A latest cosmological simulation study has systematically revealed, for the first time, how the interaction between dark matter and dark energy significantly influences the rotation and shape alignment of dark matter halos in the universe. This research provides a new perspective for understanding the formation of large-scale cosmic structures and lays an important foundation for interpreting observational data from future sky survey projects such as the China Space Station Telescope (CSST).

The research paper, titled "Halo spin and orientation in Interacting Dark Matter Dark Energy Cosmology," was recently published in Physical Review D .

Although the current ΛCDM (cosmological constant and cold dark matter) model describing the universe has achieved great success, it still faces some observational challenges, such as the Hubble Tension and the S8 Tension. The Interacting Dark Matter-Dark Energy (IDE) model is one of the candidate theories to solve these problems. In this model, dark matter and dark energy do not evolve independently but undergo mutual energy transfer.

This study focuses on two typical IDE scenarios: in the IDE I model, dark matter decays and transforms into dark energy; whereas in the IDE II model, the process is reversed, with dark energy converting into dark matter. These two processes alter the effective mass of dark matter particles, thereby profoundly affecting the formation history and dynamical properties of dark matter halos.

This study utilized a dedicated N-body numerical simulation program named ME-GADGET to conduct high-precision simulations of cosmic structure formation under both IDE models and the standard ΛCDM model. Using numerical simulations, the study found that in the IDE I model where dark matter decays, the alignment consistency between the shapes of dark matter halos and the direction of the tidal field from the surrounding cosmic filamentary structures is significantly stronger than in the ΛCDM model. In the IDE II model where dark matter proliferates, the completely opposite trend was observed， shape alignment weakened.

As shown in the figure above, the evolution of matter density in the context of large-scale filamentary structures is illustrated from left to right. In the IDE I model, the dark matter halos are loose and elongated, whereas in the IDE II model, they are compact and round.

Corresponding author Zhang Jiajun said, "It's actually easy to understand this change. If dark matter converts into dark energy, the dark matter halo becomes looser and thus more susceptible to environmental influences, whereas the conversion of dark energy into dark matter has the opposite effect. It's like two people: if one exercises regularly, with tight muscles, they move more freely and have sufficient strength to resist gravity. The other doesn't exercise properly, with weight mainly coming from fat, so they obviously collapse more easily onto the sofa under gravity. This research not only tells us the distribution patterns of dark matter in the universe but also reminds us to pay attention to exercise, convert more into muscle, to maintain health."

In terms of observational significance, it provides the first detailed depiction of the Intrinsic Alignment (IA) signal of halos in IDE cosmology. IA is a key source of systematic error in weak gravitational lensing observations, arising from the non-random alignment between galaxy shapes and between galaxies and large-scale cosmic structures. In the future, for large-scale weak gravitational lensing survey projects like the China Space Station Telescope (CSST), precise calibration of the IA effect is a prerequisite for successfully constraining cosmological parameters, especially for detecting the properties of dark matter and dark energy.

Zhang Jiajun pointed out, "Our work provides the necessary physical foundation and fitting formulas for constructing more accurate IA calibration models that include IDE effects. This will directly contribute to extracting cleaner cosmological signals from the future data of CSST."

Paper Link： Halo spin and orientation in interacting dark matter dark energy cosmology | Phys. Rev. D

Scientific contact：ZHANG Jiajun   jjzhang@shao.ac.cn