[Paper Review] The Nature of Dark Matter

Joel R. Primack|arXiv (Cornell University)|Dec 11, 2001

Scientific Research and Discoveries1 references7 citations

TL;DR

This paper reviews the Cold Dark Matter (CDM) paradigm as the standard model of cosmological structure formation, assessing its successes on large scales and addressing persistent small-scale issues—such as cored vs. cusped halo density profiles and overpredicted satellite galaxy counts. It concludes that while CDM faces challenges on sub-kiloparsec scales, recent data suggest these discrepancies may be less severe than previously thought, and alternatives like self-interacting (SIDM) or warm dark matter (WDM) introduce new problems, leaving CDM as the most viable framework despite its unresolved small-scale tensions.

ABSTRACT

Cold Dark Matter (CDM) has become the standard modern theory of cosmological structure formation. Its predictions appear to be in good agreement with data on large scales, and it naturally accounts for many properties of galaxies. But despite its many successes, there has been concern about CDM on small scales because of the possible contradiction between the linearly rising rotation curves observed in some dark-matter-dominated galaxies vs. the $1/r$ density cusps at the centers of simulated CDM halos. Other CDM issues on small scales include the very large number of small satellite halos in simulations, far more than the number of small galaxies observed locally, and problems concerning the angular momentum of the baryons in dark matter halos. The latest data and simulations have lessened, although not entirely resolved, these concerns. Meanwhile, the main alternatives to CDM that have been considered to solve these problems, self-interacting dark matter (SIDM) and warm dark matter (WDM), have been found to have serious drawbacks.

Motivation & Objective

• To evaluate the current status of the Cold Dark Matter (CDM) paradigm in cosmology, particularly its agreement with large-scale observations.
• To examine persistent small-scale problems in CDM, including cored vs. cusped halo density profiles and overabundance of small satellite halos.
• To assess alternative models such as self-interacting dark matter (SIDM) and warm dark matter (WDM) in light of recent data and simulations.
• To determine whether CDM remains the most viable model despite unresolved tensions on small scales.
• To explore whether revised CDM models with a tilted primordial power spectrum (n ≈ 0.9) can alleviate small-scale discrepancies.

Proposed method

• Analysis of observational data from cosmic microwave background (CMB), large-scale structure, Lyα forest, and galaxy rotation curves to test CDM predictions.
• Comparison of simulated CDM halos with observed rotation curves and halo density profiles, focusing on the cusp-core problem.
• Evaluation of alternative models: SIDM and WDM, using N-body simulations and constraints from lensing and reionization.
• Use of the ΛCDM model with a tilted primordial power spectrum (n ≈ 0.9) to assess its impact on halo concentration and cusp profiles.
• Incorporation of reionization effects to assess whether they suppress satellite galaxy formation and reconcile CDM predictions with observations.
• Application of analytic models to predict halo concentration in tilted ΛCDM, validated against simulation results.

Experimental results

Research questions

• RQ1 To what extent do CDM predictions for large-scale structure agree with current observational data?
• RQ2 Why do observed rotation curves in dark-matter-dominated galaxies show flat or rising profiles, while CDM simulations predict steep inner density cusps?
• RQ3 Can self-interacting dark matter (SIDM) resolve the cusp-core discrepancy without introducing new inconsistencies?
• RQ4 Does warm dark matter (WDM) reduce the number of small satellite halos and better match observed galaxy counts?
• RQ5 Can a tilted primordial power spectrum (n ≈ 0.9) in ΛCDM reduce halo concentrations and improve agreement with rotation curve data?

Key findings

• The ΛCDM model remains in good agreement with large-scale observations, including CMB anisotropy, large-scale structure, and Lyα forest data.
• The cusp-core problem—where CDM predicts steep inner density cusps but observations show flatter cores—remains unresolved, though recent data suggest discrepancies may be smaller than previously thought.
• Self-interacting dark matter (SIDM) is likely ruled out due to inconsistencies with lensing and halo structure, despite its potential to resolve the cusp problem.
• Warm dark matter (WDM) reduces the number of small halos and leads to lower-concentration halos, but may predict too few satellites and fails to fully resolve the cusp issue.
• A tilted primordial power spectrum with n ≈ 0.90 in ΛCDM reduces halo concentration by about half compared to standard ΛCDM, improving agreement with observed rotation curves.
• Reionization effects may suppress star formation in small halos, potentially reconciling the observed number of satellite galaxies with CDM predictions, reducing the need for alternative models.