Astrophysicists simulate a fuzzy galactic halo of dark matter

Slices of density zooming into a blurry halo of dark matter. The graph on the right shows the reconstructed fuzzy dark matter wave function with a self-consistent interference pattern and a central solitonic core using the newly presented Gaussian beam method in the innermost and highly resolved region of the halo. Credit: Schwabe & Niemeyer.

Dark matter is a type of matter in the universe that does not absorb, reflect, or emit light, making it impossible to detect directly. In recent years, astrophysicists and cosmologists around the world have attempted to indirectly detect this elusive type of matter, in order to better understand its unique characteristics and composition.

One of the most promising candidates for dark matter is “fuzzy dark matter”, a hypothetical form of dark matter that is thought to be made of extremely light scalar particles. This type of material is known to be difficult to simulate, due to its unique characteristics.

Researchers from the University of Zaragoza in Spain and the Institute of Astrophysics in Germany recently proposed a new method that could be used to simulate fuzzy dark matter forming a galactic halo. This method, presented in an article published in Physical examination lettersis based on the adaptation of an algorithm that the team introduced in its previous work.

“The numerical challenge for studies focused on fuzzy dark matter is that its distinctive features, the granular density fluctuations in halos and collapsed filaments, are orders of magnitude smaller than any sufficiently large cosmological simulation box. to accurately capture the dynamics of the cosmic web.” Bodo Schwabe, one of the researchers who conducted the study, told “So for years people have tried to combine efficient numerical methods capturing large-scale dynamics with algorithms that are computationally demanding but able to accurately scale these density fluctuations.”

In their recent study, Schwabe and his colleague Jens C. Niemeyer adapted and improved an algorithm they had introduced in their previous work. So far, the method they have developed is the only one that can be successfully used to perform simulations of fuzzy dark matter cosmology.

Thanks to their adapted algorithm, the researchers were able to simulate the collapse of the web of the cosmos into filaments and halos. This was achieved using the so-called “n-body method”, which splits the “initial density field” into small particles which move freely under the force of gravity.

“The n-body method is a very stable, well-tested, and efficient method, but it does not capture the density fluctuations of the interfering fuzzy dark matter field in filaments and halos,” Schwabe explained. “In a tiny sub-volume of our simulation box plotting a pre-selected halo in the center, we therefore switched to a different algorithm, known as the finite difference method, which directly scales the fuzzy wave function of the dark matter and can thus capture its interference modes giving the characteristic fluctuations of granular density.

While n-body and finite-difference methods are widely used by astrophysics worldwide to perform cosmological simulations, they have rarely been used together. To perform their simulations, Schwabe and Niemeyer combined these two methods, relying on moderation between them on the subvolume surface.

Specifically, the method they used promotes n-body particles into coherent wave packets known as “Gaussian beams.” The superimposition of these elements led to a fuzzy dark matter wave function at their intersection, which ultimately allowed their simulations to be performed.

“Our successful combination of n-body and finite-difference methods paves the way for realistic simulations of cosmological fuzzy dark matter,” Schwabe added. “These simulations may include the collision of two or more halos of fuzzy dark matter, the evolution of star clusters inside a halo, or their interaction with the central solitonic core whose random walk can potentially heat or even disrupt the star cluster.”

This is what a “fuzzy” universe could have looked like

More information:
Bodo Schwabe et al, Deep zoom-in simulation of a galactic halo of fuzzy dark matter, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.128.181301

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