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This paper is dedicated to assessing modified cosmological settings based onthe gravitational Standard-Model Extension (SME). Our analysis rests upon theHubble tension (HT), which is a discrepancy between the observationaldetermination of the Hubble parameter via data from the Cosmic MicrowaveBackground (CMB) and Type Ia supernovae, respectively. While the latterapproach is model-independent, the former highly depends on the model used todescribe the physics of the early Universe. Motivated by the HT, we take intoaccount two recently introduced cosmological models as test frameworks of thepre-CMB era. These settings involve local Lorentz and diffeomorphism violationparameterized by nondynamical SME background fields $s_{00}$ and $s^{ij}$,respectively. We aim at explaining the tension in the measured results of thecosmic expansion rate in early and late epochs by resorting to these twomodified cosmologies as potential descriptions of the pre-CMB era. As long asthe HT does not turn out to be a merely systematic effect, it can serve as acriterion for exploring regions of the parameter space in certain pre-CMBnew-physics candidates such as SME cosmologies. By setting extracted limits onSME coefficients into perspective with already existing bounds in theliterature, we infer that none of the aforementioned models are suitablepre-CMB candidates for fixing the HT. In this way, new physics arising from theparticular realizations of Lorentz and diffeomorphism violation studied in thispaper does not explain the HT. Our paper exemplifies how to exploit thisdiscrepancy as a novel possibility of refining our description of the earlyUniverse.

Hubble tension as a guide for refining the early Universe: Cosmologies with explicit local Lorentz and diffeomorphism violation