More than 20 years ago, NASA's COBE (Cosmic Background Explorer) spacecraft captured the first full-sky map of cosmic microwave background radiation still visible from the Big Bang. It was an enlightening moment that also came with a riddle: some parts of the microwave frequencies were inexplicably brighter than others.
Astronomers puzzled over the source behind these "anomalous microwave emissions (AME)," suspecting they were linked to organic compounds known as polycyclic aromatic hydrocarbons (PAH). Abundant throughout the universe, PAHs are associated with not only star and exoplanet formation, but also theorized as a possible starting material for the earliest forms of life. But doubts were cast on the link to the bright patches when researchers discovered that the unusual microwave radiation didn't always coincide with dust clouds rich in PAHs.
A multifrequency all-sky image of microwave radiation as composed using data from the ESA's Planck spacecraft. The central band is the plane of our galaxy. A large portion of the image is dominated by the diffuse emission from its gas and dust. (Photo: ESA/ LFI & HFI Consortia)
Glitterati in the cosmos
A chance observation by researchers at Cardiff University and The University of Manchester may have finally solved the mystery. The curious microwave radiation is not derived from organic compounds but by clouds of fine diamond dust encircling newly formed stars.
"This is a cool and unexpected resolution to the puzzle of anomalous microwave radiation," Dr. Jane Greaves, co-author of a new study published in Nature Astronomy about the discovery, said in a statement. "It's even more interesting that it was obtained by looking at protoplanetary disks, shedding light on the chemical features of early solar systems, including our own."
Greaves and her team came across the surprising diamond correlation while studying the rings of dust and gas surrounding 14 young stars located some 500 light-years from Earth. Three of the 14 stars were found to not only produce AMEs, but were also the only ones to carry the IR spectral signature of hydrogenated nanodiamonds. "In fact, these are so rare," Greaves noted, that "no other young stars have the confirmed infrared imprint."
The diamond particles swirling around these stars are so small, 0.75 to 1.1 millionths of a millimeter long, that they spin exceptionally fast, emitting electromagnetic radiation in the microwave range that allows it to stand out from other wavelengths.
In an interview with The Guardian, Greaves said that despite their individually miniscule size, these diamond particles stacked together would likely equal the mass of the planet Mercury.
Why diamonds are a cosmologist's best friend
The discovery of the AME's source is likely to have a big impact on efforts to further isolate a more complete picture of the universe's early beginnings.
"This is good news for those who study polarization of the cosmic microwave background, since the signal from spinning nanodiamonds would be weakly polarized at best," Brian Mason, an astronomer at the National Radio Astronomy Observatory and coauthor on the paper said in a statement. "This means that astronomers can now make better models of the foreground microwave light from our galaxy, which must be removed to study the distant afterglow of the Big Bang."
As co-author Anna Scaife from Manchester University concluded, it's also just really cool.
"It is an exciting result," she said. "It's not often you find yourself putting new words to famous tunes, but 'AME in the Sky with Diamonds' seems a thoughtful way of summarizing our research."