Structure formation in warm dark matter cosmologies: Top-Bottom Upside-Down

Abstract: The damping on the fluctuation spectrum and the presence of thermal velocities as properties of warm dark matter particles like sterile neutrinos imprint a distinct signature found from the structure formation mechanisms to the internal structures of halos. Using warm dark matter simulations we explore these effects on the structure formation for different particle energies and we find that the formation of structure is more complex than originally assumed, a combination of top-down collapse and hierarchical (bottom-up) clustering on multiple scales. The degree on which one scenario is more prominent with respect to the other depends globally on the energy of the particle and locally on the morphology and architecture of the analyzed region. The presence of shells and caustics in warm dark matter halos is another important effect seen in simulations. Furthermore, we discuss the impact of thermal velocities on the structure formation from theoretical considerations as well as from the analysis of the simulations. We re-examine the assumptions considered when estimating the velocity dispersion for warm dark matter particles that have been adopted in previous works for more than a decade and we give an independent estimation for the velocities. We identify some inconsistencies in previous published results. The relation between the warm dark matter particle mass and its corresponding velocity dispersion is strongly model dependent, hence the constraints on particle mass from simulation results are weak. Finally, we review the technical difficulties that arise in warm dark matter simulations along with possible improvements of the methods.

• The paper reveals that warm dark matter structure formation is a hybrid of bottom-up and top-down processes, unlike the purely hierarchical cold dark matter model.
• It scrutinizes the role of thermal velocities and phase space density, emphasizing their pivotal influence on WDM particle gravitational behavior and resultant structures.
• The study identifies inconsistencies in previous simulation assumptions and highlights the need for refined models that accurately incorporate thermal velocities and complex phase space dynamics.

Structure Formation in Warm Dark Matter Cosmologies: An Overview

The study titled "Structure Formation in Warm Dark Matter Cosmologies Top-Bottom Upside-Down," authored by Sinziana Paduroiu, Yves Revaz, and Daniel Pfenniger, examines the intricacies of structure formation in warm dark matter (WDM) cosmologies, with emphasis on sterile neutrinos as particle candidates. This paper sheds light on the complexities involved in modeling structure formation under the WDM framework, as opposed to the more traditional cold dark matter (CDM) paradigm.

Key Findings and Methodologies

The paper outlines several phenomena associated with WDM, primarily focusing on how it impacts the formation and evolution of cosmic structures, both at small and large scales. Using N-body simulations, the authors explore how the damping of fluctuation spectra and thermal velocities inherent to WDM particles manifests differently compared to CDM.

1. Hybrid Structure Formation: The study reveals that WDM structure formation is not purely hierarchical (bottom-up) as in CDM, nor is it strictly top-down, but rather a combination of both. The prominence of each mechanism varies depending on the energy of the WDM particles and the morphology of specific regions in the universe.
2. Thermal Velocities and Phase Space Density: The role of thermal velocities is scrutinized, with a re-examination of their influence on structure formation. The authors challenge prior assumptions and provide independent estimations of velocity dispersions. The study emphasizes how the phase space density and thermal velocities of WDM particles are pivotal in shaping their gravitational behavior and resultant structures.
3. Numerical and Theoretical Analysis: The paper identifies inconsistencies in previous literature regarding the assumption of vanishing initial velocities in simulations, which it argues is mathematically inconsistent. The study also challenges some of the established methods of estimating velocity dispersion in WDM scenario.
4. Presence of Shells and Caustics: The simulations highlight observable features such as shells and caustics in WDM halos, which are more visible than in CDM models, offering new insights into the internal structure of dark matter halos.
5. Limitations and Constraints: The research recognizes the challenges in simulating neutrino-like WDM particles accurately due to their complex phase space dynamics and the methodological shortcomings when applying traditional N-body codes.

Implications and Future Directions

The implications of this research are twofold: practical and theoretical. Practically, the work necessitates a reevaluation of cosmic structure simulations, prompting more refined models that incorporate thermal velocities from the onset. Theoretically, it provokes further investigation into the nature of dark matter and its role in cosmic evolution, especially within the context of existing discrepancies in the CDM model, such as the well-documented "missing satellites" problem.

Looking forward, advancements in computational techniques and the integration of quantum statistical mechanics into simulations could yield deeper insights. Additionally, the intersection of baryonic physics with WDM models holds promise for more accurately depicting the universe's evolution. The study opens new avenues for exploring the mass constraints of dark matter particles and reinforces the importance of precise initial conditions in simulations.

In summary, the paper highlights the need for nuanced approaches to understanding WDM's impact on cosmic structure formation. The intricate interplay between different formation mechanisms calls for a broader application of high-resolution simulations to reconcile differences between observed galactic phenomena and theoretical models.