than Newton's law of gravity would predict. The cause of this "Pioneer effect" is hotly debated, but the rate of deceleration is about what you would expect if Jacob Bekenstein's theory were correct.
Despite decades of searching, the "dark matter" thought to hold galaxies together is still nowhere to be found... Some physicists think it makes more sense to change our theory of gravity instead.
Originally posted by zhangmaster
I'm not sure how many of you read Discover Magazine, but I was going through my stack at home and came across something published in August of last year that was extremely interesting. Dark Matter is sometimes used as a sort of "fudge factor" to explain strange events, like the observation that outer stars in a galaxy don't move slower than the inner stars, something that contradicts Newton's laws. To fix the error,
"TeVeS does everything," says Mario Livio with enthusiasm. A self-described agnostic in the MOND debate, but one with an obvious love for the underdog, Livio says that Bekenstein's work is "a phenomenal........