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In the Kaluza–Klein model, where one extra compactified spatial dimension is added to usual spacetime, one can consider hadronic states emerging from extra dimensional excitations. These can be associated with e.g., strangeness or higher mass states appearing in the nuclear equation of state, with a non-trivial modification to the speed of sound, especially in the conformal limit [1].
Macroscopic neutron star observables using a multi-dimensional zero-temperature Fermi gas equation of state with a linear repulsive potential have been calculated and compared to observational data. A regime for constraining extra-dimension sizes based on compact star mass measurements has been determined [2,3]. Microscopic effects of a strong gravitational field on the equation of state are investigated [4,5].
[1] A. Horváth, E. Forgács-Dajka, G.G. Barnaföldi: ”Speed of sound in Kaluza-Klein Fermi gas”, https://arxiv.org/abs/2502.04974
[2] A. Horváth, E. Forgács-Dajka, G.G. Barnaföldi: "Application of Kaluza-Klein Theory in Modeling Compact Stars: Exploring Extra Dimensions", MNRAS, https://doi.org/10.1093/mnras/ (2024)
[3] A. Horváth, E. Forgács-Dajka, G.G. Barnaföldi: "The effect of multiple extra dimensions on the maximal mass of compact stars in Kaluza-Klein space-time", IJMPA, https://doi.org/10.1142/ (2025)
[4] A. Horváth, A. Wojnar, G.G. Barnaföldi: “The effects of strong gravity on the dispersion relation of massive particles in the Kaluza–Klein theory”, https://arxiv.org/pdf/2510.16631
[5] A. Horváth, A. Wojnar, G.G. Barnaföldi: "Modified Dispersion Relation in Kaluza–Klein Theory", Particles 2026, 9, 87. https://doi.org/10.3390/particles9030087