-Koopa:
@ the comic...
The perihelion of Mercury is hardly an anomaly--it's explained fully by special relativity. Same goes for guidance satellites. The GPS is adjusted to compensate for the effects of special relativity's predictions. Also, it's kind of a moot question why the laws of the universe are so, because they would be incredibly hard to discover and have very little bearing on our world.
I know that general relativity (not special btw; special relativity doesn't actually say anything about gravity) explains the perihelion of mercury.
It is an anomaly to
classical, Newtonian physics, which works fine at low speeds and requires space to be euclidian and not curved like it really is. Which is what I tried to say, but the internal references in that particular sentence were a bit mixed up:
We just know that this stuff seems to work... as long as the space stays pleasantly eucleidean. If it doesn't, we start having anomalies like the perihelion of Mercury, guidance satellites throw us dozen kilometres off-target etc. etc."This stuff" was supposed to refer to Newtonian physics, but looking back to the text I can see that I wouldn't understand it that way even myself. My bad.
Your explanation of dark matter makes no sense. Dark matter is simply matter we cannot see because it does not emit what we use to see it. It may consist of particles we do not know of yet, but it is otherwise ordinary matter. Even if, as you say, it does not physically interact with most matter (like neutrinos don't) they would, according to my admittedly limited worldview, still collapse to a point. If they are affected by gravity, then they will eventually "hit" each other and eventually cease movement, because even if they keep passing through one another they lose energy each time. If dark matter does not collide with other matter including itself, then it will form a point of very little volume indeed, whose infinitesimality is limited only by the Pauli exclusion principle.
Herra, I have to agree with Agent Koopa for the moment. If dark matter does not interact with itself or baryonic matter through strong nuclear or electromagnetic forces, there'd be even less to stop dark matter from collapsing into black holes than baryonic matter.
Well, weakly interactive massive particles (WIMPs, and I didn't make this up!) are just one possible candidate for dark matter. But if we consider how a cloud of WIMPs would behave, you must remember that it only interacts through weak interaction and gravity. That means that the most profound thing we tend to associate with matter is not there: you cannot touch dark matter. Touch is dealt solely via electromagnetic interaction, and if dark matter lacks it, it will be totally different from matter as we know it.
For example, if dark matter really consists of WIMPs, it really wouldn't concentrate on any kinds of blobs. The formation of mass concentrations requires that the particles can hit each other. Normal matter works like this - if you put a gas or dust cloud into space, it will start falling towards common center of gravity, and eventually particles start hitting a surface and larger and larger sphere forms etc. etc.
But what would happen if the particles can't touch each other? The answer is that they would simply fall through the center of gravity - not necessarily at the same time, mind you - and start oscillating at complex pattern around the cloud's center of gravity. There's nothing to stop them at the center of gravity. Of course it is possible for these particles to form an event horizon (black hole), but it's not as likely as you might think. It would only happen if sufficient amount of particles happened to fall into small enough area at the same time.
You can ask why won't neutrinos collapse into black holes. The universe is full of them and they do have a small mass. The answer is, there's no concentrations to collapse to, since they can't affect each other via electromagnetic interaction like most particles.
Anyway, weakly interacting massive particles are just one hypothetical solution to the problem of dark matter. Wiki has quite intereesting article about Dark Matter and it says about what I could say about the subject, so I'm going to quote a part of the possible explanations here:
(...) to explain structure in the universe, it is necessary to invoke cold (non-relativistic) dark matter. Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo objects, or "MACHOs". However, studies of big bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter.
At present, the most common view is that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electrons, protons, neutrons, and known neutrinos. The most commonly proposed particles are axions, sterile neutrinos, and WIMPs (Weakly Interacting Massive Particles, including neutralinos). None of these are part of the standard model of particle physics, but they can arise in extensions to the standard model. Many supersymmetric models naturally give rise to stable WIMPs in the form of neutralinos. Heavy, sterile neutrinos exist in extensions to the standard model that explain the small neutrino mass through the seesaw mechanism.
Pauli's exclusion principle doesn't really concern sizes of particle concentrations, it just states that two fermions with same quantum properties cannot share the same space. But if there's different quantum properties involved, you can easily pack several particles to occupy same space. Simplest example of this is the electron shells of atoms. The electrons in the shell occupy the same space, but have slightly different quantum properties. Not that Pauli's exclusion principle would have anything to do with whether or not weakly interactive massive particles will/can form black holes. They can, but they won't do that automatically.
->Mika: Sorry, I don't really know that much of the actual methods of background radiation research. But the thing with background radiation is that it appears the same everywhere as far as I know. What little changes perhaps are caused by the movement of the probe during the measurements, they would most likely be insignificant compared to the diameter of the observable universe.
