Dark Matter Around the Corner?

The meaning of the title of Clara Moskowitz’s new article for the public, “Dark Matter Mystery May Soon Be Solved“, all lies in the word “may”.  It may.  It may not.

According to the article, “the answer to this cosmic mystery could come within the next three or four years, scientists say.”

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.   And “the answer… could come…”? Interpreted literally it is content-free: yes, the answer could come in the next few years, or not — but you don’t need any scientists to tell you that.  If one interprets it more optimistically — that it is intended to imply that the answer will very likely come within the next three or four years — then I think it is far from clear what fraction of the experts will agree with that statement.

Rather than debate the claim, let’s start with the physics.  What will determine how long it takes to discover what dark matter is made from?

Well, it is possible this has already started to happen; remember this hint of a discovery of dark matter?  This is still alive; here’s a recent update from the Resonaances blog.  Whether this is a real signal of dark matter or an experimental problem or subtlety, only time will tell. But such a discovery could occur, in the optimistic scenario that dark matter particles are abundant and that, when they find each other near the center of the galaxy, they often annihilate in spectacular fashion, to a pair of photons.  It would almost be — and it may be — too good to be true.

Alternatively, an extremely pessimistic scenario is possible: that dark matter is made from something that only interacts with ordinary matter via gravitational forces, which are exceedingly weak.  In this case there is no possibility that any of the on-going searches for dark matter particles will turn up anything, because all of them assume, by necessity, that the particles they are looking for do interact with ordinary matter in some much stronger, non-gravitational fashion.

And even if dark matter is made from particles that do interact with ordinary matter in a non-gravitational way, how long it takes to find them will depend on how rare they are, and on how strong those non-gravitational forces are.  We know roughly how much dark matter there is out in the universe, but (assuming it is even made from particles, which we don’t know for sure) it might be made from a great abundance of low-mass particles or a lesser abundance of particles with a large mass.  All other things being equal, it’s harder to find the particles if there are fewer of them around.  And we can infer how these particles will interact with ordinary matter only by making certain simplifying assumptions about how the dark matter particles were left behind as the universe cooled during the Big Bang.  These assumptions are just that: assumptions, ones which are intelligent but not necessarily correct.

So we can’t guess whether insights into the nature of dark matter have already been obtained, whether they won’t be obtained for centuries, or whether the next few years will dramatically change our knowledge.

What is true — and this is really the content, properly interpreted, of the article I referenced at the beginning — is that the next few years will allow for some of the most popular possibilities for the nature of dark matter to be ruled out or confirmed.  (If you read the last half of the article, you’ll find this is actually its meaning, despite the optimistic words in the first half of the article.) Of course, just because something is popular doesn’t mean it is more likely.  And that means that when you’ve ruled out the popular options, you can’t assume the unpopular options are wrong; you still have to go check.  [You've heard me say this, even recently, about supersymmetry, extra dimensions, and the like; that's because it's always true when you're searching.  Notice what Lance Dixon says in the article accords with this point of view.]  So although one can be hopeful, the stark reality is that this process could take a few years, decades, or millenia.

As Dan Bauer says at the end of the article, “I guess it’s the natural optimism of physicists to think [dark matter] is something we might actually be able to find.”  I have nothing against optimism, as long as we don’t confuse our own optimism that nature behaves in such and such a way for high likelihood that nature behaves that way. Optimism is a good thing: most of the things we look for — decay of the proton, neutrino oscillations, dark matter, etc. — don’t come with a guarantee that they are discoverable.  [The Higgs particle was very unusual; we had a very strong reason to believe it or something like it had to be discoverable at the Large Hadron Collider, because without it our equations for the already-known particles were inconsistent.  We are rarely so confident.]  Pessimism would keep us from doing these experiments; and although most such experiments do come up empty (proton decay, for instance — at least so far), a few exceptional ones do not (neutrino oscillations) and they permanently change our understanding of the world.  Maybe dark matter will be discovered soon, maybe it won’t; but it is a good thing that we’re looking for it.   And maybe a better title for Ms. Moskowitz’s article would have been “Searchers for Dark Matter Optimistic About Near Term.”  That at least would have been undeniable.