In the heart of a galaxy cluster 200 million light-years away, astronomers have failed to detect hypothetical particles called axions.
This puts new constraints on how we think these particles work, but it also has some pretty serious implications for string theory and the development of The Theory of Everything, which describes how the physical universe works.
Scientists have come up with some pretty good theories when it comes to understanding how the universe works. One of them is general relativity, which describes how physics works at the macro level. The other is quantum mechanics, which describes how things behave at the atomic and subatomic level.
The big problem is that the two theories don't get along. General relativity cannot be reduced to the quantum level, and quantum mechanics cannot be extended. There have been many attempts to get them to become friends by developing the so-called Theory of Everything.
One of the most promising candidates for resolving the difference between general relativity and quantum mechanics is so-called string theory, which involves replacing point particles in particle physics with tiny, vibrating one-dimensional strings.
In addition, many string theory models predict the existence of axions, ultra-low-mass particles first hypothesized in the 1970s to address the question of why strong atomic forces follow what is called charge parity symmetry, when most models say they don't. . As it turns out, string theory also predicts more particles that behave like axions, called axion-like particles.
One of the properties of axion-like particles is that they can turn into a photon when they pass through a magnetic field; conversely, photons can turn into axion-like particles when they pass through a magnetic field. The likelihood of this happening depends on a number of factors, including the strength of the magnetic field, the distance traveled, and the mass of the particle.
Scientists used the Chandra X-ray Observatory to study the active nucleus of the galaxy NGC 1275, which lies about 237 million light-years away at the center of a cluster of galaxies called the Perseus cluster.
Their observations over eight days ended up with little to no knowledge of the black hole. But then they realized that the data could be used to search for axion-like particles.
“X-ray light from NGC1275 must pass through the hot gas of the Perseus cluster, and this gas is magnetized,” Reynolds explained.
The magnetic field is relatively weak (10,000 times weaker than the magnetic field on the Earth's surface), but photons must travel a great distance through this magnetic field. This means that there is ample opportunity for converting these photons into axion-like particles (provided that the axion-like particles have a sufficiently low mass). '
Since the probability of conversion depends on the wavelength of the photons, observations should reveal distortion, since some wavelengths are converted more efficiently than others.
It took the researchers about a year of painstaking work, but in the end, no such distortion was found.
This means that scientists can rule out the existence of axions in the mass range to which their observations were sensitive – up to one billionth the mass of an electron.
“Our study does not rule out the existence of these particles, but it certainly does not help string theory,” said astronomer Helen Russell of the University of Nottingham in the UK.
The study was published in the Astrophysical Journal.
Sources: Photo: NASA / CXC / SAO / E.Bulbul, et al.
