How isotropic is dark energy?

Richard A. Battye; Adam Moss

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arXiv · 2601.22351

How isotropic is dark energy?

Richard A. Battye·Adam Moss

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Abstract

Tensions in late-time expansion data have renewed interest in models beyond $\Lambda$CDM. We ask: \emph{how isotropic must dark energy be?} Working in Bianchi~I, we allow time-dependent anisotropic stress and introduce a parameterisation that enforces a vanishing line-of-sight integral of the shear, thereby satisfying the CMB ISW quadrupole bound by construction. Using Pantheon+SH0ES SNe together with DESI BAO distances, single-bin (constant) and five-bin anisotropic models improve the fit over $w$CDM by $\Delta(-2\ln L_{\rm iso})=14.8$ and $26.6$ respectively, but both violate the quadrupole constraint. In contrast, a five-bin constrained model achieves $\Delta(-2\ln L_{\rm iso})=15.4$ while remaining compatible with the quadrupole limit. The fit improvement arises from two sources: capturing directional structure in the Pantheon+ SNe data, and partially alleviating the tension between the SH0ES $H_0$ value and DESI BAO distances.

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Richard A. Battye, Adam Moss. 2026-01-29. How isotropic is dark energy?. https://arxiv.org/abs/2601.22351

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