Three thermodynamically based parametrizations of the deceleration parameter

We propose and constrain with the latest observational data, three model-independent parametrizations of the cosmic deceleration parameter q(z). They are well behaved and stay finite at all redshifts. We construct them by fixing the value of q at high redshift, q(z1)=1/2 (as demanded by cosmic structure formation), and at the far future q(z=-1)=-1, and smoothly interpolating q(z) between them. The fixed point at z=-1 is not arbitrarily chosen; it readily follows from the second law of thermodynamics. This fairly reduces the ample latitude in parametrizing q(z). © 2012 American Physical Society.