  In Poincare gauge theory of gravity, in addition to mass-energy content, spin
is also a source for gravitational interactions. Although the effects of spin
are negligible at low energies, they can play a crucial role at very early
universe when the spin density was very high. In this paper by choosing a
suitable Lagrangian for Poincare gauge theory of gravity, and suitable
energy-momentum and spin density tensors, we show that the effects of spin and
torsion can lead to a inflationary phase without the need for any additional
fields. No fine tuning of parameters is required in this setup. We also
calculate the scalar spectral index at the end of inflation and show that it
agrees with the most recent observational data.
