A 2021 relativistic MOND, built from the start to pass gravitational-wave observations.
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.
Looping ambient scene for Modified Gravity / MOND. 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.
§1 · The claim, 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.
§2 · Why it might be true
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.
§2.5 · Evidence
- The theory was built to match the GW170817 gravitational-wave observation, where gravity and light arrived together to one part in 10^15, a constraint that earlier MOND extensions like TeVeS violated
- Recovers the radial acceleration relation seen in real galaxy rotation curves
- Reduces to general relativity in the strong-field, high-acceleration limit, matching the solar-system tests that GR passes
§3 · What you'd need to test it
- 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
§4 · Where it breaks
- 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
Go deeper
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.
▸§5 · Who built it, and when(2 sources, 2 established)
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