Simulations Predict Where We Can Find Dark Matter 61
p1234 writes with this excerpt from the Max Planck Institute for Astrophysics:
"Simulations by the Virgo team show how the Milky Way's halo grew through a series of violent collisions and mergers from millions of much smaller clumps that emerged from the Big Bang. ... If Fermi does detect the predicted emission from the Milky Way's smooth inner halo, then it may, if we are lucky, also see gamma-rays from small (and otherwise invisible) clumps of dark matter which happen to lie particularly close to the Sun. ... The largest simulation took 3.5 million processor hours to complete. Volker Springel was responsible for shepherding the calculation through the machine and said: 'At times I thought it would never finish.' Max Planck Director, Professor Simon White, remarked that 'These calculations finally allow us to see what the dark matter distribution should look like near the Sun where we might stand a chance of detecting it.'"
We discussed a related simulation a few months ago.
Re:Obvious (Score:1, Informative)
there is no such thing as dark matter.
Re:Hope it works out for you (Score:5, Informative)
Don't bother reading the paper. They discovered that they could find dark matter "in space" if it exists.
Did you read the same article I did? Before this simulation their best guess was to look at other galaxies and target their centers. However, the simulation revealed that they have a better chance if they look at our own galaxy, but 10-30 degrees of center, where they should be able to detect gamma rays caused by dark matter.
Re:Hmmm (Score:4, Informative)
N-body simulations require a high degree of communications between processing nodes, something "@home" systems don't provide.
Re:Hope it works out for you (Score:5, Informative)
Anytime the underlying assumptions of a computer input or algorithm are faulty, even the 3 1/2 million hours of computation on a supercomputer will not lead to the discovery of the scientific truth. The principle of garbage in garbage out still applies. The bottom line is simple: the dark matter emperor is as naked as a newborn.
They have a theory about how dark matter should work. They expended 3.5M hours of computing to make a prediction based on that theory. Now they'll try to confirm the prediction empirically. If the prediction doesn't pan out, the theory will be jettisoned or patched. If the empirical observations agree with the prediction, the theory is left standing until such time as new evidence shows it to be squidgy around the edges. That's how science works.
Re:Hope it works out for you (Score:3, Informative)
"Has anybody ever scientifically demonstrated that it is possible to generate such electromagnetic radiation by means OTHER than moving electrical charges?"
Yes. Go to a large hospital that has a PET scanner. They routinely generate gamma rays inside human bodies through the annihilation of positrons and electrons.
Gamma rays are generated by interactions that involve atomic nuclei or particle-particle interactions. Because of that, in most processes the gammas all have specific energies. For example, in the electron-positron annihilation, usually two gamma rays are produced, each at 511 keV.
Accelerating electrical charges tend to radiate across all or a large portion of the spectrum unless the acceleration on all of them is identical. The article says the researchers recommend looking for large regions of smoothly varying gamma intensity. That's not a pattern that would be easy to produce with acceleration, particularly not at a uniform energy.
Big Assumptions (Score:3, Informative)
This assumes that dark matter generates gamma rays, a form of electromagnetic radiation. Has anybody ever scientifically demonstrated that it is possible to generate such electromagnetic radiation by means OTHER than moving electrical charges?
You are quite correct. They are making a huge assumption that dark matter can annihilate with itself. Whether it can do this depends on the type of dark matter - in some models it can in others it cannot. This is not pointless though - their work will either see it or at least put a limit on how well it does annihilate which we may be able to use to give us a better idea where to look at the LHC.
One way to avoid having charges produce photons is to have your particle partly consist of the EM field. For example, although the experiment has never been done, I think (if you are a particle theorist feel free to correct me!) you could annihilate two Z bosons to give two photons because the Z consists partly of an EM field.
If they do detect such gamma radiation, how will they ascertain that this radiation is NOT caused by an electrical phenomena rather than some unknown action of undiscovered dark matter?
When you have an annihilation like this the photons produced are mono-energetic. Hence you will get a large spike in the emissions at a particular energy value. Since dark matter has a mass in the 10's of GeV minimum (for most models) this would be extremely hard to produce by other means and certainly not in a mono-energetic fashion. Of course, this is not concrete proof that it comes from dark matter but if all the physics we know about cannot explain it then it would have to be dark matter or something else completely new.
Re:places devoid of any stars (Score:3, Informative)
I got the impression what they meant was that instead of looking at the core of a galaxy (where the dark matter is most concentrated but there are also guaranteed to be lots of stars) you stand a better chance of detecting it if you look out a little further where there are still some decent clumps but there are many fewer stars. If you get really lucky you might even find a clump that has very few stars in it.
Re:places devoid of any stars (Score:3, Informative)
Don't read too much into that sentence. They're just saying that if they found a clump without stars inside it, then one can immediately rule out a star-based source of gamma radiation.
If they don't rule that out, then it will be hard to argue that the gamma rays are from dark matter, and not some other more mundane source.
Re:Hope it works out for you (Score:3, Informative)
"Moving electrical charges" and "electrical effects" suggests you're talking about Maxwell-law electrodynamics. There are many other sources of electromagnetic radiation, including atomic state transitions (e.g., fluorescence) and particle interactions (e.g., radioactive decay, particle annihilation, etc.).
Saying that electromagnetic radiation is produced by "electrical" effects is something of a tautology, as electromagnetic radiation is, as you might guess, an electromagnetic effect. However, it's well-known that there are ways of producing photons other than Maxwell electrodynamics.