Shadow of a Spinning Black Hole in an Expanding Universe.

We study the influence of the cosmic expansion on the size of the shadow of a spinning black hole observed by a comoving observer. We first consider that the expansion is driven by a cosmological constant only and build the connection between the Kerr-de Sitter metric and the FLRW metric. We then calculate the angular size of the shadow for an observer comoving with the cosmic expansion. Furthermore, by adopting the approximate method proposed in \cite{Bisnovatyi-Kogan:2018vxl} we extend the study to the general multi-component universe case. It is interesting to find that the oblateness of the black hole shadow, namely the ratio of the horizontal and vertical angular radii, becomes significant in the case that the black hole is at a high redshift.