Beyond ΛCDM with low and high redshift data: implications for dark energy

Assuming that the Universe at higher redshifts (z similar to 4 and beyond) is consistent with Lambda CDM model as constrained by the Planck measurements, we reanalyze the low red-shift cosmological data to reconstruct the Hubble parameter as a function of redshift. This enables us to address the H-0 and other tensions between low z observations and high z Planck measurement from CMB. From the reconstructed H(z), we compute the energy density for the dark energy sector of the Universe as a function of redshift without assuming a specific model for dark energy. We find that the dark energy density has a minimum for certain redshift range and that the value of dark energy at this minimum rho(min)(DE) is negative. This behavior can most simply be described by a negative cosmological constant plus an evolving dark energy component. We discuss possible theoretical and observational implications of such a scenario.