Vera Rubin and Dark Matter It is assumed that matter orbits about the center of a galaxy owing to a centripetal force which is the gravitational attraction of other matter in the galaxy. Assuming all other matter in the galaxy is luminous, astrophysicists cannot account for the centripetal accelerations observed. These can be accounted for, however, if additional matter is present. Consequently, Rubin's measurements were of fundamental importance as empirical evidence for dark matter. Rubin determined the velocities as a function of distance from the galactic center of clouds of ionized hydrogen (in astrophysical terminology, HII regions). This was done by measurement of the Doppler shift of their H-alpha emission lines. The hydrogen clouds move with the stars and other visible matter in the galaxies. Their velocities are more easily and directly measured than other visible matter. Rubin found that the velocities of the clouds did not decrease with increasing distance from the galactic center, and in some cases even increased a little. This is in striking contrast to the decrease in velocity with radius predicted by Keplerian motion, which would occur if all the mass of the galaxy were concentrated in the center of the galaxy. In this case, one has the result that the velocity decreases inverseley as the square root of the radius (distance from center); i.e., Newton's law of gravity yieldsIn other words, the further from the centre of a galaxy (and most of the mass) one would expect to see slower moving stars. After all, faster moving stars would be able to break free of the weaker gravitational pull and this is certainly what one sees with the motion of the planets around the sun. Why should galaxies be any different? Maybe dark matter. Okay so there's dark matter, what of it? Well, for one thing nobody really knows what it is or what it is made up of. Candidates for the constituents of dark matter tend to fall into one of two categories which for cuteness reasons have been dubbed MACHOs and WIMPs. MACHOs (Massive Compact Halo Objects) are made up of ordinary matter, stuff that lurks about in the darkness like clouds of dust and objects like "brown dwarves" which are planet-like objects much more massive than Jupiter and yet too small to ignite and become stars. In order to explain the strange rotational behaviour of galaxies, dark matter is postulated to surround galaxies in great spherical "halos". The next problem that then arises is to explain why MACHOs don't behave like ordinary matter and don't just fall in to become part of the galactic disc like everything else. The other problem with MACHOs is that clouds of gas are really not all that invisible or hard to detect. If they were really that prevalent we would be lucky if we were able to see any distant stars and galaxies at all. WIMPs (Weakly Interacting Massive Particles), on the other hand, are not ordinary matter and are by their natures much harder to detect. WIMP dark matter is thought to be contained in one or many kinds of stable, massive, exotic, particles that don’t respond to either the electromagnetic or the strong nuclear interactions. These particles are thought to be all-pervasive yet elusive even to the extent that vast quantities of them constantly zip through our bodies undetected. They may attract matter through gravity but every in other respect they don't interact with it. The problem of dark matter is that by definition it is very hard to test for. A more thorny problem for science is that once proposed it is even harder to find a test that can conclusively refute it. This wouldn't be so bad if it wasn't for the fact that current estimates of dark matter are running at something like 90% of all the matter in the Universe!1 If Isaac Newton had rocked up touting a universal theory of gravitation that required this much fudging to make it work properly, it seems highly unlikely that it would have been received as warmly by his contemporaries. Instead of being the toast of England, Newton would have simply been toast. And as if things weren't already bad enough, there's still those pesky space probes, Pioneers 10 and 11 to think about. Spacecrafts pulled by mystery force (BBC) Mysterious "Force" affects Pioneer 10 (Telegraph) Puzzling hyper-gravity proves weighty mystery (CNN) Three spacecraft reveal unexplained motion (Los Alamos National Laboratory)
Of course not all the mass of the galaxy is located in the center. Generally, however, there is a cental region where most of the visible matter is located, as the picture below of galaxy NGC 4414 shows. The region is called the central bulge. If we assume the mass in the galaxy is distributed as are the luminous regions, the velocities would still be expected to decrease with increasing radius at large radii, though the decrease would not be as rapid as if all the mass were located in the center. Spiral Galaxy NGC 4414Thus astrophysicists have been led to believe that there is matter outside the bulge which has not been observed but exerts a gravitational force and 'raises' the velocities of the clouds above that expected from the known distribution of visible matter.
Space oddity A mysterious force is tugging on Nasa's ancient Pioneer spacecraft, retarding them as they streak out towards the distant stars. It amounts to a mere 10-billionth of a "g" – gravitational force. Yet it could bring the entire edifice of physics tumbling down. Pioneer 10 and 11 have been in space for an astonishing three decades. They were launched on 2 March 1972 and 4 December 1973, respectively, with Pioneer 10 the first spacecraft to fly by the giant planet Jupiter, and Pioneer 11 the first to visit both Jupiter and Saturn. "After those encounters, we thought the mission was over," says the Pioneer astronomer John Anderson of Nasa's Jet Propulsion Laboratory (JPL) in Pasadena. "How wrong we were." Anderson and his colleagues continued to track the probes using the giant dishes of Nasa's Deep Space Network. "By 1980, when Pioneer 10 was 20 times farther from the Sun than the Earth – halfway between Uranus and Neptune – it was clear that the probes were not where our calculations said they should be." Both probes were feeling a tiny force in addition to gravity. In both cases, it was of the same magnitude and directed towards the Sun, despite the fact that Pioneer 10 and 11 were travelling in roughly opposite directions. Today, the probes are way beyond the outermost planet, Pluto. For a long time, Anderson did not publicise the "Pioneer anomaly". Everything changed, however, in the mid-1990s, with the involvement of Michael Martin Nieto, a physicist at Los Alamos National Laboratory, New Mexico. Nieto was interested in how well we know gravity within the solar system. Someone told him to talk to Anderson, which he did. "When John told me the size of the Pioneer discrepancy, I almost fell off my chair," he says. Nieto and Anderson began working together, joined by Slava Turyshev of the JPL. The team tracked down generation-old data about the space probes from retired Nasa personnel, and painstakingly examined it, trying to find a mundane explanation of the Pioneer anomaly. An obvious possibility was a heat leak. It would require only 70 watts of heat escaping in a direction opposite to the Sun to push the 241kg probes sunward at the observed acceleration. But was this amount of heat available? Each probe carries a plutonium heat source on the end of a long boom. A measly 70 watts was being delivered to the body of each spacecraft. "To explain the anomaly, all of that would have to be radiated in one direction, which is very unlikely," says Anderson. The other obvious possible explanation of the Pioneer anomaly is a fuel leak. But there is no obvious evidence from spacecraft data of any such leak. More seriously, a fuel leak would have come about by accident. "It's hard to imagine such a leak happening on both probes at the same time in such a way as to produce an identical acceleration," says Anderson. According to Anderson, the most likely explanation is still some unknown but mundane effect. Other physicists agree. "For now, in my role as Old Grumpy, I'm just waiting for the whole thing to go away," says the Nobel prizewinner Steven Weinberg of the University of Texas at Austin. Some physicists, on the other hand, are willing to stick their necks out. "I think the acceleration anomaly is real," says Bernard Haisch of the California Institute for Physics and Astrophysics in Palo Alto. One obvious - though dramatic - possibility is that Newton was wrong and that the force of gravity does not weaken with the well-known inverse-square law in the outer solar system, but drops away more slowly with distance. There is certainly evidence of such an effect on cosmic scales. For instance, the stars orbiting the centre of spiral galaxies like ours, and galaxies orbiting within galaxy clusters appear to be in the grip of stronger-than-expected gravity. The standard explanation is to postulate the existence of a huge amount of invisible, or "dark", matter that enhances the gravity. But an alternative, Modified Newtonian Dynamics (Mond), has been proposed by the Weizmann Institute's Mordehai Milgrom. "Maybe the Pioneer anomaly is the first hint of Mond," says Nieto.MOND you say? Now that sounds interesting...
Dark-Matter Heretic - An interview with Mordehai Milgrom Mordehai Milgrom thinks that the mass discrepancy observed in galactic systems is more elegantly explained by modifying Newtonian dynamics than by postulating the existence of exotic dark matter. The amount of matter that astronomers can detect with their instruments doesn’t seem to be nearly enough to explain why some of the big-ticket items in the cosmos behave the way they do. Spiral galaxies spin faster than they should, and clusters of galaxies stick together even though the velocities of their constituent galaxies suggest they should be flying apart. The standard solution to the problem posits the existence of some hidden mass in the universe—often called dark matter (sometimes abbreviated DM)—that’s holding everything together by the force of gravity. Most astronomers believe that dark matter exists—even though it has never been seen, and no one knows what it might be. But a 20-year-old alternative to dark matter has been getting an increasing amount of attention lately. The idea, called MOND (for Modified Newtonian Dynamics), was proposed back in 1983 by physicist Mordehai Milgrom, now at the Weizmann Institute of Science in Rehovot, Israel. Milgrom had seen the data like everyone else, but instead of thinking about new kinds of matter, he decided to question the physical laws that describe how ordinary matter behaves. The laws in question are the law of gravity and Newton’s second law, f = ma, which simply tells us that the amount of force you need to accelerate a mass increases linearly as you increase the mass or its acceleration. MOND says that Newton’s law still holds true for the kinds of accelerations that take place inside our solar system, but that it’s different when the mass is accelerating very, very slowly - precisely the way that bodies in galaxies and systems of galaxies move. MOND employs an acceleration constant, a0, roughly equal to 10-8 centimeters per second per second, and it kicks in when the accelerations are this slow. With this modification to standard Newtonian dynamics, MOND has proved to be every bit as successful at explaining the astronomer’s observations as dark matter has. Yet it has relatively few adherents. Why? The question of how scientific ideas get accepted or rejected is one that philosophers and historians of science have been exploring for decades, of course. But what is it like for a scientist who is in the middle of such a debate? I interviewed Milgrom this past November to get his thoughts on why MOND has gotten such a cold (and dark) reception [More...]There's lot's more to be found about MOND on Milgrom's web site. Milgrom also makes a glancing reference to the mysterious accelerations of Pioneers 10 and 11 in MOND - A Pedagogical Review.