We study the optical appearance of Schwarzschild--de Sitter and Reissner--Nordstr\"om--de Sitter black holes viewed by distant observers inside cosmological horizons. Unlike their asymptotically flat counterparts, due to the positive cosmological constant, there are outermost stable circular orbits in the spacetimes, resulting in significant outer edges in the images. Besides this, when the Reissner--Nordstr\"om--de Sitter black hole has a stable Cauchy horizon, the photons from the preceding companion universe can be received by the observer in our universe. These rays create a multiring structure in the image. Since the stable Cauchy horizon violates the strong cosmic censorship conjecture, this novel image sheds some light on the test of the conjecture by astronomical observations.
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Cao et al. (2024) studied this question.
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