Papers › H₀ Tension, Phantom Dark Energy and Cosmological Parameter Degeneracies
H₀ Tension, Phantom Dark Energy and Cosmological Parameter Degeneracies
G. Alestas, L. Kazantzidis, L. Perivolaropoulos
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Phantom dark energy can produce amplified cosmic acceleration at late times, thus increasing the value of H₀ favored by CMB data and releasing the tension with local measurements of H₀. We show that the best fit value of H₀ 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 H₀ + 30.93 w - 36.47 = 0 (H₀ in km sec⁻¹ Mpc⁻¹). This equation is derived by assuming that both Ω_(0 m)h² and d_A=∫₀^(z_(rec))dz/(H(z)) remain constant (for invariant CMB spectrum) and equal to their best fit Planck/ΛCDM values as H₀, Ω_(0 m) and w vary. For w=-1, this linear degeneracy equation leads to the best fit H₀=67.4 km sec⁻¹ Mpc⁻¹ as expected. For w=-1.22 the corresponding predicted CMB best fit Hubble constant is H₀=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 Ω_(0 m)h² is fixed. We verify the above H₀-w degeneracy equation by fitting a wCDM model with fixed values of w to the Planck TT spectrum showing also that the quality of fit (χ²) is similar to that of ΛCDM. However, when including SnIa, BAO or growth data the quality of fit becomes worse than ΛCDM when w< -1. Finally, we generalize the H₀-w(z) degeneracy equation for w(z)=w₀+w₁ z/(1+z) and identify analytically the full w₀-w₁ parameter region that leads to a best fit H₀=74 km sec⁻¹ Mpc⁻¹ in the context of the Planck CMB spectrum. This exploitation of H₀-w(z) degeneracy can lead to immediate identification of all parameter values of a given w(z) parametrization that can potentially resolve the H₀ tension.
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