When astronomers first peered at the cosmos in microwave light, they knew they had stumbled on a window into the universe’s earliest moments. After all, the cosmic microwave background—that hazy afterglow of the big bang released when the universe was a mere 380,000 years old—has allowed scientists to answer fundamental questions about where we came from. But microwave light has also raised an intriguing mystery closer to home. In 1996 astronomers noticed an inexplicable excess of microwaves emanating from our own galaxy. For over 20 years, this so-called anomalous microwave emission has remained an enigma—until today. A new study published in Nature Astronomy suggests spinning nano-diamonds might be the culprit.
Ten years ago, while studying nascent planetary systems forming in whirling disks of gas and dust around young stars, Cardiff University astronomer Jane Greaves noticed a few of those systems seemed to be faintly glowing with microwaves. She initially attributed the glow to flaws in her data but later reconsidered after hearing a colleague’s talk about anomalous microwave emission. Returning to the telescope, she and her collaborators monitored 14 young star systems for mysterious microwave emissions, finally finding three radiating that telltale glow. Those same three systems, it turns out, are also the only three within Greaves’s sample known to host nano-diamonds—pint-size, pyramid-shaped crystals containing only hundreds of carbon atoms, all sheened with an atoms-thin gloss of frozen hydrogen likely accumulated from the interstellar medium. “This really is a clue of nature telling us nano-diamonds are what is responsible” for the anomalous microwaves, she says.
But how can objects so tiny emit microwaves so mighty that they can be glimpsed across hundreds of thousands of light-years? The trick is that our galaxy is a turbulent place, in which tides and winds raised by the motions and activities of stars make any small object—be it a puny dust grain, a hefty molecule or even a wee diamond—jiggle and spin as it is jostled by other particles bumping into it. Should that object possess an asymmetrical electric charge (where one side has slightly more charge than the other), its spin could emit electromagnetic radiation in the form of microwaves. Disks around newborn stars host particularly speedy particles, further amplifying this effect.
Initially, astronomers suspected the minuscule objects responsible for the glow were organic molecules called polycyclic aromatic hydrocarbons (PAHs)—essentially, the cosmic equivalent of soot, albeit produced by aging stars rather than smokestacks. Bruce Draine, an astronomer at Princeton University who was not involved in the study, had been a proponent of PAHs as the leading candidates for the microwave anomaly, but he knew that explanation lacked proof. So, he and his colleagues set out to find it, comparing distribution maps of PAHs and the anomalous microwaves throughout the Milky Way. An overlap between the high-density and low-density regions on both maps would have been smoking-gun evidence that PAHs were the culprit. “To our surprise, no such connection was seen,” Draine says. His 2016 study declared PAHs innocent, and the emission became a mystery once again. At least, that is, until Greaves and her colleagues reported their new findings. Draine finds the nano-diamond hypothesis appealing, but notes the correlation between the three stars’ microwave glows and nano-diamonds in their disks might be mere coincidence.
Although Greaves and her colleagues calculated the odds of a chance association at just 0.01 percent, that calculation assumes all the stars were observed on equal footing, without any possible bias. But Aigen Li, an astronomer at the University of Missouri–Columbia who did not take part in the work, worries a bias may in fact exist due to the fact not all stars are the same temperature. Nano-diamonds are usually only visible to Earthbound astronomers when they circle extremely hot stars, he says, which means there could easily be other nano-diamond–hosting stars within Greaves’s sample that fail to emit anomalous microwaves. Clive Dickinson, an astronomer at the University of Manchester in England, also not involved in the work, expresses similar concerns. He argues hot stars tend to ionize the gas around them to create plasma—clouds of charged particles that can also emit microwave radiation as they whiz through their orbits around the star. Without very careful modeling of this effect, it could lead to a case of mistaken identity and be associated with anomalous microwave emission. “Assuming that has been done correctly, then this is quite exciting—it’s quite a cool result,” Dickinson says.
To bolster their nano-diamond hypothesis, Greaves and her team next will try to observe both the anomalous microwave emission and nano-diamonds in colder, less-suspect environments, such as the frigid clouds of interstellar gas and dust that dot our galaxy.
If ultimately validated as the true source of anomalous microwaves, maps of nano-diamonds throughout the Milky Way will become crucial for scientists hoping to scrub out its contaminating effects to perform deeper, more precise studies of the cosmic microwave background, revealing untold secrets of the universe’s genesis. In some sense, says co-author Anna Scaife, an astronomer at Manchester, the microwave anomaly’s vital importance for such studies makes astronomers’ past disregard of nano-diamonds as its source all the more surprising. “A lot of the time in astrophysics we’re narrowing down the details of things where we already understand the big picture whereas this is a completely new association,” she says. “This really is a step-change in our thinking, rather than just an incremental advance.”