Distant galaxies recede faster than light in the standard cosmological description without breaking relativity: recession measures how the distance between widely separated places changes. It is not the speed of a galaxy passing a nearby observer. Locally, light still travels at the speed of light in a vacuum, and a massive object cannot overtake it.
The distinction matters whenever a headline describes an extremely distant galaxy as moving faster than light. The statement can be physically meaningful, but only after specifying what “distance” and “speed” mean.
Two different measurements of motion
Imagine measuring a spacecraft as it flies past your laboratory. You compare its position and time using clocks and rulers in your local frame. Relativity limits that local motion.
Cosmologists use another measurement for distant galaxies: their separation at the same cosmic time, in a model that averages over the universe’s large-scale structure. If that separation grows, its rate of change is called recession velocity. There is no single local laboratory stretching from here to the distant galaxy.
Ned Wright’s cosmology tutorial explains this distance convention. Thinking of a chain of neighboring observers helps: each measures a small part of the separation, while cosmology combines those parts into one distance. The resulting rate is not a spacecraft speed measured at one location.
A calculation that shows the difference
For objects following the smooth cosmic expansion, the Hubble–Lemaître relation is:
Recession velocity = expansion rate × proper distance.
Use an illustrative present-day expansion rate of 70 kilometers per second per megaparsec. A megaparsec is about 3.26 million light-years. At a proper distance of 5,000 megaparsecs, the calculation gives:
70 × 5,000 = 350,000 kilometers per second.
That exceeds light’s roughly 300,000 kilometers per second. It describes increasing separation in the cosmological model, not a galaxy locally accelerating through the light barrier. The chosen expansion rate is a rounded teaching value, not a new measurement or a resolution of disagreements over the Hubble constant.
Distance itself needs care. A present-day separation is different from the time light spent traveling to us; our guide to measuring cosmic distances introduces the measurements behind these estimates.
What a galaxy’s redshift actually tells us
A spectrum provides a measurement of shifted wavelengths, not a direct reading from a speedometer beside the galaxy. NASA’s explanation of cosmological redshift shows how expansion stretches light during its journey.
To turn that observation into the galaxy’s present distance and recession velocity, astronomers use an expansion model. This is why quoting a travel time, a present-day distance, and a speed as if they were interchangeable can create contradictory-looking numbers. The distinction in Davis and Lineweaver’s analysis is especially important for distant galaxies: a wavelength shift alone does not specify every quantity in the Hubble–Lemaître calculation.
Can we see a galaxy that recedes faster than light?
Yes. The distance where recession equals light speed is called the Hubble sphere. It is not generally the boundary of the observable universe.
In their analysis of cosmological horizons, Tamara Davis and Charles Lineweaver show how light from a region with faster-than-light recession can eventually reach us as the expansion history changes the geometry of its journey. At every point along that journey, a nearby observer measures the photon moving at light speed.
This does not mean every signal from every galaxy will arrive. Whether light emitted now can ever reach us is a different question, involving a cosmological event horizon and the universe’s future expansion.
What this allows—and what it does not
Cosmological recession provides no method for sending a spacecraft or message faster than a nearby beam of light. It also does not mean the Solar System expands in proportion to the distances between remote galaxies; gravitationally bound systems require their own dynamical description, as Wright’s cosmology FAQ explains.
When reading a claim about a galaxy’s speed, ask whether it describes local motion, cosmological recession, or an apparent motion inferred from an image. Those are different measurements, and substituting one for another creates the apparent paradox.
