Dark Matter Question

Factual Questions
1

My very limited understand of dark matter (and yes I realize no one knows for sure what, if anything, it is) is that it is supposed to interact only gravitationally with normal matter so it’s even less interactive than neutrinos which also feel the weak force.

Given that we have no quantum-GR theory, how can any theory we have predict how often this interaction will occur or is it all done purely by gravitational calculations (which wold seem strange for particle physicist)? I’m asking this question in the context of experiments which have claimed to have spotted a few dark matter “collisions” (I guess you’d call them). How do we have any idea what they would be like to know if the data is consistent with the theory?

2

It was my understanding candidates for dark matter can have an effect on the weak force too.

3

We know what the collisions would be like but we don’t know what dark matter is, fundamentally, in terms of particles. In other words, we see these galaxies colliding and it there is more matter there than we can see based on observed gravitational effects, so we say there is dark matter. “Data is consistent with the theory” is like saying the observations of the gravitational effects is consistent with relativity. Well, of course they are consistent, because we are using relativity to explain gravitational effects, and we need dark matter to have that explanation. Sound circular? It is, but until we have some independent way to identify dark matter or we have something better than relativity that doesn’t need it, then we’ll use it to make things consistent.

4

This is where the hiccup is. The dark matter that terrestrial “direct detection” experiments are looking for interacts via some force other than gravity. That force has to be very weak to fit cosmological observations, but not so weak that it would have to be undetectable. In many models dark matter simply interacts via the usual “weak force”.

5

I think it’s misleading to say that the explanation is circular.

“Data is consistent” here means that the hypothesis makes predictions, and so far those predictions have been validated, so the hypothesis has not been falsified.

Now, granted, those “predictions” are crude, and the hypothesis is nowhere near a complete model. But still we can do science at this level and various competing hypotheses have been ruled out by subsequent empirical data (or indeed detailed analysis of existing empirical data).

6

Most models for dark matter have it interacting via the weak force as well as gravity, primarily because we lack the theoretical framework necessary for constructing any model that does not. It’s certainly conceivable, perhaps even likely (though I don’t know how you’d estimate likelihood) that there’s dark matter out there that’s purely nothing but gravitational, or even interacts via gravity and via other forces we know nothing of, but if there is, there’s not really much we can say about it.

7

Perhaps the question was brought on by some recent observations hinting at a sterile neutrino (see e.g. here), which indeed would be a dark matter candidate not even weakly interacting. Here, the observation is not of the neutrino itself, but of what it decays into, namely, an ordinary neutrino and a photon; it’s the latter which has (possibly) been detected.

8

It’s not that we don’t have the theoretical framework necessary. We can construct any number of well-formulated models with new forces in them. It’s just that there hasn’t been any experimental evidence requiring such forces, so it’s hard to motivate adding them.

Some additional background for the thread: If you postulate that a weakly-interacting massive particle (WIMP) is the dark matter, you can ask what properties that particle should have in order to match observations. Intriguingly, a particle with a “reasonable” mass[sup]1[/sup] and an interaction strength on par with the usual weak force[sup]2[/sup] works perfectly. This so-called “WIMP miracle” is the motivation behind the last two decades of direct searches[sup]3[/sup] for dark matter. From the experimental side, though, you can’t tell the nature of the force, only its strength.[sup]4[/sup] Thus, direct detection experimental results are generally presented without any particular underlying force assumed. This leads to plots like this one, where the possible dark matter particle is characterized by a mass and a “cross section” (i.e., interaction strength), and experimental data are shown as limits (or in the case of detection, preferred regions) in this plane.

As the limits on WIMP interaction strength get pushed lower and lower due to non-observation[sup]5[/sup], the simplest assumption that the particle is interacting via the standard weak force gets more strained, as you start having to explain why the force seems to be weaker than weak. Thus, in just the last few years, real attention has been directed toward dark matter models that are more complex, involving multiple dark matter particles and/or multiple new forces proposed. It’s not the wild west, of course – some semblance of pattern-matching with the “regular” particles is usually present. Indeed, we have a rich zoo of regular matter and regular forces, so why not a similarly rich “dark” sector of particles? Some folks are running with that philosophy and coming up with interesting possibilities and predictions.

In any case, the “WIMP miracle” has been, and still is, a strong motivator for a lot of the experimental effort, but other possibilities remain, including ones where the dark matter doesn’t interact with regular matter at all.[sup]6[/sup]
[sup]1[/sup]given what we know of the Standard Model and potential extensions to it
[sup]2[/sup]as in the Standard Model’s “weak force”, not just any weak force or gravity
[sup]3[/sup]“Direct” here is a special word, meaning “detect a dark matter particle interacting with a detector directly”. There are several other avenues for looking for dark matter (beyond the obvious gravitational one that is well established).
[sup]4[/sup]This statement assumes the detectors all use the same detection material. They don’t, so you can ask questions like whether the relevant force pays attention to the quantum mechanical “spin” of the detection material (nuclei).
[sup]5[/sup]barring the hints that have popped up here and there
[sup]6[/sup]Although, “not at all” sometimes means “so little that you’d never detect it directly”. If the dark matter particle can be produced via regular particles or can decay to regular particles, then you can generally detect it with a detector made of regular particles, although that detector might need to be of impractical, astronomical (literally) size.

9

I should have phrased that more clearly. We can construct such models, but we don’t have the framework for them. They’re squishy, spineless. You can throw any old set of equations onto the wall and they’ll stick, with no basis for choosing one over another.