Dark energy constraints from the cosmic age and supernova

Using the low limit of cosmic ages from globular cluster and the white dwarfs: t(0) > 12 Gyr, together with recent new high redshift supernova observations from the HST/GOODS program and previous supernova data, we give a considerable estimation of the equation of state for dark energy, with uniform priors as weak as 0.2 < Omega(m) < 0.4 or 0.1 < Omega(m)h(2) < 0.16. We find cosmic age limit plays a significant role in lowering the upper bound on the variation amplitude of dark energy equation of state. We propose in this Letter a new scenario of dark energy dubbed quintom, which gives rise to the equation of state larger than -1 in the past and less than -1 today, satisfying current observations. In addition we have also considered the implications of recent X-ray gas mass fraction data on dark energy, which favors a negative running of the equation of state. (C) 2004 Elsevier B.V. All rights reserved.