Since the 1930s, scientists have been aware of an enigmatic form of matter that does not reflect light, which they named dark matter. According to our current understanding, this entity interacts with other particles primarily through gravity and accounts for about 85% of the mass in the Universe.
Scientists introduced the concept of dark matter into their theories because they observed that the quantity of ordinary matter was not enough to explain some astronomical observations. These ranged from higher than expected velocities of stars within galaxies to anomalies in the cosmic microwave background, and the rapid formation of galaxies.
Despite there being more dark matter in space than any other form of matter, of its composition remains a mystery. Neutrinos, black holes, and weakly interacting massive particles known as WIMPs have all been considered as its constituents, but so far none of these hypotheses has been confirmed.
Axions could be an answer
This uncertainty has prompted researchers to explore other candidates, with one particularly interesting candidate being the axion, a hypothetical particle that theorists believe could not only solve the mystery of dark matter, but could resolve an important issue in the theory of strong interactions.
“Axions are an especially interesting dark matter candidate because they solve two mysteries: the nature of dark matter and the strong charge-parity (CP) problem,” explained Elisa Todarello of Turin University in an email. “The strong CP problem is the unexplained lack of violation of the parity and charge conjugation symmetries in the strong interactions. Axion particles were first proposed in the context of a solution to this puzzle. As it turns out, they also have all the right characteristics to be dark matter.”
But to confirm axions as candidates for dark matter particles, physicists need to know their mass and strength of interaction, something current theories are unable of predicting. This leaves experiment to determine these parameters.
To do this, Todarello analyzed the intensity of light coming from five nearby dwarf galaxies to determine if any of the emitted photons originated from the decay of axions into pairs of these particles — a phenomenon that is predicted to occur, according to the theory of axions, even if it does occur very rarely.
A telescope sheds light
In a recent study published in Annalen der Physik, Todarello used data collected by the Multi Unit Spectroscopic Explorer (MUSE), which is one of the Very Large Telescope of the European Southern Observatory’s instruments working in the visible range.
“Axions are predicted to interact feebly with light,” said Todarello. “The strength of this interaction is represented by a quantity that we call ‘g’. This interaction allows an axion to transform into two photons. If axions are the dark matter that holds together galaxies, they are present in extremely large quantities, for example in the dwarf galaxies observed by MUSE.
“When dark matter axions decay into photons, they cause the galaxy to ‘glow’ at a specific frequency — half the axion mass [in certain units]. We analyze MUSE data to search for this glow. Its presence would indicate the existence of axions, while its absence allows us to rule out axion dark matter within specific ranges of the axion mass and g.”
Todarello’s analysis of the radiation from five dwarf galaxies did not reveal any peculiarities that indicated the decay of the axion. However, this result is still very important, since it constrains the possible parameters of the axion, thereby limiting the range of parameters in which this particle should be sought in the future.
“By observing five dwarf galaxies with MUSE, we are able to exclude a dark matter axion with a mass between 2.65 to 5.27 electronvolts [which is approximately a hundred thousand times lighter than an electron] and a certain level of interaction with photons, larger than a threshold that depends on MUSE’s sensitivity,” explained Todarello. “We find consistent results across the five galaxies we consider. Since we don’t see the diffuse glow from axion decay into photons, we can conclude that there’s no dark matter axion with such mass and such interaction strength.”
The author believes that the method she used can be improved and used to search for axions with a different mass and a different strength of interaction with photons, making it possible to determine whether dark matter indeed consists of these particles — or something else entirely.
“In the future, we might explore different instruments and telescopes to extend our bounds in frequency, for example to the near-infrared,” concluded Todarello. “As the coupling strength is expected to be lower for lighter axions, we need to find an instrument with excellent sensitivity and focus on targets that can provide a large flux, for example, more massive dwarf galaxies.”
Reference: Elisa Todarello, Updated Bounds on Axion-Like Particle Dark Matter with the Optical MUSE-Faint Survey, Annalen der Physik (2023), DOI: 10.1002/andp.202300042
Feature image credit: ESA/Hubble & NASA