Dark matter discovery: behind the corner?

Clara Moskowitz did a pretty good job

Two days ago, I read a popular article about dark matter:

Dark Matter Mystery May Soon Be Solved

It was written by Clara Moskowitz, an important editor of Space.com, and I thought that the article was unusually good.

The article sketched what dark matter is, what it is composed of according to the most convincing theories (WIMP or axion, and speculations on hidden dimensions properly identified as a cherry on top of the pie), the arguments in favor and against of the superpartners as dark matter particle candidates, and the ongoing experiments as well as those that are getting started such as LUX that are trying or will be trying to directly detect the dark matter particle.




In my opinion, the content of the article was attractive yet balanced and the author collected the voices of some highly relevant scientists.

Matt Strassler wrote an unexpectedly critical text about Clara Moskowitz's article, however. His criticism focused on the main proposition of the article from the title, namely that the discovery of the composition of dark matter is probably imminent according to some scientists. He complained:

I have to admit that this kind of phraseology, which one often sees in the press in reports about science, drives me a bit nuts.  Which scientists? How many of them?  You can’t tell from this line whether this is something that a group of three or four mavericks are claiming, or whether it is conventional wisdom shared by most of the community.  

It is understandable he wants the assertions to be accurate enough. However, he suggests what the articles about science should emphasize – some kind of "consensus" – and I completely disagree with that. There is clearly no consensus about questions that aren't quite settled yet and more importantly, science isn't the search for consensus. Whether the dark matter has been detected and if it has not, whether it will be detected soon is a question that no one really knows for sure and Clara Moskowitz didn't try to claim otherwise. It's not important how many scientists would answer Yes or No in a poll. Science is simply not done in this way. A person aligned with majorities may be close to an "average scientist" but good science is primarily done by "good scientists" which is something else.

But she wrote that there's a good case to believe that we will be probing the "last places" of the parameter space where the dark matter particles could be hiding. I think it's a fair appraisal and what I expect from a fair science-oriented article isn't some counting of heads who don't really know what to say and most of whom are affected by their environment but rather a balanced presentation of the arguments in favor or against the propositions that are being made.

So when it is being said that the dark matter is likely to be discovered in years or earlier, the only "catch" could be if someone has good enough arguments that this proposition and the known arguments supporting it are wrong. If such arguments aren't available on the market, the author of the popular article isn't obliged to report any "votes" to the contrary or struggle to count the "supporters" of a particular proposition. In science, where there are no arguments, the counting of heads is simply irrelevant. It's unfortunate if Matt disagrees with this proposition. And I am kind of scared by his recommendation to write about "mavericks". It's absolutely not a science writers' business to label scientists "mainstream people" and "mavericks" or "seers" and "craftsmen" or to attach any other emotionally loaded, propagandist adjectives to scientists and theories, for that matter. Their task is to impartially inform about the known theories and conceivable proposed theories, about the discoveries and exclusions.

I would quantify my subjectively felt probability that the dark matter particle will be discovered in 5 years as 70 percent. It's more likely than not and I think that if you start with this number, it's already a remarkable situation and it justifies the title at Space.com, too.

Matt is right that the crucial property of the dark matter particle that decides about its looming (non)discovery is whether it has nonzero non-gravitational i.e. detectably strong interactions. If it only interacts gravitationally, we will probably not detect the particle directly here on Earth. I would say the odds may be 20% that the dark matter is composed of purely gravitationally interacting particles, 10% that it is not composed of isolated particles at all (including the possibility that there's no dark matter and the right explanation of the galactic curves etc. is totally different).

This still leaves those 70% for the "detectable" scenarios and if you cover some parameter spaces by some expected distributions, you will see that it's extremely likely that the experiments will see it over there. And I think it's legitimate for popular science writers to write about these expectations because they're exciting and science fans in the public should simply be interested in them – and, in fact, many of them are surely interested in them.

Matt Strassler usually writes about the events "half a year after the funeral" so it's unlikely that the people who want to know "what is shaking" are spending too much time with his blog. He didn't understand that the OPERA neutrino claims had been proved invalid even 3 months after everyone knew that they had been invalid, and he had to wait for July 2012 although all insiders knew that because of the (combined) 4+ sigma bumps at the LHC and other arguments, the existence of the 125+ GeV Higgs boson had been known since December 2011.

There's one important point, completely ignored both by Moskowitz and Strassler, namely that several experiments are claiming that they have already detected a dark matter particle. The "Is Dark Matter Seen" war has brought us lots of battles and salvos that were covered on this blog. I think that the probability may be something like 40% that the positive side's claims are right and the great discovery has already been made. Of course that if these claims are right, it's very likely that the claims will get settled by "significantly stronger" experiments that will supersede the existing ones in coming years.

Let me dedicate a few words to the title. Recall that Moskowitz's title was

"Dark Matter Mystery May Soon Be Solved."

Strassler proposes the following alternative with a supportive slogan:

"Searchers for Dark Matter Optimistic About Near Term."  That at least would have been undeniable.

I actually think that Moskowitz's title – and even her original title, "Dark Matter Running Out of Places to Hide", is more informative, fair, and impartial than Matt's proposed replacement. Her title informs us that the following years are expected to have an unusually high probability per year to find the dark matter particle which is true. She isn't claiming it's certain or anything of the sort.

The reason why I find Matt's alternative title problematic is that the word "optimistic" introduces emotional biases. Clearly, he relates "optimism" to the "discovery". But that's not how science should proceed. As Moskowitz's title described more correctly, the scientists' task is to answer questions and solve mysteries and have no a priori "preferences" about what the answers should be. So even though the discovery of a new particle brings more fame to the experimenters than a null result (and meaningful experimental projects should have high enough probabilities of finding a result that isn't null), it's just not a job for the "searchers" to be biased and prefer one result over another. If they find out that the dark matter can't exist in the range of parameters that their experiment may probe, i.e. if they can rule out some precise types of the dark matter particle, it's an insight and a contribution to science, too. So as long as their experiment will work, they should be optimistic, whatever the results of the experiment will be!

If you understand the scientific optimism in the impartial way, Matt's proposed title is vacuous. Moskowitz's title has beef and the beef reproduces the scientific arguments on the market.