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 electromagnetic 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
Evidence - The Cosmic Evolution Survey
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.
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 2 square degrees of sky in the constellation Sextans in visible light (red). On the right hand side is an image of dark matter (in blue) using the gravitational lensing technique for the exact same area. 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. 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 must exist seems unavoidable. Top
Cluster Evidence
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.
This structure is actually two clusters of galaxies passing through one another. As the two clusters crossed at a speed of 10 million miles per hour, the luminous matter in each cluster interacted with the luminous matter in the other cluster and slowed 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 (blue) in the lead and luminous matter (red) lagging behind.
To detect this separation of dark and luminous matter, researchers compared x-ray images of the luminous matter with measurements of the cluster's total mass. To learn the total mass, they took measurements of gravitational lensing, which occurs when the cluster's gravity distorts light from background galaxies. The greater the distortion, the more massive the cluster.
By measuring these distortions with the Hubble Space Telescope, the Magellan Telescopes and the Very Large Telescope, the team mapped out the location of all the mass in the Bullet Cluster. They then compared these measurements to x-ray images of the luminous matter taken with the Chandra X-ray Observatory and discovered four separate clumps of matter: two large clumps of dark matter (blue) speeding away from the collision, and two smaller clumps of luminous matter (red gas) trailing in their wake.
In another cluster image to the left, Galaxy MACS J0025.4-1222, is also the result of two galaxies having previously collided similar to the Bullet Galaxy above. It is also a composite image from optical and x-ray telescopes showing that visible matter (red) has slowed down resulting from the collision.
Meanwhile, as with the Bullet Cluster, most of the mass of the dark matter (blue) does not slow down and passes right through the visible matter (red) creating two blue halos that are moving away from the central collision.
This galaxy collision is so similar to the Bullet Cluster that it demonstrates that the Bullet Cluster is not an anomaly. A few more red/blue collisions have since been discovered.
Proponents of dark matter consider these cluster collisions to be the most compelling evidence that dark matter really exists and demonstrates dark matter's exotic behavior.
However, critics say the Bullet Cluster story matches what we observe, but it is oversimplified. (See the section in the next page Dark Matter Disbelievers.) (Fix) Top
What Might Dark Matter Be Composed Of?
Shown in the photo at the left is a galaxy surrounded by a halo of dark matter (shown in blue) inferred by the use of gravitational lensing. In general, dark matter may be either like ordinary matter (baryonic in scientific language) in a form which makes it invisable or 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 WIMPS (weak interactive massive particles).
Non-ordinary matter (non-baryonic) is often characterized into two categories: 1) Hot dark matter (HDM) are 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. 2) Cold dark matter (CDM) which was non-relativistic at the early stage (CDM particles are also referred to as WIMPS - weak 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.
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.
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." Top
NASA Estimates Milky Way Dark Matter
NASA estimated the amount of Dark Matter in our Milky Way Galaxy using the Hubble Space Telescope and the European Space Agency’s Gaia satellite as part of an estimate of the Galaxy's total mass. The Milky Way weighed in about 1.5 trillion solar masses (one solar mass is the mass of our Sun), according to the latest measurements. Not knowing the mass of the Milky Way presented a problem for a lot of cosmological simulations. Only a small portion of the total galaxy mass, 16.7 percent, is contributed by the approximately 200 billion stars in the Milky Way which also includes a 4-million-solar-mass supermassive black hole at the center.
The new mass estimate puts our galaxy on the beefier side, compared to other galaxies in the universe. The lightest galaxies are around a billion solar masses, while the heaviest are 30 trillion, or 30,000 times more massive. The Milky Way’s mass of 1.5 trillion solar masses is fairly normal for a galaxy of its brightness.
The Hubble and Gaia observations are complementary. Gaia was exclusively designed to create a precise three-dimensional map of astronomical objects throughout the Milky Way and track their motions. It made exacting all-sky measurements that include many globular clusters. Hubble has a smaller field of view, but it can measure fainter stars and therefore reach more distant clusters.
The new study augmented Gaia measurements for 34 globular clusters out to 65,000 light-years, with Hubble measurements of 12 clusters out to 130,000 light-years that were obtained from images taken over a 10-year period. When the Gaia and Hubble measurements are combined as anchor points, like pins on a map, astronomers can estimate the distribution of the Milky Way’s mass out to nearly 1 million light-years from Earth. (Author's note: NASA, publishing our Galaxy's total mass including Dark Matter, implies that NASA believes Dark Matter is real. However, nowhere do they come right out and say that.)
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