From Gineipaedia, the Legend of Galactic Heroes wiki
The "Three Graces" (Japanese: 三美神) are the foundation for modern interplanetary travel and colonization, consisting of subspace jump theory, which enables faster-than-light travel; gravitational control technology; and inertial control technology.
Developed sometime between the late 28th and early 29th centuries CE, the Three Graces gave humanity the stars, and ushered in a new golden age of deep space exploration and colonisation. This wave of expansion resulted in the birth of the Galactic Federation, and ultimately the expansion of humanity to the furthest reaches of the galaxy. (Legend of Galactic Heroes Volume 1: Dawn)