Dark Matter
Feb 28th, 2008 by admin
For more than 70 years Scientists have been perplexed because galaxy clusters have too much gravity to be explained by the amount of visible matter alone. Something strange and unseen must be causing the gravity; otherwise they should have flown apart by now. This traditional explanation is that a form of dark invisible matter makes up the majority of the mass in the galaxy. An alternative theory is that gravity behaves differently over large distances (which are typically a few million light years in size), than it does on Earth and in our solar system. Dark matter has remained the dominant theory until recently, as astronomers where unable to test the theories of alternate gravity.
The fact that dark matter doesn’t emit radiation makes it difficult to spot. It also renders it in a shroud of mystery. Indeed, no one has detected it and not all scientists are convinced of its existence. While regular matter is the stuff that makes up stars, humans, planets and everything else on Earth, clumps of dark matter are the seeds for the formation of galaxies. The best and probably the simplest evidence of dark matter is that there are stars in our sky. If there was no dark matter, the stars would be flying off into space. Dark matter holds the stars in their given places as they move around the galaxy.
Kepler and Newton managed to weigh the sun just by knowing the location of Earth and how fast it was moving in relation to the sun. Astronomers did the same the thing to determine the weight of dark matter only that this time it was in three dimensions. They measured the position and speed of a large number of stars in several dwarf galaxies orbiting the Milky Way.
The results were surprising to say the least, aside from the speed; the researches calculated the smallest clump of dark matter that existed, was 1000 light years across. The results imply that dark matter is hotter than previously predicted, which means that what astronomers call “cold dark matter” may not be so cold after all. At 10,000 degree Celsius it’s still cool by astronomical standards but warm enough to solve two glitches that have plagued standard models of how galaxies form. The first is that there are too few dwarf galaxies; the second is that dark matter has not concentrated in the center of galaxies.
Scientists were initially skeptical of the results, which together seemed too simple to be conceivable. The discovery of a super-dim galaxy by Dr. Beth Willman from New York University gave astronomers an opportunity to successfully test its predictions.
Small amounts of dark matter continuously pass through Earth on a regular basis. The clouds may be remnants of the first structures to form after the big bang, and could possibly be detected by future missions. Dark matter is seven times more abundant in the universe and interacts gravitationally with the normal matter. Unfortunately physicists have no idea what this mysterious matter is made of or how exactly it got distributed into space. Nonetheless, they have formulated a probable number of hypothetical dark matter particles that were created in the big bang.
These particles formed the Universe’s first structures, where mysterious “quantum seeds” caused matter to clump more densely in certain areas. Dark matter is said to have slid into theses structures and later merged to form giant clouds or halos with trillions of times the mass of the sun. Previous computer simulations have modeled these giant halos, which appear to surround galaxies and also to cluster galaxies.
In October of 2003 the identity of Universe’s dark matter was finally discovered. A team of researchers from England and France claimed that gamma rays emitted form the center of our galaxy show traces of these ghostly particles. Dark mater “feels” like ordinary visible matter, so it is a fair bet that it clumps at the center of our galaxy. The team studied a distinctive pattern of gamma rays that were coming from the center of the Milky Way. The sharp signal, with energy of 511 kiloelectronvolts (keV), is believed to be due to the annihilation of electrons and positrons, the antimatter equivalent of electrons. However, researchers were unable to prove where the electrons and positrons came from.
Everything from blast waves of a “hypernova” to neutron stars and black holes were looked at as being explanations, however, none of these explanations seemed satisfactory. Astronomers suspected that the electrons and positron were the by-products of dark matter and their antimatter annihilations. But to produce a sharp line at 511 keV which is the “rest” energy of an electron the electron and positron must be slowed down to a virtual standstill before the annihilation ensues. This ruled out dark matter at the large masses that most researchers expect.
Heavy dark matter would be producing high energy electrons, besides it would be almost impossible to bring them to a standstill. This caused researchers to zoom in on a surprisingly light dark matter. To test their idea, the researchers looked at observations from the Integral, the European Space Agency’s gamma ray telescope. The brightness map fitted exactly with the expected distribution of light dark matter particles. If dark matter is really made of light matters then every cubic centimeter of space in the vicinity of the Earth must contain a few tens of them.
In June of 2004 a dim galaxy, Andromeda IX was discovered. It was thought to provide clues to the mysterious dark matter that appears to push regular matters around out in space. The theory that galaxies are embedded in dark matter stems from the observation of stars in spiral galaxies. These galaxies, including the Milky Way are flattened spiral–shaped disks of stars and gas. Since most of the visible matter in a spiral galaxy is concentrated at the center, the stars there would be expected to move quickly than the stars on the outskirts, according to the laws of gravity.
