Since September 14, 2015, we have been exploring a previously unknown side of the universe with the help of ripples in spacetime: gravitational waves. The major gravitational-wave observatories – LIGO, Virgo, and, more recently, KAGRA – have detected several hundred signals during four observing runs, released by the orbiting and merging of various binary systems. The results are astonishing: the universe is full of black holes, which come in various sizes and tell different stories of their origins. The dance of neutron stars has also already been observed.
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