Since the mainstream news media, in their reporting on the new result from the LUX experiment I wrote about Wednesday, insists on confusing the public with their articles and headlines, I thought I’d better write a short post reminding my readers what we do and don’t know about dark matter.
- Do we know dark matter exists?
Scientists are, collectively, pretty darn sure, though not 100% certain. Certainly something is out there that acts a lot like a dark form of matter (i.e. something that gravitates and clumps, but doesn’t shine, either in visible light or in any other form of electromagnetic waves). There have been some proposals that try to get around dark matter, by modifying gravity, but these haven’t worked that well. Meanwhile the evidence that there really is dark stuff out there that really behaves like matter continues to grow year by year, and every claim that it actually isn’t there (such as this one I wrote about — see the second half of the article) has turned out to be wrong. Dark matter is needed to explain features of the cosmic microwave background, to explain how galaxies form, to explain why we see certain types of gravitational lensing, etc. etc. No one alternative can explain all of these things. And dark matter easily arises in many particle physics theories, so it’s not hard to imagine it might be created in the early universe and be a dominant player today.
- Do we know dark matter is made from particles (i.e. ultra-microscopic objects with uniform properties)?
No, that’s not certain. Particles would do the job, but that’s not a proof it is made from particles.
- If dark matter is made from particles, do we know these are Weakly Interacting Massive Particles (WIMPs) — to be precise, particles that interact with the Standard Model via the weak nuclear force or the Higgs force or something else we already know about?
No. Dark matter could be WIMPs. Or dark matter could be made from a very different type of particle called “axions”. Or dark matter could be made from particles that aren’t of either of these types. This could include particles that only interact with ordinary matter through the force of gravity, which could make them very, very hard to detect.
- Do most scientists believe dark matter is made from WIMPs? (This was claimed to be true in several news articles.)
As far as I can tell, most experts do not know what to think; some have a bias toward one idea or another, but when pressed admit there’s no way to know. Many scientists think WIMPs are a good candidate, but I’ve never heard anyone say they are the only one.
- So why are so many experiments (there’s a partial list here) looking for WIMPs?
Partly because they can. Sometimes science involves looking under the lamppost for your keys. You look where you can because you can look there, and you may get lucky — it has happened many times before in history. That’s fine as long as you remember that’s what you are doing.
Not that WIMPs are the only things that people are looking for. They can also look for axions, and there are experiments doing that search too. Looking for other types of dark matter particles directly is sometimes very difficult. Some of these other types of particles could be found by the experiments at the Large Hadron Collider [LHC] (and people are looking.) Others could be found by experiments such as FERMI and AMS, through the effect of dark matter annihilation to known particles (and people are looking; there’s even a hint, not yet shown to be wrong). Still other possible types of dark matter particles are completely inaccessible to modern experiments, and may remain so for a long time to come.
- If we don’t know dark matter is particles, or that those particles are WIMPs, then why do the headlines say “dark matter search in final phase” in reference to the new result from LUX, even though LUX is mainly only looking for WIMPs?
Don’t ask me. Ask the editors at CBS and the BBC why their headlines about science are so often inaccurate.
The search for dark matter will end when some type of dark matter is found (or somehow shown convincingly not to exist), not before. The former could happen any day; the latter will not happen anytime soon. The only thing that is currently approaching its end is the search for WIMPs as the dark matter (and even that search will not, unfortunately, end as soon or as conclusively as we would like.) If WIMPs aren’t found, that just probably means that dark matter is something else on the list I gave you above: some other type of particle, or some other type of thing that isn’t a particle. Or it could mean that dark matter forms clumps, rather than being smoothly distributed through our galaxy, and that we’re unlucky enough to be in an empty zone. Certainly, if LUX and XENON1T and the other current experiments don’t find anything, we will not be able conclude that dark matter doesn’t exist. Only those who don’t understand the science will attempt to draw that conclusion.
- So why is the LUX experiment’s result so important?
Well, it’s important, but not amazingly important, because indeed, (a) they didn’t find anything, and (b) it’s not like they ruled out a whole class of possibilities (e.g. WIMPs) all at once. But still, (i) they did rule out a possibility that several other experiments were hinting at, and that’s important, because it settles an outstanding scientific issue, and (ii) their experiment works very, very well, which is also important, because it means they have a better chance at a discovery in their next round of measurements than they would have otherwise. In short: they deserve and will get a lot of praise and admiration for their work… but their result, unlike the discover of the Higgs particle by the LHC experiments, isn’t Nobel Prize-worthy. And indeed, it’s not getting a front-page spread in the New York Times, for good reason.