One of the biggest ideas in physics is the possibility that all known forces, particles, and interactions can be connected in one framework. String theory is arguably the best-known proposal for a "theory of everything" that would tie together our understanding of the physical universe.
Despite having many different versions of string theory circulating throughout the physics community for decades, there have been very few experimental tests. Astronomers using NASA's Chandra X-ray Observatory, however, have now made a significant step forward in this area.
By searching through galaxy clusters, the largest structures in the universe held together by gravity, researchers were able to hunt for a specific particle that many models of string theory predict should exist. The researchers were searching for a type of "axion," an as-yet-undetected particle that should have extraordinarily low mass. Some scientists think that axions could explain the mystery of dark matter, which accounts for the vast majority of matter in the universe.
A team of astronomers examined over five days of Chandra observations of X-rays from material falling towards the supermassive black hole in the center of the Perseus galaxy cluster. The long observation and the bright X-ray source gave a spectrum — that is, the amount of X-ray emission at different energies — with enough sensitivity to have shown distortions that scientists expected if axion-like particles were present. However, no signs of axion-like particles were found.
The lack of detection allowed the researchers to rule out the presence of most types of axion-like particles below about a millionth of a billionth of an electron's mass. While the non-detection does not rule out string theory, it does deliver a blow to certain models within that family of ideas. This study, however, does demonstrate that Chandra and future X-ray observatories may have an important role to play in testing this theory of everything.
Revised: May 19, 2022