  Recent astrophysical data indicate that our universe might currently be in a
de Sitter (dS) phase. The importance of dS space has been primarily ignited by
the study of the inflationary model of the universe and the quantum gravity. As
we know Einstein's theory of gravitation (with a nonzero cosmological constant)
can be interpreted as a theory of a metric field; that is, a symmetric tensor
field of rank-2 on a fixed de Sitter background. It has been shown that the
massless spin-2 Fierz-Pauli wave equation (or the linearized Einstein equation)
is not conformally invariant. This result is in contrary with what we used to
expect for massless theories. In this thesis we obtain conformally invariant
wave equation for the massless spin-2 in the dS space. This study is motivated
by the belief that conformal invariance may be the key to a future theory of
quantum gravity.
