Does the Hubble constant tension call for new physics?
  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.
