Astrophysicists cast doubt on link between excess positrons and dark matter

Apr 16, 2010

(PhysOrg.com) -- Astrophysicists are looking everywhere - inside the Large Hadron Collider, in deep mines and far out into space - for evidence of dark matter, which makes up about 25 percent of the energy density of the universe.

Despite the recent tantalizing observation of excess high-energy positrons - thought to be due to dark matter - UC Irvine researchers say we're not quite there yet.

Models predict that when dark-matter particles collide, they'll annihilate some of the time into and positrons, said Manoj Kaplinghat, physics & astronomy associate professor. Scientists working on a satellite experiment called PAMELA recently identified a large excess of positrons, causing a flurry of excitement about having detected dark matter.

Kaplinghat - working with Jonathan Feng, UCI physics & astronomy professor, and Hai-Bo Yu, postdoctoral researcher - evaluated the dark-matter explanation for the PAMELA finding. "What we concluded is that the detection of so many positrons makes it unlikely they're all from dark matter," Kaplinghat said.

The UCI study sharpens predictions of what scientists can expect to detect from the annihilation of dark-matter particles in our galaxy. While it shows that currently popular models cannot account for the excess positrons observed, it leaves open the possibility that discovery of dark-matter evidence could be right around the corner, perhaps within reach of current and planned experiments.

The UCI team's study was published April 15 in Physical Review Letters and is suggested reading by the editor.

Worldwide and at UCI, the hunt for dark matter is in full swing. In the 17-mile-long tunnel of the near Geneva, Switzerland, scientists are smashing together subatomic particles at nearly light speed in a quest for insight into the nature of the universe.

In dark mine shafts, researchers are registering faint hints of subatomic particles that they surmise could be dark matter. There are satellite, balloon-borne and ground-based endeavors to find high-energy gamma rays, neutrinos and antimatter that could be signatures of dark-matter particles.

Success in these multipronged efforts would bring astronomers closer to identifying the invisible material that constitutes a quarter of the world and determines the architecture of the visible universe. And physicists would have the first evidence supporting the theory that all forces of nature can be unified under one mathematical expression.

"For now, we're left with no convincing dark-matter explanation for excess ," Yu said. "Data from the LHC and experiments deep underground, on the Antarctic ice and in space will throw more light on these issues in the next couple of years - and perhaps we'll even detect the particle."

Explore further: Physical constant is constant even in strong gravitational fields

Provided by University of California - Irvine

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deatopmg
3.5 / 5 (4) Apr 16, 2010
"Astrophysicists are looking everywhere ... dark matter, which makes up about 25 percent of the energy density of the universe."

If you read further than the summary, the UCI release is abundantly clear that so called dark matter is far from being a done deal.
baudrunner
2.3 / 5 (3) Apr 16, 2010
"Models predict that when dark-matter particles collide, they'll annihilate some of the time into electrons and positrons"

Bubble-chamber exposures of high speed proton-proton collisions have demonstrated the occasional spontaneous generation of electron-positron pairs from what is thought to be the photon background. Photons are not thought to have mass, but photonic energy must have, because electrons and positrons do. The vast volume of space in which massive objects appear to be gravitationally affected by so-called dark matter (observed at extreme distances) is comprised on its most fundamental level of the photon background, therefore it is not inconceivable that space itself is the candidate for dark matter and therefore dark energy, or more plausibly, the spontaneously created virtual particles in that space, which are no more than confined bundles of energy, however temporary their existence.
deatopmg
1 / 5 (2) Apr 16, 2010
my understanding is that the need for the dark stuff vanishes if gravity diminishes to zero at ca. galactic size distances.

@baudrunner These spontaneously generated virtual sic particles of the SM are identical to the electron/positron pairs that boil up out of the discarded (by Bohr et al) negative (binding) energy solutions to Dirac's 1928 equation. Their "virtualness" (oh, don't ask it just happens) an excellent way for SMers to continue to ignore the other half of Dirac's work.
Could it be that the negative energy portion of our surroundings (as electron/positron pairs) IS dark matter and energy?
Slotin
2.1 / 5 (7) Apr 16, 2010
Frankly, I never understood the models, in which antiparticles should form from dark matter. In aether theory particles of antimatter should exhibit antigravity, but they should be attracted by dark matter instead. Antiparticles are behaving like bubbles of vaccum and they should be trapped into dark matter preferably - well, in simmilar way, like bubbles at watter surface.

http://preview.ti.../y2cvcnu

After all, the amount of dark matter is sufficient to explain all existing inbalance of matter-antimatter, observed in our Universe after inflation. We could kill a two birds in single shot in such a way.
seneca
1.7 / 5 (6) Apr 16, 2010
Note the apparent connection of positronic dark matter to Alfvén's Plasma Universe model.

http://en.wikiped...osmology

Of course, Hannes Alfvén was plasma expert, so he tended to see plasma everywhere - but this simple example illustrates, no crazy theory may be crazy enough.
seneca
1.8 / 5 (5) Apr 16, 2010
In general relativity the black hole should always exhibit the highest intensity of gravity field (gravitational force) at central singularity. But from Newton's theory follows, the highest intensity of gravity field is just at the surface of object - as we expect, the gravitational field is zero at the center of Earth. By quantum mechanics gravity is always repulsive, so that aether theory is somewhere in between of both models.

Therefore the gravity field of massive objects has a bell shape profile with positive curvature of space at center of object, but slightly negative outside of massive body. This negative curvature is the source of accelerated expansion of objects at 40+ MPc distance scale. Antiparticles are objects of negative curvature like bubbles of vacuum fluid - so they're attracted & collected by negative curvature of space-time around massive objects, forming dark matter, rather then by "normal" gravity field of positive curvature.
Parsec
4.2 / 5 (5) Apr 17, 2010
Photons are not thought to have mass, but photonic energy must have, because electrons and positrons do.


Hmmm not so fast baudrunner. Photons can easily create electron/positron pairs has been observed for decades even tho photons are massless. Good old E=MC2 allows conversion of energy into mass, which is what happens when a very high energy gamma ray photon passes near a nucleus and the electron/positron pair are created.
seneca
2 / 5 (4) Apr 17, 2010
Photons are not thought to have mass, but photonic energy must have... Hmmm not so fast baudrunner
Are you saying here, photons have no energy? How solar cells are working, after then? Like perpetual mobiles?

I can experience for many persons in discussion, they tend to occupy negative stance at any price, as it enables them to dignify in discussion. In such way the discussion becomes turbulent, marginal and never-ending.
Could it be that the negative energy portion of our surroundings (as electron/positron pairs) IS dark matter and energy?
Yes, you're basically right, these concepts are related together.
Shootist
5 / 5 (1) Apr 17, 2010
Photons are not thought to have mass, but photonic energy must have... Hmmm not so fast baudrunner
Are you saying here, photons have no energy? How solar cells are working, after then? Like perpetual mobiles?

I can experience for many persons in discussion, they tend to occupy negative stance at any price, as it enables them to dignify in discussion. In such way the discussion becomes turbulent, marginal and never-ending.
Could it be that the negative energy portion of our surroundings (as electron/positron pairs) IS dark matter and energy?
Yes, you're basically right, these concepts are related together.


photons have no mass, but they do have momentum.
SpiffyKavu
5 / 5 (2) Apr 18, 2010
I have not seen a single quantum mechanical formulation of gravity that has been seriously accepted by any number of physicists.

Anti-particles are exactly like normal particles. There are certain values attached to them which are opposite their normal counterpart, but mass is not one of them. Anti-particles behave just like anything else when subjected to gravity.
Parsec did not say that photons have no energy, he did say that only some photons have enough energy to create electron/positron pairs. The total energy of two colliding photons must be greater than or equal to the rest mass energy of the electron and positron for the photons to generate these two particles. Each particle (electron/positron) have a mass around 511 keV. This means each colliding photon must have at least this much energy. These photons can only be gamma rays; they have extremely high energy. If a thermal source emitted these photons, the source would be over 5 billion degrees.
seneca
2 / 5 (4) Apr 18, 2010
Anti-particles are exactly like normal particles. Anti-particles behave just like anything else when subjected to gravity.
If antiparticles are like bubbles of vacuum, they should exhibit a repulsive force, i.e. the antigravity. It would explain the lack of dark matter in proximity of normal matter and the observation of a expanding universe.

http://dx.doi.org...88930074
kevinrtrs
not rated yet Apr 19, 2010
Just an ignoramus flashing his teeth:
Let me see if I can get a grasp on all this:
There's lots of matter in the universe but if big Bang Theory is true to the laws of physics, there should be an equal amount of anit-matter?
We can't find the anti-matter anyhere we've searched in the universe. So it must be in things we cannot see or detect using normal electromagnetic methods. Thus it's called DARK matter and DARK energy. It's out there, hiding from us in deep dark allys which we cannot traverse?
Man, this universe, it's a dangerous place to be.

We'd better be careful not to disturb it when we find it - it might annihilate us all and we'll go back to nothing! I say we should stop looking for this stuff, it's just too dangerous!!!! ;-))))
ralph_wiggum
Apr 19, 2010
This comment has been removed by a moderator.
broglia
2.3 / 5 (3) Apr 20, 2010
..there should be an equal amount of anti-matter?...
Of course - I cannot understand, why contemporary physicists are so surprised by the lack of dark matter in observable universe and by its relative abundance in dark matter clouds at the same time.

Until they're not complete trolls, indeed...