Preposterous Universe

Friday, May 07, 2004
Dark Light

Nature has a feature known as "concepts essays," in which they ask highly respected (or at least "willing") scientists to write short reflective pieces about specific concept of importance to their field. The idea is to go slightly beyond a standard pedagogical introduction to a subject and allow for the kind of discussions that scientists might have around coffee but would never put into a journal article. (I.e. it's an old-media version of a blog.)

I was invited to write such an essay about dark matter, which has now appeared. (That's a pdf version on my site; there is an html version on the Nature site, but it's not as pretty and might require registration.) I couldn't help but mention dark energy as well, so in the final version the "concept" includes both dark matter and dark energy. (Here's a very short intro to both subjects.)

The idiosyncratic angle I chose to take was to ask how interesting the dark sector could be -- in particular, whether there could be interactions between dark matter particles (or dark matter and ordinary matter, or dark matter and dark energy) that might allow some sort of structures to form, even intelligent life. We might ask, for example, whether there could be some weakly-coupled massless abelian gauge boson that mediates interactions between dark matter particles: "dark light."

As I say in the article, probably not. We don't know as much as we would like about the distribution of dark matter, but we do know something, and it appears to be much more smoothly distributed than the ordinary matter in the universe. See for example this computer reconstruction of the dark matter density in a cluster of galaxies, using gravitational lensing. The simple explanation for this smoothness is that dark matter is probably collisionless. When atoms of ordinary matter bump together, they can emit light and cool, and this dissipation process allows the ordinary stuff to condense into the center of galaxies. But the interactions that give rise to dissipation are exactly those necessary for making structures and life. Of course, all we have are upper limits; it's still possible that there is life out there in the dark matter, but characterized by much larger sizes and much longer timescales than anything in our experience. Perhaps a dark heartbeat takes millions of years to complete.

Now I am wondering whether the goofy illustration chosen by the editors to accompany the article (shown above) might feature the most dramatic decolletage ever to appear in a major scientific journal. Anyone have any other candidates?

Ideas on culture, science, politics.
Sean Carroll

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