Svet pogovorov
http://www.gape.org/cgi-bin/yabb/YaBB.pl General >> Duhovnost in znanost >> Dark matter http://www.gape.org/cgi-bin/yabb/YaBB.pl?num=1079554098 Message started by Kali on 17.03.2004 at 21:08:18

Title: Dark matter Post by Kali on 17.03.2004 at 21:08:18 Dark matter could be light Gamma rays from galaxy centre may signify less massive missing particles. 17 March 2004 PHILIP BALL The INTEGRAL satellite searches the skies for gamma rays. © ESA Gamma rays streaming from the centre of our galaxy could be the signature of elusive dark matter, astrophysicists claim. The rays support an exotic theory about dark matter: that it consists of very light particles. Physicists know that a large proportion of the universe's mass cannot be accounted for by objects we can see, such as stars and planets. In galaxies such as our own, there could be as much as ten times more dark matter than normal matter. One popular idea suggests that the 'missing' dark matter consists of as yet unidentified subatomic particles that are much heavier than entire atoms of normal matter but that hardly interact with it, except through gravity. They are called weakly interacting massive particles, or WIMPs. But Céline Boehm of Oxford University in England and her colleagues think that dark matter particles need not be massive at all. Instead, they think they could be between ten and a thousand times lighter than a hydrogen atom. Integral research Their evidence comes from data collected by a satellite called INTEGRAL, operated by the European Space Agency, which searches the skies for gamma rays. The satellite has mapped out the cosmic sources of gamma rays with an energy of 511,000 electronvolts (511 keV). Such rays are about 200,000 times more energetic than visible light rays and are produced when electrons and their antimatter equivalents, positrons, annihilate one another. Last January, the INTEGRAL team announced that the 511 keV gamma rays come from a source that is evenly distributed throughout the central bulge of our galaxy1. Intriguingly, dark matter is known to be concentrated in our galaxy's central bulge, thanks to observations of how the missing mass affects the orbit of stars. Boehm's team says that if dark matter were made up of particles with a low mass, these particles could generate positrons and electrons when colliding with antimatter. When these products collide, they generate gamma rays. The researchers calculate that the number of such particles needed to produce the intensity of 511 keV gamma rays seen by INTEGRAL fits well with the amount of dark matter that the galactic bulge is estimated to contain. "The numbers are really reasonable," says Boehm. They report their findings in the current issue of Physical Review Letters2. "It would be very exciting if it turns out to be real", says gamma-ray astronomer Jürgen Knödlseder of the Centre d'Etude Spatiale des Rayonnements in Toulouse, France, who works with INTEGRAL data. But Knödlseder cautions that it is not yet clear if Boehm's dark-matter theory is really needed. The source of the positrons could be exploding stars called supernovae, rather than exotic particles. "They are still the most plausible source," he says. He suggests that very accurate measurements of the distribution of the 511 keV gamma-ray emissions might enable researchers to work out whether the source is dark matter or exploding stars. References   1. Knödlseder, J. et al. Astronomy and Astrophysics , (in the press). Preprint: http://www.arxiv.org/abs/astro-ph/0309442|Article|;   2. Boehm, C., Hooper, D., Silk, J., Casse, M. & Paul, J. J. Phys. Rev. Lett, 92, 101301, doi:10.1103/PhysRevLett.92.101301 (2003).|Article| © Nature News Service / Macmillan Magazines Ltd 2003 http://www.nature.com/nsu/040315/040315-6.html

Title: Re: Dark matter Post by Kali on 25.03.2004 at 23:20:34 Researchers Suggest That 'Dark-Matter Highway' May Be Streaming Through Earth TROY, N.Y.-- Astrophysicist Heidi Newberg at Rensselaer Polytechnic Institute and her colleagues suggest that a "highway" of dark matter from another galaxy may be showering down on Earth. The findings may change the way astronomers look for mysterious cosmic particles, long suspected to outweigh known atomic matter. http://www.sciencedaily.com/releases/2004/03/040325071616.htm http://news.bbc.co.uk/1/hi/sci/tech/3564273.stm še tale link o članku ki govori o Theory of matter may need rethink

Title: Re: Dark matter Post by Kali on 29.03.2004 at 17:43:41 sm dala kar sm Earth on the 'Wimp highway' By Roland Pease Earth would be sitting in this stream of Wimps (Image by David Low) Mysterious sub-atomic particles from another galaxy could be raining down on planet Earth, according to a collaboration of astronomers. If so, it could explain controversial results from a particle-detection experiment deep inside mountains to the east of Rome. The story concerns Wimps - standing for weakly interacting massive particles - which astronomers think may make up the bulk of the Universe. For every kg of material made up from atoms like the ones we have in our bodies, or which make up the stars, there are up to 20kg of something completely different, whose principal quality is that it has never been actually observed directly by scientists. Which is why they call it dark matter. But they know it is there because its effect on the movements of galaxies can be weighed. If Wimps exist, they would fill the spaces between the stars, and would interact with normal matter so weakly that they would pass right the way through the Earth. Seasonal variation The Dama (DArk MAtter) experiment, at the underground Gran Sasso facility in Italy, is one of several around the world hoping to spot the tell-tale signs of rare collisions between Wimps and ordinary atoms. Dama has been reporting possible detections, and what is more, it is seeing more events in summer than in winter.       Along with the stars being ripped out of the Sagittarius dwarf galaxy, there would be a large amount of dark matter and that would provide a Wimp highway that's coming right down on to the Earth Prof Katherine Freese This seasonal variation has been a particular puzzle. Should cosmic particles obey the cycles of the Earth? The solution could come from a completely different branch of science. In 1994, UK astronomers discovered a small galaxy just on the other side of our Milky Way galaxy, about to swallowed up by its larger neighbour. Since then it has become clear that this Sagittarius dwarf galaxy, as it is been called, is actually orbiting over the poles of our own galaxy, with long streams of stars preceding and trailing the main body. It was when dark matter expert Katherine Freese, from the University of Michigan, heard astronomer Heidi Jo Newberg, from the Rensselaer Polytechnic Institute, talk about these so-called tidal streams that an explanation for the puzzling seasonal Dama Wimp result started to crystallise. "Part of this stream of stars is coming past our part of the galaxy, close to the Solar System," explains Heidi Newberg. Stream direction And this is what excited Katherine Freese. "Along with the stars being ripped out of the Sagittarius dwarf galaxy, there would be a large amount of dark matter and that would provide a Wimp highway that's coming right down on to the Earth," she said. The argument is that we are stuck in the middle of a fast-moving stream of Wimps, billions passing through every square metre of the Earth (and our bodies) each second at speeds of over a million km/h. The seasonal variation in detection would then depend on whether the Earth's orbit around the Sun is taking us upstream or downstream in this flow of extragalactic debris. Writing in the journal Physical Review Letters, the scientists say their theory should be provable, if dark-matter detectors could see a variation in the energy of atom-Wimp collisions from winter to summer. But confirming that Wimps exist would only be the start of a bigger search - for the identity of what they are actually made from. http://news.bbc.co.uk/1/hi/sci/tech/3573041.stm

Title: Re: Dark matter Post by Kali on 01.07.2004 at 17:16:27 http://physicsweb.org/article/news/8/6/14 New light on dark energy 24 June 2004 Cosmologists in the US have made the most accurate measurements ever of how dark energy varies with time -- and found that it remains constant. Yun Wang at the University of Oklahoma and Max Tegmark at the University of Pennsylvania performed numerical simulations on observational data from supernovae, the cosmic microwave background and galaxy clusters. The results lend further support to the existence of dark energy (Phys. Rev. Lett. 92 241302). The acceleration of the universe is driven by a force that has repulsive rather than attractive gravitational interactions. But although this so-called "dark energy" is thought to account for around two-thirds of the universe, no one knows what it is made of. The first evidence for dark energy came from supernovae observations in 1998. Further evidence arrived in 2002 from a survey of 250,000 galaxies and later from observations of gravitational lensing. Some explanations for dark energy -- such as quintessence, modified gravitational theories that include extra dimensions, or string physics -- suggest that dark energy could change with time. If dark energy became progressively weaker, the universe would eventually tear apart in a "big crunch". If it became stronger, on the other hand, the universe would collapse in on itself in a "big rip". Tegmark and Wang used a novel model-independent approach to measuring the dark-energy density. They analysed data from type 1a supernovae, recorded with the Hubble Space Telescope; the cosmic microwave background (CMB) taken with the Wilkinson Microwave Anistropy Probe (WMAP) and the Sloan Digital Sky Survey (SDSS); and from large-scale galaxy cluster observations. The results agree with recent observations on supernovae that suggested that dark energy remains constant with time. Moreover, the physicists calculated that if the dark energy density were to change with time, a big crunch or big rip could not occur for at least 50 billion years for models that allow such events. "I'm struck by the fact that the dark energy seems so 'vanilla'," Tegmark told PhysicsWeb. "Theorists have invented scores of elegant models where it increases or decreases its density over time, yet even with this new improved measurement, it remains perfectly consistent with Einstein's Lambda model where its density is a mere constant." Author Belle Dumé is Science Writer at PhysicsWeb

Title: Re: Dark matter Post by mauk on 24.08.2004 at 22:04:38 Po spominu se mi zdi, da je "temna materija" tista, ki ne seva nobene svetlobe... to so menda po domače črne luknje, ki imajo tako maso, da gravitacija preprečuje da bi svetloba ušla čez dogodkovni horizont. Menda, pa so ugotovili, da črne luknje niso povsem črne, ker fizikom še ni uspelo povezati splošne relativnostne teorije in kvantne mehanike. Po kvantni mehaniki zaradi načela nedoločenosti, bi bilo možno, da črna luknja seva, da oddaja energijo, in prav to sevanje so menda tudi že našli... Načelo nedoločenosti je povezano s tem, da ne moreš neke stvari opazovati ne da bi to stvar zmotil. To postane očitno pri zelo lahkih delcih, recimo pri elektronih, če hočeš ugotoviti kje je elektron ali kakšno hitrost ima, ga moraš z nečim obsevati... npr. s fotoni, ampak vsak foton ki se zadane v elektron vpliva tudi na njegovo pozicijo in hitrost, tko da nastane cela tragedija... Menda je tako, da če veš natančno pozicijo delca prav nič ne veš o njegovi hitrosti in obratno. Sicer mi ta trenutek ni prav nič jasno, zakaj bi to načelo nedoločenosti omogočalo, da kaj uide črni luknji... pišem po spominu lahko da se tud motim. Skratka temna materija bi lahko prispevala k temu, da se bo naše vesolje nekoč začelo spet krčiti... zdej se menda širi in če bi upoštevali samo vidne zvezde je menda gostota materije taka, da se bo vesolje širilo v neskončnost. Če pa je zadosti te temne materije pa se lahko zgodi, da se bo proces širjenja vesolja enkrat obrnil in se bo začelo vse skupaj krčiti... vse do velikega antipoka ko bo šlo vse v eno točko z neskončno gostoto. Znanstveniki se zlo trudijo za veliko teorijo poenotenja, kjer bi združili kvantno mehaniko in splošno teorijo relativnosti. Zdej so menda nekje pri 11 dimenzionalnih matematičnih modelih, večina vizualnih predstav in razmišljanj razpade že pri četrti dimenziji, tako da se mi zdi res noro to kar počnejo... na kakšen način lahko kretivni um deluje pri 11 dimenzijah. Mislim da človeštvo potrebuje res nekoga zares norega, da bi lahko stvar poenotil v teh abstraktnih matematičnih modelih. Moje mnenje je, da so šli v tej smeri znanstveniki predaleč, ker se mi zdi, da poizkušajo dosečt stvari samo še na podlagi poizkusov in napak, se pravi mogoče se bodo pa na tak in tak način enačbe izšle... dost nekreativno... Mislim da znanost rabi enega genija, saj Hawking je svaka mu čast, samo poenotenja pa še nimamo.

Title: Re: Dark matter Post by Kali on 24.02.2005 at 02:04:25 Astronomers find star-less galaxy http://news.bbc.co.uk/1/hi/wales/south_east/4288633.stm Astronomers Find Star-less Galaxy By BBC News Feb 23, 2005, 16:55       Astronomers say they have discovered an object that appears to be an invisible galaxy made almost entirely of dark matter. The team, led by Cardiff University, claimed it is the first to be detected. A dark galaxy is an area in the Universe containing a large amount of mass that rotates like a galaxy, but contains no stars. It was found 50 million light years away using radio telescopes in Cheshire and Puerto Rico. The unknown material that is thought to hold these dark galaxies together is known as 'dark matter', but scientists still know very little about what that is. The five-year research has involved studying the distribution of hydrogen atoms throughout the Universe, estimated by looking at the rotation of galaxies and the speed at which their components moved. Hydrogen gas releases radiation that can be detected at radio wavelengths. In the Virgo cluster of galaxies, they found a mass of hydrogen atoms a hundred million times the mass of the Sun. The mysterious galaxy has been called VIRGOHI21. Similar objects that have previously been discovered have since turned out to contain stars or be remnants of two galaxies colliding. However, the scientists from the UK, France, Italy and Australia found no visible trace of any stars, and no galaxies nearby that would suggest a collision. Dr Robert Minchin, of Cardiff University, said: "From its speed, we realised that VIRGOHI21 was a thousand times more massive than could be accounted for by the observed hydrogen atoms alone. "If it were an ordinary galaxy, then it should be quite bright and would be visible with a good amateur telescope." The astronomers say it is hard to study the universe's dark, hidden objects because of the Earth's proximity to the Sun. They liken it to looking out at the darkest night from a well-lit room - it is easy to make out street lights but not trees, hedges and mountains. Astronomers say it marks an important breakthrough because, according to cosmological models, dark matter is five times more abundant than the ordinary (baryonic) matter that makes up everything we can see and touch. Another of the Cardiff team, Dr Jon Davies, added: "The Universe has all sorts of secrets still to reveal to us, but this shows that we are beginning to understand how to look at it in the right way. It's a really exciting discovery."

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