Skordis-Złośnik Aether Scalar-Tensor
A 2021 relativistic MOND theory constructed to ensure gravitational waves propagate at light speed. The most active line of relativistic-MOND research.
Placeholder for a 3D visualisation of Modified Gravity / MOND. The interactive scene will land in Phase 3. Modified-gravity programs share a single editorial commitment: instead of postulating invisible particles (dark matter) or a cosmological term (dark energy), modify general relativity at the scales where ΛCDM invokes them. MOND's original observation is that galaxy rotation curves obey a tight phenomenological law tied to a universal acceleration scale a_0 ≈ 10^-10 m/s². This pattern is too clean to be coincidence and is not derived from first principles by ΛCDM. The hard part is extending the same idea to clusters, the CMB, and large-scale structure. Whether the same principle scales to clusters and cosmology is the open empirical question. Cluster and cosmological data favor ΛCDM cleanly; galaxy-scale data favor MOND-style patterns just as cleanly. Both empirical signals are real.
In one sentence
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.
The claim
TeVeS predates GW170817 and generically violates it. Skordis and Złośnik (2021 PRL 127) built a new relativistic-MOND theory specifically designed to pass GW170817 from the start. The construction adds a scalar field and an 'Aether' (unit timelike) vector field to the metric, but the coupling structure is chosen so that tensor modes of the metric, the gravitational waves we detect, propagate at exactly the speed of light, regardless of the scalar and vector field configurations.
The same scalar and vector fields produce MOND-like modifications to galaxy dynamics in the appropriate weak-field limit. The theory recovers the radial acceleration relation through the scalar field's contribution to gravitational potentials at low accelerations, while the vector field handles the propagation properties of various perturbation modes. Cosmological behavior is constructed to resemble ΛCDM at zeroth order, with specific deviations at the level of structure formation and CMB acoustic-peak details.
Skordis-Złośnik is the live frontier of relativistic-MOND theory in 2024-2026. Follow-up work by the authors and collaborators (Bataki-Skordis-Złośnik 2024) has elaborated the Hamiltonian formalism, cosmological perturbations, and consistency with structure-growth data. A full global fit to CMB plus BAO plus supernovae plus galaxy data has not been completed yet; the program is young. Skeptics expect tensions similar to earlier relativistic-MOND attempts to emerge once the analysis is thorough.
The family stance
Galaxies don't need invisible matter; they need modified gravity at low accelerations. The radial acceleration relation is real and tight, and any successful theory of the dark sector must reproduce it. Whether the same principle scales to clusters and cosmology is the open empirical question. Cluster and cosmological data favor ΛCDM cleanly; galaxy-scale data favor MOND-style patterns just as cleanly. Both empirical signals are real.
Predictions
- Galaxy dynamics reproduce MOND at low accelerations, including the radial acceleration relation
- Tensor mode propagation speed equals c exactly, by construction (no GW170817 conflict)
- CMB acoustic-peak structure and large-scale-structure growth differ from ΛCDM at observable levels; specific predictions await global cosmological fits
Evidence
- Theory is by construction consistent with GW170817 (tensor modes propagate at light speed)
- Recovers the radial acceleration relation in galaxy-scale dynamics
- Reduces to GR in the appropriate strong-field, high-acceleration limit
Counterpoints
- 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 deep-MOND galaxy fits inherited from MOND-original carry the same challenges at cluster scales (Bullet Cluster) and CMB scales
- Without a unique microscopic motivation for the specific field content, the theory is hard to falsify in a sharp sense
Variants in this family
▸Go deeperTechnical detail with proper terminology
Field content: physical metric g_μν, a unit timelike Aether vector field A^μ with g_μν A^μ A^ν = -1, and a scalar field φ. The action includes a kinetic term for A coupled to the scalar in a specific way that gives MOND-like behavior at galaxy scales while keeping c_T = c.
The construction generalizes TeVeS by carefully arranging the kinetic terms so that tensor perturbations decouple from the vector field's velocity-dependent contributions, in contrast to TeVeS where such mixing changes c_T.
Cosmological dynamics: at the background level, the theory mimics ΛCDM (effective dark-matter-like fluid emerging from the scalar-vector sector). At the perturbation level, growth of structure differs measurably from ΛCDM; this is where the program will succeed or fail observationally.
Bataki-Skordis-Złośnik 2024 follow-up: canonical Hamiltonian analysis, confirming the constraint structure is consistent, and laying groundwork for numerical cosmology in the theory.
References
Last reviewed May 17, 2026
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