Ancient light might be sketching a boundary. Not a wall of brick and mortar, but a finite bubble of spacetime whose size and curvature could, in principle, be read straight off the sky. The cosmic microwave background, a bath of photons released when the universe first turned transparent, already functions as a full‑sky survey of that geometry, carrying imprints of how space itself has stretched and bent.
The bold claim is simple. The angles of those faint hot and cold patches could reveal whether space closes in on itself like the surface of a sphere or flattens only over a limited volume before global curvature takes over. General relativity predicts that mass‑energy fixes the metric of spacetime, and that metric in turn dictates how photon geodesics thread across the cosmos, so even tiny departures from perfect flatness would shear the background pattern in statistically specific ways.
Skeptics argue the data already favor a nearly flat cosmos. Yet the current error bars still leave room for a large but finite bubble, just big enough to masquerade as endless. Precision measurements of angular power spectra and gravitational lensing in the microwave background act like a cosmic ruler and protractor, testing whether the largest observable scales subtly misalign. If they do, the sky would stop being an infinite canvas and become a map with edges implied, if not directly drawn.
