  In the presence of a gravitational field decay rates may significantly differ
from flat space equivalent. By studying mutually interacting quantum fields the
decay rates can be calculated on a given spacetime. This paper presents the
calculation of the transition probability for the decay of a massive scalar
particle in a stiff matter dominated universe. We find that due to the precence
of a gravitational field a finite correction to the transition probability is
added which depends inversely on the mass. Moreover the decay rate is smaller
and lifetime of the particles is longer compared to flat space. The mass
dependence is such that the lifetime of lighter particles is prolonged more
compared to heavier particles. This result may be of significance when studying
cosmological situations involving stiff matter.

General Relativity and Quantum Cosmology;Cosmology and Nongalactic Astrophysics;High Energy Physics - Theory