For most of history, we viewed our world as a flat plane until observation triumphed over intuition. Today, we face a similar crossroads: space appears flat to our instruments, but could it possess a curvature so immense that it is imperceptible from our single vantage point? This episode explores the geometry and global structure of the cosmos, moving from Einstein's vision of a finite universe without boundaries to modern attempts to find "circles in the sky".

Our most powerful tool in this search is the Cosmic Microwave Background (CMB). By measuring the apparent size of hot and cold spots in this 13.8-billion-year-old light, cosmologists create a "cosmic triangle".

While the simplest tests for the universe's shape have come up empty, they set a new minimum scale for the cosmos. Any possible curvature or finiteness lies hidden beyond our current cosmic horizon.