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What Is Dark Matter?
What is dark matter? That is a question that can not really be answered at the present time; hypotheses about the nature of dark matter and what it is will have to suffice for now. The existence of dark matter has been hypothesized to explain certain effects in the Universe, especially gravitational ones, within the current framework of physics that could not otherwise be explained. One very basic example is that of rotating galaxies – the amount of matter which is observed in galaxies is insufficient to keep the rotational force of galaxies from making them fly completely apart. There has to be a much larger amount of matter for galaxies to even exist. Another example is observations of the effect of gravitational lensing, where very strong gravitational fields cause the bending of light from stars, galaxies and other celestial objects. The extent of the gravitational lensing effect from distant celestial bodies in the Universe cannot be explained by the observed amount of matter of these bodies – a much greater amount of matter must be present.
There are a number of other examples of effects in the Universe which cannot be explained unless large amounts of dark matter are present. Some of these are the cosmic microwave background, which is a remnant from the early formation of the Universe, the evolution and formation of galaxies since the amount of observable matter is insufficient to allow galaxies to even exist, the gravitational effects of galactic collisions, and the movement of galaxies within galactic clusters such as the Virgo Supercluster or even our own Local Group of galaxies, which include Andromeda, the Milky Way, the Large and Small Magellanic Clouds, and Triangulum.
For current theoretical models in physics to work properly, about 85 percent of the matter and 25 percent of the energy density in the Universe would have to be in the form of dark matter. For this reason, most scientists now think that dark matter is ubiquitous in the Universe even playing a crucial role in the development of the structure and evolution of the Universe itself. In the current Lambda-CDM cosmological model, the Universe consists of about 5 percent ordinary matter and energy, 27 percent dark matter, and 68 percent of dark energy, which is also a completely unknown type of energy. If we take dark matter and dark energy together, they constitute about 95 percent of the total amount of mass and energy in the Universe. The existence of both dark matter and dark energy is contingent on current theoretical models of physics being largely correct.
Even the name for this unseen and undetected matter is enigmatic – dark matter. Dark matter is called dark because it is completely unseen and so far at least undetectable – it does not seem to interact at all with light, or for that matter with any part of the electromagnetic spectrum. It is completely invisible to all forms of detection that we have available in our current technology.
Dark matter does not seem to interact with ordinary matter or radiation to any significant extent – the only evidence we have for it is the aforementioned gravitational effects based on current theoretical models of physics and cosmology. Many scientists now believe that dark matter must consist largely, if not completely, of as yet undiscovered subatomic particles. There are many experiments now underway to try and detect dark matter particles, but as of now none of them have succeeded. Despite this fact, the hypothesized existence of dark matter is currently accepted by the mainstream scientific community.
Even so, some theoretical physicists, having observed some effects which don’t fit the dark matter theory, are now arguing for some modifications of the General Theory Of Relativity(not to be confused with the Special Theory Of Relativity) which would not require dark matter subatomic particles to exist. In other words, with these modifications, ordinary matter might be sufficient to explain the effects we see in the Universe.
Dark Matter History
Dark Matter Early History
Contrary to what one might think, the first ideas of dark matter go back to 1884 when Lord Kelvin, a Scots-Irish mathematical physicist, gave a talk in which he made an estimate of the number of dark bodies in our galaxies based on observations of the velocity of star dispersions orbiting around the galactic center. From these observations, he was able to estimate the total mass of our galaxy which was much more than that suggested by the number of visible stars, thus concluding that there must be a significant number of dark bodies in our galaxy.
In 1922 the Dutch astronomer Jacobus Kapteyn suggested the existence of dark matter and in 1932 another Dutch astronomer Jan Oort also hypothesized the existence of dark matter; based on his observations of nearby galaxies he concluded that their mass must be greater than the number of visible stars and so there must be some unseen matter which accounted for this discrepancy.
The Swiss astrophysicist Fritz Zwicky hypothesized about the existence of dark matter in 1933 while working at the California Institute of Technology. He was studying the kinetic energy of the Coma Cluster of galaxies, a large cluster of over 1000 galaxies, and came to the conclusion that this cluster had about 400 times more mass than could be observed visually. He estimated the total mass by studying the motions of galaxies at the edge of this cluster compared to their brightness and the number of galaxies. The gravitational effects he observed was much less than what was needed to hold these fast-moving galaxies in their orbits in the cluster. So he concluded that there was a great unseen amount of matter which provided the mass and resultant gravitational force to hold the cluster together and keep it from flying apart.
In 1939 Horace Babcock, an American astronomer, noticed some unusually rapid rotation in the edge region of the Andromeda Galaxy that could be attributed to some unseen dark matter, since the visible matter didn’t account for it. In 1940 Jan Oort made similar conclusions about an elliptical galaxy, NGC 3115, that he discovered, believing there must be a very large invisible halo of matter surrounding it.
Dark Matter Recent History
In the 1960s and 1970s astronomers Kent Ford, Ken Freeman, and Vera Rubin did work on galactic rotational curves with a more advanced type of spectrography to find the rotational velocities of galaxies with greater accuracy than was possible before. In 1980 a very important paper was published by Vera Rubin and Kent Ford showing that most of the galaxies observed must contain about six times as much unseen, or dark matter, as the matter that was actually visible. These findings showed that by this time there was a need for large amounts of dark matter to account for inconsistencies in galactic mechanics that were actually observed – this was now a major unsolved problem in celestial mechanics, and astronomy in general.
Roughly at the same time that all this was happening, radio astronomers were using a new generation of radio telescopes to map the wavelengths of hydrogen in nearby galaxies; interstellar hydrogen is distributed to a much greater galactic radius than that which is observed by optical telescopes and thus with a greater sampling available significantly more accurate measurements of the dynamics of galactic rotation can be made, resulting in greater accuracy.
Early mapping of the Andromeda Galaxy in this way, using radio telescopes(the Green Bank and Jodrell Bank radio telescopes) to map the distribution of atomic hydrogen, showed that rotational velocities did not follow what was expected from celestial mechanics, suggesting the presence of dark matter. With even more sensitive radio telescopes, astronomers Morton Roberts and Robert Whitehurst further refined this tracing of galactic rotational velocity to confirm what seemed to be the presence of dark matter. In 1972 David Rogstad and Seth Shostak published some interferometry studies of five spiral galaxies showing the rotational curves of all of them were much too flat to be accounted for by the observed matter(mass), thus also suggesting the presence of dark matter.
In the 1980s many more observations were made of cosmological phenomena such as the gravitational lensing of distant stars by large galaxy clusters, cosmic microwave background anomalies, and the temperature distribution of the gases present in galaxies. These observations and studies all suggested the presence of some type of dark matter which, they surmised, must be composed of some type of as yet undiscovered subatomic particles. Currently, particle physics researchers are intensely searching for these particles, so far without any success.
Dark Matter Definition
Since dark matter is by definition not only invisible but completely undetectable(so far) by any means currently available from our state of technology the question comes to mind what exactly is dark matter – how is it defined? Well, to answer this question as completely as possible, we have to look into the standard cosmological model and define what matter in general is(i.e. ordinary matter). In this cosmological model, matter is anything whose energy density is inversely proportional to the cube of the scaling factor. A simple example of this is a known quantity of matter(which has a certain energy potential), such as a particle, in a cubical box; increase the length of the side of the cube by a factor of 2(doubling it), and the energy density will decrease by 2 cubed, or to the third power, which is equal to 8. So since the energy density is inversely proportional to the cube, it is then 8 times less – energy density divided by 2 cubed, or 8. Therefore, by definition dark matter must follow this inverse cube law, since it is still a form of matter, although of a type we are unfamiliar with and do not as yet understand.
Dark Matter Evidence
Galaxy Rotation Curves
Galaxy rotation curves are a very important piece of evidence supporting some form of dark matter. This relates to the fact that the arms of a spiral galaxy rotate around the galactic center and the luminous mass density of these spiral galaxies will decrease as the distance from the center of the galaxy increases, moving towards the outermost edges. This would lead to a conclusion from classical physics, Kepler’s Second Law in particular, that the rotational velocity should decrease as the distance from the galactic center increases, going towards the edge of the galaxy.
This is not at all what happens – instead, the rotational velocity relating to the galaxy rotation curve remains more or less constant as this distance from the galactic center increases. If we assume that Kepler’s Second Law of motion is correct, then this inconsistency can be solved with the assumption that there are large amounts of dark matter in the outermost regions of the galaxy.
Galactic Velocity Dispersion
Another type of observational evidence which is suggestive of the presence of dark matter is the velocity dispersion within a galaxy. In classical physics, there is a theorem, known as the virial theorem, which relates over a period of time the average of the kinetic energy of a stable system consisting of a defined number of particles which are bound together by potential forces with the total amount of potential energy in the system.
Astronomers can use this theorem along with the measured velocity dispersion in a galaxy to calculate what the mass distribution should be in a gravitationally bound system, such as a galaxy. Actual observations of this galactic velocity dispersion do not match what the mass distribution should be according to our current understanding of celestial mechanics. Again, if our understanding of celestial mechanics is correct, then this is only resolved if a large quantity of dark matter is present.
The mass of a galaxy cluster can be calculated in several ways – from observing the radial velocities of these galaxies, by estimating the temperature and density of the hot gas of the galaxies in the cluster from spectrographic studies of the x-ray emissions from these gases which then in turn determine the pressure of the gases from which an estimate of the mass can be made, and by observing the gravitational lensing effect of celestial objects such as distant stars by the huge mass of the galactic cluster. After much study through recent years, all of these methods are suggestive of a huge amount of dark matter – roughly 5 times as much dark matter as ordinary matter.
As mentioned previously, gravitational lensing is the effect, predicted by Einstein’s General Theory Of Relativity, that the light from a distant object, commonly a star, is bent by the gravitational force resulting from the huge mass of something like a galaxy cluster, or perhaps even a large black hole. The degree of the bending of the light can be precisely predicted if the amount of mass from the galaxy cluster, for example, is known. When the visible portion of the mass of the galaxy cluster is calculated and compared to the gravitational lensing effect observed, there is a very large discrepancy which once again suggests that a much greater amount of dark matter is present than ordinary matter alone.
Cosmic Microwave Background
Ordinary matter and dark matter, although they are both matter, behave in different ways and these differences cause anomalies in the cosmic microwave background emanating from the early Universe which can only be explained in the current cosmological model if dark matter is present in huge quantities. The ordinary matter we see all around us reacts very strongly with electromagnetic radiation, especially the shorter wavelengths of the electromagnetic spectrum which will cause ionization in this visible type of matter. As we discussed previously, dark matter does not seem to react to any parts of the electromagnetic spectrum, including the ionizing effects of the shorter wavelengths, at all.
But dark matter does interact with gravity, especially on a larger scale, much as ordinary matter does. This gravitational interaction between dark matter and ordinary matter will have an effect on the velocity dispersion of various types of observable matter, including more massive ones, such as galaxies, where the effect would be much more evident.
These differences in the way that dark matter and ordinary matter interact with electromagnetic radiation show up as puzzling anomalies in the cosmic microwave background from the early Universe which can then be explained by the presence of large quantities of dark matter.
Formation Of The Universe
The formation of the Universe and the resulting evolution of its structure is highly dependent on the existence of dark matter, at least within the constraints of theoretical physics as we know it today and current cosmological models of the Universe.
The General Theory Of Relativity suggests that the state of the very early Universe after the Big Bang was mostly homogenous in nature, with very few, if any, density perturbations that would allow the Universe to form structures(stars, galaxies, etc.) as we know them today. Ordinary matter interacts quite easily with electromagnetic radiation, which is believed to have been the predominant state of the very early Universe, so any density perturbations that might occur would be easily ‘washed’ out by the interaction with this radiation.
This is where the existence of dark matter is believed to have entered into the picture. Dark matter does not interact with the radiation but does interact strongly with gravity, so the density perturbations of the dark matter would not be washed out in the same way that those of ordinary matter would. So the dark matter could form into larger structures of mass, which in turn would interact through gravity waves with the visible matter of the Universe, resulting in density perturbations of ordinary matter that could coalesce into the visible matter and resultant structures, such as stars and galaxies, that we see today in the Universe. This is another strong argument in favor of the existence of dark matter.
Expansion Of The Universe
The measurement of galactic distances by astronomers, for example using certain types of supernovae, has been used to show how fast the Universe has been expanding from the past to the present time. From these studies, it seems that the Universe is expanding at an accelerating rate, which would not be possible based on the amount of visible matter observed in the Universe – this may indicate the presence of dark matter, but it somewhat more complicated than that.
Dark energy, a type of energy not detectable at the present time with our current state of technology and which we will go into more detail in a later article, seems to actually be slowing down the expansion of the Universe. The way this may be happening is through some mechanism of the conversion of dark matter into dark energy so that as the volume of the Universe increases the density of dark matter will decrease while the density of dark energy will stay more or less constant, suggesting this type of conversion. Please note that this is still hypothetical in nature and much more research needs to be done – dark matter and dark energy, if they exist, are still great mysteries today.
Redshift Distortions In The Universe
Redshift is a phenomenon in physics where electromagnetic radiation, including light, a narrow range of the electromagnetic spectrum, shifts toward longer wavelengths. The name ‘redshift’ comes from this shift in visible light which shifts towards the red part of the visible light portion of the radiation spectrum, which has longer wavelengths which also corresponds to a decrease in the energy of the photons.
This has relevance in our discussion of dark matter because of the degree of redshift observed, for example, when studying the radial velocities of galaxies in superclusters(as well as other celestial objects) which show redshift anomalies which can not be explained by only the presence of ordinary matter.
For example, in a three-dimensional map of galaxy distribution of a given supercluster of galaxies, the supercluster is expanding at a slower rate than it should from the gravitational effects of the visible matter by itself – this would imply the presence of some sort of dark matter. In addition to this, galaxies in front of the supercluster have higher radial velocities and redshifts than their distance from each other would seem to imply, and galaxies behind the supercluster have lower radial velocities and redshift values than their distance from each other would imply. This effect was confirmed by a decisive measurement in 2001 by the 2dF Galaxy Redshift Survey, which was conducted by the Anglo-Australian Observatory between 1997 and 2002. This is in agreement with current cosmological models and again strongly suggests the presence of dark matter.
What Is Dark Matter Made Of?
What is dark matter made of – that is still a great mystery at the present time. Dark matter could be any substance which interacts with ordinary matter through the gravitational force – at the present time, most scientists believe that it is composed of some as yet undiscovered subatomic particles.
Some types of hypothetical particles which might compose dark matter include sterile neutrinos, a type of neutrino only interacting with gravity and none of the other fundamental forces of the current model of particle physics, axions, weakly interacting massive particles, gravitationally interacting massive particles, supersymmetric particles, or primordial black holes, an exotic type of black hole which theoretically could have formed soon after the Big Bang and which could have a remarkably small size, invisible to our observations.
If dark matter is composed of weakly interacting particles it would be unlikely to form large structures, such as stars or planets because 1) it would not have a sufficient way to lose energy, and 2) it would not have the range of interactions within the standard model of theoretical particle physics needed to form these structures.
Primordial black holes are still being considered as a possibility for the composition of dark matter but the idea of minuscule, atomic-sized primordial black holes making up dark matter has pretty much been ruled out by measurements of the Voyager 1 spacecraft of positron and electron fluxes emanating from the Sun’s heliosphere.
Detecting Dark Matter
If dark matter is composed of some sort of exotic subatomic particle, then many billions(or even more) of them could be passing through every square inch of space, including the Earth, every second. Many experiments are being devised to detect dark matter based on this hypothesis.
Experiments designed for direct detection will try to detect very low energy movements of atomic nuclei which are caused by the hypothetical movement of these gigantic amounts of particles through the Earth and their interaction with these nuclei. These experiments would have to be conducted under very stringent conditions with extremely low background radiation since such interactions, if they exist, would be very slight.
Experiments designed for indirect detection will look for the decay or annihilation products of dark matter particles in outer space. Such products could be in the form of electromagnetic radiation in the form of gamma rays, protons, or anti-protons which might be emanating from high-density regions of our Milky Way Galaxy or other galaxies. One problem inherent with this detection method would be the very high background radiation which would seem to make any definitive detection unlikely.
Still another type of experiment to detect dark matter would be to use something such as the CERN Large Hadron Collider to detect dark matter particles which might result from the collision of proton beams being accelerated at very close to the speed of light. Interactions with electron collisions could also be studied. Although the possible interaction of dark matter particles with ordinary matter particles would be very slight(if at all), missing energy and momentum which is not explained by ordinary matter particle interactions could be some preliminary sort of evidence of the existence of dark matter, although such evidence would have to be confirmed by the aforementioned direct or indirect experiments.
Is Dark Matter Real?
As we have seen, there seems to be much compelling evidence for the existence of dark matter, but is this real evidence or is it only a delusion – an artifact of current models of theoretical physics and cosmology which will be determined to be incorrect or incomplete in the future, similar to how Quantum Theory complemented(but did not replace) classical Newtonian physics in the early part of the twentieth century. After all, the hypothetical supposition of the existence of dark matter is predicated completely on these current theoretical models being correct.
The very experiments being devised in their various forms to detect dark matter have to make the assumption that dark matter has some interaction with visible matter aside from just gravity, if only very slight. But what if dark matter does not interact with ordinary matter at all, except through gravity? Then it’s detection will probably continue to be elusive for many years to come. In fact, it may never be detected at all. Or perhaps our current theoretical models of physics and cosmology will become obsolete and be replaced by newer and better, maybe even revolutionary, theories and the idea of the existence of dark matter will become a thing of the distant past. There is really no way to know what direction the search for dark matter will take, we’ll just have to wait and see.