A New Way of Modeling the Growth of Dark Matter
By Ali Fazal
Christmas morning may have been a few months ago, but NASA still feels like it got the biggest present of all.
The James Webb Space Telescope (JWST) launched on the morning of December 25, 2021 in its quest to make more advanced observations about the history of the universe. One of the projects in JWST’s observation cycle will be to uncover the nature of dark matter – an invisible component that aids in universal expansion – by testing models of cold dark matter (CDM), which is theorized to move slowly and form into smaller structures, or halos, that later merge with others to become large and clumpy.
One such model deals with the growth of both individual dark matter halos, as well as groups of halos, by incorporating new fitting functions called Diffmah for the former and DiffmahPop for the latter. Andrew Hearin, Jonas Chaves-Montero, Matt Becker, and Alex Alarcon – researchers from the Argonne National Laboratory in the U.S. and the Donostia International Physics Center in Spain – posited a model that uses mass assembly history, which is characterized by fast initial halo growth and slow growth later on. The model also incorporates merger trees, which describe structures that combine with the main halo. Together, these principles helped compare the model with already-published simulations that use only gravity or hydrodynamics, the motion of fluids.
For the growth of individual halos, the model discussed the evolution of peak halo mass growth, which notably depends on a slope that varies depending on whether the time of growth is early or late, as well as the transition time between the early and late stages. The model fitted the peak halo mass equation to both gravity-only and hydrodynamic simulations. Additionally, it validated core principles of dark matter halo growth from the standard cosmological model, but with better accuracy and less residual error. Not only did it show that the peak halo growth reached an asymptotic value, but it also proved that the mass growth rate decreased over time.
By also simulating different merger trees of halos, the data confirmed a correlation between halo formation and the density of the environment in which it evolves. This is known as halo assembly bias.
The model explored groups of dark matter halos by describing the assembly distribution of halos that have a peak mass at the present-day age of the universe. Just as in the discussion of individual halo growth, the model fit the data of gravity-only simulated halos to an increased accuracy. While the specific slope changed depending on the value of the present-day peak mass, the average assembly history for halo populations appeared to involve an increase of mass over time in addition to an average decrease of mass accretion rates over time.
While the model represents an increased accuracy in modeling cold dark matter halo formation, there are some restrictions. Given the measure of peak mass over time, instantaneous mass – which involves other astrophysical effects – is not captured. The results of the study are also only limited to fixed cosmological parameters. Further studies, however, can potentially use this model as a foundation to predict the rate at which dark matter halos merge together, or how the mass of a subhalo changes over time.
With the next couple of decades of astrophysics research underway, dark matter is undoubtedly one of the areas scientists are putting at the forefront of astronomical discovery. As JWST attempts to examine dark matter halos more closely, scientists hope such models help to provide answers to one of the universe’s most mysterious phenomena.
- awelox. (2009, July 12). DM Halo Assembly. https://www.youtube.com/watch?v=Lej5n3lusD8
- Hearin, A. P., Chaves-Montero, J., Becker, M. R., & Alarcon, A. (2022). A Differentiable Model of the Assembly of Individual and Populations of Dark Matter Halos. ArXiv:2105.05859 [Astro-Ph]. https://doi.org/10.21105/astro.2105.05859
- O’Keefe, M. (n.d.). How JWST will test models of cold dark matter. Symmetry Magazine. Retrieved April 25, 2022, from https://www.symmetrymagazine.org/article/how-jwst-will-test-models-of-cold-dark-matter