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Physicists propose new way to detect dark matter
After decades of theories without observable results, astrophysicists persist in their search for answers to a fundamental question about the origin and evolution of the Universe
ALPs are hypothetical particles proposed by theoretical physicists, promising to compose dark matter – Photo: Daniel Cid/Pexels
Ordinary matter, which composes everything we know, such as planets, stars, and living beings, is only a fraction of the surrounding cosmos; 85% of the Universe is made up of dark matter, elementary particles that don’t emit light or interact with the environment in any perceptible way. Gravitational evidence indicates that it influences cosmic dynamics and the movements of galaxies, but its nature remains a mystery to science.
Researchers at USP São Carlos Institute of Physics (IFSC) are proposing an innovative way of looking for this invisible matter. The group simulated the interaction between two elements: axion-like particles (ALPs) – a model predicted by physicists that is a strong candidate for the composition of dark matter – and cosmic rays, ultra-energetic protons that come from space. The research suggests that, in very dense astrophysical environments, this process produces faint flashes of gamma rays, which can be detected by modern telescopes.
Igor Reis, a doctoral student at IFSC, authored the research in partnership with the University of Paris-Saclay (France). Victor Gonçalves, a researcher at the Federal University of Pelotas (UFPel), and Aion Viana, a professor at the IFSC, contributed to the work. “If cosmic rays travel through space interacting with dark matter, even weakly, it’s very likely that we have a gamma ray signal,” Viana told Jornal da USP.
The research combined extensive theoretical models with advanced computational techniques. He explains the simulation: “We take the theory and bring it into a real environment, to deduce what the observable phenomenon would be,”. “This way, we can predict how a telescope should detect a dark matter signal, and what aspects that signal would have.”
Aion Viana - Photo: Lattes
The CTAO is one of the telescopes still being developed to detect gamma rays, which in turn have the potential to confirm the existence of dark matter - Photo: CTAO/Freepik
Viana is part of the Apoema (particle astrophysics with the most energetic observatories in the Universe) research group, which aims to explore various cosmic phenomena. However, instead of using particle accelerators on Earth, the group focuses on analyzing elementary particles from natural accelerators in the Universe, such as supernova remnants and environments around black holes.
“We use telescopes or detectors of high-energy particles coming from the Universe to study the nature of these cosmic rays – where they come from and how they are generated,” he comments. He adds that “we are also trying to understand the fundamental interactions during these processes in environments with energy regimes, density, and speeds much higher than those in laboratories on Earth.”
A brief history of dark matter
The first evidence of dark matter emerged in the 1930s, in observations made by Swiss astronomer Fritz Zwicky (1898-1974). He measured the mass of a cluster of galaxies and showed that the mass of the whole was much greater than the sum of the masses of the individual galaxies, and named this difference dark matter. In 1970, American astronomer Vera Rubin (1928-2016) revolutionized the field of astrophysics by officially stating that most of the matter in galaxies is dark.
The Standard Model of physics emerged in the 1970s and remains the best description of subatomic particles to date, though it still has many gaps. For instance, it offers no explanation for why gravity is so much weaker than the other fundamental forces. Another major challenge is the asymmetry between matter and antimatter in the Universe: in theory, the Big Bang should have produced equal amounts of both, yet astrophysicists have detected very few antiparticles. The nature of dark matter and dark energy, the mass of neutrinos (particles with no electric charge), and the reconciliation of general relativity with quantum mechanics also remain unresolved.
Vera Rubin studied the movement of stars and gases in galaxies and discovered the rotation curves of galaxies - Photo: KPNO/NOIRLab/NSF/AURA - Wikimedia Commons
Thus emerged the field of Physics Beyond the Standard Model, which encompasses theories for all these inconsistencies. Among BSM’s proposals is the axion, a hypothetical elementary particle described in 1978 by Roberto Peccei and Helen Quinn. Currently, astrophysicists hypothesize that axion-like particles (ALPs) may be scattered throughout the Milky Way and are strong candidates to make up dark matter.
Since its theorization, the search for dark matter has generated major scientific and technological advances, including new materials, hypersensitive sensors, cryogenics, and algorithms for high-performance supercomputers. “It’s difficult to study phenomena occurring millions of light-years away, so a large part [of the work of particle astrophysics] is to deduce what is happening,” Viana told Jornal da USP.
Ultra telescopes versus satellites
Viana is the IFSC representative in the SWGO (Southern Wide-field Gamma-ray Observatory) collaboration. He is also a member of the international consortium CTAO (Cherenkov Telescope Array Observatory), which operates dozens of telescopes in two hemispheres and map the sky in gamma rays. “The idea of developing techniques to observe gamma rays coming from the Universe began in the 1980s, with the first telescopes, such as the 10-meter Whipple telescope,” he says. “It’s an ongoing project, and we keep pushing the observational instrumental limits.”
Gamma rays can arise from various ultra-energetic phenomena, and traditional satellites look for signals from these astroparticles directly in space. According to the professor, the biggest problem is that gamma rays are extremely penetrating and can pass through the satellite without being detected by the sensors, even under very high-speed conditions.
Fermi, for example, is a satellite that has already tried to measure gamma rays from dark matter when observing dwarf galaxies, but the signals obtained were so weak that they were within the noise range of the devices - Photo: NASA/Picryl
The upside is that gamma rays interact with the Earth’s atmosphere in the form of extensive air showers, which extend for kilometers of particles and radiation. “The gamma ray generates a cascade of charged particles, because the atmospheric layer breaks the particles above the speed of light in that medium,” Viana explains. At the moment of energy loss, the particles emit a type of ultraviolet light known as Cherenkov radiation. Ground-based telescopes attempt to detect this light, which is produced by particle showers in Earth’s atmosphere.
“The satellite can detect gamma rays in an energy regime where there are many other astrophysical sources of radiation, while telescopes detect that same signal in an energy regime where the sky is much cleaner,” he comments. The satellites detect gamma rays with energies ranging from mega-electron volts (10⁶ eV) to hundreds of giga-electron volts (10⁹ eV), while telescopes, which are more sensitive, detect gamma rays of up to tera-electron volts (10¹² eV) and peta-electron volts (10¹⁵ eV).
The super-technological telescopes under construction have resulted from several international efforts over the last few decades. Some particle models, for example, could have been detected by the HESS (High Energy Stereoscopic System), which has been active since 2004. “As HESS didn’t detect it, we ruled out such models, because if they were feasible, we would have seen the signs. And we continue to test stronger theoretical models,” the scientist says.
The expectation is that the next few decades will be a turning point for identifying dark matter. “Both CTAO and SWGO have the potential for essential discoveries, and we hope they can reach this detection limit,” he told Jornal da USP. “If they don’t find any signals, we’ll start to question the hypothesis of dark matter as a particle.”
If the expected detection occurs, the greatest challenge for astrophysicists will be interpreting the results, and this study offers a framework for understanding the signal. “It’s one of the biggest unsolved problems in physics today,” he concludes.
The article Probing axionlike particles through the gamma-ray production from cosmic-ray scattering in the Milky Way dark matter halo is available online and can be read here.
More information: aion.viana@ifsc.usp.br, with Aion Viana
*Intern supervised by Fabiana Mariz
**Intern under the supervision of Moisés Dorado
English version: Nexus Traduções, edited by Denis Pacheco
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A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.
A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.