Dark Energy and Fate of the Universe
  We explore the ultimate fate of the Universe by using a divergence-free
parametrization for dark energy $w(z)=w_0+w_a({\ln (2+z)\over 1+z}-\ln2)$.
Unlike the CPL parametrization, this parametrization has well behaved, bounded
behavior for both high redshifts and negative redshifts, and thus can genuinely
cover many theoretical dark energy models. After constraining the parameter
space of this parametrization by using the current cosmological observations,
we find that, at the 95.4% confidence level, our Universe can still exist at
least 16.7 Gyr before it ends in a big rip. Moreover, for the phantom energy
dominated Universe, we find that a gravitationally bound system will be
destroyed at a time $t \simeq P\sqrt{2|1+3w(-1)|}/[6\pi |1+w(-1)|]$, where $P$
is the period of a circular orbit around this system, before the big rip.
