The Nightmare Scenario for Dark Matter is Inching Closer
Physicists have spent decades searching for dark matter particles, but what if dark matter only has a gravitational pull and nothing else?
The idea of dark matter is almost a century old, yet physicists still haven't found out what it is or what else explains their observations. There is one possibility which they don't like to talk about: the nightmare scenario—a particle which is so hard to detect that we'll never catch it, and it's becoming increasingly plausible. Everything we know about dark matter we only know from its gravitational pull. Dark matter supposedly hovers in a halo around galaxies, and its extra mass makes the galaxies rotate faster. Dark matter also supposedly lingers around in galaxy clusters and makes their constituent galaxies move faster. Astrophysicists also believe that it makes gravitational lenses stronger and that it generally accelerates the formation of structures in the universe. All of this comes merely from its gravitational pull.
Particle physicists, not so surprisingly, believe that what's creating this extra gravitational pull is really a type of particle. If they were bakers, they'd be convinced the universe was held together by cosmic croissants. But the dark matter particle can't be any of the particles we already know of because they interact with each other too much or aren't heavy enough. The dark matter particle, therefore, needs to be a new particle. The trouble is that the observations tell us basically nothing about the new particle specifics. In the lack of data, particle physicists have wishes. They wish for a dark matter particle that does something else besides having a gravitational pull so that they can catch it in a particle detector or indeed produce it in a particle collider. Unfortunately, this has not happened despite decades of trying.
So what if dark matter just has this gravitational pull and that's it—nothing else? This is called gravitationally coupled dark matter. It's a nightmare scenario because if this hypothesis was correct, we might just never be able to tell that it's correct. Or to be precise, since we can't verify a hypothesis, we might never find a way to falsify it. And that's exactly the way things have been going. Take, for example, WIMPs—weakly interacting massive particles, not to be confused with the particles that make up most internet comment sections. That they're weakly interacting doesn't just mean not strong; it means they interact with approximately the strength of the weak nuclear force. At least, that's what it used to mean. Unlikely as that sounds, astrophysicists thought this was correct because of a numerical coincidence known as the WIMP miracle. If you have particles that interact with approximately the strength of the weak nuclear force and they have approximately the mass of the Higgs boson, then they'd be produced approximately in the early universe in the right amounts to explain our observations.
Sounds intriguing? Think again. There are a lot of numbers in physics, and if you multiply sufficiently many in various iterations, you'll always find some coincidences of this sort. It would be surprising if there were no such coincidences. It still boggles my mind that almost all astrophysicists actually believed that this WIMP miracle meant something. They believed it so much that they commissioned dozens of experiments to look for these WIMP particles, and one after the other found nothing. Each time an experiment found nothing, physicists came up with some explanation for why the particles are just a little more weakly interacting. In case you find that mindboggling too, better hold your head because they're still looking for those particles. The current generation of detectors is now eight orders of magnitude more sensitive than the ones that were supposed to find those particles in the first place. We now know that the WIMP miracle is not fulfilled, so even the original motivation is gone. Still, no one wants to say that the idea of WIMPs is just wrong. They keep building bigger detectors.
Physicists keep chasing elusive dark matter particles, but sometimes the boring answers are the correct ones.
After repeated experiments found nothing, physicists came up with explanations for why the particles are just a little more weakly interacting. If you find that mindboggling, better hold your head because they're still looking for those particles. The current generation of detectors is now eight orders of magnitude more sensitive than the ones that were supposed to find those particles in the first place. We now know that the WIMP Miracle is not fulfilled, so even the original motivation is gone. Still, no one wants to say that the idea of WIMPs is just wrong, and they keep building bigger detectors. I wonder in what other discipline you can possibly do this—get a prediction wrong by a factor of at least 100 million. If weathermen were this persistent, they'd still be predicting the great flood of Noah.
It doesn't look better for other particles that supposedly make up dark matter, notably Axions. Axions too are supposed to have a particular interaction, in this case with the magnetic field. They've been ruled out over and over again for decades, and still, physicists keep looking for them. They're now leaving magnets on lab fridges just in case the Axions want to leave a note.
So, if not WIMPs and if not Axions, then what? Well, maybe the dark matter particles just don't interact with normal matter at all except for gravity. From a theoretical point of view, this idea is both appealing and unappealing. It's appealing because it's easy to make theories for such dark matter particles; pretty much any theory that completes the standard model will throw a few into the pool. It also doesn't really matter much exactly what properties these particles have since you can't measure them anyway. It's unappealing because there isn't much you can say about them, and since there isn't much to say, you don't hear much about it. Gravitationally coupled dark matter particles are the obvious in-your-face solution, but they're incredibly uninteresting. It's much more interesting to talk about the implausible, contrived solutions. It's basically the same reason why conspiracy theories are so interesting. It's not because we believe they're true—at least most of us don't—it's because they're so interesting. That Biden had COVID is about as boring as it gets; he died and was replaced by an AI—now we're talking. Yes, it's not just the case in politics but sadly in science too that we talk about speculative ideas just because there's more to talk about. But more often than not, it's the boring answers that are correct, and the truth is just dark to me.
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