Dark matter appears to explain the bulk of matter in our cosmos. While it has clearly been observed to exert influence over normal matter – nuclei, electrons, light – through gravitation, it has never been observed through any other interaction. Dark matter does not appear to directly emit light, nor respond to light; it has never been conclusively observed to scatter off a nucleus in a terrestrial detector, so far ruling out interactions through the strong or weak nuclear forces. A great deal of effort is therefore spent speculating on its properties, in order to design experiments that can measure them.
I’ve been thinking about freeze out, a property of particles which is often exploited in order to make statements about dark matter. “Freeze out” is when a particle species is unable to interact with itself or other matter because either the probability of an interaction is very small or because the likelihood of running into other matter is very small, or both. One can think of freeze out in a very human way.
Let’s consider a social situation, in which you are invited to a party where you don’t know anybody (except maybe the host). Let’s also consider a parameter, your “social cross-section”, which simply is a measure of how willing you are to approach a complete stranger and begin, then sustain, a conversation.
At the beginning, the party is hot. People are densely packed, the music is loud and thumping, you’re walking around with a drink in your hand and bumping into complete strangers. If you have a high social cross-section, you’ll take every bump and jab as a chance to flash a smile and start a conversation. With a sufficiently high social cross-section, you’ll dance with people and even as the party cools off (time goes on, people leave, the music gets less interesting) you’ll continue to stick with party-goers and socialise. You’ll keep meeting up with people, running into them, dancing, forming small social cliques of equally interactive people. Even when the party is completely wound down, you’ll stick with the hard-core types, the most social – maybe even go and have an after-party somewhere else. A high social cross-section can overcome the cooling effect on the party, allowing people to stick together and form new relationships despite a thinning population density at the party.
Consider instead a person with a low social cross-section. They may be comfortable starting a conversation but not sustaining it past small-talk. They certainly don’t like to dance, unless they really get prodded into it. Maybe they really don’t even like to start conversations but they feel obliged to go to the party because of the host. So they go, and the party starts off real hot. They get bumped and jostled, drink spilling, and maybe they flash a smile and start a conversation. But the interactions are weak and rare, sustained in frequency only by the density of the people at the party. Eventually the party cools, people start leaving, high social cross-section folks are cliquing and grouping. The low cross-section people are off by themselves, tapping a foot by the side of the dance floor or standing by the buffet, but not forming new relationships.
This is dark matter, and this is how dark matter “freezes out” of interactions. When the party is hopping, it almost doesn’t matter what your cross-section is. You’re caught up in the fray, forced to interact by the sheer number density of party-goers. But as the party cools and the crowd thins, the inclination to interact remains the same but the opportunity drops precipitously. Now imagine a party where 85% of the party-goers left over when the cooling happens are low cross-section types. That’s our universe. High cross-section people are forced to clump and group in between the low cross-section types, forming little groups in this sea of non-interacting people. Welcome to our cosmos. Nerds rule!
Of course, the trick here is that it’s possible that the low cross-section types are only low on verbal and physical communication. Imagine a party where the low cross-section types are super-techy, and prefer to interact through social media. They’re standing around, mobile phones out and tapping away on their Facebook or Twitter streams, while the high cross-section people have phones put away and are gabbing and laughing and dancing. “This party is lame,” tweets one low cross-section type. “Let’s leave,” types another. “But the dip is good and the beer is free,” says a third. “Good call, let’s stay,” says a fourth. They are actually communicating all the time but the talkers are unaware, because they are not tapped into this social stream.
The promise of studying the dark sector, dark matter and all of its friends, is that we might tap into how they communicate with one another. Maybe every now and then one of the low cross-section types laughs at a Tweet, and a nearby high cross-section type hears them and asks, “What’s so funny?” “Oh, it’s just that my friend PwnN00bs1984 noticed that girl in the corner is so drunk she is mistaking some random guy for the guy she came in with.” “That IS funny,” says the high cross-section type, suddenly exposed to a whole conversation going on behind the scenes.
Let’s hope that eventually, we find a way to tap into the dark sector’s own little conversation. Maybe, then, we’ll figure out what 85% of these people are doing at this party.