Dark Matter vs. MOND: How Do We Explain Galaxy Rotation Curves?

[Armend Xani]

Welcome to the Astronomy section of Cosmos!

One of the greatest unsolved mysteries in modern cosmology is the Galactic Rotation Problem. When we observe spiral galaxies, the orbital velocities of stars at the outer edges do not drop off as predicted by standard Newtonian dynamics or General Relativity based solely on visible (baryonic) matter.
Instead, outer stars rotate at nearly constant speeds, forming a “flat rotation curve.”
To explain this anomaly, astrophysicists have proposed two competing paradigms:

1. Cold Dark Matter (Lambda-CDM Model)

The Core Concept:** Galaxies are embedded inside massive, spherical halos of unseen, non-baryonic matter (Dark Matter) that interacts only via gravity and potentially the weak force.
&Key Evidence: Gravitational lensing observations, the Cosmic Microwave Background (CMB) power spectrum, and galaxy cluster collisions like the **Bullet Cluster**.
Current Challenges: Particle physics experiments (LUX-ZEPLIN, XENONnT, CERN) have not yet directly detected candidate particles like WIMPs (Weakly Interacting Massive Particles) or Axions.

2. MOND (Modified Newtonian Dynamics)

The Core Concept:** Originally proposed by Mordehai Milgrom, MOND suggests that Newton’s second law ($F = ma$) breaks down at extremely low gravitational accelerations ($a < a_0 \approx 1.2 \times 10^{-10} \text{ m/s}^2$).
Key Evidence: Accurately predicts galaxy rotation curves and the **Baryonic Tully-Fisher Relation (BTFR)** using only visible matter, without adjusting arbitrary halo parameters.
Current Challenges: Struggles to explain cosmological scales, galaxy cluster dynamics, and the CMB acoustic peaks without requiring additional unseen mass (e.g., sterile neutrinos).

💬 Discussion Questions for the Community:

1. Do you believe future space telescopes (JWST, Euclid, Roman Space Telescope) will find direct proof of Dark Matter particles, or will gravity theories need fundamental revision?
2. How do you view the Bullet Cluster evidence—does it rule out MOND, or can relativistic extensions of MOND (like TeVeS) account for it?

Share your theoretical insights, data analyses, or questions below!**

• This topic was modified 6 hours, 5 minutes ago by Armend Xani.