  Many of the essential features of the evolution of scientific research are
imprinted in the structure of citation networks. Connections in these networks
imply information about the transfer of knowledge among papers, or in other
words, edges describe the impact of papers on other publications. This inherent
meaning of the edges infers that citation networks can exhibit hierarchical
features, that is typical of networks based on decision-making. In this paper,
we investigate the hierarchical structure of citation networks consisting of
papers in the same field. We find that the majority of the networks follow a
universal trend towards a highly hierarchical state, and i) the various fields
display differences only concerning their phase in life (distance from the
"birth" of a field) or ii) the characteristic time according to which they are
approaching the stationary state. We also show by a simple argument that the
alterations in the behavior are related to and can be understood by the degree
of specialization corresponding to the fields. Our results suggest that during
the accumulation of knowledge in a given field, some papers are gradually
becoming relatively more influential than most of the other papers.
