Light from a remote galaxy has only had a limited time to travel, yet the galaxy’s current distance can far exceed that light‑travel time measured in light‑years. The key is that space itself has been expanding while the light was en route. A light‑year is a measure of distance assuming a fixed stage, but cosmology plays out on a moving, stretching stage.
General relativity treats gravity as the curvature of space‑time, allowing the metric that defines distance to change. During the journey of a photon, the underlying metric expands, so the comoving distance to its source grows beyond the simple product of speed and travel time. This is captured in models that use the Friedmann equations and a scale factor to track how much the universe has stretched.
When astronomers quote a galaxy more than 46 billion light‑years away, they refer to its current proper distance, not how long its light has traveled. The observed cosmological redshift encodes the integrated expansion history: higher redshift means the light left when the scale factor was smaller. Look‑back time and present‑day distance diverge because expansion has been compounding the gap the entire way.
