WIMPs

OK, guys, the time has come to bite the “dark matter” bullet. Weakly Interacting Massive Particles (WIMPs) were once upon a time a favoured type of “dark matter”, but not any more, since according to most physicists, “experiment” has “proved” that WIMPs “don’t exist”. Now, as a Weakly Interacting Mathy Person, I can tell you that us WIMPs do exist, it’s just that you completely fail to notice us.

What is a WIMP, basically? A neutron is a WIMP, because it has no electric charge, so does not interact electrically, but it undergoes beta decay into a proton, electron and anti-neutrino, so interacts “weakly” (to use the technical term). Why are neutrons ruled out as WIMPs that could explain “dark matter”? How, in other words, do we “know” that interstellar space isn’t completely full of free neutrons just milling about, clogging space up, and making things much heavier than they seem?

How we “know” this is because neutrons decay with a half-life of about 10 minutes, so that over the lifetime of the universe (or even of a galaxy) they would all have disappeared. So either we need a method of stopping them from decaying, or a method of replenishing the supply. I will address both questions, because in both cases the experimental evidence indicates that the theory may not be correct.

So first let us address the half-life of neutron decay. There is a persistent 3 sigma discrepancy in measurements of this half-life, depending on the method used to measure it. Hmmm. What does that mean? It means that we do not know what the half-life of neutron decay is, even in extremely tightly controlled experimental environments on the Earth. How, therefore, have we got a snowball’s chance in Hell of knowing what the half-life of neutron decay is in inter-galactic space? Firstly, we can’t see the neutrons in the first place, and secondly we can’t detect the anti-neutrinos that are the signature of the decay. To a first approximation, I would say that we have no experimental evidence that such neutrons decay at all, and if they do, the half-life could be a billion years, or a billionth of a second.

But maybe we can do better than that. If they exist, *and* they decay, then the resulting electrons and protons will create magnetic fields on an intergalactic scale. Do we see the evidence of magnetic fields on such an inconceivably large scale? Well, actually, yes we do. Galaxies tend to line up, spinning in the same direction on a vast axis on a scale of tens or hundreds of millions of light years. This could be caused by beta decay of intergalactic neutrons, but if you say this explanation is impossible, please tell me what your “better” explanation is?

Even if the intergalactic neutrons exist, and their decay causes these huge magnetic fields, we still don’t know how long it takes for this decay to happen, and for the fields to build up to the required strength. And we still don’t know how many neutrons there were in the first place, or how many there are left, or how many get replenished every time a neutron-rich star explodes. But there are some clues in the experimental record. One is the critical acceleration of MOND, which describes a phase transition between Newton-Einstein gravity and an entirely different inverse-linear regime.

What could this acceleration have to do with WIMPs in general, or neutrons in particular? It could, for example, be a cut-off point below which free neutrons are completely stable. One could hypothesise that a certain minimum acceleration is required in order to shake up the neutrons enough for them to fall apart. One could hypothesise that as the acceleration increases, the half-life decreases. One could design experiments to test such hypotheses. One could hypothesise that in regions of the universe in which the gravitational pull of stars and galaxies falls below this value, the universe is full of neutrons that are completely undetectable via electromagnetic and weak interactions. Such a hypothesis would explain why these WIMPs (neutrons) have not been detected – they are in principle undetectable.

OK, so this gives us a candidate WIMP for baryonic dark matter: free neutrons that are too cold to decay. This candidate emerges from an analysis of the experimental evidence, and has nothing whatever to do with hypothetical WIMPs dreamt up in speculative theories by imaginative theorists. It may not be correct, but it is a reasonable conjecture.

Now if you put the wide binary test in front of me, and say that this test actually rules out this type of WIMP, I will have to think more deeply. But I think you are wrong to say that these WIMPs are ruled out by this test: what is ruled out is a galaxy-scale uniform dark matter halo. But the observations that support MOND indicate that “dark matter”, whatever it is, closely tracks the ordinary matter that we can detect. If it consists of neutrons, then this property follows from standard theory anyway, so that the analysis that supports a quasi-Newtonian regime with an enhanced value of G appears to be consistent with this hypothesis.

Anyway, food for thought. I’ve got some more WIMP candidates if you manage to rule this one out.