ETHOS - An Effective Theory of Structure Formation: Dark matter physics as a possible explanation of the small-scale CDM problems (1512.05349v2)

Abstract: We present the first simulations within an effective theory of structure formation (ETHOS), which includes the effect of interactions between dark matter and dark radiation on the linear initial power spectrum and dark matter self-interactions during non-linear structure formation. We simulate a Milky Way-like halo in four different dark matter models and the cold dark matter case. Our highest resolution simulation has a particle mass of $2.8\times 10^4\,{\rm M}_\odot$ and a softening length of $72.4\,{\rm pc}$. We demonstrate that all alternative models have only a negligible impact on large scale structure formation. On galactic scales, however, the models significantly affect the structure and abundance of subhaloes due to the combined effects of small scale primordial damping in the power spectrum and late time self-interactions. We derive an analytic mapping from the primordial damping scale in the power spectrum to the cutoff scale in the halo mass function and the kinetic decoupling temperature. We demonstrate that certain models within this extended effective framework that can alleviate the too-big-to-fail and missing satellite problems simultaneously, and possibly the core-cusp problem. The primordial power spectrum cutoff of our models naturally creates a diversity in the circular velocity profiles, which is larger than that found for cold dark matter simulations. We show that the parameter space of models can be constrained by contrasting model predictions to astrophysical observations. For example, some models may be challenged by the missing satellite problem if baryonic processes were to be included and even over-solve the too-big-to-fail problem; thus ruling them out.

• The paper demonstrates that integrating dark matter-dark radiation interactions with self-interactions effectively modifies subhalo abundance and internal structures.
• The ETHOS simulations reveal that an altered primordial power spectrum produces diverse dwarf galaxy velocity profiles, offering insights into observed rotation curves.
• The study suggests that combining these novel dark matter interactions in simulations addresses major small-scale CDM challenges, including the missing satellite and too-big-to-fail problems.

An Effective Theory of Structure Formation: Addressing Small-Scale Problems in Cold Dark Matter Models

The paper, credited to Mark Vogelsberger and collaborators, evaluates dark matter (DM) models using the Effective Theory of Structure Formation (ETHOS) framework. This research primarily analyzes how interactions between dark matter and dark radiation, along with dark matter self-interactions, can address longstanding small-scale discrepancies within the Cold Dark Matter (CDM) paradigm. The paper is motivated by issues such as the "missing satellite problem" (MS), "too-big-to-fail problem" (TBTF), and "core-cusp" discrepancies, all of which challenge the traditional CDM framework when considering galactic and sub-galactic scales.

Simulation Methodology

The researchers employ a series of high-resolution N-body simulations incorporating various DM models, focusing on a Milky Way-like halo across four distinguished ETHOS models alongside a conventional CDM scenario. Each ETHOS model is distinguished by specific interactions and initial power spectrum characteristics aimed at replicating key galactic observations. These attributes include the introduction of a primordial power spectrum cutoff, an innovative alternative to the typical CDM power spectrum, influencing structure formation at smaller scales. Various cross-sections for DM self-interactions were also modeled, covering significant velocities relevant to galactic scales.

Key Findings

1. Subhalo Abundances and Internal Structures: The ETHOS models, particularly ETHOS-4, demonstrate an ability to alleviate the MS problem by moderating subhalo numbers more closely to observed values, unlike CDM predictions. However, some ETHOS scenarios overcorrect, leading to an artificial scarcity of substructure, implying a delicate balance in DM interaction strengths. The self-interacting DM (SIDM) aspect is also a determinant factor, influencing core densities and halo core sizes, although less impactful than the primordial power cutoff.
2. Velocity Profile Diversification: Significantly, certain model configurations generate a more extensive range of circular velocity profiles in dwarf galaxies, a feature lacking in contemporary CDM models. Such diversity may provide insights into the observed heterogeneity of dwarf galaxy rotation curves, a point of contention in astrophysical modeling.
3. Implications on Core-Cusp and TBTF Issues: While SIDM alone can alter inner density profiles, combining it with alterations in the initial power spectrum presents viable corrections to several small-scale CDM challenges, such as the TBTF and core-cusp issues. ETHOS outcomes suggest cores forming in dwarf galaxies that align more with observations than the steep cusps inherent to CDM halos.

Theoretical and Practical Implications

ETHOS, as conveyed in the paper, represents a significant theoretical advancement in simulating and understanding dark matter physics beyond the narrow scope of CDM. By integrating early-universe dark radiation interactions and later-time DM self-interactions, researchers can explore a multifaceted picture that better aligns with empirical data. These findings suggest that dark matter properties could indeed harbor complexities that vary by scale and epoch, providing more nuanced constraints on DM models within the ETHOS framework.

Future Directions

The exploration into the behvior of DM through ETHOS offers substantial potential for refining our models of galaxy formation. Future work should include baryonic processes to further benchmark the ETHOS model predictions and address the broader implications of these dynamics on cosmic structure formation. Moreover, this paper intimates future studies focusing on the unique power spectrum signatures of ETHOS models as they apply to large-scale distribution constraints, such as those from Lyman-alpha forests.

In conclusion, the research by Vogelsberger et al. highlights that resolving small-scale cosmological discrepancies necessitates a holistic approach considering novel interactions in dark matter physics, effectively expanding the cosmological modeling horizon beyond conventional paradigms.