Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions (1112.3960v2)

Abstract: A wealth of astronomical data indicate the presence of mass discrepancies in the Universe. The motions observed in a variety of classes of extragalactic systems exceed what can be explained by the mass visible in stars and gas. Either (i) there is a vast amount of unseen mass in some novel form - dark matter - or (ii) the data indicate a breakdown of our understanding of dynamics on the relevant scales, or (iii) both. Here, we first review a few outstanding challenges for the dark matter interpretation of mass discrepancies in galaxies, purely based on observations and independently of any alternative theoretical framework. We then show that many of these puzzling observations are predicted by one single relation - Milgrom's law - involving an acceleration constant (or a characteristic surface density) of the order of the square-root of the cosmological constant in natural units. This relation can at present most easily be interpreted as the effect of a single universal force law resulting from a modification of Newtonian dynamics (MOND) on galactic scales. We exhaustively review the current observational successes and problems of this alternative paradigm at all astrophysical scales, and summarize the various theoretical attempts (TeVeS, GEA, BIMOND, and others) made to effectively embed this modification of Newtonian dynamics within a relativistic theory of gravity.

• The paper presents MOND as an alternative to dark matter by modifying gravitational dynamics at low accelerations to reproduce flat galaxy rotation curves.
• It evaluates relativistic extensions such as TeVeS, which incorporate additional fields to address gravitational lensing and align with general relativity.
• The paper identifies challenges in reconciling MOND with Cosmic Microwave Background data and large-scale structure, prompting the need for further theoretical refinements.

Overview of "Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions"

The paper authored by Benoit Famaey and Stacy McGaugh provides a comprehensive analysis of Modified Newtonian Dynamics (MOND), its observational successes, and its challenges as an alternative to the dark matter paradigm in explaining mass discrepancies in the universe. This review covers not only the phenomenology of MOND but also its theoretical extensions aiming to incorporate relativity.

MOND as an Alternative Framework

MOND was introduced to address the limitations of Newtonian dynamics and General Relativity (GR) observed at galactic scales without resorting to dark matter. The core premise of MOND is the proposal of a modification to the dynamical laws at low accelerations, characterized by the acceleration constant $a_0$, approximating $10^{-10} \mathrm{m~s^{-2}}$. This modification predicts flat rotation curves of galaxies, a behavior traditionally attributed to dark matter's gravitational effects.

The paper emphasizes that MOND effectively reproduces several galactic phenomena that are challenging for the standard Cold Dark Matter (CDM) paradigm, such as the baryonic Tully-Fisher relation, which is an empirical relation between a galaxy's rotation velocity and its baryonic mass. Moreover, the observed distribution of dark matter inferred from the mass discrepancy in spiral galaxies intriguingly mimics the predictions made by MOND.

Theoretical Extensions and Relativistic MOND

Several attempts have been made to extend MOND into a relativistic theory capable of addressing gravitational lensing and cosmology. One such effort is the Tensor-Vector-Scalar (TeVeS) theory, which introduces additional fields to account for relativistic effects. These modifications aim to reconcile MOND with observations requiring GR, such as lensing, while explaining the observed dynamics without dark matter.

TeVeS and similar theories illustrate the significant conceptual leap required to bolster MOND's predictive success. Despite their innovative formulations, challenges remain, particularly regarding the theory's predictions on cosmological scales.

Cosmological Challenges and Observational Implications

A point of contention for MOND is its ability to reconcile with the detailed measurements of Cosmic Microwave Background (CMB) anisotropies and the large-scale structure of the Universe. While MOND naturally explains galactic dynamics without dark matter, the CMB acoustic peaks' heights and the power spectrum still imply the presence of additional unseen mass or energy influencing the early universe's evolution.

In extension theories like TeVeS, auxiliary fields are posited to augment MOND's effects in cosmological models, resembling some aspects of dark matter behavior. These fields contribute to structure growth and influence the observed CMB peaks, although often requiring adjustments that introduce complexities, such as additional degrees of freedom or coupling parameters.

Conclusion and Future Directions

Famaey and McGaugh's paper thoroughly articulates that MOND remains a foundational yet contentious proposition within astrophysics. While MOND handles galactic phenomena with remarkable efficacy, its extensions into a complete cosmological model prove challenging. The tension between MOND's galactic success and its cosmological difficulties underscores the need for further theoretical advancements.

Future endeavors may focus on refining these theories or developing novel models to incorporate MOND-like behavior, emphasizing gravitational lensing and cosmic structure formation. Finally, the observational advances promised by upcoming astronomical surveys and experiments (e.g., those directly probing the neutrino masses or searching for traces of baryonic dark matter) have the potential to either solidify MOND's standing or necessitate a shift towards alternative explanations within the cosmic paradigm.