Is Dark Matter Really Disproved? A New Look at MOND Theory
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Chapter 1: Understanding Dark Matter
Dark matter is a crucial element of the prevailing cosmological framework, yet its enigmatic nature continues to puzzle scientists. Numerous investigations have been conducted, but no dark matter particles have been detected to date. Various candidates have emerged, including axions, sterile neutrinos, and even the elusive Weakly Interacting Massive Particles (WIMPs), which have historically been favored by string theorists. Recently, dark photons have also been suggested as potential candidates. However, particle physicists have yet to identify a definitive option. This uncertainty has led some researchers to explore alternatives such as Modified Newtonian Dynamics (MOND) or modified gravity models.
For the first time, researchers have investigated the interrelations between dark matter, modified gravity, and modified inertia by analyzing both the inner and outer regions of galactic rotation curves collectively and individually.
— Kyu-Hyun Chae
The majority of visible matter in galaxies is predominantly found at their centers, while galaxy clusters exhibit high temperatures due to plasma fields. The interplay of gravity, magnetism, and substantial black holes contributes to this phenomenon, inhibiting cooling, promoting star formation, and facilitating complex clustering. Interestingly, the end of every galaxy is characterized by its demise, with gravity and magnetism aiding in the absorption of the entire galaxy into the supermassive black hole at its core. Given the intense gravitational pull at a galaxy’s center and the concentration of visible matter there, one would expect stars located near the center to have higher rotational velocities compared to those situated further away, akin to planets in our solar system.
In reality, however, observations reveal that stars within a galaxy rotate at nearly uniform velocities.
Section 1.1: The Galactic Spin Mystery
To understand galactic rotation, we require a modification that varies with distance. This necessity arises because the average mass and velocity of celestial bodies in galaxies does not significantly exceed those in solar systems. This intricate situation fuels the need for dark matter, which aligns with predictions from the standard cosmological model.
The rotation curve of the typical spiral galaxy M33 (depicted with yellow and blue points and error bars) and its predicted curve based on visible matter distribution (shown as a white line). The disparity between these curves is attributed to the addition of a dark matter halo surrounding the galaxy. Credit: Mario De Leo, Wikimedia Commons
Under the standard Big Bang cosmology model (ΛCDM), which incorporates dark matter and dark energy, general relativity is assumed to be the accurate theory of gravity on cosmological scales. The resulting rotation curve is expected to be smooth without any abrupt changes.
According to Newton's second law, a significant deviation in the outward rotation curve should be anticipated, as Newtonian calculations based on visible matter necessitate this. To address this discrepancy, some scientists propose replacing Newton's law of universal gravitation with dark matter, which accounts for the missing gravitational force predicted by the standard model.
In 1983, Mordehai Milgrom posited that the issue (the crisis in cosmology) might not lie with a mysterious entity, but rather with the inadequacies of our gravitational model. This hypothesis challenges the foundational assumptions of the ΛCDM framework.
Section 1.2: The Birth of MOND
The concept of Modified Newtonian Dynamics (MOND) originated from observations of galaxy rotation. Initially articulated by Israeli physicist Mordehai Milgrom in 1983, this hypothesis aimed to elucidate why the velocities of stars in galaxies appeared greater than predictions based on traditional Newtonian mechanics. MOND suggests that gravitational acceleration is modified based on its intensity. In scenarios involving strong accelerations, such as within the solar system, MOND behaves similarly to Newtonian physics, maintaining that the force acting on an object is proportional to its acceleration (F = ma). However, for extremely weak accelerations, the relationship becomes more complex, resembling the square of the acceleration. This alteration implies that for objects far from a galactic center, the force they experience is independent of distance, resulting in a smooth rotation curve.
In essence, MOND attempts to address the issue by augmenting Newton's gravity rather than relying on dark matter, as proposed by the ΛCDM model. However, incorporating adjustments to Newton's gravity necessitates modifications to Einstein's equations as well. Consequently, MOND has been expanded in various ways, including the AQUAL model (A Quadratic Lagrangian), which is rooted in MOND principles yet employs a Lagrangian framework. Both AQUAL and the standard ΛCDM model are capable of explaining observed galactic rotation curves, though they exhibit subtle differences.
The study published by the American Astronomical Society in The Astrophysical Journal examined high-resolution rotation curves of 152 galaxies sourced from the Spitzer Photometry and Accurate Rotation Curves (SPARC) database.
In the ΛCDM model, which is considered the standard cosmological framework, the rotation curves are expected to be smooth. Nevertheless, data from the SPARC database, analyzing 152 galaxies, revealed a slight kink in the curves.
Interestingly, the ΛCDM framework robustly predicts that the inner radial acceleration relation (RAR) will fall below the outer RAR, aligning with AQUAL predictions. Moreover, it is theoretically feasible for halo models to be adjusted to match the inner and outer RARs observed in the SPARC data. Notably, the AQUAL theory can naturally align with the RARs without requiring fine-tuning.
— Kyu-Hyun Chae
The AQUAL model is too subtle to be detected within a single galaxy; however, statistically, there should be a minor shift between the inner and outer velocity distributions. The kinks identified in the rotation curves from the SPARC database are consistent with simulations based on MOND's AQUAL framework.
When Chae et al. (2020, 2021) analyzed the SPARC database using a simplified spherical MOND model from Famaey & McGaugh (2012), they were surprised to find that the external fields inferred from internal dynamics were in agreement with expectations derived from cosmic conditions.
— Kyu-Hyun Chae
The findings are indeed promising. This study lends credence to MOND as a plausible alternative to dark matter. Could it be that our understanding has been fundamentally flawed? While it's premature to declare dark matter definitively disproven, this research warrants serious consideration as we advance our comprehension of the cosmos.
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Reference: Kyu-Hyun Chae, Distinguishing Dark Matter, Modified Gravity, and Modified Inertia with the Inner and Outer Parts of Galactic Rotation Curves. 2022 December 12, Published by the American Astronomical Society. The Astrophysical Journal. DOI: 10.3847/1538–4357/ac93fc *