Posted on 10/02/2003 12:55:26 PM PDT by Ernest_at_the_Beach
The identity of the Universe's dark matter may finally have been discovered. In what seems to be the most convincing claim for dark matter so far, researchers in England and France say gamma rays coming from the centre of our galaxy show hallmarks of these ghostly particles.
The research has only just been made public, so the team is still waiting for a response from other dark matter experts. But though the researchers are cautious, there is no hiding their excitement. "I've dropped everything else to work on this," says Dan Hooper of the University of Oxford. "We're really excited," adds his colleague Céline Boehm, also of Oxford. "I'm cautious but it's surprising everything fits so well."
The identity of the Universe's dark matter, which outweighs the visible stuff by at least a factor of seven, is the outstanding mystery of modern astronomy. Scientists think it must exist because its gravity affects the way galaxies hold together. But the particles do not emit any electromagnetic radiation so they have never been detected directly. No one knows what the particles are like, or exactly how they are distributed.
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Mystery cloud |
However, because dark matter "feels" gravity like ordinary visible matter, it is a fair bet that it clumps in the centre of our galaxy. So the team turned their attention to a distinctive pattern of gamma rays coming from the centre of the Milky Way (see graphic). The sharp signal, which has an energy of 511 kiloelectronvolts (keV), is believed to be due to the annihilation of electrons and positrons the antimatter equivalent of electrons.
Virtual standstill
But where did the electrons and positrons come from? People have speculated that the source is anything from the blast waves of a "hypernova" a super-powerful supernova to a neutron star or black hole. "But none of the explanations have seemed satisfactory," says Hooper.
(Excerpt) Read more at newscientist.com ...
That's exactly my point. There are no primordial positrons, yet we see positrons, hence there must be a method of producing positrons. The same argument applies to dark matter antiparticles. Unless I'm missing your point...
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'Still a little tough to grasp undetectable matter that should be coupling to gravity, locally.
Best to You.
That's true, but we can see evidence of distant anti-muon production and even anti-proton production, so such energies are available. (The mass of a muon is of the same order of magnitude as this supposed dark matter particle.)
OTOH, if instead dark matter is commonly distributed with anti-dark matter, the annhiliations that would be regularly produced should be easily observable.
That depends on how strongly coupled it is to electromagnetic particles like electrons. If the coupling is small enough, there could be a fantastic holocaust of annihilation occurring all about you--even through you--and you'd never know it.
Your statements, taken as a pair are contradictory; what I think we are supposed to think is that the math demands that there be a mass large enough to explain the part we do see and dark matter is what we currently call it.
And, if it exists, it can't all be balled up in one little corner to fit the current notions.
Wasn't it Faraday who said that space had to be either completely empty or completly full?
No, they are not. Whither the neutrino, before Cowen and Reines?
He was good at lab experiments and demonstrations, maybe the best ever, but he was not a theoretical physicist and had little math.
As I recall, the neutrino was noticed precisely because the energies of neutron decay didn't consistently add up. It became clear that something else was being produced, something that was carrying away some of the energy, even if we couldn't see it directly. Shouldn't something like that have happened already for the dark energy particle?
Well, to be fair, they didn't say that. I merely deduced it.
Even if they don't bang into other particles very readily, wouldn't the mass/energy disappearing into the production of these things have been noticed by now?
That's essentially where I was going with the "ASP" signature, but it's a tricky measurement even for neutrinos. Whether we should have seen it or not depends on the size of the coupling constant, but they should have enough data to calculate what that is.
[Geek Alert: the ASP (Anomalous Single Photon) detector was an electron-positron collision experiment at the PEP collider at SLAC. (Coincidentally, my boss at Penn, Prof. Robert Hollebeek, was the spokesman for the ASP Collaboration.) The detector was optimized for low-angle photon detection. Its purpose was to count the number of invisible particle species being produced in e+e- collisions at (IIRC) 29 GeV. It does this through a quantum process known as Bremsstrahlung radiation. As the electron and positron come into very close proximity, there is a very large chance that one of them will emit a very energetic photon. The distribution of these photons is sharply peaked along the beam axis. (Double Geek alert: this is also called "initial-state radiation". There can also be intermediate-state radiation, which is the real money winner for getting large transverse momentum.) All collisions have some probability of emitting such photons, but if charged particles, hadrons or photons are produced in the annihilation, they will be detected. If neutrinos (or other invisible particles) are produced, the photon alone will be detected. Since the backgrounds are calculable and the three neutrino cross-sections are known, the rate of single photon production (and its angular distribution) can be accurately predicted. Any unknown process that produces undetectable particles will make itself known in this experiment by producing an anomalous distribution of single photons, if the coupling is large enough for a given sample size.]
As I recall, the neutrino was noticed precisely because the energies of neutron decay didn't consistently add up. It became clear that something else was being produced, something that was carrying away some of the energy, even if we couldn't see it directly. Shouldn't something like that have happened already for the dark energy particle?
Unfortunately, there are no spontaneous decays that would produce these particles, in analogy to the decay spectra in which the existence of neutrinos was first noticed.
Is the problem that in particle collisions you can't tell the input energies with the necessary precision, as was possible when the input energy was simply the mass of a known particle? I had been assuming that some kind of precise accounting was possible: mass A at velocity n, mass B at velocity m. Splat! Then various products smacking with measured energies into detectors.
The long answer I will have to compose.
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