Astrophysicists devised a new method to simulate a 'fuzzy' dark matter galactic halo

Dark matter accounts for roughly 85 percent of all the matter in the universe.
Chris Young
Density slices zooming into a fuzzy dark matter halo.Schwabe & Niemeyer

Astrophysicists have long sought to detect dark matter, the elusive, invisible force that does not reflect or emit light but accounts for a vast amount of all the matter — roughly 85 percent according to some estimates — in the universe.

One promising avenue of research is the concept of "fuzzy dark matter", a hypothetical form of the mysterious matter that is theorized to consist of extremely light scalar particles.

The type of matter is known to be difficult to simulate due to its unique characteristics. Still, scientists at the Universidad de Zaragoza in Spain and the Institute for Astrophysics in Germany have recently proposed a method to simulate fuzzy dark matter forming a galactic halo.

Their method, outlined in a paper in Physical Review Letters, improves upon an algorithm the team introduced in a previous study.

"The numerical challenge for studies focusing on fuzzy dark matter is that its distinguishing features, the granular density fluctuations in collapsed halos and filaments, are orders of magnitude smaller than any cosmological simulation box large enough to accurately capture the dynamics of the cosmic web," Bodo Schwabe, one of the researchers who carried out the study, explained to

Astrophysicists devised a new method to simulate a 'fuzzy' dark matter galactic halo
Density slices zooming into a fuzzy dark matter halo from the researchers' simulation. Source: Schwabe & Niemeyer.

"Thus, for years people have tried to combine efficient numerical methods capturing the large-scale dynamics with algorithms that are computationally demanding but can accurately evolve these density fluctuations," Schwabe continued.

Uncovering the mystery of dark matter

Schwabe and a colleague, Jens C. Niemeyer, believe the method they developed is the only one currently capable of successfully conducting fuzzy dark matter cosmology simulations. Using their algorithm, they said they were able to simulate the collapse of the cosmos web into filaments and halos using what is known as the "n-body method". The n-body method divides the "initial density field" into small particles that evolve with the effects 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 tracing the center a pre-selected halo, we, therefore, switched to a different algorithm, known as the finite difference method, which directly evolves the fuzzy dark matter wave function and can thus capture its interfering modes yielding the characteristic granular density fluctuations."

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Schwabe and Niemeyer combined both the n-body and the finite difference methods, both of which are widely used but rarely combined to perform cosmological simulations. This promoted the n-body particles to the state of coherent wave packages known as "Gaussian beams", leading to a fuzzy dark matter wave function that allowed them to perform their simulations. The researchers believe their method will help the global scientific community to better understand dark matter as a whole.

Though large telescope projects such as NASA's James Webb aim to help uncover the mysteries of dark matter and dark energy, new methods for simulating the elusive forces on massive scales will still be required to make sense of their findings in the coming years.

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