A Tale of Two Paradigms: the Mutual Incommensurability of LCDM and MOND (1404.7525v2)

Abstract: The concordance model of cosmology, LCDM, provides a satisfactory description of the evolution of the universe and the growth of large scale structure. Despite considerable effort, this model does not at present provide a satisfactory description of small scale structure and the dynamics of bound objects like individual galaxies. In contrast, MOND provides a unique and predictively successful description of galaxy dynamics, but is mute on the subject of cosmology. Here I briefly review these contradictory world views, emphasizing the wealth of distinct, interlocking lines of evidence that went into the development of LCDM while highlighting the practical impossibility that it can provide a satisfactory explanation of the observed MOND phenomenology in galaxy dynamics. I also briefly review the baryon budget in groups and clusters of galaxies where neither paradigm provides an entirely satisfactory description of the data. Relatively little effort has been devoted to the formation of structure in MOND; I review some of what has been done. The amplitude ratio of the first to second peak in the CMB power spectrum was correctly predicted a priori, but the third peak is more natural to LCDM. MOND anticipates that structure forms more quickly than in LCDM. This motivated the prediction that reionization would happen earlier in MOND than originally expected in LCDM, as subsequently observed. This also provides a natural explanation for massive, early clusters of galaxies and large, empty voids. However, it is far from obvious that the mass spectrum of galaxy clusters or the power spectrum of galaxies can be explained in MOND, two things that LCDM does well. Critical outstanding issues are the development of an acceptable relativistic parent theory for MOND, and the reality of the non-baryonic dark matter of LCDM. Do suitable dark matter particles exist, or are they a modern aether?

• The paper analyzes the conflicting strengths and weaknesses of the LCDM and MOND paradigms, highlighting how LCDM excels at large scales but fails at galaxy scales, while MOND succeeds at galaxy scales but lacks a cosmological framework.
• Observational evidence supports LCDM for cosmic structure and MOND for galaxy dynamics, revealing systematic mass discrepancies that fit MOND's acceleration-based predictions better than CDM's scale-free model.
• Both paradigms face significant theoretical challenges, suggesting the observed mass discrepancies may require fundamental new physics or refined hybrid models beyond current LCDM and MOND frameworks.

Overview of "A Tale of Two Paradigms: the Mutual Incommensurability of $\Lambda$ and MOND"

Stacy S. McGaugh's paper "A Tale of Two Paradigms: the Mutual Incommensurability of $\Lambda$ and MOND" scrutinizes the contrasting frameworks of the $\Lambda$ Cold Dark Matter (CDM) cosmology and Modified Newtonian Dynamics (MOND), focusing on their respective successes and limitations. While $\Lambda$CDM broadly accounts for the universe's large-scale structure and evolution, it flounders when describing galaxy dynamics on smaller scales. Conversely, MOND excels in explaining galactic behavior but lacks comprehensive cosmological big-picture applicability.

Contradictory Cosmological Views

The paper opens by emphasizing the mutual exclusivity between these paradigms, highlighting $\Lambda$CDM's inadequacies in small-scale phenomenology and MOND's silence on universal evolution. McGaugh catalogs the considerable observational evidence supporting $\Lambda$CDM, decisively predicting phenomena like the accelerating universe's expansion confirmed by Type Ia supernovae and the geometry of cosmic microwave background (CMB) fluctuations. Notably, the implementation of non-baryonic cold dark matter (CDM) is seen as indispensable for maintaining a coherent cosmological model, albeit the hypothetical particles remain experimentally elusive.

In contrast, MOND consistently predicts galactic behaviors such as rotation rates in low surface brightness galaxies, with observations corroborating its phenomenological basis. MOND applies at low acceleration regimes, explaining phenomena where standard gravity theories infer unseen matter. Despite these empirical successes, a formal relativistic underpinning for MOND that integrates well with broader cosmological observations remains undeveloped.

Insights on Galaxy Dynamics and Structure Formation

The discrepancies in universal mass evaluations underpin much of the discussion. McGaugh notes that conventional models, like $\Lambda$CDM, consistently require unseen mass (dark matter) to rationalize anomalous galactic velocities. Conversely, MOND suggests that a modification of gravitational laws may fit galactic velocity dispersions without invoking dark matter. Observational data exhibits systematic mass discrepancies correlating with acceleration, a pattern fundamentally at odds with CDM's scale-free predictions yet naturally aligned with MOND's force law modifications.

Diving into cosmological challenges, McGaugh presents both paradigms’ strengths in large-scale structure formation. $\Lambda$CDM naturally models matter distribution fluctuations in early recombination eras that expand into cosmic web structures. Alternatively, MOND hypothesizes a faster formation rate, providing a rationale for early reionization and initial stellar cluster and void formations in these epochs.

Implications for Cosmology and Future Directions

McGaugh discusses how both paradigms face unique obstacles—$\Lambda$CDM's dependence on unconfirmed particles and MOND's missing relativistic theory framework. A significant challenge is $\Lambda$CDM's failure to predict without flexible baryon density parameters, contrasting MOND's empirical fidelity but lack of theoretical universality.

McGaugh suggests that the mass discrepancy problem underscores our fundamental understanding, hinting at a possible need for a new physics principle akin to resolving the historical \ae ther debate. Both paradigms offer substantial descriptive power, yet appear to converge only in certain domains, pointing to an underlying complexity in our comprehension of the universe.

In conclusion, this research piece thoughtfully delineates open-ended questions in cosmology, challenging the community to reconcile empirical observations with theoretical constructs. As observational capabilities expand and theoretical boundaries are pushed, nuanced refinements or hybrid models may emerge, guiding future cosmological inquiries.