How do you think the dark matter problem is solved?” Vera C. Rubin urgently asked me, within minutes of being introduced at a 2009 Women in Astronomy conference. To this day, I can’t remember what I said in response. I was awestruck: the famed astronomer who had won the National Medal of Science for her work finding the first conclusive evidence for dark matter’s existence was asking me, a twentysomething Ph.D. student, for my opinion.
So far none of these strategies has turned up the missing matter. We still don’t know if dark matter can talk to regular matter in any way beyond gravity. It may be impossible to produce in the accelerators we can build or to detect in the experiments we can construct. For this reason, astronomical observations—cosmic probes of dark matter—are one of our best hopes.
WIMPs were the most highly favored dark matter candidates for a long time, particularly in the U.S. Opinions have shifted in recent years, though, as evidence for WIMPs has failed to show up at the Large Hadron Collider or in any of the direct and indirect detection experiments. Axions had been proposed in the 1970s by Hertzberg’s Ph.D. adviser at M.I.T., Frank Wilczek, one of the first to realize that one consequence of a model proposed by Helen Quinn and the late Roberto Peccei was a particle, which Wilczek named “axion” after a brand of laundry detergent. Thus, Hertzberg was already quite familiar with axions. I, on the other hand, was relatively new to this idea. I had spent most of my career focused on other questions, and I had to get up to speed.
The same year our paper came out, another group was looking into other interesting implications of axionlike particles. A team led by Hsi-Yu Schive of National Taiwan University published computer simulations of certain axionlike particles that are often referred to as “ultralight axions” or “fuzzy dark matter,” so named because they have a very low mass and would act like blurred-out waves rather than pointlike particles.
Clues in the Sky In astronomy we are relatively passive observers. We can choose our instruments, but we cannot design a galaxy or a stellar process and watch it unfold. Cosmic phenomena rarely happen on human-friendly time scales—galaxy formation takes billions of years, and the cosmic processes that might emit dark matter particles do so over tens to hundreds of years.
We can also learn more about the nature of dark matter by studying the best evidence we have for its existence so far—the cosmic microwave background radiation. This light is a radio signal that originated in the early universe, and it is inescapably everywhere, all around us. It provides a snapshot of a moment early in cosmic history, and the patterns we see in the frequencies of its light reflect the makeup of the universe when it was created.
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