Dark Matter Discovery
Black holes of course are dark matter because they can not be seen. However, there is a more general definition of dark matter. Dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and cannot be directly detected by electo-magnetic sensors.
The first person to provide evidence of the presence of dark matter was astro-physicist Fritz Zwicky, of Caltech in 1933. Zwicky estimated the total mass of the Coma Cluster of Galaxies based on the speed of the galaxies near its edge. He then compared that estimate to one based on the number of galaxies in the cluster. He found there was about 400 times more estimated mass than was observable. Zwicky inferred that there must be some non-visible form of matter which would provide enough mass, and therefore gravity, to hold the cluster together.
In 1975 Vera Rubin, a young female astronomer at the Department of Terrestrial Magnetism in Washington, DC, announced that based on her spectrographic data, most stars in spiral galaxies orbit at roughly the same speed. This implied that their masses were the same. These results suggested that either Newtonian Gravity does not apply universally or that more than 50% of the mass of these galaxies was dark matter. Met with much skepticism, Rubin insisted that her observations were correct. Eventually other astronomers corroborated her work and it became accepted that most galaxies were in fact dominated by "dark matter". Recent calculations indicate that in the total universe, dark matter mass makes up about 23% of the total mass.
The image above is known as the Ring Of Dark Matter in the galaxy cluster Cl 0024+17. The blue map of the cluster's dark matter distribution is superimposed on a Hubble image of the cluster. The ring is one of the strongest pieces of evidence for the existence of dark matter, an unknown substance that pervades the universe. Top
What Evidence Is There For Dark Matter?
Dark matter is particularly elusive as it does not emit, absorb or reflect light, but makes itself apparent only through gravitational attraction. To locate this mysterious matter a technique known as gravitational lensing can be used. The bending of light rays from distant galaxies as they pass through the dark matter's gravitational field reveals the mass of the dark matter. By carefully plotting the way that the distant images are distorted, it is possible to quite accurately map the dark matter and estimate its mass. For a fuller explanation of gravitational lensing see the Bending Of Light section in the Relativity section.
The duo pictures at the left are from the Cosmic Evolution Survey by NASA and ESA. On the left hand side is a shot of the galaxy in visible light (red). On the right hand side is an image of dark matter (in blue) using the gravitational lensing technique. The brightness of clumps corresponds to the density of mass. The maps cover an area of sky nine times the a diameter of the full moon, and is the largest sample of the distribution of dark matter ever obtained. It demonstrates how normal matter, including stars, galaxies and gas, is built within an underlying scaffolding of dark matter.
There does not appear to be any alternative scientific explanation that can explain dark matter on all scales, so the conclusion that dark matter exists appears unavoidable. Top
The Bullet Cluster
The galaxy in the picture to the left is known as the Bullet Cluster, so named because of the red bullet shaped pocket of gas on the right side. The blue halos are believed to be dark matter and the red halos are hot gases. The Bullet Cluster is actually two clusters of galaxies having collided with one another. As the two clusters crossed (at a speed of 10 million miles per hour), the luminous matter (red halos) in each cluster interacted with the luminous matter in the other cluster causing both of them to slow down.
But the dark matter (blue halos) in each cluster did not interact at all, passing right through without disruption. This difference in interaction caused the dark matter (blue halos) to speed ahead of the luminous matter (the red gas halos), separating each cluster into two components: dark matter in the lead and luminous matter lagging behind.
Most astro-physicists consider the Bullet Cluster to be the most compelling evidence that dark matter really exists and demonstrates dark matter's exotic behavior. Top
What Might Dark Matter Consist Of?
Shown in the photo at the left are 18 galaxy clusters, each containing thousands of galaxies with giant clumps of dark matter (shown in blue) inferred by the use of gravitational lensing. All the clumps (called halos) are flattened a bit like footballs, with their horizontal axes about twice as long as the vertical axes, regardless of the shape and distribution of their galaxy clusters.
In general, dark matter may be either like ordinary matter (baryonic in scientific language) in a form which makes it much dimmer than normal stars, or else it may comprise a more exotic species of non-ordinary matter (non-baryonic). Among the more plausible of the normal matter candidates are brown dwarfs (dim low mass stars about one-tenth the mass of the sun), white dwarfs (remnants of stars which have blown themselves apart), and very massive black holes (larger than 200 solar masses). Ordinary dark matter is often called MACHO - massive compact halo object - since it is a prime candidate for explaining the dark matter within galactic clusters.
On the other hand, dark matter may be non-ordinary matter (non-baryonic). Favorite among non-ordinary contenders from particle physics are the light neutrino, the axion (predicted by promising next generation theories of particle physics), and the LSP (lightest stable particle) predicted by super-symmetric particle theories. Theoretical calculations of the ratios of hydrogen, helium and other light elements formed in the early history of the universe indicate that the total amount of ordinary matter can be no more than ten times the amount of visible matter. This is sufficient to explain the dark matter on the scale of galaxies but not enough to explain the dark matter in galaxy clusters. The existence of non-ordinary (non-baryonic) matter therefore appears inescapable. However to date, no particles with invisible mass (light passes through them), gravitational pull, and zero radiation characteristics have been discovered.
Non-ordinary matter (non-baryonic) is often characterized into two categories: Hot dark matter (HDM) is particles which were relativistic (moving at velocities close to the speed of light) at the early stage of the universe when it was so dense that it was opaque to light. Cold dark matter (CDM) which was non-relativistic at the early stage (CDM particles are also referred to as WIMPS - weakly interacting massive particles). Dark matter is likely dominated by CDM, although the best theoretical fit between the two includes both CDM and HDM in the ratio of about 2 to 1. CDM could in principle also explain dark matter within individual galaxies. Top
NASA 3D Map Of Dark Matter
An international team of 70 astronomers led by Richard Massey of the California Institute of Technology, made a three dimensional map that illustrates a web-like large scale distribution of dark matter in the universe in unprecedented detail. The map was derived from the Cosmic Evolution Survey (COSMOS) of the universe made by the Hubble Space Telescope.
The map provides the best evidence that normal matter, mostly in the form of galaxies, accumulates along the densest concentrations of dark matter. The map reveals a loose network of filaments intersecting where clusters of galaxies are located. The map is consistent with conventional theories of how structure formed in the evolving universe under the relentless pull of gravity, making the transition from an almost smooth distribution of matter into a sponge-like structure of long filaments. The map also reveals how dark matter has recently grown increasingly clumpy as it continues to collapse under gravity. Top
Dark Matter Filaments Connect Galaxies
Jörg Dietrich of the physics department at the University of Michigan, together with his co-workers, have uncovered a specific filament of dark matter connecting two neighboring Abel galaxy clusters. This was a "first" such observation of a specific "link" of a dark matter bridge between two galaxies. It provides direct evidence that the universe is filled with a latticework of dark matter filaments and that visible matter is found at its junctions.
The two Abel clusters each have about 150 galaxies, are separated by about 400 million light years, and are about 2.4 billion light years from Earth. Our Milky Way is about 110 thousand years wide, so these two galaxies are a fair distance apart even by astronomical standards. In the image to the left, a filament of dark matter connects the galaxy clusters Abell 222 and Abell 223. The blue shading and yellow contour lines represent the densities of the dark matter. "We found dark matter filaments and for the first time we can "see" one of them," said Jörg Dietrich.
Scientists believe the structure of the current dark matter filaments is a remnant of the initial plasma fluctuations which dominated the universe shortly after the Big Bang. These dark matter filaments are very large and long, and like a highway guide galaxies toward their destinations. Once a galaxy has joined a filament, it provides a path for it to join a "galaxy cluster" which are located at the vertices of the filaments. Top
Dark Matter Distribution
Each bright "city light" in the image to the left is a galaxy, and the dark areas between the lights that appear to be empty are actually full of dark matter. An international team of astronomers from Taiwan, UK, Japan, and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) used a large sample of galaxy clusters to figure out how the density of dark matter changes from the center of a typical galaxy cluster to its outskirts.
The scientists measured the density of dark matter in fifty galaxy clusters and found that dark matter density gradually decreases from the center of these cosmic giants to their outskirts. This new evidence about dark matter that pervades our universe conforms to the predictions of cold dark matter theory, known as “CDM”.
CDM, the leading theory about dark matter, predicts that dark matter particles only interact with each other and with other matter via the force of gravity. They do not emit or absorb electromagnetic radiation and are impossible to see. The team combined measurements from observations of fifty of the most massive known galaxy clusters to calculate their concentration parameter. The concentration parameter is a single measurement of a cluster’s average density (how compact it is). They found that the density of dark matter increases from the edges to the center of the cluster, and that the concentration parameter of galaxy clusters in the near universe conforms to CDM theory. Top
A Dark Matter Signal From The AMS?
The $1.5 billion Alpha Magnetic Spectrometer (AMS) is located on the International Space Station (ISS). AMS was designed to study high energy cosmic rays before they have a chance to interact with the earth's atmosphere. Weighing 7.5 tons, too heavy for a dedicated satellite, the AMS detector uses a cylindrical magnet 1 meter in diameter and 1 meter in height to sort incoming particles according to their momentum and charge.
According to standard theory the "positron fraction", the ratio of positrons to electrons in outer space, should decrease with increasing energy. AMS searches for an excess of positrons in an energy range from 0.5 GeV (billion electron volts) to 1 TeV (trillion electron volts). Over this range, AMS is looking for an excess that could point to evidence of dark matter annihilation. This could be caused by the collision in outer space of WIMPs, the leading candidate for dark matter.
In the chart to the left, the positron fraction (in red) as measured by AMS shows a decrease from 1 to 10 GeV, then increases from 10 to 250 GeV, and thereafter seems to be leveling off. Samuel Ting, AMS principal investigator from MIT, says that the data also indicates that there are no sharp peaks, no significant variations over time, or any preferred incoming direction. Assuming an even distribution of dark matter particles, the results are consistent with positrons originating from the annihilation of WIMPs (weakly interacting massive particles) in outer space. Shown in blue and green are previous measurements of the positron fraction using less sophisticated equipment by the Pamela and Fermi experiments, which are in general agreement.
Do the measurements indicate dark matter? "Unfortunately, the answer is maybe", says astro-physicist John Wefel from Louisiana State University. "The results could have been caused by the presence of WIMPs or by other particles that are colliding with each other and emitting positrons. We just have to stay tuned and be patient." Top
Dark Matter Disbelievers
In spite of all the inferred evidence of dark matter, there are disbelievers in the astro-physics community. The total theory of the universe makes sense only if astro-physicists either modify the theories of gravity or invoke additional gravitational forces due to unseen sources of mass which are referred to as dark matter.
Some such as Mordehai Milgrom, an astro-physicist at the Weizmann Institute of Science in Rehovot, Israel, think that the theory of gravity has to be modified. Some scientists think that by now we should have found some independent evidence for dark matter and that hasn't happened. Gravitational theory predicts that spiral galaxies, like our own Milky Way, must have massive dark matter halos that provide the galaxy's missing mass. But the Milky Way's own dark matter halo has yet to be detected or inferred, even indirectly.
Particle physicists are hopeful that dark matter particles will be detected in the lab. For example, the Xenon100 experiment at Italy's National Laboratory in Gran Sasso, is built to register direct hits from particulate dark matter. Xenon100 is designed to search for the most favored dark matter particle candidate, the WIMP, by watching for signals that a WIMP has recoiled off an atom in a tank of liquid xenon. But so far no WIMP particles have been detected. The debate rages on, but most scientists believe that dark matter really does exist.
In the image above in this section, Galaxy MACS J0025.4-1222 is the result of two galaxies having previously collided similar to the Bullet Galaxy at the top of this page. It is a composite image from optical and x-ray telescopes showing that visible matter (pink) has slowed down resulting from the collision. Meanwhile, most of the mass of the dark matter (blue) does not slow down and passes right through the visible matter creating two blue halos that are moving away from the collision. This galaxy collision demonstrates that the Bullet Cluster is not an anomaly." Top
New Evidence Questions Dark Matter
See the Chandra x-ray image of galaxy cluster Abell 383 to the left. The purple halo cloud is believed to be dark matter that is inferred from gravitational effects of the cluster.
A team led by researchers from Case Western Reserve University in Cleveland has found a significant new relationship in spiral and irregular galaxies. The rotational speeds of stars inside galaxies tightly correlates with the gravitational speeds expected from visible mass only (no dark matter necessary).
The finding is consistent among 153 spiral and irregular galaxies, ranging from giant to dwarf, those with massive central bulges or none at all. It is also consistent among those galaxies composed of mostly stars or mostly gas.
In a September, 2016 paper accepted for publication by the journal Physical Review Letters, Stacy McGaugh, lead author of the research and chair of the Department of Astronomy at Case Western, says that "the relationship we have found is tantamount to a new natural law".
"We find a relation between what you see in normal matter in galaxies and what you get in their gravity," McGaugh said. "This is important because it is telling us something fundamental about how galaxies work."
Previous analyses of the orbital velocities of the stars in galaxies often depended on visible wavelengths of light. However, the stars that produce the most visible light are prone to fluctuations and may not provide the best picture of how matter is scattered throughout a galaxy. Instead, McGaugh and his colleagues analyzed near-infrared images collected by NASA's Spitzer Space Telescope over the past five years.
One possible theoretical explanation for this new observation suggests that at large scales, the laws of gravity are different from Einstein’s theory of general relativity. As mentioned above, this idea, known as Modified Newtonian Dynamics (MOND), was proposed more than 30 years ago by Israeli physicist Mordehai (Moti) Milgrom of the Weizmann Institute, pictured at the left. Specifically, Milgrom proposes that for very small accelerations, the square of the acceleration, not the acceleration itself, is proportional to the gravitational force.
The new data are exactly what MOND would have forecast. However, McGaugh and his team at this point are not pressing for any theoretical interpretation of their empirical data. They say that other models of dark matter also have the potential to explain their findings.
Please note that dark matter does do a very good job of explaining the variations in the cosmic microwave background (CMB) curve, the total mass of the Universe, and galaxy cluster halos. MOND struggles with all three of these items.
Despite decades of trying, researchers have failed to capture a single piece of dark matter, even though it is supposed to make up roughly five-sixths of all matter in the universe. "However, until we fully understand dark matter we should keep an open mind", says Abraham Loeb, a theoretical astrophysicist and chair of astronomy at Harvard University, who did not take part in the research.
Therefore, one can conclude that while dark matter is no longer necessary to explain the rotational speeds of stars in a galaxy. At this time dark matter still has its place in the cluster of galaxies, mass of the Universe, and the CMB of the Big Bang. The study and mystery of dark matter continues.Top