General relativity is a theory of gravity, where gravity emerges as a particular geometrical property of space-time. This geometrical property is called curvature. Similarly to a large sheet that warps under the weight of an object, mass and energy bend space-time and create curvature. In the absence of mass and energy the space-time is flat, and objects move in straight lines. Around a large body like the Earth, objects move following the curvature produced by the mass of the planet. Which means if you throw a stone, it eventually curves and falls to the ground, and does not move in a straight line. The elegant movements of celestial bodies in the cosmos are mostly orchestrated by mass and energy diligently following the (usually) gentle hills of space-time. Since moving mass and energy affect the curvature as well, the resulting dance is a complex, ever-changing choreography. The laws of this dance can be written in a very elegant form, called the Einstein’s Equation (\ref{einstein}), maybe one of the most beautiful equations in physics. \[\label{einstein} G_{\mu\nu}+\Lambda T_{\mu\nu}=\frac{8\pi G}{c^4}T_{\mu\nu}\]