Scientists from NCBJ have recently proposed a mechanism for the production of self-interacting dark matter, resulting from decays that occur after the end of the recombination era. The authors showed that self-interacting dark matter can simultaneously solve the problems of the ΛCDM model on small and large scales.

The currently accepted theory describing the evolution of the universe is the Standard Cosmological Model – ΛCDM (Lambda-CDM, Lambda-cold dark matter). Despite its simplicity – it has only 6 free parameters – it describes the Universe amazingly correctly over a wide range of scales. However, thanks to new, precise observations, it is known that ΛCDM does not describe some phenomena precisely enough.

Scientists from the National Center for Nuclear Research have recently proposed a mechanism for the production of self-interacting dark matter, created as a result of the decay of additional unstable particles that took place after the formation of atoms (recombination), which took place around 370,000 years after the Big Bang. It is known that self-interacting dark matter can solve the problems of the ΛCDM model on small scales – e. g. the problem of the dwarf galaxy deficit or the problem of the diversity of galaxy rotation curves – which arise when it is assumed that dark matter does not interact strongly enough with either particles of known matter or known matter with herself. At work Self-interacting dark matter from late decays and the H₀ tension, published in Physical Review D and recently presented at the international EPS-HEP conference, NCBJ Assistant Professor Dr Andrzej Hryczuk and NCBJ PhD student Krzysztof Jodłowski showed that self-interacting dark matter can simultaneously solve both these problems and the recently identified problems of the ΛCDM model in large scales – including the so-called Hubble tension *.

The mechanism investigated in the above-mentioned work is the introduction of unstable dark matter, which has a very long lifetime, because its decay is possible only due to a minimal violation of symmetry in the scalar field sector. This particle breaks down mainly into self-interacting dark matter and to a lesser extent into radiation. „Due to the fact that radiation and matter lose energy at different rates as the universe expands, such a mechanism leads to an increase in the H₀ value obtained from the CMB, which is in line with the value obtained from supernova observations,” says Jodłowski. The proposed mechanism „simultaneously reduces the parameter, σ8, which measures the growth rate of structures in the universe, which improves compliance with the observations,” adds Dr Hryczuk. Finally, the work also shows that the production of self-interacting dark matter ~ 4 million years after the Big Bang can also provide a solution to the problems of the ΛCDM model at small scales.

* Hubble Parameter – a parameter that determines the speed of the expansion of the universe. The Hubble parameter, H₀, is a derivative value obtained from measurements made of phenomena from both the very distant and recent past. Recently, the precision of these measurements has reached a very high level, allowing accurate determination of H₀ by various independent methods. It turns out that the Hubble parameter obtained on the basis of the phenomena from the early universe is significantly smaller than the value obtained from the phenomena of the late universe. This discrepancy is known as the so-called Hubble tension.

Additional information:

Where did the concepts of dark matter and dark energy come from?

1. Accelerated expansion of the Universe and dark energy

Dark matter and dark energy are two mysterious entities that define the evolution of the universe. From Edwin Hubble’s measurements of the escape velocity of galaxies, we know that the universe is expanding. We’ve also known since the late 1990 s that the pace of expansion of the universe increases with time. This amazing fact has been discovered through observations of Type Ia supernovae. These observations showed that the light from distant large redshift supernovae is less bright than expected assuming the universe expands at a constant rate. According to the ΛCDM model, a hitherto unknown phenomenon, which has been called „dark energy”, acting in opposition to gravity, is responsible for accelerating the expansion of the Universe.

2. Microwave background radiation and dark matter

Although dark energy is now the dominant component of the universe (the era of dark energy began around 10 billion years after the Big Bang), its influence Was completely negligible at the time of recombination, when the first atoms were formed. This is because at that time the universe Was about 1000 times smaller than it is now, and the energy of dark energy depends linearly on the volume of the universe. The remainder of the recombination process is microwave background radiation (CMB), the first detection of which in the 1960 s allowed – together with other observations – to formulate the ΛCDM model. According to him, it is the influence of the second mysterious component of the universe – dark matter – that is key to understanding the evolution of the early universe, including the formation of structures in the universe. There are alternative theories to dark matter, such as the modified Newtonian dynamics. However, this is a purely phenomenological approach that has not yet developed into a coherent theory such as the ΛCDM model. Moreover, these types of alternatives often require the existence of some form of dark matter in order to correctly describe phenomena on scales not only of galaxies but also of clusters. Therefore, research on dark matter is one of the leading research directions of astrophysicists and physicists around the world, including numerous NCBJ scientists employed in the Department of Astrophysics and the Department of Theoretical Physics.

3. The unusual speed of rotation in galaxies

Besides the CMB, there are several strong arguments for the existence of dark matter. The simplest and at the same time historically opening the era of modern research in this field comes from the observation of galaxy rotation curves. According to the theory of gravity, the speed at which the stars, dust and gas on the outskirts of the galaxy orbit the galaxy’s core should decrease the further the object is from the core. However, observations have shown that this speed is relatively constant. It would seem that this is due to an unknown type of matter that does not glow, but acts only by gravity and makes galaxies more „stiff”. This matter is called „dark matter”.

Sources:
https: //journals. aps. org/prd/abstract/10.1103/PhysRevD. 102.043024
https: 102.043024//arxiv. org/abs/2110.11622