From NASA Chandra: “Chandra Data Tests ‘Theory of Everything'”

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Astronomers used Chandra to perform a test of string theory, a possible “theory of everything” that would tie all of known physics together.

The researchers were looking for a type of particle known as an “axion” and other similar particles.

Galaxy clusters with their strong magnetic fields and X-ray emission can be excellent places to search for evidence for axions.

The team looked at the Perseus galaxy cluster for over 5 days with Chandra, but did not find signals of any axion-like particles.

Astronomers using NASA’s Chandra X-ray Observatory have made one of the first experimental tests of string theory, a set of models intended to tie together all known forces, particles, and interactions. As described in our latest press release, researchers used Chandra to look for signs of an as-yet undetected particle predicted by string theory. The lack of a detection in these Chandra observations helps rule out some versions of string theory.

The team looked for extraordinarily low-mass “axion-like” particles in the Perseus galaxy cluster, shown in a Chandra image in the main panel of this graphic (red, green and blue colors are low, medium and high X-ray energies respectively). Galaxy clusters, the largest structures in the Universe held together by gravity, offer an excellent opportunity to search for these particles. In a galaxy cluster, X-ray photons from an embedded or a background source can travel through a large amount of hot gas permeated with magnetic field lines. Some of the X-ray photons may undergo conversion into axion-like particles, or the other way around, along this journey. A simplified illustration shows this process, with shorter wavelength X-ray photons (in blue) converting into axion-like particles (yellow) and back to photons, as they travel across magnetic field lines (grey) in the cluster. Longer wavelength X-ray photons (red) are converting into axion-like particles, but not back into photons. Such conversions would cause a distortion in the X-ray spectrum (the amount of X-rays at different energies) of a bright or embedded source of X-rays.

Illustration Credit: Amanda Smith/Institute of Astronomy/University of Cambridge

Astronomers obtained a long Chandra observation, lasting over five days, of the central supermassive black hole in the center of the Perseus galaxy cluster (shown in the inset.) The spectrum of the region around the black hole showed no distortions, allowing the team to rule out the presence of most types of axion-like particles in the relatively low mass range their search was sensitive to.

Here the Chandra spectrum (red) of Perseus’ central black hole shows the intensity of X-rays as a function of X-ray energy, along with an example (black) of a model X-ray spectrum predicted if axion-like particles were actually being converted from and into photons. To highlight the distortions that could have been detected, the data divided by the example model are also shown.

Credit: NASA/CXC/Cambridge Univ./C.S. Reynolds

One possible interpretation of this work is that axion-like particles do not exist. Another possible interpretation is that the particles undergo conversion from and into photons less easily than some particle physicists have expected. They also could have higher masses than probed with the Chandra data.

There has been a surge of interest in studies of these particles in recent years for three reasons: First, despite a lot of work, there continues to be no detection of Weakly Interacting Massive Particles (WIMPs), either with gamma-ray observations, or earth-based experiments that could explain the nature of dark matter. These particles are predicted to interact with normal matter only via the weak force, and have been considered to be one of the strongest candidates for dark matter. Secondly, scientists have realized that axions and axion-like particles are predicted by string theory. Finally, there are a large number of experiments or observations that can be done to search for these particles.

A paper describing these results appeared in the February 10th, 2020 issue of The Astrophysical Journal. The authors are Christopher Reynolds (University of Cambridge, UK), David Marsh (Stockholm University, Sweden), Helen Russell (University of Nottingham, UK), Andrew C. Fabian (University of Cambridge), Robyn Smith (University of Maryland in College Park, Francesco Tombesi (University of Rome, Italy), and Sylvain Veilleux (University of Maryland).

March 19, 2020
Media contacts:
Megan Watzke
Chandra X-ray Center, Cambridge, Mass.

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NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.