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MOND, Original Phenomenological Law vs Skordis-Złośnik Aether Scalar-Tensor

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	MOND, Original Phenomenological Law Frontier	Skordis-Złośnik Aether Scalar-Tensor Frontier
Proposed	1983	2021
Key figures	Mordehai Milgrom, Stacy McGaugh, Federico Lelli	Constantinos Skordis, Tom Złośnik
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.	Skordis and Złośnik's 2021 relativistic MOND theory is constructed from the start so that tensor modes propagate at light speed. It reproduces MOND galaxy-scale phenomenology while passing GW170817 by design.
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 dynamics reproduce MOND at low accelerations, including the radial acceleration relation Tensor mode propagation speed equals c exactly, by construction (no GW170817 conflict) The cosmic microwave background's acoustic peaks and the way galaxies cluster on large scales should show measurable deviations from ΛCDM; the exact numbers await fitting the theory to all the major datasets at once
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	A full global fit to CMB plus BAO plus supernovae plus galaxy data has not been completed; the framework is too young to know if it can match all data simultaneously Like TeVeS, the theory adds extra fields and free functions; some critics see it as replacing [[dark matter]] with an equally elaborate modified-gravity sector The galaxy-scale success is real, but it is inherited from MOND-original, and so are MOND's unsolved problems: the theory still faces the cluster-scale mass gap (Bullet Cluster) and the CMB acoustic-peak fit, neither of which Skordis-Złośnik has yet shown it resolves Without a unique microscopic motivation for the specific field content, the theory is hard to falsify in a sharp sense
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 untested-at-scale problem: no one has yet checked the Skordis-Złośnik theory against all the big cosmological datasets at once, the cosmic microwave background, galaxy surveys, supernovae, and sound-wave imprints (BAO), so we cannot tell if it explains everything ΛCDM does.
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MOND, Original Phenomenological Law

1983 · Frontier

Skordis-Złośnik Aether Scalar-Tensor

2021 · Frontier

Proposed

1983

2021

Key figures

Mordehai Milgrom, Stacy McGaugh, Federico Lelli

Constantinos Skordis, Tom Złośnik

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.

Skordis and Złośnik's 2021 relativistic MOND theory is constructed from the start so that tensor modes propagate at light speed. It reproduces MOND galaxy-scale phenomenology while passing GW170817 by design.

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 dynamics reproduce MOND at low accelerations, including the radial acceleration relation
Tensor mode propagation speed equals c exactly, by construction (no GW170817 conflict)
The cosmic microwave background's acoustic peaks and the way galaxies cluster on large scales should show measurable deviations from ΛCDM; the exact numbers await fitting the theory to all the major datasets at once

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

A full global fit to CMB plus BAO plus supernovae plus galaxy data has not been completed; the framework is too young to know if it can match all data simultaneously
Like TeVeS, the theory adds extra fields and free functions; some critics see it as replacing [[dark matter]] with an equally elaborate modified-gravity sector
The galaxy-scale success is real, but it is inherited from MOND-original, and so are MOND's unsolved problems: the theory still faces the cluster-scale mass gap (Bullet Cluster) and the CMB acoustic-peak fit, neither of which Skordis-Złośnik has yet shown it resolves
Without a unique microscopic motivation for the specific field content, the theory is hard to falsify in a sharp sense

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 untested-at-scale problem: no one has yet checked the Skordis-Złośnik theory against all the big cosmological datasets at once, the cosmic microwave background, galaxy surveys, supernovae, and sound-wave imprints (BAO), so we cannot tell if it explains everything ΛCDM does.

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No votes yet