Cosmic Antimatter Excess Confirmed 113
sciencehabit writes "In 2008, the Italian satellite PAMELA picked up an unusual signal: a spike in antimatter particles whizzing through space. The discovery, controversial at the time, hinted that physicists might be coming close to detecting dark matter, an enigmatic substance thought to account for 85% of the matter in the universe. Now, new data from NASA's Fermi Gamma-ray Space Telescope confirm the spike (abstract)."
Re:Dark matter or antimatter? (Score:5, Interesting)
"Theorists generally believe that when two dark matter particles collide, they should annihilate each other to produce ordinary particles, such as an electron and its antimatter twin, a positron. I suspect that the author doesn't know that "dark matter" isn't a synonym for "antimatter". The above paragraph, if true, would make the universe a very explode-y place.
Some dark matter candidates are, according to theory, their own anti-particle. The only reason it is not a more explode-y space is that dark matter interacts very weakly with other matter, including itself, and therefore has not been identified yet.
Re:Dark matter or antimatter? (Score:2, Interesting)
If that statement were true dark matter would be anything but dark. You would haver massive strongly interactive particles dumping a huge amount of matter, antimatter, and energy (not to be confused with dark energy) into the universe. Then the matter and antimatter reacts, releasing still more energy. Such a reaction would be pretty easily observed. By comparison, WIMPs should be near massless, but abundant, and hence dark.
Alternate explanation (Score:3, Interesting)
Antimatter galaxies (or at least clumps) seem like they could also plausibly explain this. That's not an outright endorsement of the theory but I can't help but think that they've got insufficient evidence to show causal link.
They're saying:
1. lots of energy released (presumably when dark matter interacts)
2. anti-matter is created
An equally plausible interpretation is:
1. the anti-matter already exists
2. the interactions with small amounts of matter cause the energy release
I may have missed it, but I don't see anything to rule that possibility out. The primary objection to the anti-matter galaxy theory is that we don't see a lot of annihilation events; This could just as easily be those exact events.
Re:Dark matter or antimatter? (Score:4, Interesting)
Some dark matter candidates are, according to theory, their own anti-particle. The only reason it is not a more explode-y space is that dark matter interacts very weakly with other matter, including itself, and therefore has not been identified yet.
That would make dark matter very lonely. If it interacts weakly, wouldn't there need to be more of it to account for the effects that dark matter was invented to explain?
Gravity is incredibly weak for individual particles. The reason we notice it in everyday life is because there are a lot of particles in the Earth pulling us the same way and all those little bits add up. This is the bit that we rely on to explain the galactic rotation curves (and to explain the cold spots in the CMB). If the dark matter only interacted gravitationally then it would almost impossible for us to make any direct detection of this sort (but it is also difficult to explain how so much of it was produced).
The idea of the WIMP is that the dark matter, in addition to interacting gravitationally, also interacts via another force called the weak force. While these interactions would have to be somewhat small so that the dark matter did not all explode, or collide too much with itself, it would still be much much stronger than the gravitational interactions on a per particle basis -- but would not "add up" the same way. [As a very simple analogy, the electric forces between protons and electrons are very strong compared to their gravitational attraction, but on large distances matter is almost neutral because opposite charges attract]
This idea is appealing to physicists because
1) if true, we have hope of detecting the dark matter and verifying its existence and
2) it tells us (very broadly) that we would produce the right amount of dark matter as the universe was cooling (the so-called WIMP miracle)