Introductory Chapter: Introduction to Dark Matter

1. Introduction

Dark matter is a subject defined by the observational context. It, whatever it may be, is currently something only indirectly observed as the puzzling motions of distant galaxies and the unpredictable rotational velocities of stars in neighboring galaxies. Attempts to observe dark matter as something like traditional particles almost always fail leaving this earthly topic as a major observational problem. This is a very different situation from that of ordinary matter that can be mutated, split into components, destroyed and resurrected at will by physicists, given enough energy, enough time and plenty of money.

Dark matter was first postulated by astronomers in the mid-twentieth century to explain the seemingly non-Newtonian character of intergalactic dynamics. The first, widely broadcast, discrepancy was observed by Fritz Zwicky for the relative velocities of three spiral galaxies circulating within a distant, smallish galactic group. To calculate the relative velocities of these galaxies, Zwicky made the assumption that the galactic masses are proportional to the luminosities. In this case, the galaxies behave as if they are much more densely packed with matter than one can explain in simple terms. Since that report, it seems that dark matter of some form is often required to properly explain the paths and velocities of large galaxies within galactic groups [1]. Though intriguing, I do not think Zwicky necessarily meant that a new type of matter was warranted but only that the observed rotational velocities are much faster than expectation and that much of the matter in these galaxies is not luminous.

Dark matter remained just a thought, not even considered a serious question for many years, until publication of the relative velocities of some bright stars of the Andromeda galaxy [2]. The velocities of very luminous stars, which are outside the dense interior so could be observed without interfering light, do not follow the Newton-Kepler laws as these rotate around the galactic center. According to classical gravity, as supported by observations of our planetary neighbors, objects more distant from a large central object should travel more slowly than those closer to the galactic center. This correlation does not hold for stars of the Andromeda galaxy though; astronomers found that past a critical distance from the center, all stars circulate at the similar angular velocities—meaning distant stars travel faster, in the relative sense, than those closer to the galaxy center. This is a real surprise (and seemingly an insurmountable challenge). This and more recent observations of star motions in other spiral galaxies have been interpreted as necessitating the presence of something like dark matter [3]. To bring this mystery closer to home, the Dutch astronomer Oort observed that nearby stars in our Milky Way seem to be traveling too fast when one considers only the mass calculated for our luminous neighbors.

It was when these latter observations were published that the problem of dark matter woke up some astronomers and physicists. The outstanding, current problem with dark matter is how to relate these interstellar and intergalactic observations to something on earth. Answering this question has been the goal of many young particle physicists with dozens of aspiring theoreticians proposing a bevy of suggestions often in the form of new microscopic particle types. These ideas include many brands of super-symmetry, where the borders of the standard model of particle physics are enlarged to encompass the new particle types. The most important type might (or may not) be WIMPS (Weakly Interacting Massive ParticleS). But other than exhibiting gravitational attraction, predictions of WIMPS properties are nearly impossible to constrain.

Unfortunately, observations of any new, special particle that only interacts via gravity but not electromagnetically, as required to be WIMPS, are very rare and have not been confirmed. A few observations supporting a new microscopic particle type have been claimed by the DAMA/NaI experiment of Italy, which have been searching for new microscopic particles for the past two decades [4]. This sensitive method uses underground detectors of Th-doped-NaI crystals housed in a radiation dampening enclosure to detect novel events over many months. In this respect, the detector(s) is designed in a manner similar to neutrino detectors, expecting very rare, low-energy interactions with normal matter. DAMA has spawned related experiments, ANAIS-112 in Spain and COSINE-100 in Korea, in related attempts to observe WIMPS. Every once in a while, DAMA reports successful observations of signals that indicate the presence of some type of WIMPS [5].

It has been pointed out by others that DAMA investigators observe a seasonal signal variation that can be simply explained by the annual difference in the earth’s velocity as we circle the sun—loosely analogous to the Michelson-Morley experiment. It has also been reported that these claims of particle collisions with the NaI detector cannot be duplicated. If these serious counterclaims are true, this means that concrete, worldly evidence attributing dark matter to microscopic particles has evaporated [6]. Good-bye WIMPS.

There are other mechanisms whereby the motions of stars and galaxies attributed to dark matter can be explained. The exaggerated gravitational redshifts might be the results of orbital decay of innumerable axions in a galaxy, which is translated into a gravity-like effect. One may check the validity of such models, presuming the presence of axions, against the behaviors of highly luminous stars in galaxies with well-mapped translations and observe if the model(s) properly describe the data.

Another very likely explanation for dark matter, which seems rather obvious, is abundant dark H and He, which remains undetected, therefore unaccounted, residing in intra- and intergalactic space. Because most of this H and He reside in the ground state, both are dark and remain undetected, except by gravity, because we lack the technical ability for detection. In line with this thought is the likely possibility that dark matter really is very dark; much is located in black holes. This suggestion seems very reasonable since black holes are a strong “magnet” for matter, perturbing the spacetime within and between galaxies rather than indicating the presence of dark matter. Perhaps both mechanisms are in play?

Perhaps, we take the Friedmann approximation of the Einstein Equation, as the FLRW interpretation, too seriously and too direct, and this is part of the problem. The level of accuracy with which one considers the distances from here to many supernovae emissions is exaggerated invalidating the FLRW model. A universe that is not isotropic and homogeneous should not be expected to be predictable. Also, the Ω_k term of the FLRW model is often overinterpreted as representing spacetime curvature. The Ω_k term rather only presents the remainder of universe after the Ω_m term is calculated and not really spacetime curvature [5, 7]. That is, we cannot trust our distance estimates or trust the claim of flat spacetime and violation of the Newton-Kepler gravitational laws should rather be attributed to poor distance estimates.

Finally, recent observations of many black holes of much smaller mass than well-known supermassive black holes, by LIGO/VIRGO, may require more explicit math than the Kepler approximation to explain the stellar orbital motions. These observations also indicate that “smaller” black holes are much more abundant than previously thought. Such a situation should be observed as stars (and planets) follow non-Keplerian orbits due to the unequal gravitational pull within galaxies. This should also be observed as more rapidly rotating galaxies within galactic groups than predicted from luminosity measurements.