  We discuss the concept of measurement in cosmology from the relativistic and
quantum mechanical points of view. The uncertainty principle within the
particle horizon, excludes the momentum of particles to be less than $\pi\hbar
H/c$. This effect modifies the standard thermodynamics of early universe for
the ultra-relativistic particles such that the equation of state as well as
dependence of energy density and pressure to the temperature. We show that this
modification to the thermodynamics of early universe is important for energies
$E>10^{17} GeV$. During the inflation, the particle horizon inflates to a huge
size and makes the uncertainty in the momentum to be negligible.
