Modern physics has explained some of the most complex features of our Universe, yet the field has not been able to reach a single, all-encompassing theoretical framework, the almost mythical “Theory of Everything”. This theory should be able to describe all that exists in the universe and finally find an agreement between quantum mechanics and general relativity. String theory is often suggested as a possible solution for this universal explanation, but it is very difficult to test. 

In string theory, every particle is a minuscule vibrating string, with the ways the string vibrates corresponding to a particular mass, charge, etc. The mathematical framework allows for this to work as a "Theory of Everything" but we can't test for the existence of the strings directly. On top of that, string theory is not a single theory but one with allowed ranges, each with certain predictions for the existence of certain facets of our universe. Testing these particular predictions is necessary to decide which aspects are correct and which are not. However, most aspects of the theory are beyond our current ability to investigate. Luckily for us, we are being given a cosmic hand.

Researchers using NASA's Chandra X-ray Observatory have tested for the existence of specific particles predicted to exist in some (but not all) models of string theory. These particles are supposed to be tiny, on the order of 100 billionths of the mass of an electron, and can be useful not just in string theory but to explain dark matter.

As reported in The Astrophysical Journal, the team looked at a galaxy cluster known for its energetic X-ray photons (particles of light). Einstein's famous E=mc² equation notes that matter is equivalent to energy. This means that it is possible to not only turn matter into energy, like what happens in a nuclear reactor, but also energy (in this case photons) can be used to form particles. In particular, the X-ray photons are energetic enough and abundant enough that they could potentially turn into axions. So if these particles exist, we should be able to see them.

The paper did not detect any axions for a particular range of energies. While this is not a complete blow to the whole of string theory, it reduces the possible versions that are correct in our Universe.

"Our research doesn't rule out the existence of these particles, but it definitely doesn't help their case," co-author Helen Russell of the University of Nottingham in the UK, said in a statement. "These constraints dig into the range of properties suggested by string theory, and may help string theorists weed their theories."

The work has three possible interpretations: The axions don’t exist, they exist but they do not turn into photons as much as predicted, or they exist but have different ranges of mass.

It is an exciting question with no solution at the moment, but future X-ray telescopes might help with an answer.

"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," added Christopher Reynolds of the University of Cambridge in the United Kingdom, who led the study. "If these particles are eventually detected it would change physics forever."