  We investigate the holographic quantum entanglement of a visible universe in
an inflationary cosmology. To do so, we consider an AdS space with a dS
boundary which represents an expanding space in time. In an inflationary
cosmology, there exists a natural entangling surface called a cosmic event
horizon which divides a universe into visible and invisible parts. In this
model, cosmic event horizon monotonically decreases and approaches a constant
value proportional to the inverse of Hubble constant. We show that the quantum
entanglement between the visible and invisible universes divided by cosmic
event horizon decreases monotonically in time. After an infinite time
evolution, it finally approaches a constant value which is proportional to the
inverse square of the Hubble constant for a four-dimensional dS space.
