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arXiv · 2608.02458

Isotropic universes with a preferred direction

Jose Beltrán Jiménez·Shinji Tsujikawa
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Abstract

We present a class of cosmological scenarios in which a preferred spatial direction in the matter sector coexists with an exactly homogeneous and isotropic Friedmann--Lema\^{\i}tre--Robertson--Walker (FLRW) geometry. The Cosmological Principle is realized on shell, with the vector-field equations enforcing a vanishing momentum density and suitable interactions eliminating the anisotropic stress. Consequently, the construction yields an entire FLRW branch without tuning the matter-field initial conditions. The preferred direction reemerges in perturbations, producing direction-dependent propagation and mixing among scalar, vector, and tensor modes already at linear order. In particular, the mixing opens a linear channel through which perturbations in the scalar sector can in principle source gravitational waves. Our construction thus reveals a new route by which preferred-direction physics can leave observable cosmological signatures while remaining hidden in the background geometry.

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Jose Beltrán Jiménez, Shinji Tsujikawa. 2026-08-03. Isotropic universes with a preferred direction. https://arxiv.org/abs/2608.02458

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