1. E.H.T. Collaboration et al., Astrophys. J. Lett. 875(1), L1 (2019)
2. F.W. Dyson, A.S. Eddington, C. Davidson, Philos. Trans. R. Soc Lond. Ser. A Contain. Pap. Math. Phys. Charact. 220, 291 (1920)
3. A. Vilenkin, E.P.S. Shellard, Cosmic Strings and Other Topological Defects (Cambridge University Press, Cambridge, 2000)
4. M. Barriola, A. Vilenkin, Phys. Rev. Lett. 63(4), 341 (1989)
5. W. Rindler, M. Ishak, Phys. Rev. D 76(4), 043006 (2007)