  We consider radiation from cosmological apparent horizon in
Friedmann-Lemaitre-Robertson-Walker(FLRW) model in a double-null coordinate
setting. As the spacetime is dynamic, there is no timelike Killing vector,
instead we have Kodama vector which acts as dynamical time. We construct the
positive frequency modes of the Kodama vector across the horizon. The
conditional probability that a signal reaches the central observer when it is
crossing from the outside gives the temperature associated with the horizon.
