Ask Ethan: How can ultra-distant galaxies move so fast? – Image for illustrative purposes only (Image credits: Pixabay)
Astronomers have long observed that the most remote galaxies recede from Earth at apparent speeds greater than the speed of light. This observation stems from measurements of redshift in their light, which indicate rapid expansion of the space between them and us. The finding does not contradict Einstein’s theory of relativity, because the motion arises from the stretching of space itself rather than from travel through space at superluminal velocities.
Hubble’s Law and the Expanding Universe
Edwin Hubble’s 1929 discovery established that galaxies farther away move away faster, a relationship now known as Hubble’s law. The constant that quantifies this expansion has been refined over decades through observations of supernovae and the cosmic microwave background. Today’s value places the recession velocity of a galaxy at roughly 70 kilometers per second for every megaparsec of distance. At sufficiently large separations, this cumulative effect produces apparent speeds above light speed while local physics remains unchanged.
Why Relativity Still Holds
Special relativity prohibits objects from moving faster than light through space, yet the universe’s expansion operates on a different principle. Space itself stretches uniformly, carrying galaxies along without imparting any local velocity. A photon traveling between two points experiences no violation because the metric of spacetime expands around it. Calculations based on the Friedmann-Lemaître-Robertson-Walker metric confirm that no information or causal influence travels faster than light in this framework.
Observable Consequences Today
The most distant galaxies detected by the James Webb Space Telescope already show recession velocities well above light speed. Their light left them when the universe was far younger and smaller, yet the intervening space has since expanded dramatically. This means astronomers see these objects as they existed billions of years ago, while their current positions lie beyond our cosmic horizon. Future surveys will map even earlier epochs, tightening constraints on the expansion history without altering the underlying rules of relativity. The continued refinement of these measurements underscores that the universe’s large-scale behavior follows consistent physical laws, even when everyday intuition about speed and distance no longer applies.