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Topic: Hot dark matter

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  Hot dark matter - Wikipedia, the free encyclopedia
Hot dark matter is a form of dark matter, which consists of particles that travel with relativistic velocities.
Dark matter is matter that cannot be detected by electromagnetic radiation.
Hot dark matter therefore is nowadays always discussed as part of a mixed dark matter theory.
en.wikipedia.org /wiki/Hot_dark_matter   (258 words)

 Dark matter - Wikipedia   (Site not responding. Last check: 2007-10-08)
Dark matter refers to material that can't be detected by their emitted radiation but whose presence can be inferred from gravitational effects on visible matter, like stars and galaxies.
Determining the nature of dark matter is also known as the dark matter problem or the missing mass problem, and is one of the most important problems in modern cosmology.
Dark matter is not to be confused with white matter or gray matter.
wikipedia.findthelinks.com /da/Dark_matter.html   (734 words)

 Dark matter - Wikipedia, the free encyclopedia
In cosmology, dark matter refers to matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence may be inferred from gravitational effects on visible matter such as stars and galaxies.
Dark matter is crucial to the Big Bang model of cosmology as a component which corresponds directly to measurements of the parameters associated with Friedmann cosmology solutions to general relativity.
Large computer simulations of billions of dark matter particles have been used to confirm that the cold dark matter model of structure formation is consistent with the structures observed in the universe through galaxy surveys, such as the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest.
en.wikipedia.org /wiki/Dark_matter   (3153 words)

 dark matter
Within a spiral galaxy, the stars move as if large quantities of dark matter exist around the galaxy's disk; similarly, within clusters of galaxies, the individual galaxies move as if many times more matter is present than that visible in the form of stars and interstellar gas and dust.
Another surprise is that the particles of which dark matter is made are moving on average with a speed of about 9 km/s, corresponding to a temperature of about 10,000 K – much higher than expected.
Whatever dark matter is, it’s the primary source of gravitation in the universe; so it must have played a crucial role in determining the structure of everything from galaxies to the large scale arrangement of superclusters (clusters of clusters) of galaxies.
www.daviddarling.info /encyclopedia/D/darkmat.html   (1976 words)

 Dark Matter - Crystalinks
In cosmology, dark matter consists of matter particles that cannot be detected by their emitted radiation but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies.
However, dark matter and dark energy are together said to account for 96% of all matter in the universe.
An alternative to dark matter is to suppose that the inconsistencies are due to an incomplete understanding of gravitation.
www.crystalinks.com /darkmatter.html   (1582 words)

 Dark Matter
The nature of this dark matter, and the associated "missing mass problem", is one of the fundamental cosmological issues of modern astrophysics.
Hot dark matter is composed of particles that have zero or near-zero mass (the neutrinos are a prime example).
The difference between cold dark matter and hot dark matter is significant in the formation of structure, because the high velocities of hot dark matter cause it to wipe out structure on small scales.
csep10.phys.utk.edu /astr162/lect/cosmology/darkmatter.html   (890 words)

 dark matter. The Columbia Encyclopedia, Sixth Edition. 2001-05
Dark matter may consist of dust, planets, intergalactic gas formed of ordinary matter, or of MACHOs [Massive Astrophysical Compact Halo Objects], nonluminous bodies such as burned-out stars, fl holes, and brown dwarfs; these are the so-called hot dark matter and would be dispersed uniformly throughout the universe.
This suggests that the slower, cold dark matter might form the smaller clumps associated with the galaxy while the faster, hot dark matter might form the larger clumps associated with the galactic cluster.
Unlike cold dark matter, which has mass but virtually no velocity or temperature, or hot dark matter, which has mass and is highly energetic, warm dark matter has mass and a low temperature corresponding to an extremely low velocity.
www.bartleby.com /65/da/darkmatt.html   (474 words)

 The Search for the Missing Matter   (Site not responding. Last check: 2007-10-08)
Dark Matter is not visible to the human eye but it is known to exist because of the relative movement of galaxies.
Hot dark matter is made up of particles that have zero mass so that the particles must move at very high velocities and create hot gases.
Discovering Dark Matter - One of the biggest problems that modern cosmologists continue to struggle with today is the nature of “missing mass”, or dark matter.
www.kn.sbc.com /wired/fil/pages/listdarkmatam.html   (2104 words)

 How do we know dark matter exists?
Dark matter is the name scientists have given to the particles which we believe exist in the Universe, but which we cannot directly see!
Dark matter was initially called "missing matter" because astronomers could not find it by observing the Universe in any part of the electromagnetic spectrum.
The hot gas in the cluster is held in the cluster primarily by the gravity of the dark matter, so the distribution of the hot gas is determined by that of the dark matter.
starchild.gsfc.nasa.gov /docs/StarChild/shadow/questions/question59.html   (723 words)

Hot dark matter consists of quickly moving small particles such as neutrinos, a particle which may or may not have a mass and therefore may or may not contribute to the shape of the universe.
The reason many people believe the dark matter is a cold-dark-matter WIMP is precisely because the cold dark matter simulation that we can create in the computer looks a lot like the real universe, whereas every other possibility we’ve tried, including hot dark matter, has turned out to look nothing like the universe.
The fact that cold dark matter looks so good in a computer simulation doesn’t prove, of course, that it is the force shaping the universe.
www.pbs.org /wnet/hawking/mysteries/html/uns_frenk-3.html   (638 words)

 Dark Matter in the Universe   (Site not responding. Last check: 2007-10-08)
Dark Matter is recognized today to be nearly 25% of all the mass in the universe.
Dark Matterís story starts off with the human imagination proposing the existence of matter millions of light years away that we canít even see, but ends billions of years from now when the ultimate fate of the universe is decided.
Dark Matter is a part of the ìomega factorî or the story of how the universe is expanding or contracting.
astro.uchicago.edu /home/web/olinto/courses/A18200/maggie.html   (2482 words)

 Dark Matter: Overview
Dark matter is needed to explain the structure of Polar Ring Galaxies, rare structures that consist of a ring of stars around a sphere of stars.
This means that a large amount of baryonic dark matter is transformed into stars as the system evolves, and in Sc and Sd galaxies, the dark matter must be in the form of diluted H and He in order for star formation to occur.
The suggestion of tilted cold dark matter, or TDM, is an attempt to reconcile the discrepancies of CDM and HDM by using a power spectra obtained as follows: Take the power spectrum line of pure CDM in Figure 19, and rotate it clockwise about the point marked "X".
www.calstatela.edu /faculty/rcarr/a151/darkmtr.html   (11408 words)

 Access: Shedding Light on Dark Matter
Another possible form of dark matter—hot dark matter—describes a particle like massive neutrinos, which are so weakly interactive that they pass right through regular matter.
However, if most dark matter were neutrinos, matter in the universe would be quite uniformly distributed, explains Bode, and researchers know that it is not.
Because particle velocities are lower, dark matter clumps more with its surroundings than do neutrinos but not as much as CDM particles.
www.ncsa.uiuc.edu /News/Access/Stories/wdm/shedding2.html   (281 words)

 Missing "Dark" Matter
Since the galaxy was not flying apart, he reasoned that there must be enough matter inside the galaxy such that the central gravitational force was strong enough to keep the stars from escaping, much as the Sun's gravitational pull keeps a planet in its orbit.
Hot dark matter - postulated to be fast moving particles floating throughout the universe, neutrinos, tachions etc.
The "missing matter" proposal attempts to balance the equation by increasing one of the variables (one of the mass terms).
www.electric-cosmos.org /darkmatter.htm   (2092 words)

 Open Questions: Dark Matter
While there may be quite a lot of matter around that isn't part of stars, the proportion of different elements in that matter should not be much different from what is in stars, simply because all but the oldest stars have condensed out of the available supply of baryonic matter.
Although research into the distribution of dark matter is only one of many investigations to be enabled by the LSST, the information it provides about distribution should tell us far more than we now know about where dark matter is located and how it has affected the evolution of the universe.
Studies of the motion of luminous matter in galaxies and clusters of galaxies show that most of the mass must be in a form that is not visible.
www.openquestions.com /oq-co002.htm   (5036 words)

 Curious About Astronomy: What is Hot Dark Matter theory?
I have heard of something called HDM which is a theory to explain dark matter that is no longer as popular as it used to be.
Neutrinos are the main HDM candidate for dark matter as they are very weakly interacting and exist in such large numbers in the universe.
HDM is thought to be limited to at most a few percent of dark matter if the amount is even measurable.
curious.astro.cornell.edu /question.php?number=689   (357 words)

 Munch   (Site not responding. Last check: 2007-10-08)
Previous considerations on holographic dark energy disfavor the Hubble parameter as a candidate for the IR cutoff (for spatially flat universes), since in this case the ratio of dark energy to dark matter is not allowed to vary, thus hindering a deceleration era of the universe for the redshifts z>=0.5.
Although a simple argument implies that the distribution of dark matter in galactic halos is characterized by discrete flows and caustics, their presence is often ignored in discussions of galactic dynamics and of dark matter detection strategies.
We estimate the number of dark matter flows as a function of galactocentric distance and consider the various ways in which that number can be increased, in particular by the presence of structure on small scales (dark matter clumps) and the scattering of the flows by inhomogeneities in the matter distribution.
home.fnal.gov /~bertone/DM/JC080805.html   (1542 words)

 Hot Dark Matter
The foremost HDM candidate is a particle known as the "neutrino." This elusive particle was predicted to exist as early as 1931 by physicist Wolfgang Pauli in order to account for the conservation of momentum and energy in beta decay.
HDM, as represented primarily by neutrinos, does not account for the pattern of galaxies observed in the Universe.
This is to be compared to the cold dark matter component, which is around 1/3 of the total energy in the universe.
astro.berkeley.edu /~mwhite/darkmatter/hdm.html   (776 words)

 [No title]   (Site not responding. Last check: 2007-10-08)
Salinet Points on Dark Matter: Following is an excerpt of an e-mail reply.
hot dark matter) and therefore they erase fluctuations on small scales (they free stream and fill the horizon in the early universe).
One way to accomodate this is to go to a mixed dark matter model in which you have some hot dark matter (for the large scale power) and some cold dark matter (wimps, axions, photinos, supersymmetric particles, etc) to act as a seed for galaxy formation.
zebu.uoregon.edu /text/darkmatter.txt   (665 words)

 hot dark matter Comparison Table   (Site not responding. Last check: 2007-10-08)
Matter whose presence is inferred from dynamical measurements but which has no optical counterpart.
However, the mass-luminosity ratio in the outer halos of many spiral galaxies is 100 or more; one sees the brightness fall off with distance from the center of the galaxy but considerable mass is present.
Any form of dark matter which was relativistic at its point of decoupling.
www.site.uottawa.ca:4321 /astronomy/hotdarkmatter_table.html   (183 words)

 Chapter 14: Dark Matter and Large Scale Structure
The matter density is one of the fundamental parameters of the universe; in the standard (lambda = 0) models, the matter density determines the geometry of the cosmos.
At the scale of individual galaxy disks, much of the dark matter can be attributed to stellar ashes such as white dwarfs, neutron stars, and fl holes, as well as to extremely faint objects such as very low-mass stars and brown dwarfs.
The dark matter is also inextricably connected with the formation of galaxies and galaxy clusters, since it must dominate the gravity of the universe.
astsun.astro.virginia.edu /~jh8h/Foundations/chapter14.html   (1564 words)

 Cosmology - Dark Matter
We know this Dark Matter exists, and the only way we can identify it is by its affect on known objects.
Hot dark matter is thought to be near zero-mass moving near the speed of light.
One interesting effect of Dark Matter surrounding a galaxy is that objects that lay behind the galaxy in the line of sight of an observer will witness what is called a gravity lens.
astronomyonline.org /Cosmology/DarkMatter.asp   (542 words)

 3.2 Non-baryonic Dark Matter
The non-baryonic candidates are classified as either hot or cold dark matter.
Hot implies the matter was relativistic in the early universe.
An example of hot dark matter is the neutrino.
relativity.livingreviews.org /Articles/lrr-2002-4/node8.html   (288 words)

 Hot Dark Matter
The 'hot' of Hot Dark Matter refers to the velocity of the particles involved, specifically at or close to the speed of light.
The most prominent candidate for a hot dark matter particle is the 'neutrino'.
On the other hand, the physicist Wolfgang Pauli postulated the existence of a small particle which had no charge, possibly no mass, and that travelled at relativistic speeds (that is speeds approaching the velocity of light) was also emitted during beta decay.
www.angelfire.com /electronic/isolderadford/hotdm.html   (369 words)

 Chapter 22: Dark Matter & Fate of Universe   (Site not responding. Last check: 2007-10-08)
Dark Matter only reveals itself to us through the gravitational force if it is truly dark.
This implies the presence of dark matter surrounding the galaxies in order to speed up their rotation in the outer arms.
An example of dark matter may be the neutrino.
www.accd.edu /sac/astronom/astr1370/benet22n.htm   (767 words)

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