  We study glueballs in the holographic gauge theories living in a curved
space-time. The dual bulk is obtained as a solution of the type IIB superstring
theory with two parameters, which correspond to four dimensional (4D)
cosmological constant $\lambda$ and the dark radiation $C$ respectively. The
theory is in the confining phase for $\lambda <0$ and small $C$, then we
observe stable glueball states in this theory. However, the stability of the
glueball states is lost when the density of the dark radiation ($C$) increases
and exceeds a critical point. Above this point, the dark radiation works as the
heat bath of the Yang-Mills theory since the event horizon appears. Thus the
system is thermalized, and the theory is in a finite temperature deconfinement
phase, namely in the QGP phase. We observe this transition process through the
glueball spectra which varies dramatically with $C$. We also examined the
entanglement entropy of the system to find a clue of this phase transition and
the role of the dark radiation $C$ in the entanglement entropy.
