The relationship between cause and effect. Typically, we assume that a given event is a result of events that came before it in time.
A specific type of Binary system in which both members are compact (meaning they are White Dwarfs, Neutron Stars, or Black Holes) and have roughly equal mass.
A long, heavy object from Quantum Field Theory or String Theory, which is very thin. They may have been created in the early life of the Universe, and would now stretch across the entire Universe. No cosmic string has ever been observed. They may or may not exist.
A mathematical device used by Einstein to keep the Universe from falling in on itself. He later called this his "greatest blunder", because it kept him from predicting the expansion of the Universe. Astrophysicists now believe there may be a use for the cosmological constant.