Distant galaxies recede faster than light — and we see them

Every galaxy beyond a redshift of roughly 1.4 to 1.5 is receding from us faster than the speed of light today, and we can still see them. No law is broken: nothing outruns light through space, but the expansion of space between us and those galaxies has no speed limit.

How can a galaxy recede faster than light?

Because special relativity caps only motion measured locally, against nearby space. A cosmological recession velocity is something else: the rate at which the distance between us grows as the universe expands. As UCLA cosmologist Ned Wright notes, that distance and the age of the universe are not the quantities special relativity deals in, so there is no contradiction. The exact threshold depends on the model: z ≈ 1.46 in Tamara Davis and Charles Lineweaver’s analysis, z ≈ 1.41 in Wright’s.

Can we see galaxies receding faster than light?

Yes, despite what some books say. In their paper Expanding Confusion, cosmologists Tamara Davis and Charles Lineweaver collected misleading statements from popular books and astrophysics textbooks, including the claim that galaxies receding faster than light must be invisible. Light such a galaxy sends our way initially loses ground, swept backwards by expansion. But the Hubble sphere, the distance at which recession equals light speed, grows over time. Once it overtakes the photons, they sit in slower-receding space and begin closing in. Any light we now see that was emitted in the universe’s first five billion years started out in a region receding faster than light.

The extreme case is the cosmic microwave background. The matter that emitted it is receding at about 3.2 times the speed of light today, and was receding at roughly 58 times light speed when the light left it, in a model of 30% matter and 70% dark energy.

The effect is testable. Treating redshifts as ordinary special-relativistic Doppler shifts would make distant supernovae the wrong brightness, and Davis and Lineweaver rule that reading out at a confidence level of 23 sigma.