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Evaluating the Viability of Solutions to the Hubble Tension

Junhe Chen

The model dependent and independent measurements of the Hubble constant H0 are in a 4.4�? tension, which increases to 5.3 �? after the inclusion of independent time-delay cosmography data. This tension has been one of the most important discordance in cosmology, suggesting unknown systematic errors or potentially new physics beyond the standard model of cosmology. We analyze the current literature of independent experimental results, the reproduciblility of which leads to the conclusion that theoretical solutions would be more promising than unknown systematic errors. Several exotic theoretical scenarios not affecting the �?�?CDM model are then evaluated and ruled out, leaving modifications to �?�?CDM to be the more likely category. As a result of the tight constraints imposed by the CMB data, it is concluded that new pre-recombination mechanisms would be more promising than alterations of late universe expansion history as predicted by �?�?CDM. Then, two relatively promising early universe modifications are evaluated, giving the conclusion that ��?Neff models are weakly preferred over EDE models and could therefore be a route for future research, though it is noted that neither theories can yet fully resolve the Hubble tension. For definitive confirmation or rejection of most models, data from the next generation experiments with a higher precision would be needed.