In physics, there's an idea about the universe — some more theoretical than others — that every force, particle, and interaction is connected in one network described by a single, elegant theory. String theory is one popular proposal for such a "theory of everything," and — while there are many takes on string theory throughout the physics community — few exist with experimental bases.
Axions are crucial for string theory
Researchers of the study sought a unique particle called "axion." No one had yet detected these particles of exceptionally low mass. No one even knew the range of masses these particles should span. But — while non-detection doesn't cancel their theoretical viability completely — it strikes a critical blow to some models within the family of ideas about string theory.
"Until recently I had no idea just how much X-ray astronomers bring to the table when it comes to string theory, but we could play a major role," said the University of Cambridge's Christopher Reynolds, who led the study. "If these particles are eventually detected it would change physics forever."
One wild property of these ultra-low-mass particles is their capacity to sporadically convert into photons (they mean light) as they (or it) move(s) through magnetic fields. But the opposite happens too: sometimes circumstances turn photons back into axions. How often conversions happen depends on their "convertibility."
However, some scientists have suggested the existence of a broader class of ultra-low-mass particles — similar in properties to axions — but more flexible.
Axion-like particles, the flexible alternative
Where ordinary axions would have one lone convertibility value at each specific mass, these alternative "axion-like particles" would display a range of convertibility values at the same mass.
"While it may sound like a long shot to look for tiny particles like axions in gigantic structures like galaxy clusters, they are actually great places to look," said David Marsh, co-author of the study, from Stockholm University in Sweden, according to phys.org. "Galaxy clusters contain magnetic fields over giant distances, and they also often contain bright X-ray sources. Together these properties enhance the chances that conversion of axion-like particles would be detectable."
Throwing shade on the 'theory of everything'
Looking for signs of axion-like particle conversion, the team of astronomers assessed more than five days of Chandra observations of X-rays from material falling helplessly toward the supermassive black hole at the center of the Perseus galaxy cluster. The team's study of the bright X-ray source unveiled a spectrum with a sensitivity at which distortions were predicted to occur in the presence of axion-like particles.
Since no distortions of this kind were detected, the researchers ruled out the presence of most kinds of axion-like particles in the requisite mass range — roughly one-millionth of a billionth of an electron's mass.
While their research doesn't rule out the existence of axion-like particles completely, this represents an "underrated tweet" of science: progress made by disconfirming scientific theories is just as important as (although less exciting than) affirmative discoveries. It will be less exciting, but more significant when scientists declare the COVID-19 coronavirus is no longer a threat — than it was when the novel virus first swept across the planet.