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MOND, Original Phenomenological Law vs Moffat MOG (Scalar-Tensor-Vector Gravity)

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	MOND, Original Phenomenological Law Frontier	Moffat MOG (Scalar-Tensor-Vector Gravity) Frontier
Proposed	1983	2006
Key figures	Mordehai Milgrom, Stacy McGaugh, Federico Lelli	John Moffat
In one sentence	Milgrom proposed in 1983 that Newton's law of gravity breaks down at extremely low accelerations, below a universal scale a_0 ≈ 10^-10 m/s². Below this scale, gravity effectively gets stronger than Newton predicts, and galaxies rotate as observed without invoking dark matter.	Moffat's 2006 Modified Gravity (MOG, also called STVG) adds a vector field and scalar-field\|scalar fields to general relativity, effectively making Newton's gravitational coupling run with scale. It claims to explain galaxy rotation curves and cluster dynamics without dark matter, but the analyses are largely confined to one research group.
Predictions	Radial acceleration relation: in every galaxy, how fast stars actually accelerate as they orbit is fixed by how much visible matter is present. The observed centripetal acceleration (the inward pull that keeps an orbit curving) tracks the Newtonian acceleration predicted from baryonic matter (ordinary stars and gas) through one universal scale a_0, with no separate dark-halo parameter tuned per galaxy The baryonic Tully-Fisher relation: a spinning galaxy's flat rotation speed to the fourth power tracks its visible (baryonic) mass, V^4 ∝ M_baryon · a_0, with none of the scatter you would expect if dark-halo properties varied, because in MOND there is no halo Wide binaries at separations above ~10^4 AU have orbital velocities probing the deep-MOND regime; specific Newton-Kepler deviations are predicted	Galaxy rotation curves reproduced with effective scale-dependent gravity, no dark halos required Cluster mass profiles and light-bending (lensing) patterns should be explained by visible matter plus MOG's scale-dependent gravity, with no dark matter, the real test being whether one fixed set of MOG parameters works across many clusters rather than being retuned for each Specific deviations from GR at intermediate (galactic, cluster) scales; near-GR behavior in the solar system
Where it breaks	Clowe et al 2006 Bullet Cluster: weak-lensing mass is spatially separated from baryonic gas after a cluster collision, requiring some dark component beyond what MOND alone provides CMB acoustic-peak structure is fit precisely by ΛCDM with cold dark matter; no MOND-derived cosmological framework reproduces the CMB without invoking some particle-like dark component Pittordis & Sutherland 2022 wide-binary analysis using Gaia EDR3 found a preference for GR over MOND, contradicting earlier claims of MOND-like tension; subsequent analyses are ongoing Galaxy cluster dynamics (X-ray temperature profiles, lensing mass) require more mass than baryons alone, even in MOND, undercutting the 'no dark matter' motivation at cluster scales MOND has no fundamental motivation: a_0 is fit empirically, not derived from first principles, and the interpolating function between Newtonian and deep-MOND regimes is also free-form	Many MOG fits use system-specific parameter values; critics argue this is parameter-fitting rather than unique theoretical prediction The [[bulk]] of MOG papers come from one research group; independent reproductions of the claimed cluster and lensing fits are scarce in the broader literature Cosmological survey pipelines (DESI, KiDS, DES, Euclid) do not include MOG as a baseline analysis; mainstream cosmology has not adopted the framework Like other modified-gravity proposals, MOG faces the Bullet Cluster (Clowe et al 2006) and CMB acoustic-peak challenges shared across the family
Key unresolved problem	The cluster-scale failure: MOND fits single galaxies beautifully, but in galaxy clusters like the Bullet Cluster the bending of light reveals far more mass than the visible matter and MOND together can explain, so some hidden dark component still seems necessary.	The independent-reproduction gap: almost all of MOG's successful fits to galaxy clusters, gravitational lensing, and collisions come from Moffat's own group, and outside teams have not confirmed them, so the wider community has not adopted the theory.
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MOND, Original Phenomenological Law

1983 · Frontier

Moffat MOG (Scalar-Tensor-Vector Gravity)

2006 · Frontier

Proposed

1983

2006

Key figures

Mordehai Milgrom, Stacy McGaugh, Federico Lelli

John Moffat

In one sentence

Milgrom proposed in 1983 that Newton's law of gravity breaks down at extremely low accelerations, below a universal scale a_0 ≈ 10^-10 m/s². Below this scale, gravity effectively gets stronger than Newton predicts, and galaxies rotate as observed without invoking dark matter.

Moffat's 2006 Modified Gravity (MOG, also called STVG) adds a vector field and scalar-field|scalar fields to general relativity, effectively making Newton's gravitational coupling run with scale. It claims to explain galaxy rotation curves and cluster dynamics without dark matter, but the analyses are largely confined to one research group.

Predictions

Radial acceleration relation: in every galaxy, how fast stars actually accelerate as they orbit is fixed by how much visible matter is present. The observed centripetal acceleration (the inward pull that keeps an orbit curving) tracks the Newtonian acceleration predicted from baryonic matter (ordinary stars and gas) through one universal scale a_0, with no separate dark-halo parameter tuned per galaxy
The baryonic Tully-Fisher relation: a spinning galaxy's flat rotation speed to the fourth power tracks its visible (baryonic) mass, V^4 ∝ M_baryon · a_0, with none of the scatter you would expect if dark-halo properties varied, because in MOND there is no halo
Wide binaries at separations above ~10^4 AU have orbital velocities probing the deep-MOND regime; specific Newton-Kepler deviations are predicted

Galaxy rotation curves reproduced with effective scale-dependent gravity, no dark halos required
Cluster mass profiles and light-bending (lensing) patterns should be explained by visible matter plus MOG's scale-dependent gravity, with no dark matter, the real test being whether one fixed set of MOG parameters works across many clusters rather than being retuned for each
Specific deviations from GR at intermediate (galactic, cluster) scales; near-GR behavior in the solar system

Where it breaks

Clowe et al 2006 Bullet Cluster: weak-lensing mass is spatially separated from baryonic gas after a cluster collision, requiring some dark component beyond what MOND alone provides
CMB acoustic-peak structure is fit precisely by ΛCDM with cold dark matter; no MOND-derived cosmological framework reproduces the CMB without invoking some particle-like dark component
Pittordis & Sutherland 2022 wide-binary analysis using Gaia EDR3 found a preference for GR over MOND, contradicting earlier claims of MOND-like tension; subsequent analyses are ongoing
Galaxy cluster dynamics (X-ray temperature profiles, lensing mass) require more mass than baryons alone, even in MOND, undercutting the 'no dark matter' motivation at cluster scales
MOND has no fundamental motivation: a_0 is fit empirically, not derived from first principles, and the interpolating function between Newtonian and deep-MOND regimes is also free-form

Many MOG fits use system-specific parameter values; critics argue this is parameter-fitting rather than unique theoretical prediction
The [[bulk]] of MOG papers come from one research group; independent reproductions of the claimed cluster and lensing fits are scarce in the broader literature
Cosmological survey pipelines (DESI, KiDS, DES, Euclid) do not include MOG as a baseline analysis; mainstream cosmology has not adopted the framework
Like other modified-gravity proposals, MOG faces the Bullet Cluster (Clowe et al 2006) and CMB acoustic-peak challenges shared across the family

Key unresolved problem

The cluster-scale failure: MOND fits single galaxies beautifully, but in galaxy clusters like the Bullet Cluster the bending of light reveals far more mass than the visible matter and MOND together can explain, so some hidden dark component still seems necessary.

The independent-reproduction gap: almost all of MOG's successful fits to galaxy clusters, gravitational lensing, and collisions come from Moffat's own group, and outside teams have not confirmed them, so the wider community has not adopted the theory.

Reader vote

No votes yet