Dark Matter


There is perhaps no current problem of greater importance to astrophysics and cosmology than that of 'dark matter'.

The controversy, as the name implies, is centered around the notion that there may exist an enormous amount of matter in the Universe which cannot be detected from the light which it emits.

This is 'matter' which cannot be seen directly. So what makes us think that it exists at all? Its presence is inferred indirectly from the motions of astronomical objects, specifically stellar, galactic, and galaxy cluster/supercluster observations.

The basic principle is that if we measure velocities in some region, then there has to be enough mass there for gravity to stop all the objects flying apart. When such velocity measurements are done on large scales, it turns out that the amount of inferred mass is much more than can be explained by the luminous stuff. Hence we infer that there is dark matter in the Universe. There are many different pieces of evidence on different scales. And on the very largest scales, there may be enough to "close" the Universe, so that it will ultimately re-collapse in a Big Crunch.


Various means of weighing the universe lead us to believe in the presence of dark matter. There is evidence from different astronomical objects, in order of increasing size:

Stellar Motions:

Dutch astronomer Jan Oort first discovered the presence of dark matter in the 1930's when studying stellar motions in the local galactic neighborhood. By observing the Doppler shifts of stars moving near the galactic plane, Oort was able to calculate how fast the stars were moving. Since he observed that the galaxy was not flying apart he reasoned that there must be enough matter around that the gravitational pull kept the stars from escaping, much as the sun's gravitational pull keeps the planets in the solar system in orbit. He was able to determine that there must be three times as much mass as is readily observed in the form of visible light. Hence, Oort's calculations yielded an M/L ratio of 3 for the region of the immediate galactic neighborhood. The M/L ratio increases by several orders of magnitude as larger astro-physical phenomena come under similar scrutiny.


Luminous regions of galaxies

The luminous region of a galaxy extends over a radius of about 10 kpc. The sun is on the outskirts of the Milky Way galaxy, and about this distance from the center of the galaxy. One measures the total mass interior to the orbit of the sun from the sun's rotation speed around the galaxy and its galactocentric distance: this gives the centrifugal force, which must be balanced by the gravitational force due to all the mass interior to the sun's orbit. One finds that this mass is 10^11 M(Sun), while the cumulative luminosity of all the stars in the Milky Way is about 10^10 L(Sun). The ratio of mass to luminosity is therefore equal to 10, so that the average star is about half the mass of the sun. This is not a great surprise: the solar neighborhood contains younger, relatively more massive and luminous stars as do other spiral arm regions as compared with the galaxy as a whole. When we add up the luminosity from ale the stars in all the galaxies in the universe we find that the mass is far less than that required to close the universe. It is also significantly less than the mass density implied by Big Bang Nucleosynthesis. This deficit indicates that there may be "baryonic" dark matter (although not enough to make the universe recollapse), as well as the more exotic "particle" dark matter.


Galactic Rotation Curves

The galaxy "M51". Messier 51 is also known as NGC 5194 and sometimes called the Whirlpool galaxy. It is the prototypical "Grand Design" spiral (i.e. very symmetrical and regular arms). The galaxy is of type Sc and it is very nearly face-on. The distance to M51 is about 9 Mpc (or about 30 million light years), and it is moving away from us at about 500 km/s. The other object is the lenticular companion galaxy NGC 5195. This particular image was taken at near-infrared wavelengths. Picture provided by Rosa Gonzalez and Jim Graham.

Evidence of dark matter has been confirmed through the study of galactic rotation curves. These measurements are on a smaller scale than the galaxy clusters, but give more detail about the way the dark matter is distributed.

To make a rotation curve one calculates the rotational velocity of stars along the length of a galaxy by measuring their Doppler shifts, and then plots this quantity versus their respective distance away from the galactic center.

Invariably, it is found that the stellar rotational velocity remains constant, or "flat", with increasing distance away from the galactic center. This result is highly counterintuitive since, based on Newton's law of gravity, the rotational velocity would steadily decrease for stars further away from the galactic center. Analogously, inner planets within the Solar System travel more quickly about the Sun than do the outer planets (e.g. the Earth travels around the sun at about 100,000 km/hr while Saturn, which is further out, travels at only one third this speed). One way to speed up the outer planets would be to add more mass to the solar system, between the planets. By the same argument the flat galactic rotation curves seem to suggest that each galaxy is surrounded by significant amounts of dark matter. It has been postulated,and generally accepted, that the dark matter would have to be located in a massive, roughly spherical halo enshrouding each galaxy.

The rotation curve for the galaxy NGC3198 from Begeman 1989

The first real surprise in the study of dark matter lay in the outermost parts of galaxies, known as galaxy halos. Here there is negligible luminosity, yet there are occasional orbiting gas clouds which allow one to measure rotation speeds and distances. The rotation speed is found not to decrease with increasing distance from the galactic center, implying that the mass distribution of the galaxy cannot be concentrated, like the light distribution. The mass must continue to increase: since the rotation speed satisfies v^2=GM/r, where M is the mass within radius r, we infer that M increases proportionally to r. This rise appears to stop at about 50kpc, where halos appear to be truncated. We infer that the mass-to-luminosity ratio of the galaxy, including its disk halo, is about 5 times larger than estimated for the luminous inner region, or equal to about 50. Many people believe that the galactic halos are composed of particle dark matter. The Center Direct Detection Group is actively searching for evidence of this dark matter.


Galaxy Clusters

While Oort was carrying out his observations of stellar motions, Fritz Zwicky of Caltech discovered the presence of dark matter on a much larger scale through his studies of galactic clusters. A galactic cluster is an group of galaxies which are gravitationally bound. Our own galaxy, the Milky Way, is a member of a small cluster known as the Local Group. Using the same method employed by Oort, Zwicky determined the Doppler shifts of individual galaxies in one particular system, the Coma cluster--about 300 million light years away. Zwicky found nearly 10 times as much mass as observed in the form of visible light was needed to keep the individual galaxies within the cluster gravitationally bound. It was clear to Zwicky, as it had been to Oort, that a large sum of mass was extant which was simply not visible. At the time, astronomers referred to the material as "missing mass". However, this was deemed a misnomer as the mass was clearly present, but simply not visible. Hence, the more appropriate term "dark matter" came to supercede "missing mass". Since Zwicky's efforts, more recent measurements have found that certain galaxy clusters (and binary galaxies) have M/L ratios up to 300.

The mass-to-light ratio can also be evaluated by studying galaxy pairs, groups, and clusters. In each case, one measures velocities and length-scales, leading to a determination of the total mass required to provide the necessary self-gravity to stop the system from flying apart. The inferred ratio of mass-to-luminosity is about 100 in galaxy pairs, which typically have separations of about 100 kpc, and increases to 300 for groups and clusters of galaxies over a length scale of about 1 Mpc.


Superclusters

The largest scale on which the mass density has been measured with any precision is that of superclusters. A supercluster is an aggregate of several clusters of galaxies, extending over about 10 Mpc. Our local supercluster is an extended distribution of galaxies centered on the Virgo cluster, some 10-20 Mpc distant, and our Milky Way galaxy together with the Andromeda galaxy forms a small group (the Local Group) that is an outlying member of the Virgo Supercluster. The mass between us and Virgo tends to decelerate the recession of our galaxy, as expected according to Hubble's law by about ten percent. This effect is seen as a deviation from the uniform Hubble expansion of the galaxies and provides a measure of the mean density within the Virgo Supercluster. One again finds a ratio of mass-to-luminosity equal to 300 over this scale, which amounts to about 20Mpc.


Dark matter has important consequences for the evolution of the Universe. According to standard cosmological theory, the Universe must conform to one of three possible types: open, flat, or closed. A parameter known as the "mass density" - that is, how much matter per unit volume is contained in the Universe - determines which of the three possibilities applies to the Universe. In the case of an open Universe, the mass density (denoted by the greek letter Omega) is less than unity, and the Universe is predicted to expand forever. If the Universe is closed, Omega is greater than unity, and the Universe will eventually stop its expansion and recollapse back upon itself. For the case where Omega is exactly equal to one, the Universe is delicately balanced between the two states, and is said to be "flat".

In the figure above we show graphically some of the measurements of the density of the universe which we have discussed above. What is plotted is the density of the universe, both visible matter and the inferred "dark matter", as a function of the "scale" at which the measurement was made, from the local neighbourhood up to the largest scales. On the smallest scales, probed by Oort, the visible matter and three times as much dark matter give Omega about 1/1000. As we go to larger and larger scales the inferred value of Omega increases, although the measurements become harder and progressively more uncertain. The next point to the right is the mass in galaxies, which moves to the position of the higher dot if we include the dark matter inferred from rotation curves. Then on larger scales we have the measurements from the motions of clusters of galaxies and the cosmic microwave background. The yellow band indicates the amount of matter that can reside in "normal" matter, or baryons, as inferred from Nucleosynthesis. If there is more matter in the universe than this, as the measurements appear to be telling us, then it must be made up of some strange particle which is not familiar to us here on earth.

There is also a somewhat philosophical idea that makes Omega=1 attractive. The point is that as the Universe expands the value of Omega changes. In fact the value 1 is unstable, and the Universe would prefer to evolve towards one of the two natural values: 0, if the expands forever further apart until the Universe is almost totally empty ; and infinity, if the matter recollapses to a state of higher and higher density. Then the observation that Omega is fairly close to 1 today, means that it must have been even closer to 1 in the past. It is unsatisfying to believe that we just happen to live at the time when Omega is just starting to depart from 1 by a small factor. It is much more appealing to consider that we do not live at a special epoch, so that Omega is still close to 1 today. But then we need to explain why Omega started out very close to 1 in the early universe. The theory of inflation provides just such a justification - it predicts that the early Universe was driven extremely close to flat, and that it is still very close to flat today. If this is so, then at least 90% of the mass of the Universe is dark. Dark matter (DM) candidates are usually split into two broad categories, with the second category being further sub-divided:

  • Baryonic
  • Non-Baryonic - hot dark matter (HDM) and cold dark matter (CDM),

    depending on their respective masses and speeds. CDM candidates have relatively large mass and travel at slow speeds (hence "cold"), while HDM candidates include minute-mass, rapidly moving (hence "hot") particles.

    - Encyclopedia Britannica


    NEWS ARTICLES


    More Evidence that Dark Matter Rules the Universe November 2002 - Space.com


    Scientists Map Dark Matter, Prove Einstein Right

    May 13, 2000 - Space.com

    Eighty-four years after Albert Einstein introduced the world to his theory of general relativity, scientists are seeing that he was right all along about measuring what we now call dark matter.

    Astronomers supported by the National Science Foundation have found the first evidence of an effect called cosmological shear, a phenomenon predicted by Einstein�s theory, in which light from distant cosmic objects bends due to gravitational forces. What�s more, the detection of cosmological shear has allowed astronomers to track down significant amounts of dark matter, non-luminous matter whose presence in the universe has been predicted, but scantly detected until now.

    "This marks a totally new way of finding dark matter," said Max Tegmark, a physicist at the University of Pennsylvania. "It�s going to revolutionize our ability to map out where all the dark matter is."

    The results on cosmological shear were published in this week's issue of the journal Nature.

    Observable matter takes up no more than about 10 percent of the total amount of matter predicted to exist in the universe. The rest � dubbed dark matter because it can�t be "seen" or detected in the same way that gas, dust, and other observable matter can � remains largely a mystery to astronomers precisely because it�s so difficult to find. Now, researchers have a new tool for uncovering the elusive stuff.

    On the left is an optical image of a small cluster of galaxies. The center image shows a 1-degree image of the same field. The right image represents a mass map of the same patch of sky.

    "The existence of dark matter has been known for a long time, and has even been mapped in various places over the past decade," explained astrophysicist David Wittman of Bell Labs, Lucent Technologies. "But the places that had been mapped were small areas of the sky. What�s new here is that we have studied a fair sample of the universe � and so were able to deduce some properties of the universe in general."

    Cosmological shear helps astronomers "see" dark matter because it makes the light from distant galaxies appear distorted. A distant spherical-shaped galaxy, for example, will appear elliptical to astronomers back on Earth. This is because dark matter existing in the path between the galaxy and Earth exerts a gravitational pull on the light, causing it to bend.

    Wittman and a team of researchers analyzed 145,000 distant galaxies in order to find evidence of cosmological shear, also known as "weak gravitational lensing." They used a special camera called a charged couple device or CCD and the 4-meter Blanco telescope at the Cerro Tololo Interamerican Observatory in Chile.

    The recent observations have also shed new light on the eventual fate of the universe. Astrophysicists currently predict that the total amount of matter present in the universe will determine whether the universe will continue to expand, or whether it will eventually slow down, or even begin to contract. According to Wittman, the scientists� observations of cosmological shear have suggested that the overall density of matter in the universe is "too low to stop the expansion of the universe."

    At the same time, astronomers admit that their new method for finding dark matter has not yet been tested enough to allow experts to make a definitive generalization about the fate of the universe. "Since our approach is new, it�s not very precise yet," said Wittman. "Really strict tests of the theory will come in the next few years as astronomers measure the [weak] lensing more and more accurately."

    "Ultimately, we�d like to be able to map the whole sky to see just how much dark matter is out there," added Tegmark. For now, the use of cosmological shear to uncover dark matter in one region of the universe at a time will have to suffice. But as Bell Labs astrophysicist Anthony Tyson joked, "The future looks bright for dark matter."


    Astronomers Map 'Dark Matter'

    By analyzing the light from 200,000 distant galaxies, indicated above in blue, astronomers are creating a map of the interconnecting dark matter, the invisible material that keeps our universe together. This numerical simulation reveals filaments of dark matter, shown here in red and white, which are invisible even to the largest telescopes.

    March 14, 2000 - Discovery

    Trying to find invisible matter may seem an impossible task, but with scientific theory postulating that 90 percent of what makes up our universe is unaccounted for, the quest for so-called dark matter is far from academic.

    A multinational team of cosmologists, astrophysicists, statisticians, technicians and other experts revealed they have developed a map of dark matter distributions across a 2-square-degree section of the sky.

    To detect the undetectable, the scientists used a high-resolution, wide-field imaging camera on the Canada-France-Hawaii Telescope in Hawaii to analyze the light from 200,000 distant galaxies, looking for tiny distortions in the light caused by the gravitational effects of intervening dark matter.

    The analysis showed a vast, interconnected web of dark matter, raising hopes that age-old questions about how the universe formed and what its ultimate fate will be may one day be answerable.

    To build accurate mathematical models of the universe scientists need to have an idea of how much matter it contains, according to Yannick Mellier, of the Institut d'Astrophysique de Paris and the Observatoire de Paris.

    "Since around 90 percent of this matter is invisible, it's hard for us to get a precise reading on this. Also, to test our models to see if they accurately describe the universe, we need to look at what is actually out there," says Mellier, who heads the French-based research team.

    Telescope director Greg Fahlman called the team's results a preliminary view of what may be achieved with a new, more sensitive wide-field camera currently under development.

    "Our goal is to help create the first distribution maps of dark matter across the sky, similar to the maps you currently see for galaxies," says Fahlman.

    
    
    
    
    
    
    
    
    
    
    
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