Small-Scale Challenges to the $Λ$CDM Paradigm (1707.04256v2)

Abstract: The dark energy plus cold dark matter ($\Lambda$CDM) cosmological model has been a demonstrably successful framework for predicting and explaining the large-scale structure of Universe and its evolution with time. Yet on length scales smaller than $\sim 1$ Mpc and mass scales smaller than $\sim 10^{11} M_{\odot}$, the theory faces a number of challenges. For example, the observed cores of many dark-matter dominated galaxies are both less dense and less cuspy than naively predicted in $\Lambda$CDM. The number of small galaxies and dwarf satellites in the Local Group is also far below the predicted count of low-mass dark matter halos and subhalos within similar volumes. These issues underlie the most well-documented problems with $\Lambda$CDM: Cusp/Core, Missing Satellites, and Too-Big-to-Fail. The key question is whether a better understanding of baryon physics, dark matter physics, or both will be required to meet these challenges. Other anomalies, including the observed planar and orbital configurations of Local Group satellites and the tight baryonic/dark matter scaling relations obeyed by the galaxy population, have been less thoroughly explored in the context of $\Lambda$CDM theory. Future surveys to discover faint, distant dwarf galaxies and to precisely measure their masses and density structure hold promising avenues for testing possible solutions to the small-scale challenges going forward. Observational programs to constrain or discover and characterize the number of truly dark low-mass halos are among the most important, and achievable, goals in this field over then next decade. These efforts will either further verify the $\Lambda$CDM paradigm or demand a substantial revision in our understanding of the nature of dark matter.

• The paper highlights key small-scale discrepancies in the ΛCDM paradigm, including the cusp/core, missing satellites, and too-big-to-fail problems.
• It compares observational data with model predictions below 1 Mpc and masses under 10¹¹ M☉ to reveal critical inconsistencies.
• The study suggests that refining baryonic feedback mechanisms or exploring alternative dark matter models could resolve these tensions.

Small-Scale Challenges to the ΛCDM Paradigm

The paper "Small-Scale Challenges to the $\Lambda$CDM Paradigm" by James S. Bullock and Michael Boylan-Kolchin critically examines the prevailing issues confronting the $\Lambda$CDM model, particularly at smaller scales. The $\Lambda$CDM model, which stands for Lambda Cold Dark Matter, has been successful in explaining the large-scale structure and evolution of the Universe. However, discrepancies arise when its predictions are scrutinized against observations on scales smaller than approximately 1 Mpc or mass scales below $10^{11} M_\odot$.

Key Challenges and Issues

1. Cusp/Core Problem: Observations indicate the central regions of dark-matter-dominated galaxies are less dense and exhibit flatter density profiles than those predicted by the $\Lambda$CDM model. While the model forecasts cuspy centers due to cold dark matter dynamics, several observed galaxies reveal cores with a more uniform density.
2. Missing Satellites Problem: The $\Lambda$CDM model predicts a significant number of small subhalos around galaxies like our Milky Way, which should host dwarf galaxies. However, the number of observed satellites is markedly lower than the predicted count. This discrepancy suggests either an inefficiency in star formation in these small halos or the possibility that some halos remain completely dark.
3. Too-Big-To-Fail Problem: There's an observed paucity of galaxies with central densities corresponding to $10^{10} M_\odot$ halos. According to the model, these should form stars efficiently, yet they are conspicuously absent in observations, suggesting a fundamental misunderstanding of galaxy formation processes at these scales or potential inaccuracies in the model's assumptions.

Theoretical and Observational Implications

These issues suggest that the simplistic treatment of baryonic (normal) matter interactions and processes in the $\Lambda$CDM model might require revisiting. The resolution could involve better understanding and incorporating complex baryonic feedback mechanisms such as supernova-driven winds, cosmic-ray heating, and various other baryonic processes that can alter the density profiles of dark matter halos.

Moreover, the discrepancies open the door for considering alternative models or additional components that might complement cold dark matter, such as self-interacting dark matter (SIDM) or warm dark matter (WDM). These alternatives propose different interaction strengths or thermal properties of dark matter particles, potentially smoothing out the dense cores predicted by the $\Lambda$CDM.

Future Directions

The paper highlights future opportunities in observational campaigns, particularly those aimed at discovering faint, distant dwarf galaxies and accurately measuring their masses and density structures. These could provide crucial tests for potential solutions like adjusted baryonic physics or new dark matter models. Additionally, efforts to detect starless, low-mass dark matter halos could provide significant evidence for the nature of dark matter and validate or challenge the foundation of the $\Lambda$CDM model.

In conclusion, while the $\Lambda$CDM model remains a central pillar of cosmology, the paper underscores the importance of addressing these small-scale challenges, which might necessitate refined understanding of both baryonic physics and the fundamental nature of dark matter. As astronomical techniques advance, new data may offer the key to resolving these longstanding tensions.