expression, noun
event horizon
Astronomical term for the boundary around a black hole beyond which events cannot affect an outside observer.
ブラックホールの事象の地平線を越えると、光さえも脱出できない。
Once you cross the event horizon of a black hole, not even light can escape.
事象の地平線 is the boundary of a ブラックホール, not the black hole itself.
Calque of English 'event horizon', composed of 事象 (event, phenomenon) + の (possessive particle) + 地平線 (horizon).