A simplified approach for Chaplygin-type cosmologies
  A new class of accelerating cosmological models driven by a one-parameter version of the general Chaplygin-type equation of state is proposed. The simplified version is naturally obtained from causality considerations with basis on the adiabatic sound speed $v_S$ plus the observed accelerating stage of the universe. We show that very stringent constraints on the unique free parameter $\alpha$ describing the simplified Chaplygin model can be obtained from a joint analysis involving the latest SNe type Ia data and the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations (BAO). In our analysis we have considered separately the SNe type Ia gold sample measured by Riess et al. (2004) and the Supernova Legacy Survey (SNLS) from Astier et al. (2006). At 95.4% (c.l.), we find for BAO + \emph{gold} sample, $0.91 \leq \alpha \leq 1.0$ and $\Omega_{\rm{M}}= 0.28^{+0.043}_{-0.048}$ while BAO + SNLS analysis provides $0.94 \leq \alpha \leq 1.0$ and $\Omega_{\rm M}=0.27^{+0.048}_{-0.045}$. 
J.A.S. Lima (Univ. de Sao Paulo), J.V. Cunha (Univ. de Sao Paulo),   J.S. Alcaniz (Observatorio Nacional)