Observational constraints on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:mi>f</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mi>T</mml:mi><mml:mo stretchy="false">)</mml:mo></mml:math> theory

The f(T) theory, which is an extension of teleparallel, or torsion scalar T, gravity, is recently proposed to explain the present cosmic accelerating expansion with no need of dark energy. In this Letter, we first perform the statefinder analysis and Om(z) diagnostic to two concrete f(T) models, i.e., f(T)=α(−T)n and f(T)=−αT(1−epT0/T), and find that a crossing of phantom divide line is impossible for both models. This is contrary to an existing result where a crossing is claimed for the second model. We, then, study the constraints on them from the latest Union 2 Type Ia Supernova (Sne Ia) set, the baryonic acoustic oscillation (BAO), and the cosmic microwave background (CMB) radiation. Our results show that at the 95% confidence level Ωm0=0.272−0.032+0.036, n=0.04−0.33+0.22 for Model 1 and Ωm0=0.272−0.034+0.036, p=−0.02−0.20+0.31 for Model 2. A comparison of these two models with the ΛCDM by the χMin2/dof (dof: degree of freedom) criterion indicates that ΛCDM is still favored by observations. We also study the evolution of the equation of state for the effective dark energy in the theory and find that Sne Ia favors a phantom-like dark energy, while Sne Ia + BAO + CMB prefers a quintessence-like one.

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