Exploring the low redshift universe: two parametric models for effective
  pressure
  Astrophysical observations have put unprecedentedly tight constraints on
cosmological theories. The $\Lambda$CDM model, mathematically simple and fits
observational data-sets well, is preferred for explaining the behavior of
universe. But many basic features of the dark sectors are still unknown, which
leaves rooms for various nonstandard cosmological hypotheses. As the pressure
of cosmological constant dark energy is unvarying, ignoring contributions from
radiation and curvature terms at low redshift, the effective pressure keeps
constant. In this paper, we propose two parametric models for non-constant
effective pressure in order to study the tiny deviation from $\Lambda$CDM at
low redshift. We recover our phenomenological models in the scenarios of
quintessence and phantom fields, and explore the behavior of scalar field and
potential. We constrain our model parameters with SNe Ia and BAO observations,
and detect subtle hints of $\omega_{de}<-1$ from the data fitting results of
both models, which indicates possibly a phantom dark energy scenario at
present.
