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HUBBLE VISUALIZES DARK MATTER

An equally intriguing possibility is that dark matter interacts with dark energy. Most existing theories treat the two as disconnected, but there is no real reason they must be, and physicists are now considering how dark matter and dark energy might affect each other. One hope is that couplings between the two might mitigate some cosmological problems, such as the coincidence problem—the question of why the two have comparable densities. Dark energy is roughly three times as dense as dark matter, but the ratio might have been 1,000 or a million. This coincidence would make sense if dark matter somehow triggered the emergence of dark energy.

Couplings with dark energy might also allow dark matter particles to interact with one another in ways that ordinary particles do not. Recent models allow and sometimes even mandate dark energy to exert a different force on dark matter than it does on ordinary matter. Under the influence of this force, dark matter would tend to pull apart from any ordinary matter it had been interlaced with. In 2006 Marc Kamionkowski of the California Institute of Technology and Michael Kesden, then at the Canadian Institute for Theoretical Astrophysics in Toronto, suggested looking for this effect in dwarf galaxies that are being torn apart by their larger neighbors. The Sagittarius dwarf galaxy, for example, is being dismembered by the Milky Way, and astronomers think its dark matter and ordinary matter are spilling into our galaxy. Kamionkowski and Kesden calculate that if the forces acting on dark matter are at least 4 percent stronger or weaker than the forces acting on the ordinary matter, then the two components should drift apart by an observable amount. At present, however, the data show nothing of the sort.

Another idea is that a connection between dark matter and dark energy would alter the growth of cosmic structures, which depends delicately on the composition of the universe, including its dark side. A number of researchers, including one of us (Trodden) with collaborators Rachel Bean, Éanna Flanagan and Istvan Laszlo of Cornell University, have recently used this powerful constraint to rule out a large class of models.

Despite these null results, the theoretical case for a complex dark world is now so compelling that many researchers would find it more surprising if dark matter turned out to be nothing more than an undifferentiated swarm of WIMPs. After all, visible matter comprises a rich spectrum of particles with multiple interactions determined by beautiful underlying symmetry principles, and nothing suggests that dark matter and dark energy should be any different.