Dark Matter Remains Elusive, Yet New Signs Are Emerging

A mysterious entity

Dark matter is a mysterious form of matter which represents about 27 % of the universe, although it cannot be observed directly. Indeed, unlike ordinary matter, which composes the stars, the planets and ourselves, the dark matter emits or reflects light, which makes it invisible to telescopes. Its presence is detected indirectly, mainly by its gravitational influence on visible matter.

Concretely, the mystery of dark matter emerges from the theory of general relativity (RG) of Einstein, which predicts how the mass modifies the curvature of space-time. However, by observing distant galaxies, astronomers noted that their rotation curves were incompatible with the observed mass. The general relativity having been confirmed repeatedly, astronomers therefore hypothesized that there should be an additional invisible mass in the cosmos. Observations also show that this mysterious entity is distributed in a diffuse manner through the universe, and that it plays a crucial role in its structure.

What are the potential candidates?

Although its exact nature remains unknown, several hypotheses have been put forward to explain dark matter. Among the most studied are the Wimps (Weakly Interacting Massive Particles or Massive Particules with low interaction). These theoretical particles would be heavy, electrically neutral and would interact very weakly with ordinary matter, which would explain their invisibility. Wimps have been wanted for decades via ultra -sensitive underground experiences, but none has yet been detected.

Another hypothesis is based on the ALP (axion-like parties or particles of the axion type). Unlike Wimps, the ALPs would be extremely light and could interact with electromagnetic fields by producing specific signals.

Other models propose that dark matter is not made up of exotic particles, but rather invisible astrophysical objects, such as primordial black holes. These black holes would have formed shortly after the Big Bang and would be numerous enough to explain the missing mass of the universe. However, their detection remains complex.

Finally, some theories suggest that dark matter could be an illusion due to changes in the laws of gravity. Approaches such as modified gravity (Mond) try to explain the anomalies observed in the movement of galaxies without requiring dark matter. However, these theories are struggling to account for all cosmological observations.

Credits: manual_adorf/istock

Chase the invisible

More recently, experiences have made it possible to better understand the functioning of this mysterious mass. A recently published study, led by researchers from the Metropolitan University of Tokyo, notably establishes new constraints on the lifespan of these famous particles candidate for dark matter called ALP (Axion type particles).

To do this, the researchers used a new spectrographic technique in order to observe the light of two dwarf galaxies, Leo V and Tucana II, thanks to the 6.5 m Magellan Telescope in Chile.

These observations are not intended to directly detect dark matter, but rather to identify indirect signs of the presence of the ALP (Axion-Like Particles). According to theoretical models, these hypothetical particles could disintegrate by emitting a specific light in the near infrared part of the spectrum. However, this region of the spectrum is often disturbed by noise sources such as zodiacal light, interstellar dust and atmospheric interference.

In previous work, these same researchers had proposed a technique based on a particular disintegration process producing a radiation in a narrow range. To test this approach, they used the Winered spectrograph, capable of extreme dispersion and sensitivity, mounted on the Magellan Clay telescope. This advanced instrument allowed them to precisely measure the close infrared light emitted by Leo V and Tucana II.

Their results have not shown no disintegration, thus making it possible to set limits higher than the frequency of these events or a limit lower than the life of the ALPs, estimated at ten to one hundred million times the age of the universe.

A quest that continues

Although these results are the strictest obtained to date, they also suggest intriguing “excesses”, opening promising prospects for future research. Indeed, even if the researchers did not directly detect the signs of disintegration of the ALP, they have observed slight anomalies or variations in the data, which are not yet explained.

These “excesses” could simply be noise or interference, but they could also indicate a new phenomenon, perhaps linked to dark matter. This is why these anomalies are intriguing: they are not yet evidence, but they offer interesting tracks to explore for future research.

Source: Physical Review Letters