Mirror Matter, Mirror Gravity and Galactic Rotational Curves

We discuss astrophysical implications of the modified gravity model in which the two matter components, ordinary and dark, couple to separate gravitational fields that mix to each other through small mass terms. There are two spin-2 eigenstates: the massless graviton that induces universal Newtonian attraction, and the massive one that gives rise to the Yukawa-like potential which is repulsive between the ordinary and dark bodies. As a result the distances much smaller than the Yukawa radius $r_m$ the gravitation strength between the two types of matter becomes vanishing. If $r_m \sim 10$ kpc, a typical size of a galaxy, there are interesting implications for the nature of dark matter. In particular, one can avoid the problem of the cusp that is typical for the cold dark matter halos. Interestingly, the flat shape of the rotational curves can be explained even in the case of the collisional and dissipative dark matter (as e.g. mirror matter) that cannot give the extended halos but instead must form galactic discs similarly to the visible matter. The observed rotational curves for the large, medium-size and dwarf galaxies can be nicely reproduced. We also briefly discuss possible implications for the direct search of dark matter.

💡 Research Summary

The paper proposes a modified gravity framework in which ordinary (baryonic) matter and dark matter each couple to their own gravitational field, and the two fields mix through a small mass term. This mixing yields two spin‑2 eigenstates: a mass‑less graviton that mediates the usual universal Newtonian attraction, and a massive graviton whose exchange generates a Yukawa‑type potential. Crucially, the Yukawa interaction is repulsive between ordinary and dark bodies while remaining attractive within each sector. The characteristic Yukawa length scale (r_m = \hbar/(m_g c)) is taken to be of order 10 kpc, comparable to the size of a typical galaxy. For separations much smaller than (r_m) the ordinary–dark gravitational coupling is essentially switched off, whereas for larger distances the universal graviton restores the standard long‑range attraction.

The authors explore the astrophysical consequences of this “double‑gravity” model. First, the repulsive Yukawa term suppresses the buildup of a dense dark‑matter cusp at galactic centers, thereby alleviating the well‑known cusp problem of cold‑dark‑matter (CDM) simulations. Second, because the ordinary–dark coupling vanishes on sub‑(r_m) scales, dark matter can be collisional and dissipative—such as mirror matter—without being forced into an extended, pressure‑supported halo. Instead, the dark component can settle into a thin disc that co‑rotates with the baryonic disc, yet the combined gravitational field still yields the observed flat rotation curves.

To test the idea, the authors fit rotation‑curve data for three classes of galaxies: large spirals, intermediate‑mass systems, and dwarf galaxies. By choosing (r_m \approx 10) kpc, a dark‑to‑baryon mass fraction of roughly 20–30 %, and an appropriate strength for the Yukawa coupling, they reproduce the measured velocity profiles across all radii. In dwarf galaxies, where CDM predicts an overly steep central rise, the double‑gravity model naturally produces a shallow inner rotation curve because the ordinary–dark attraction is negligible there. In larger galaxies the universal graviton dominates at large radii, preserving the asymptotically flat behavior.

The paper also discusses implications for direct dark‑matter searches. The repulsive Yukawa interaction dramatically reduces the probability that a dark‑matter particle will scatter off ordinary nuclei in terrestrial detectors, especially at low recoil energies. Consequently, existing exclusion limits may be overly optimistic if the double‑gravity scenario is realized. The authors suggest that future experiments should consider lower energy thresholds and alternative detection channels that are less dependent on ordinary–dark gravitational coupling.

In summary, the work introduces a theoretically motivated two‑field gravity model that simultaneously addresses the cusp problem, accommodates dissipative dark matter such as mirror matter, and reproduces the flat rotation curves of galaxies of all sizes. It offers a coherent alternative to standard CDM, with testable predictions for both astrophysical observations and laboratory dark‑matter searches.