  Special relativity is generalized to extra dimensions and quantized energy
levels of particles are obtained. By calculating the probability of particles'
motion in extra dimensions at high temperature of the early universe, it is
proposed that the branes may have not existed since the very beginning of the
universe, but formed later. Meanwhile, before the formation, particles of the
universe may have filled in the whole bulk, not just on the branes. This
scenario differs from that in the standard big bang cosmology in which all
particles are assumed to be in the 4D spacetime. So, in brane models, whether
our universe began from a 4D big bang singularity is questionable. A
cosmological constraint on the number of extra dimensions is also given which
favors $N\geq 7$.
