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 string theory predicts should exist. While the resulting non-detection does not rule out string theory altogether, it does deliver a blow to certain models within that family of ideas.
“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 Professor Christopher Reynolds of Cambridge's Institute of Astronomy, who led the study. “If these particles are eventually detected it would change physics forever.”
The particle that Reynolds and his colleagues were searching for is called an axion. These as-yet-undetected particles should have extraordinarily low masses. Scientists do not know the precise mass range, but many theories feature axion masses ranging from about a millionth of the mass of an electron down to zero mass. Some scientists think that axions could explain the mystery of dark matter, which accounts for the vast majority of matter in the universe.
One unusual property of these ultra-low-mass particles would be that they might sometimes convert into photons, or particles of light, as they pass through magnetic fields. The opposite may also hold true: photons may also be converted into axions under certain conditions. How often this switch occurs depends on how easily they make this conversion, in other words on their 'convertibility.'
Some scientists have proposed the existence of a broader class of ultra-low-mass particles with similar properties to axions. Axions would have a single convertibility value at each mass, but 'axion-like particles' would have a range of convertibility at the same mass.
Image: Perseus: A galaxy cluster located about 240 million light years from Earth
Credit: NASA/CXC/Univ. of Cambridge/C. Reynolds et al
Reproduced courtesy of the University of Cambridge