46.5 Billion Light-Years: The Redshift Boundary
The observable universe has a radius of 46.
Most people assume "observable" means "technologically observable"—a measurement problem we could solve with better telescopes. But push harder and the assumption inverts.
The observable universe's actual boundary isn't set by how far light has traveled. It's set by whether that light can still reach us at all. When the universe expands, distant galaxies don't just move away—the space between us and them stretches. Redshifts their light toward infrared, toward undetectability.
This redshift from expansion is lethal in a way time-of-light travel isn't. A photon that left a galaxy 30 billion light-years away started its journey in a more densely packed universe. As expansion proceeded, that photon got stretched—wavelength increased, energy decreased. Eventually it drops below what any detector can measure, no matter how sensitive.
The observable universe's actual boundary isn't set by how far light has traveled. It's set by whether that light can still reach us at all.
”Edwin Hubble measured cosmic expansion in 1929 using Cepheid variables in Andromeda. He wasn't looking for this limit—he was measuring recession velocity. But his data contained the answer to a question he wasn't asking. The 46. 5-billion-light-year radius marks where light becomes undetectable, not the edge of the universe but the edge of what can still communicate with us.
