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The Evidence for Dark Matter

Updated on June 18, 2011

Dark matter is the name given to the matter that holds together the Milky Way and other galaxies. It cannot be seen using telescopes as it does not emit light or other radiation. However, there is significant evidence for its presence through its gravitational interactions with ordinary matter.

Discovery of Dark Matter

In 1933, astronomer Fritz Zwicky was observing the motions of galaxies at the edge of the Coma galaxy cluster. From these measurements, he calculated the mass of the cluster, but his calculations gave a result which was about 400 times greater than the mass of observable matter in the cluster.

Further Evidence: Rotation Curves of Spiral Galaxies

The stars in spiral galaxies lie in a rotating spiral structure. Most of the stars are clustered together in the centre, with fewer stars lying further out along the arms of the spiral.

The galaxy is held together by gravitational attraction. Newton's law of gravitation states that the attraction between two objects decreases as they get further apart. So if most of the galaxy's mass is concentrated in the centre, stars that are near the middle will experience a stronger attraction than those further out.

The speed of a star's orbit is determined by the strength of the gravitational force it experiences. Imagine tying an object to the end of a length of string and spinning it around. The faster you spin, the tighter you must hold on to the string to keep the object rotating and prevent it from flying off into the distance. The same principle applies here: a star on the galactic periphery that experiences only a weak gravitational pull must travel slowly if it is to stay in orbit rather than flying off into space.

The upshot is that we would expect stars that are further out to have a lower velocity than those near the middle of the galaxy. But there was a surprise in the results! On the graph below, curve A is what we would expect for a galaxy whose density is greatest in the centre, but the observations (curve B) showed that the stars at all distances from the centre travel at approximately the same speed. This is what we would expect if the density was the same at all locations in the galaxy.

So is there more matter lurking in the outer regions of the galaxy than what we can see? Most astrophysicists think so, and have coined the term "dark matter" to describe it

Yet More Evidence: Gravitational Lensing

Gravitational lensing is a beautiful and fascinating phenomenon in which light from a distance source is bent by a strong gravitational field. This is the result of space-time curving around a massive object as predicted by Einstein's theory of General Relativity. By looking at the distinctive patterns of gravitational lensing, astronomers can map out how matter is distributed. The results show that large amounts of dark matter are present.

Gravitational Lensing in the Galaxy Cluster Abell 2218.  The circular streaks of light are gravitational lensing effects.
Gravitational Lensing in the Galaxy Cluster Abell 2218. The circular streaks of light are gravitational lensing effects. | Source

topquark works as a researcher in theoretical particle physics and blogs about research at The Particle Pen.

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    • Spirit Whisperer profile image

      Xavier Nathan 

      8 years ago from Isle of Man

      I like the way you have explained this and presented the information in such a simple and easy to follow way. Thank you.

    • profile image

      Dan Echegoyen 

      8 years ago

      BLACK HOLES, EXPANSION, AND DARK ENERGY

      In the continuum of space and time, exists the dichotomy of matter and energy. All things exist as both matter and energy, but are experienced as one or the other.

      As energy, all things exist as wave patterns. Most wave patterns are interferences of simpler wave patterns. The simplest wave forms are those that do not interfere with other waves. These simplest wave forms hold their shape as they propagate. There are three such wave forms.

      The first such wave form is seen in three dimensions as the spherical expansion wave of a bomb blast, and in two dimensions as the circular wave of expansion on the water where a rock was tossed in. The second wave form is seen in three dimensions as the cone of sonic boom following an aircraft traveling faster than sound, and in two dimensions as the V-wake on the water where the boat is traveling faster than the water wave. The third wave form is seen in three dimensions as the propagation torus of a smoke ring and is seen in two dimensions as the double vortexes of an oar stroke on the water.

      The Torus is a particle of discrete exchange, from one point to another. The object exchanges position and momentum. While the spherical wave shows position, and the conic wave shows momentum, the torus shows both at the same time, and has a dynamic finite unbounded reality. The volumes of the cone, sphere, and torus are mathematically related as static objects.

      The Universe is a local density fluctuation. (a wave pulse) On this local density fluctuation wave, lesser wave forms may exist. All simple wave forms are also local density fluctuations, and as such are indeed universes in their own right, where other waves may exist.

      Consider the torus as a universe. Einstein said that gravity is indistinguishable from acceleration. There is both linear acceleration and angular acceleration. Although the torus as a whole travels in a straight line, every local point on the torus travels in a circle and experiences angular acceleration.

      The rubber sheet model of gravity and curved space translates directly to the propagating torus with angular acceleration. Acceleration is downward on the rubber sheet and outward on the torus. The tension field that separates the inside of the torus from the outside holds its shape as a simple two dimensional field of space and time just as the rubber sheet does.

      Experimentally verifiable is that a big fat slow smoke ring generated in a room with very still air will eventually possess a bulge that travels in a circle on the surface of the smoke ring. This bulge, being a gravitational depression, gathers more of the energy of the field toward itself. Finally the bulge gathers enough material to collapse the field and eject a new, smaller smoke ring out in the same direction as the first torus. This collapse is a black hole to the first torus, and a white hole to the second torus, where the axes of space and time in that second torus have reversed.

      While gravity tends to draw depressions together locally on a dynamic torus, even to the point of field collapse, other areas on a torus expand and contract globally as the torus propagates along without regard to local phenomenon on the surface. This is quintessence. The inertia of the torus to propagate is its dark energy. This is a two-dimensional example of the process that we experience in three dimensions.

      From structureofexistence.com by Dan Echegoyen 951-204-0201

      --

      Dan Echegoyen

      author of StructureOfExistence.com

      (951) 204-0201

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