Phantom dark energy ( w < -1 ) can produce amplified cosmic acceleration at late times , thus increasing the value of H _ { 0 } favored by CMB data and releasing the tension with local measurements of H _ { 0 } .
We show that the best fit value of H _ { 0 } in the context of the CMB power spectrum is degenerate with a constant equation of state parameter w , in accordance with the approximate effective linear equation H _ { 0 } +30.93 w - 36.47 = 0 ( H _ { 0 } in km sec ^ { -1 } Mpc ^ { -1 } ) .
This equation is derived by assuming that both \Omega _ { 0 m } h ^ { 2 } and d _ { A } = \int _ { 0 } ^ { z _ { rec } } \frac { dz } { H ( z ) } remain constant ( for invariant CMB spectrum ) and equal to their best fit Planck/ \Lambda CDM values as H _ { 0 } , \Omega _ { 0 m } and w vary .
For w = -1 , this linear degeneracy equation leads to the best fit H _ { 0 } = 67.4 km sec ^ { -1 } Mpc ^ { -1 } as expected .
For w = -1.22 the corresponding predicted CMB best fit Hubble constant is H _ { 0 } = 74 km sec ^ { -1 } Mpc ^ { -1 } which is identical with the value obtained by local distance ladder measurements while the best fit matter density parameter is predicted to decrease since \Omega _ { 0 m } h ^ { 2 } is fixed .
We verify the above H _ { 0 } - w degeneracy equation by fitting a w CDM model with fixed values of w to the Planck TT spectrum showing also that the quality of fit ( \chi ^ { 2 } ) is similar to that of \Lambda CDM .
However , when including SnIa , BAO or growth data the quality of fit becomes worse than \Lambda CDM when w < -1 .
Finally , we generalize the H _ { 0 } - w ( z ) degeneracy equation for the parametrization w ( z ) = w _ { 0 } + w _ { 1 } z / ( 1 + z ) and identify analytically the full w _ { 0 } - w _ { 1 } parameter region ( straight line ) that leads to a best fit H _ { 0 } = 74 km sec ^ { -1 } Mpc ^ { -1 } in the context of the Planck CMB spectrum .
This exploitation of H _ { 0 } - w ( z ) degeneracy can lead to immediate identification of all parameter values of a given w ( z ) parametrization that can potentially resolve the H _ { 0 } tension .