Just as I was leaving for a week’s vacation, the dark matter search experiment LZ reported its first results. Now that I’m back, I see that I didn’t miss anything. Here is their figure of merit:

The latest experimental limits on WIMP dark matter from LZ (arXiv:2207.03764). The parameter space above the line is excluded. Note the scale on the y-axis bearing in mind that the original expectation was for a cross section around 10-39 cm2, well above the top edge of this graph.

LZ is a merger of two previous experiments compelled to grow still bigger in the never-ending search for dark matter. It contains “seven active tonnes of liquid xenon,” which is an absurd amount, being a substantial fraction of the entire terrestrial supply. It all has to be super-cooled to near absolute zero and filtered of all contaminants that might include naturally radioactive isotopes that might mimic the sought-after signal of dark matter scattering off of xenon nuclei. It is a technological tour de force.

The technology is really fantastic. The experimentalists have accomplished amazing things in building these detectors. They have accomplished the target sensitivity, and then some. If WIMPs existed, they should have found them by now.

WIMPs have not been discovered. As the experiments have improved, the theorists have been obliged to repeatedly move the goalposts. The original (1980s) expectation for the interaction cross-section was 10-39 cm2. That was quickly excluded, but more careful (1990s) calculation suggested perhaps more like 10-42 cm2. This was also excluded experimentally. By the late 2000s, the “prediction” had migrated to 10-46 cm2. This has also now been excluded, so the goalposts have been moved to 10-48 cm2. This migration has been driven entirely by the data; there is nothing miraculous about a WIMP with this cross section.

As remarkable a technological accomplishment as experiments like LZ are, they are becoming the definition of insanity: repeating the same action but expecting a different result.

For comparison, consider the LIGO detection of gravitational waves. A large team of scientists worked unspeakably hard to achieve the detection of a tiny effect. It took 40 years of failure before success was obtained. Until that point, it seemed much the same: repeating the same action but expecting a different result.

Except it wasn’t, because there was a clear expectation for the sensitivity that was required to detect gravitational waves. Once that sensitivity was achieved, they were detected. It wasn’t that simple of course, but close enough for our purposes: it took a long time to get where they were going, but they achieved success once they got there. Having a clear prediction is essential.

In the case of WIMP searches, there was also a clear prediction. The required sensitivity was achieved – long ago. Nothing was found, so the goalposts were moved – by a lot. Then the new required sensitivity was achieved, still without detection. Repeatedly.

It always makes sense to look harder for something you expect if at first you don’t succeed. But at some point, you have to give up: you ain’t gonna find it. This is disappointing, but we’ve all experienced this kind of disappointment at some point in our lives. The tricky part is deciding when to give up.

In science, the point to give up is when your hypothesis is falsified. The original WIMP hypothesis was falsified a long time ago. We keep it on life support with modifications, often obfuscating (to our students and to ourselves) that the WIMPs we’re talking about today are no longer the WIMPs we originally conceived.

I sometimes like to imagine the thought experiment of sending some of the more zealous WIMP advocates back in time to talk to their younger selves. What would they say? How would they respond to themselves? These are not people who like to be contradicted by anyone, even themselves, so I suspect it would go something like

Old scientist: “Hey, kid – I’m future you. This experiment you’re about to spend your life working on won’t detect what you’re looking for.”

Young scientist: “Uh huh. You say you’re me from the future, Mr. Credibility? Tell me: at what point do I go senile, you doddering old fool?”

Old scientist: “You don’t. It just won’t work out the way you think. On top of dark matter, there’s also dark energy…”

Young scientist: “What the heck is dark energy, you drooling crackpot?”

Old scientist: “The cosmological constant.”

Young scientist: “The cosmological constant! You can’t expect people to take you seriously talking about that rubbish. GTFO.”

That’s the polite version that doesn’t end in fisticuffs. It’s easy to imagine this conversation going south much faster. I know that if 1993 me had received a visit from 1998 me telling me that in five years I would have come to doubt WIMPs, and also would have demonstrated that the answer to the missing mass problem might not be dark matter at all, I… would not have taken it well.

That’s why predictions are important in science. They tell us when to change our mind. When to stop what we’re doing because it’s not working. When to admit that we were wrong, and maybe consider something else. Maybe that something else won’t prove correct. Maybe the next ten something elses won’t. But we’ll never find out if we won’t let go of the first wrong thing.

17 thoughts on “LZ: another non-detection

  1. Until recently, the first line on XENONnT’s public web page:


    was “Dark matter has been discovered”. I have to wonder what a typical non-astrophysicist would think on reading those words. “Isn’t the goal of the XENON experiment to discover dark matter?” they would probably ask. “So why does this web page state that dark matter has ALREADY been discovered?”

    I doubt that anyone involved with the discovery of the Higgs boson would have been so irresponsible as to state “The Higgs has been discovered” before it actually had been discovered.

    Why are the standards of experimental cosmologists so different?

    I think your new piece provides the answer. If dark matter does not exist, it would be hard to justify experiments like LZ or XENON. Therefore, “Dark matter has been discovered”. Even though it hasn’t been.

    Liked by 1 person

    1. First, I note that the first paragraph of the web page you cite reads just like my thoughts in the early ’90s. That’s where most of the field came from, and where most remain stuck. It is disappointing that there is apparently no awareness that when they say “dark matter has been discovered” the more correct statement would be “there is clear evidence for acceleration discrepancies”, a phrasing that doesn’t bias the discussion by presuming the answer. Much of our woes originate from imprecise use to language.

      We have witnessed a long evolution in attitudes about this over the past half century. Early on, dark matter was a strange and dubious-sounding entity fancied by some astronomers but treated with extreme skepticism by most physicists. As it became clear that something was needed, some physicists saw the opportunity to test the idea experimentally, and the seeds for experiments like LZ were planted. The attitude in the ’80s was that astronomy was incapable of discovering dark matter; it could only indicate the need but it would require experimental proof from the laboratory to become accepted.

      Over time, people got more and more invested in the paradigm, and the standards for detection dropped. The astronomical evidence, once suspect, were gradually accepted (though not enough to award a Nobel Prize). I think when the CMB fits required Omega_CDM > 0 at high confidence, that sealed the matter in many minds. By this time, cold dark matter was a familiar friend rather than a strange abstraction, so the same minds never seem to contemplate the possibility that Omega_CDM might just be a convenient fitting parameter that glosses over our ignorance about how the universe really works.


  2. This is yet more good news for theories that dispense with the need for dark matter, as conventionally understood, to explain the dynamics of galaxies and larger structures. Looking at the many diagrams that illustrate how well MOND fits different categories of astronomical data is quite compelling. MOND is surely pointing us in the right direction for resolving these great cosmological puzzles.


    1. Indeed – and the usual way to proceed in science would be to start building from the theory that works to predict things. Instead, the current approach is to ignore MOND, pound the square peg into the round hole, and declare success.


  3. Dark matter is an idea with an old explanation for the rotation curves in galaxies. Unfortunately, there is not one formula, but many, tendency increasing.

    MOND has a simple formula (baryonic Tully-Fisher relation) but no explanation.

    Sometimes some physicists ask for new physics. Voila, there it is.

    Personally, I don’t know of any process that depends on the square root of mass….
    Also the square root of the gravitational constant looks strange.

    Liked by 1 person

    1. We want new physics, just not that kind of new physics!

      But yes – that the simple MOND formula has so much predictive power is surely teaching us something.


  4. Is there a “technological tour de force” experiment that has the potential to definitively establish the correctness of MOND, at least in the weak field limit? You have written about how JWST may find results that are not consistent with lambda CDM, but that is not the same as evidence for MOND. I know that Banik and Kroupa have proposed measuring spacecraft acceleration in deep space and Penner has described measurements in the Sun-Jupiter saddle point, as possibilities. What is your opinion? If there is such an experiment, then I would hope that the MOND community could unite behind it and push the powers that be to carry it out.


    1. It would certainly be good to send a probe to large distances from the sun, but one needs to get about 7000 AU out to even approach the MOND regime, so one needs several lifetimes of patience.


  5. I am intrigued by this sentence in the abstract of of the paper: arXiv:astro-ph/0608407v1 “By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. That word “approximately” really caught my attention, as I was under the impression that the lensing map showed perfect centering on the two galaxy clusters in the Bullet Cluster.

    This bears closer scrutiny as it could be the Achilles heel of the long touted ‘definitive’ proof of DM in this cluster. I’ve vacillated on including an alternative explanation for the Bullet Cluster’s gravitational offset in a pet theory that is nearing the finish line, mainly because I realized it would take quite a bit of research to ascertain the alternate explanation holds water. This very speculative theory posits an underlying mechanism for MOND. The current section being worked on will outline a laboratory scale experiment that, in effect, would demonstrate the reason for the gravity offset in the Bullet, and other clusters.


    1. Bullet-type clusters don’t really make sense in either theory. The discrepancy for MOND is more visually glaring, so that seems to be what people remember (plus that’s the answer they want to hear). But I too am struck by how the lensing solutions nearly trace the galaxy distribution, not some theorist’s vision of a triaxial dark matter halo.


    1. Yeah, there are some real problems there, like thin disks. It is far from obvious that these should exist in LCDM. There are others, like the ability of MOND to predict the stability properties of such things where again LCDM is unsatisfactory (dark matter halos over-stabilize low surface brightness disks, so they shouldn’t have bars and spiral arms).

      I’m struck by how the language differs. People writing reviews of LCDM or MOND talk about different things – even when their really talking about the same thing. E.g, a review of problems with rotation curves in LCDM will talk about the cusp-core problem and rotation curve diversity without engaging with how these simply follow from the observed baryon distribution plus the MOND force law.


    1. I don’t think so.
      Xenon is the heaviest non-radioactive noble gas. Being heavier (i.e. larger nucleus) than helium it provides a larger cross-section for interactions with the possible dark matter particles so it maximizes the chances to detect something.
      On the other hand, helium is much more abundant, so cheaper and could compensate its smaller cross-section by using (much) more, I guess. But then you need to have a much larger space to host the experiment.


  6. Was just reading a 2018 article on Quanta Magazine titled: “A Victory for Dark Matter in a Galaxy Without Any”, referring to galaxy NGC 1052-DF2. I’m pretty sure I read that MOND can accommodate this galaxy, but would have to check the literature. I also recall reading that Alexandre Deur’s Self-Interacting graviton theory explains this galaxy’s dynamics by its spherical shape, which effectively cancels out the self-interaction process leading to a 1/R^2 force law. In the Quanta article it’s mentioned that when the JWST is deployed that one of its tasks will be to measure the velocity of the individual stars in this diffuse galaxy virtually devoid of gas and dust. Previously, it was only possible to measure the velocity profiles of the bright globular clusters in this galaxy. It will be interesting to see what JWST reveals in the velocity dispersions of individual stars.


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