* Astronomy

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RE: Dark matter

Blobrana · L

Computer simulations have reconstructed how dark matter clumped together with normal gas in the early universe to form small galaxies and how these small galaxies merged over billions of years to create huge star systems like the Milky Way.

The simulations raised one mystery, however. They showed that the density of dark matter should spike sharply at the centre of galaxies, while observations of the motions of stars reveal that the dark matter cores of galaxies today are much more puffed out, with densities that are constant over thousands of light years.

"We have known about this problem for more than 10 years" - Sergey Mashchenko from McMaster University in Hamilton, Ontario, Canada.

Astronomers have suggested several possible fixes for this anomaly. For instance, they thought that dark matter might puff outwards because some kind of exotic force between the particles - other than gravity - makes them collide and scatter off each other like snooker balls. Now Mashchenko and his colleagues have shown that the smoothing of the density of dark matter is down to the explosions of massive stars at the end of their lives. At their peak, these supernovae can outshine their host galaxies.
Mashchenko thought that shock waves from supernovae should churn up interstellar gas in a galaxy, and that the gravitational disturbances created by this sloshing gas should in turn smooth out the spike of dark matter at the centre. To test this, his team used a supercomputer to simulate the evolution of a small, primordial galaxy that started off with a central spike in the density of dark matter. Sure enough, just 80 supernovae explosions per million years - typical of values expected in dwarf galaxies today - were enough to smooth out the dark matter spike to match observations if they continued for at least 100 million years or so (Nature, vol 443, p 539).

The same model can also solve another conundrum. The cosmos has far fewer dwarf galaxies - which contain several billion stars compared with the hundreds of billions in larger galaxies - than cosmological simulations predict. In Mashchenko's simulations, because supernovae smoothed out the spike in dark matter density at the centre, dwarf galaxies were less tightly bound together by their own gravity. Any encounters with bigger galaxies would therefore have easily torn many dwarfs apart, and this may explain their paucity.

Source

Title: Cosmological puzzle resolved by stellar feedback in high redshift galaxies
Authors: Sergey Mashchenko, H. M. P. Couchman, James Wadsley

The standard cosmological model, now strongly constrained by direct observation at early epochs, is very successful in describing the structure of the evolved universe on large and intermediate scales. Unfortunately, serious contradictions remain on smaller, galactic scales. Among the major small-scale problems is a significant and persistent discrepancy between observations of nearby galaxies, which imply that galactic dark matter (DM) haloes have a density profile with a flat core, and the cosmological model, which predicts that the haloes should have divergent density (a cusp) at the centre. Here we use numerical N-body simulations to show that random bulk motions of gas in small primordial galaxies, of the magnitude expected in these systems, result in a flattening of the central DM cusp on short timescales (of order 10^8 years). Gas bulk motions in early galaxies are driven by supernova explosions which result from ongoing star formation. Our mechanism is general and would have operated in all star-forming galaxies at redshifts z>~ 10. Once removed, the cusp cannot be reintroduced during the subsequent mergers involved in the build-up of larger galaxies. As a consequence, in the present universe both small and large galaxies would have flat DM core density profiles, in agreement with observations.

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Axion-Like Particles

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Sterile Neutrinos

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RE: Dark matter

Blobrana · L

Title: Astrophysical Effects of Scalar Dark Matter Miniclusters
Authors: Kathryn M. Zurek, Craig J. Hogan, Thomas R. Quinn (UW, Physics and Astronomy Departments)

Researchers model the formation, evolution and astrophysical effects of dark compact Scalar Miniclusters ("ScaMs"). These objects arise when a scalar field, with an axion-like or Higgs-like potential, undergoes a second order phase transition below the QCD scale. Such a scalar field may couple too weakly to the standard model to be detectable directly through particle interactions, but may still be detectable by gravitational effects, such as lensing and baryon accretion by large, gravitationally bound miniclusters. The masses of these objects are shown to be constrained by the Ly alpha power spectrum to be less than 10^4 solar masses, but they may be as light as classical axion miniclusters, of the order of 10^-12 solar masses. The researchers simulate the formation and nonlinear gravitational collapse of these objects around matter-radiation equality using an N-body code, estimate their gravitational lensing properties, and assess the feasibility of studying them using current and future lensing experiments. Future MACHO-type variability surveys of many background sources can reveal either high-amplification, strong lensing events, or measure density profiles directly via weak-lensing variability, depending on ScaM parameters and survey depth. However, ScaMs are unlikely to be responsible for apparent MACHO events already detected in the Galactic halo.
A simple estimate is made of parameters that would give rise to early structure formation; in principle, early stellar collapse could be triggered by ScaMs as early as recombination, and significantly affect cosmic reionisation.

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Title: U-boson detectability, and Light Dark Matter
Authors: Pierre Fayet

The possible existence of a new gauge boson U, light and very weakly coupled, allows for Light Dark Matter particles, which could also be at the origin of the 511 keV line from the galactic bulge. Independently of dark matter, and taking into account possible Z-U mixing effects, we show that, even under favorable circumstances (no axial couplings leading to an axionlike behaviour or extra parity-violation effects, very small coupling to neutrinos), and using reasonable assumptions (no cancellation effect in g(mu)-2, lepton universality), the U coupling to electrons can be at most as large as ~ 1.5 10^-3 (for m(U) < m(mu)), and is likely to be smaller (e.g. <~ 3 10^-6 m(U) (MeV), if the U couplings to neutrinos and electrons are similar). This restricts significantly the detectability of a light U in e+e- --> gamma U, in particular. U exchanges can still provide annihilation cross sections of LDM particles of the appropriate size, even if this may require that light dark matter be relatively strongly self-interacting.

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Date: Jul 16 18:21 2006

DARK MATTER is an invisible substante that has been here longer than any element known in the Universe. It is because it is the fabric foundation to what our Universe was formed on or in. Reviewing aspects about the Multi-Verse Theory created by Stephen Hawkins. There are many Universe's. For this to be so. The Dark Matter fabric must be an infinite fabric substante created as a form of nothingness what entails the dark emptiness to everything in it.

In Quanta Physics Theory developed by Rod Kawecki phd the Universe was formed through the accumulation of infinitesimal energies crushed by Black Holes and more so - even the Universe itself. So not only is the Universe spinning by perpetrual motion but as it does - the used up or recieved energy in a black hole or accumlated out of the Universe's massive perpetrual vortexium. The allusive energy changed by a black holes vortex is recieved at a distant area in the dark matter spectra where it accumulates and forms other Universe's. The theory also prospers that even other planets and stars are formed indivisually the same way. Formed at infinitesimal masses and accumulate other high frequency energy to grow into heavenly bodies throughout space. The Universe never dies.

Date: Jul 5 23:59 2006

DARK MATTER is a 'neutral ' element that resides in everything that exist at micro-masses and macro-masses. In and out of atomic and subatomic particles. But in space it the vast diatnce between the energy reflected by luminous suns abid. As a neutral and fifth element. It measures between ZPE/P or 10 -34cm and 10 -460cm. (critical density). All elements have a density by dark matter has no substante -chemically defineable. But that it is invisible. darkness at a distance.

Distrubances in space are activities of solar vacuum winds passing between the magnetic waves the planets are traveling on impressed on warp space. If and when dark matter shows to warp stretch or change physical form it is caused bhy a physical action such as accelerational force at high speeds and is a physical action formed not by the dark matter but the object in question. Density which renders to exhibitation of being a force. Dark matter acts accordingly.

When one thinks dark matter is dark menergy - they neglected to consider the positive plats that create the negativeness. Stephen Hawkings didn't..in ' Universe in a Nutshell..read physicsamerica.com...desitteretc.

Blobrana · L

Galaxies are born inside dark matter clumps.
Try mixing caramel into vanilla ice cream -- you will always end up with globs and swirls of caramel. Scientists are finding that galaxies may distribute themselves in similar ways throughout the universe and in places where there is lots of so-called dark matter.

Start with lots and lots of dark matter, then stir in gas. Let the mixture sit for a while, and a galaxy should rise up out of the batter.
This simple recipe for baking galaxies cannot be performed at home, but it does reflect what astronomers are learning about galaxy formation. Like baking bread with yeast, a mysterious substance in the universe called dark matter is required for a galaxy to grow.
Now, a new study from NASA's Spitzer Space Telescope is refining what is known about this essential ingredient of galaxies. It suggests that not only is dark matter necessary, but a minimum quantity of the material must be present before a galaxy can form. Any less would mean no galaxy -- the cosmic equivalent of a failed loaf of bread.

The faint red dots in the top image from the Spitzer Wide-area Infrared Extragalactic survey are dusty, distant galaxies, called ultraluminous infrared galaxies. Astronomers using the Spitzer Space Telescope have found that these galaxies form out of clumps of dark matter that are about as massive as 10 trillion suns.
Credit: NASA/JPL-Caltech/C. Lonsdale (Caltech/IPAC) and the SWIRE Team.
Position (J2000): RA: 16h09m48.99s Dec: 55d02m41.50s

Source

-- Edited by Blobrana at 19:41, 2006-06-16