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

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

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.