  We use published measurements of the transmission power spectrum of the Lyman
alpha forest to constrain several parameters that describe cosmology and
thermal properties of the intergalactic medium (IGM). A 6 parameter grid is
constructed using Particle-Mesh dark matter simulations together with scaling
relations to make predictions for the gas properties. We fit for all parameters
simultaneously and identify several degeneracies. We find that the temperature
of the IGM can be well determined from the fall-off of the power spectrum at
small scales. We find a temperature around 20.000 K, dependent on the slope of
the gas equation of state. We see no evidence for evolution in the IGM
temperature. We place constraints on the amplitude of the dark matter
fluctuations. However, contrary to previous results, the slope of the dark
matter power spectrum is poorly constrained. This is due to uncertainty in the
effective Jeans smoothing scale, which depends on the temperature as well as
the thermal history of the gas.
