Physics overturned - Scientists reveal the dark secret of the universe

The absence of a signal can, in itself, be significant. This is the core concept of a new study published in the Journal of Cosmology and Astroparticle Physics (JCAP), which proposes a different way of searching for dark matter. The research argues that scientists might not need to detect the same "traces" everywhere in the universe to understand its nature. The study focuses on a mysterious observation at the center of the Milky Way, where an excess of gamma radiation has been detected.

One possible explanation is that the signal originates from dark matter particles that annihilate each other. However, similar signals have not been clearly detected in other systems, such as dwarf galaxies. According to the new study, this absence does not necessarily exclude dark matter as the cause. Instead, dark matter may not consist of a single type of particle, but of multiple components that behave differently depending on their environment.

Excess of gamma radiation at the center of the Milky Way

Dark matter is known to exist and is thought to make up a large part of the universe, yet it has not been observed directly. Scientists infer its existence from the gravitational effects it exerts on visible matter, but its true nature remains unknown. Many theories describe dark matter as particles. In some models, when two such particles collide, they annihilate and produce high-energy radiation, such as gamma rays. Astronomers search for this radiation as a potential fingerprint.

"Right now, there appears to be an excess of photons coming from a roughly spherical region around the Milky Way's disk," explains Gordan Krnjaic, a theoretical physicist at the Fermi National Accelerator Laboratory (Fermilab) in the US and one of the study's authors. Observations from the Fermi Gamma-ray space telescope have revealed this excess, which could be linked to dark matter. However, there are other possible explanations, such as emissions from astrophysical sources, e.g., pulsars. To test these ideas, scientists examine other galaxies. "If certain theories about dark matter are correct, we should see it in every galaxy, for example in every dwarf galaxy," Krnjaic explains.

Dwarf galaxies

Dwarf galaxies are small and dim, but they contain large amounts of dark matter. They also have minimal "noise" from stars and other radiation, making them ideal for detecting clean signals. Classic models predict two main scenarios: In the simplest case, the probability of annihilation is constant and does not depend on the velocity of the particles. If this is true, then a signal in the Milky Way should also appear in other dark matter-rich galaxies, such as dwarfs.

In another scenario, the rate depends on the particle velocity. Because these move slowly in galaxies, the process would be extremely rare, making the signal difficult to detect everywhere. Based on these, the absence of a signal in dwarf galaxies makes it difficult to link the gamma-ray excess in the Milky Way to dark matter.

Two different particles

Krnjaic and his colleagues propose a different explanation. "What we are trying to show is that there can be a different dependence on the environment, even if the probability of annihilation is constant at the center of the galaxy," he explains. "Dark matter could simply consist of two different particles, which must meet in order to annihilate."

In this model, the probability depends on how much of each type of particle exists. In galaxies like our own, there may be similar quantities, increasing the chances. In dwarf galaxies, one type might dominate, reducing the meeting probability. "In this way, very different predictions for emission arise," he notes. This approach offers a more flexible alternative to existing models and allows for the explanation of why gamma rays are detected in the Milky Way but not in dwarf galaxies, without excluding dark matter.

Future observations from the Fermi Gamma-ray telescope may provide a clearer picture. Data for dwarf galaxies remains limited. The detection of gamma rays in them would suggest a more balanced dark matter composition, while continued absence might show that one type is rarer. However, none of this is definitive, and more observations are required.

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