Talk:Dark matter: Difference between revisions

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Revision as of 06:04, 25 March 2004

Removed statement that existence or non-existence of dark matter contradicts big bang. Dark matter actually has very little to do with whether big bang is true or not.

The article at Big Bang theory speaks of a dark matter problem; you may want to correct that if you're better informed than the author. Mkweise 13:05 Feb 16, 2003 (UTC)

Did so. What I suspect that the author was referring to is the fact that dark matter *helps* the big bang theory. During the 1970's, there were a number of problems (mostly deuterium abundances) that have been nicely resolved by assuming that dark matter exists.

I vaguely remember that some time in the 1990s, new observations led to the stunning (at the time) conclusion that more than 90% of the universe's matter is dark. IIRC, some previously favored theories were essentially scrapped due to that - I don't remember specifically what theories, but I think it led to a change in the estimated age of the universe by many orders of magnitude. Mkweise 16:26 Feb 16, 2003 (UTC)

Having a description of Hot Dark Matter there doesn't fit, there should be accompanying descriptions of cold and baryonic.

new content

The following content was put on the page and was very hard to read. We can try to work it in, but in the meantime I've put it here and reverted the page. --zandperl 01:45, 18 Mar 2004 (UTC)

I have just worked it over, chopping it up into paragraphs and also into sections that I think might be better suited to other articles. If nobody objects to what I did with it, I'll merge them into their final destinations tomorrow. Bryan 19:06, 20 Mar 2004 (UTC)

I just moved all the text to the various articles I'd proposed moving it to (except for the dark energy section, which was entirely redundant with the stuff already in the dark energy article; I just deleted that one). Bryan 06:04, 25 Mar 2004 (UTC)

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 I have just worked it over, chopping it up into paragraphs and also into sections that I think might be better suited to other articles. If nobody objects to what I did with it, I'll merge them into their final destinations tomorrow. [[User:Bryan Derksen|Bryan]] 19:06, 20 Mar 2004 (UTC)
+:I just moved all the text to the various articles I'd proposed moving it to (except for the dark energy section, which was entirely redundant with the stuff already in the dark energy article; I just deleted that one). [[User:Bryan Derksen|Bryan]] 06:04, 25 Mar 2004 (UTC)
-Dark matter is nonluminous matter not yet directly detected by astronomers that is hypothesized to exist because the visible matter in the universe is insufficient to account for various observed gravitational effects. In other words dark matter is anything that has mass but does not emit or reflect any form of electromagnetic radiation.
-The Question of dark matter's existence may seem irrelevant to our existence here on Earth. However, the fact of whether or not dark matter really exists determines the [[ultimate fate of the universe|ultimate fate of the present universe]]. We know the universe is now expanding because of the [[red shift]] that light from distant heavenly bodies exhibits. If there is no dark matter, this expansion will continue forever. If there is the amount of dark matter hypothesized as of now the universe's expansion will slow, and could eventually stop or even reverse (leading to an eventual [[Big Crunch]]).
-===Discovery of dark matter===
-Dark matter was first hypothesized to exist by the [[Switzerland|Swiss]] [[astrophysicist]] [[Fritz Zwicky]]. In [[1933]] Zwicky estimated the amount of mass in the galaxy (based on the number of stars and their brightness) and then found the rate at which our own Milky Way and other galaxies spin around their center. When he used a different method independent from brightness he found about 400 times more matter than he had based on number of stars and brightness. He then found the velocity to be more than twice the possible rate with the amount of mass from the brightness estimate. If the normal physical laws were applied the galaxies would be torn to shred by the high speeds because the gravity they exert would not be sufficient to hold it together. This is known as the "missing mass problem." Based on these conclusions, he stated that there must be some other form of matter existent in the galaxy which we have not detected, which provides enough of the mass and gravity to hold the galaxy together.
-From there the search for this source of the sufficient gravity has commenced.  At present, the density of the universe (excluding dark matter) is estimated to be about one hydrogen atom per cubic meter of empty space. This is not enough density for the universe to collapse on itself. However, dark matter is said to form 90-95 percent of all matter in the universe. This means that only 10-5 percent of all matter is observable.
-[[Cosmologist]]s (astronomers who study the history, origin, and future of the universe) believe there are two classes of dark matter: [[baryon]]ic (the name given to all "normal matter" composed of baryons: [[proton]]s, [[neutron]]s, and [[electron]]s) dark matter, called [[MACHO]]s (Massive Astrophysical Compact Halo Objects) and the mysterious "shadow matter" composed of unknown non-baryonic subatomic particles known as [[WIMP]]s (Weakly Interacting Massive Particles), [[neutrino]]s, and [[axion]]s. It is ironic that Wimps and Machos are the exact opposites in our language.
-===To move to the MACHO article===
-MACHOs are the less mysterious branch of dark matter. Indeed, we seem to have some form of dark matter visible in our solar system (contradicting the term dark matter). MACHOs include black holes, neutron stars, and brown dwarf stars. A black hole is a star which becomes so dense that it collapses into an extremely tiny point but contains an enormous amount of mass. Because of its huge density its gravity is so powerful that any form of matter or energy (even light) which passes its "event horizon" can never escape and is crushed into an infinitely small point called singularity. Black holes are truly black in that they emit no light and any light shined upon it is sucked in before any illumination could occur. It is believed that there is an extremely massive black hole at the center of every galaxy. Also there is thought to be a halo of black holes surrounding the galaxy. It is also a possibility that some black holes can grow to become the size of small galaxies. Could this account for much of the lost mass in the universe? Cosmologists doubt it makes up a majority of dark matter because the black holes are at isolated points of the galaxy. The largest contributor to the missing mass must be spread throughout the galaxy to balance the gravity.  The next MACHOs contradict the term dark matter although they are classified as it despite this. These are Brown Dwarfs. These are sometimes called jupiters because the planet Jupiter is a small form of this heavenly body. Brown dwarfs are semi-stars. They do not have enough mass for nuclear fusion to begin and simply glow a dull brown. Most brown dwarfs are about ten to seventy-five times the size of Jupiter. However, their composition and structure are virtually the same. MACHOs are detected through their effects on light. A black hole is detected by the halo of bight gas and dust that forms around it being pulled by its gravity. Also at times hot gas is shot out of a black hole because it cannot be absorbed quickly enough. Other unknown forms of MACHOs are seen when the MACHO eclipses a star the MACHO's gravity bends the light to a focused beam causing the star to appear smaller and brighter. A natural lens so to speak. There are thought to be in some abundance in the halo around our galaxy but the Hubble Space telescope shows only about six percent of the halo is composed of brown dwarfs. Therefore, the missing mass problem is not solved by MACHOs.
-===To move to the WIMP article===
-WIMPs are hypothetical at this point. They are considered to be significantly larger and more massive than protons and neutrons (the largest baryons) and yet cannot be detected. They are composed of some subatomic particle totally different from baryonic matter, the substances life, the earth, the sun etc. are composed of. It is thought that WIMPs physically pass through us and everything else every few seconds totally undetected, almost like a ghost. This shadow matter can be normal WIMPs or neutrinos. WIMPs are considered to be "cold" dark matter and neutrinos are called "hot" dark matter. Neutrinos are more known because they have actually been detected. Neutrinos have so little mass however that they are sometimes termed to be matter, lacking mass. These particles travel at near the speed of light and are the cause of decay in radioactive elements. WIMPs are the actual contributors of mass in the category of shadow matter. WIMPs move at relatively slow rate (thus the name cold) in contrast to the swiftness of neutrinos. Many scientists believe that WIMPs clump into galaxies in the same way baryonic galaxies do. Without neutrinos this process is very slow (billions of years). So, although the mass is contributed exclusively by the WIMPs, neutrinos are the mortar and worker which the bricks are set into place. They are then held together by their own gravity as atoms in a star are. Also there is a problem with the mechanics by which any celestial bodies formed. If atomic particles simply co-existed (being spread equidistant from one another) and exerted the same amount of gravity on each other in the time directly following the big bang, atoms could never have come together to form matter of any sufficient mass because they would simply exert their equal gravity upon each other and revolve about other atoms. Some other force would be needed to cause galactic clumping. Dark matter provides the solution to this problem. If masses of dark matter formed first then their gravity could have triggered the clumping of matter into galaxies.
-Detection of Shadow Matter: WIMPs currently cannot be detected. Some skeptical physicists say that WIMPs were simply created by cosmologists as a “quick fix” to the missing mass problem. Scientists are currently searching for proof of WIMPs. One method for WIMP detection is the looking for heat caused by the extremely slight gravity exerted by WIMPs when they pass through matter. At the Center for Particle Astrophysics a crystal is cooled to absolute zero (0°K, -273.15 °C, or -459.67°F). The crystal’s temperature is then closely monitored by an ultra-sensitive thermometer and is protected form any factor that may result in heating. If a WIMP passes through the crystal's atoms, the gravity will be transformed into a slight amount of heat detected by the thermometer. Results to the test are not available on account of the tests are still in progress.
-===To move to the Dark energy article===
-Dark energy is known about even less. It is thought to be the opposite to the force of gravity. Albert Einstein, possibly the most famous scientist to ever live, proposed something called the cosmological constant to account for the acceleration of the outward motion of the galaxies. Will this mysterious dark energy counteract the effects of dark matter’s density? Cosmologists refer to dark energy as either the cosmological constant (if we live in a static or flat universe) or quintessence (the term used by medieval scholars for the substance stars and the other heavenly bodies were composed of), if we live in an open universe (where expansion goes on forever). The mystery of this dark energy will be discussed after dark matter.