The \Lambda Cold Dark Matter model ( \Lambda CDM ) represents the current standard model in cosmology .
Within this , there is a tension between the value of the Hubble constant , H _ { 0 } , inferred from local distance indicators and the angular scale of fluctuations in the Cosmic Microwave Background ( CMB ) .
We investigate whether the tension is significant enough to warrant new physics in the form of modifying or adding energy components to the standard cosmological model .
We find that late time dark energy explanations are slightly disfavoured whereas a pre-CMB decoupling extra dark energy component has a marginally positive Bayesian evidence .
A constant equation of state of the additional early energy density is constrained to 0.086 ^ { +0.04 } _ { -0.03 } .
Although this value deviates significantly from 1/3 , valid for dark radiation , the latter is not disfavoured based on the Bayesian evidence .
If the tension persists , future estimates of H _ { 0 } at the 1 \% level will be able to decisively determine which of the proposed explanations is favoured .