Is Dark Matter a Chimera?
Ted S. Frost
Assumptions can be dangerous, especially in science. They usually start as the most plausible or comfortable interpretation of the available facts. But when their truth cannot be immediately tested and their flaws are not obvious, assumptions often graduate to articles of faith, and new observations are forced to fit them, —Prof. John S. Mattick
Hang on. We are about to go where angels fear to tread—theoretical physics. For many years, astronomers have been searching for so-called "Dark Matter," the assumed missing matter needed to hold the galaxies together.
Scientists have long known that what they see out in the firmament isn’t obeying the physical laws of gravity and inertia. Based on the amount of matter that can be detected, stars should be flying off in every direction rather than swirling around in galactic clusters. The assumption is that there is some sort of matter out there that has gravitational effects,1 even though we can’t see it. In fact, very little (5%) of our universe is capable of being directly observed by our five senses. If Dark Matter exists, it, along with Dark Energy, make up most of the universe.
| ASSUMED COMPOSITION OF THE UNIVERSE (adapted from Scientific American, October 20031) | ||
| Material | Representative Particles | Probable Contr. To Mass Of Universe |
| Stuff we can ‘see’ (baryonic matter) | Protons, neutrons & electrons | 5% |
| Dark matter | ??? | 25% |
| Dark energy | ??? | 75% |
| Other stuff | Neutrinos, background photons, etc. | 0.3% |
The major candidates for Dark Matter are: (1) — MACHOS (Massive Compact Halo Objects), such as brown dwarf stars, Jupiters, and black holes; and (2) — WIMPS (Weakly Interacting Massive Particles), which consist of hundreds of suggested exotic particles which we have not yet discovered or which have nonstandard properties.
Years of research have come up empty. We still do not know the exact nature of Dark Matter. Even if its exact properties were determined, there would still be the problem of figuring out why it seems to be distributed within galaxies in a particular way. Nevertheless, Dark Matter is still the leading theoretical picture for the formation of structure in the Universe, although the search now seems to focus mainly on intricate computer simulations involving speculative particles2. Even though the search for Dark Matter has floundered, it is so firmly imbedded in the minds of astronomers that any alternative explanation tends to be dismissed. Such was the reaction Mordehai Milgrom experienced when he proposed a different solution3.
Milgrom is an Israeli physicist who in 1983 suggested it wasn’t missing mass at all. Instead, the effect astronomers were seeing was due to a difference in the laws of gravity that he called "MOND" (Modified Newtonian Dynamics).
Milgrom proposed that at very slow accelerations the force of gravity shifts into a higher gear and increases. He deemed the cross-over point to be an acceleration termed "a0" whose value is 1.2x10-10 m.s-2. Milgrom’s a0 presumably represents a parameter of nature like the speed of light or the weight of a proton. The standard Newtonian equation for the force of gravity is expressed as follows:
Where:
F is the force (m1a1) on object 1 due to object 2
G is the gravitational constant = 6.7x10-11 m3kg-1s-2
m1 and m2 are the masses of objects 1 and 2
a1 is the acceleration of object 1
r2 is the square of the distance between 1 and 2
The m1’s in the formula offset. Therefore, the gravitational acceleration of object 1 becomes:
Thus we have the famous high school physics phenomenon of the acceleration of object 1 not being dependent on object 1’s mass so that, in a vacuum, a feather will hit the earth at the same time as a lead ball.
Milgrom modified Newton’s formula to:
Where µ(a1/a0) is a function that equals 1 when a1 is greater than a0, thereby becoming Newton’s original formula. But when a1 is less than a0, the function becomes a1, turning the left side of the formula into (a1)2 so that:
The consequence of Milgrom’s MOND formula is that, for accelerations less than a0, gravity declines linearly by distance r instead of by the square of the distance, r2. This would apply to the motion of stars in galaxies since their centripetal accelerations are in the MOND realm. Hence, the extra gravitational attraction scientists have been scratching their heads over. Eureka!
MOND has successfully predicted several subsequent technical astronomical observations. For example, MOND predicted the ‘rotational curves’ of ‘low surface brightness galaxies’ much better than Dark Matter models4.
MOND also seems to solve the so-called Pioneer Anomaly. In 1980, NASA astronomers noticed that spacecrafts Pioneer 10 and Pioneer 11 were decelerating as they left our solar system in a way that violated Newton’s gravity law. It was as though the gravitational force being exerted on them by the faraway Sun had increased.
Despite analyses looking for systematic mechanical explanations such as fuel or energy leaks, etc., the Pioneer mystery was never solved. Except that the change in acceleration turns out to be very close to what Milgrom’s MOND predicts5!
In spite of MOND’s apparent success explaining such things, it has until recently received two reactions: complete disregard for his idea, or a conviction that it couldn’t be right.
There is a significant theoretical problem with Milgrom’s concept. MOND does not relate in any way to Einstein’s general relativity field equations theory of gravitation. According to Einstein, mass warps spacetime which causes modifications to Newtonian gravity when velocities approach the speed of light or when gravitational fields are very strong. MOND does not speak to this phenomenon.
However, interest in MOND is now on the upswing6 due to a recent paper by another Israeli scientist, Jabob D. Bekenstein. Bekenstein has expanded MOND by imbuing it with something called tensor-vector-scalar (TeVeS) field theory7.
The TeVeS modifications tie MOND to Einstein’s relativity equations making it right with the world as we know it (or at least as we think that we know it). The details are much too complicated and technical for a layman such as me to comprehend, but commentaries by those who do have the necessary expertise are encouraging.
NASA’s 2004 Gravity Probe B experiment testing Einstein’s concept of gravity (distortion of space-time by Earth’s mass and dragging of space-time by its rotation) If Milgrom and Bekenstein are right, it means gravity has three ‘gears’ it can shift into: MOND for very low accelerations, Newtonian for ‘normal’ velocities and masses, and TeVeS (or Einsteinian) for general relativity scales.
Pioneer 10 in deep space: Courtesy NASA
So, the long search for Dark Matter may be coming to a close. Not because the search has been successful, but rather because scientists were looking in all the wrong places.
NASA’s 2004 Gravity Probe B experiment testing Einstein’s concept of gravity (distortion of space-time by Earth’s mass and dragging of space-time by its rotation)
Gravity, after all, is a mysterious force. Scientists have been struggling mightily to come up with equations that tie gravity to the other three forces of nature, i.e.: the quixotic search for "TOE," the theory of everything. This has led to such weirdness as String Theory which postulates we live in a world where six extra dimensions exist that we cannot physically perceive. If the world we live in is that strange, why should it be a surprise that gravity may have peculiarities in and of itself?
References:
1. “The Search For Dark Matter, D. B. Cline, Scientific American, (October, 2003).
2. “Simulations of the Formation, Evolution and Clustering of Galaxies and Quasars,” pp 629-636, V. Springel, Nature 435, (April 2, 2005).
3. “Dark Matter Heretic,” M. Szpir, American Scientist Online, (Jan-Feb 2003).
4. “Dark Matter On Galactic Scales (Or The Lack Thereof),” M. R. Merrifield, Astrophysics, abstract astro-ph/0412059, (Dec.2,2004).
5. “Nailing Down Gravity,” T. Folger, Discover Magazine, (October 3, 2003).
6. “Are There Two Types of Gravity?” M. Chown, New Scientist, issue 2483, (Jan. 22, 2005).