HIDDEN DARK MATTER? Warped galaxy clusters could indicate the presence of dark matter.
An invisible civilization could be living right under your nose.
By Lisa Randall
Even though we know that ordinary matter accounts for only about one-twentieth of the universe’s energy and a sixth of the total energy carried by matter (with dark energy constituting the remaining portion), we nonetheless consider ordinary matter to be the truly important constituent. With the exception of cosmologists, almost everyone’s attention is focused on the ordinary matter component, which you might have thought to be largely insignificant according to the energy accounting.
We of course care more about ordinary matter because we are made of the stuff—as is the tangible world in which we live. But we also pay attention because of the richness of its interactions. Ordinary matter interacts through the electromagnetic, the weak, and the strong nuclear forces—helping the visible matter of our world to form complex, dense systems. Not only stars, but also rocks, oceans, plants, and animals owe their very existence to the nongravitational forces of nature through which ordinary matter interacts. Just as a beer’s small-percentage alcohol content affects carousers far more than the rest of the drink, ordinary matter, though carrying a small percentage of the energy density, influences itself and its surroundings much more noticeably than something that just passes through.
Familiar visible matter can be thought of as the privileged percent—actually more like 15 percent—of matter. In business and politics, the interacting 1 percent dominates decision making and policy, while the remaining 99 percent of the population provides less widely acknowledged infrastructure and support—maintaining buildings, keeping cities operational, and getting food to people’s tables. Similarly, ordinary matter dominates almost everything we notice, whereas dark matter, in its abundance and ubiquity, helped create clusters and galaxies and facilitated star formation, but has only limited influence on our immediate surroundings today.
It seems very odd to assume that all of dark matter is composed of only one type of particle.
For nearby structure, ordinary matter is in charge. It is responsible for the motion of our bodies, the energy sources that drive our economy, the computer screen or paper on which you are reading this, and basically anything else you can think of or care about. If something has measurable interactions, it is worth paying attention to, as it will have far more immediate effects on whatever is around.
In the usual scenario, dark matter lacks this type of interesting influence and structure. The common assumption is that dark matter is the “glue” that holds together galaxies and galaxy clusters, but resides only in amorphous clouds around them. But what if this assumption isn’t true and it is only our prejudice—and ignorance, which is after all the root of most prejudice—that led us down this potentially misleading path?
The Standard Model contains six types of quarks, three types of charged leptons (including the electron), three species of neutrinos, all the particles responsible for forces, as well as the newly discovered Higgs boson. What if the world of dark matter—if not equally rich—is reasonably wealthy too? In this case, most dark matter interacts only negligibly, but a small component of dark matter would interact under forces reminiscent of those in ordinary matter. The rich and complex structure of the Standard Model’s particles and forces gives rise to many of the world’s interesting phenomena. If dark matter has an interacting component, this fraction might be influential too.
If we were creatures made of dark matter, we would be very wrong to assume that the particles in our ordinary matter sector were all of the same type. Perhaps we ordinary matter people are making a similar mistake. Given the complexity of the Standard Model of particle physics, which describes the most basic components of matter we know of, it seems very odd to assume that all of dark matter is composed of only one type of particle. Why not suppose instead that some fraction of the dark matter experiences its own forces?
In that case, just as ordinary matter consists of different types of particles and these fundamental building blocks interact through different combinations of charges, dark matter would also have different building blocks—and at least one of those distinct new particle types would experience nongravitational interactions. Neutrinos in the Standard Model don’t interact under the strong or electric force yet the six types of quarks do.
No one had allowed for the very simple possibility that although most dark matter doesn’t interact, a small fraction of it might.
In a similar fashion, maybe one type of dark matter particle experiences feeble or no interactions aside from gravity, but a fraction of it—perhaps 5 percent—does. Based on what we’ve seen in the world of ordinary matter, perhaps this scenario is even more likely than the usual assumption of a single very feebly or non-interacting dark matter particle.
People in foreign relations make a mistake when they lump together another country’s cultures—assuming they don’t exhibit the diversity of societies that is evident in our own. Just as a good negotiator doesn’t assume the primacy of one sector of society over another when attempting to place the different cultures on equal footing, an unbiased scientist shouldn’t assume that dark matter isn’t as interesting as ordinary matter and necessarily lacks a diversity of matter similar to our own.
The science writer Corey S. Powell, when reporting on our research in Discovermagazine, started his piece by announcing that he was a “light-matter chauvinist”—and pointing out that virtually everyone else is too. By this he meant that we view the type of matter we are familiar with as by far the most significant and therefore the most complex and interesting. It’s the type of belief that you might have thought was upended by the Copernican Revolution. Yet most people persist in assuming that their perspective and their conviction of our importance are in keeping with the external world.
Ordinary matter’s many components have different interactions and contribute to the world in different ways. So too might dark matter have different particles with different behaviors that might influence the universe’s structure in a measurable fashion.
When first studying partially interacting dark matter, I was astonished to find that practically no one had considered the potential fallacy—and hubris—of assuming that only ordinary matter exhibits a diversity of particle types and interactions. A few physicists had tried to analyze models, such as "mirror dark matter," which features dark matter that mimics everything about ordinary matter. But exemplars such as this one were rather specific and exotic. Their implications were difficult to reconcile with everything we know.
A small community of physicists had studied more general models of interacting dark matter. But even they assumed that all the dark matter was the same and therefore experienced identical forces. No one had allowed for the very simple possibility that although most dark matter doesn’t interact, a small fraction of it might.
You have no idea how cute dark matter life could be—and you almost certainly never will.
One potential reason might be apparent. Most people would expect a new type of dark matter to be irrelevant to most measurable phenomena if the extra component constitutes only a small fraction of the dark matter inventory. Having not even observed the dominant component of dark matter, concerning oneself with a smaller constituent might seem premature.
But when you remember that ordinary matter carries only about 20 percent of the energy of dark matter—yet it’s essentially all that most of us pay attention to—you can see where this logic could be flawed. Matter interacting via stronger nongravitational forces can be more interesting and more influential even than a larger amount of feebly interacting matter.
We’ve seen that this is true for ordinary matter. Ordinary matter is unduly influential given its meager abundance because it collapses into a dense matter disk where stars, planets, the Earth, and even life could form. A charged dark matter component—though not necessarily quite as bountiful—can collapse to form disks like the visible one in the Milky Way too. It might even fragment into starlike objects. This new disklike structure can in principle be observed, and might even prove to be more accessible than the conventional dominant cold dark matter component that is spread more diffusely in an enormous spherical halo.
Once you start thinking along these lines, the possibilities quickly multiply. After all, electromagnetism is only one of several nongravitational forces experienced by Standard Model particles. In addition to the force that binds electrons to nuclei, the Standard Model particles of our world interact via the weak and strong nuclear forces. Still more forces might be present in the world of ordinary matter, but they would have to be extremely weak at accessible energies since so far, no one has observed any sign of them. But even the presence of three nongravitational forces suggests that the interacting dark sector too might experience nongravitational forces other than just dark electromagnetism.