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 clarify that the notion of a comoving observer is well defined in the spacetime of a spinning black hole only in the sense of being asymptotic. The angular size of the shadow for a comoving observer is calculated. Significantly we find that the angular size approaches a non-zero finite value for a distant comoving observer, while it vanishes for a distant static observer. Furthermore, by adopting the approximate method proposed in \cite{Bisnovatyi-Kogan:2018vxl} we extend the study to the general multi-component universe case. The results show that the difference between the horizontal and vertical angular size changes a lot, while their ratio, i.e. the oblateness, keeps unchanged when the supermassive spinning black hole is at a high redshift, due to the common amplification factor exerted by the cosmic expansion. In addition, when $a=0$, our results agree with the previous studies in \cite{Perlick:2018iye,Bisnovatyi-Kogan:2018vxl}.

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