Small scale problems of the $Λ$CDM model: a short review (1606.07790v3)

Abstract: The $\Lambda$CDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem , the small-scale problems in galaxy formation, or the unexplained anomalies in the CMB. $\Lambda$CDM clearly shows difficulty at small scales, which could be related to our scant understanding, from the nature of dark matter to that of gravity; or to the role of baryon physics, which is not well understood and implemented in simulation codes or in semi-analytic models. At this stage, it is of fundamental importance to understand whether the problems encountered by the $\Lambda$DCM model are a sign of its limits or a sign of our failures in getting the finer details right. In the present paper, we will review the small-scale problems of the $\Lambda$CDM model, and we will discuss the proposed solutions and to what extent they are able to give us a theory accurately describing the phenomena in the complete range of scale of the observed universe.

• The paper reviews key small-scale problems of the Lambda-CDM model, such as issues in galaxy formation and cosmic microwave background anomalies.
• Specific small-scale challenges include the cusp-core, missing satellite, and Too Big To Fail issues in galactic structure modeling.
• Proposed solutions include astrophysical processes like supernova feedback and cosmological changes such as Warm or Self-Interacting Dark Matter or modified gravity.

Small Scale Problems of the $\Lambda$CDM Model: A Review

The $\Lambda$CDM model, recognized as a mature paradigm in cosmology, exhibits notable capacity to describe large-scale structures of the universe. However, significant challenges remain at smaller scales, including the cosmological constant problem, anomalies in the Cosmic Microwave Background (CMB), and difficulties in galaxy formation modeling. The paper under discussion, "Small Scale Problems of the $\Lambda$CDM Model: A Short Review," provides a detailed examination of these issues, focusing particularly on problems encountered at small-scales in the $\Lambda$CDM model and exploring various proposed solutions.

Key Small-Scale Problems

1. Cosmological Constant Fine-Tuning: Quantum field theories predict a significantly larger cosmological constant than observed, leading to major fine-tuning problems both today and at the Planck scale era.
2. Galactic Structure Challenges: Difficulties in describing structures at small scales, such as the cusp-core problem, missing satellite problem, and Too Big To Fail (TBTF) issue.
3. Cosmic Anomalies: Large-angle fluctuations in the CMB and anisotropies, including quadrupole-octupole alignment and cold spots.

Several significant small-scale problems highlight the limitations of $\Lambda$CDM in describing observed phenomena:

• Cusp/Core Problem: N-body simulations predict cuspy dark matter halos, which conflict with the flat density profiles observed in dwarf and Low Surface Brightness (LSB) galaxies.
• Missing Satellite Problem: The number of predicted dark matter subhalos greatly exceeds the number of observed satellite galaxies.
• Too Big To Fail Problem: Simulations predict massive and dense satellite subhalos, which are not matched in reality by corresponding luminous counterparts.

Proposed Solutions

Astrophysical/Baryonic Solutions

Numerous astrophysical processes have been proposed to resolve the small-scale problems:

• Supernova Feedback: Multiple moderately violent supernova activities can flatten the cores of galaxies by expelling baryonic material and altering the dark matter distribution.
• Dynamical Friction from Baryonic Clumps: Energy and angular momentum transfer from baryonic clumps to the dark matter component can transform cusps into core-like profiles.

In contrast to traditional approaches, recent simulations indicate that baryonic processes combined with dark matter dynamics are crucial to accurately modeling visible phenomena.

Cosmological/Modified Gravity Solutions

Besides astrophysical approaches, alternative theories explore cosmological changes to resolve discrepancies:

• Warm Dark Matter (WDM): This particle with higher thermal velocity is proposed to address the overprediction of satellite galaxies.
• Self-Interacting Dark Matter (SIDM): Post proposes complex dark matter self-interaction mechanics that prevent overly cuspy halo formation.
• Modified Gravity Theories: Theories altering gravitational equations like MOND explore fundamental changes to Newtonian dynamics to reconcile model predictions with observations, especially in galactic systems.

Implications and Future Directions

The paper implies that while modifications to the baryonic processes in structure formation show promising results, significant debates persist, particularly concerning the sufficiency of current supernova feedback models. Cosmological solutions, while theoretically intriguing, face challenges when confronted with larger astrophysical structures and empirical data from phenomena like the CMB.

Further research should leverage improved simulation techniques and expanding observational data, such as GAIA and Subaru Hyper-Supreme-Camera surveys, which offer direct insights into star movements and galactic substructures. These tools, coupled with refined models, have the potential to advance our understanding of dark matter's role in cosmology.

In conclusion, while $\Lambda$CDM provides robust large-scale structure explanations, its small-scale applicability requires refined approaches integrating both baryonic feedback and potentially novel dark matter and gravity theories. Addressing these nuances will be essential for a unified model that consistently explains the varied structures and dynamics observed across different scales in the universe.