The Redshift
The light that reaches our telescopes has been traveling for millions or billions of years. By the time it gets here, it has changed. The space it passed through has expanded, and that expansion stretches the light itself. What was emitted as blue or ultraviolet may arrive as yellow, or red, or infrared. This is cosmological redshift — not the Doppler shift of a siren speeding away, but the stretching of spacetime that carries the photon along for the ride.
How it works
Imagine a wave — a sine curve traveling through space. Crests and troughs, a wavelength $\lambda$ separating them. Now imagine that while the wave is in transit, the space it occupies expands. The wave stretches with it. The distance between crests gets longer. The wavelength increases. And because longer wavelengths mean redder light, astronomers call it redshift.
It is not the source moving through space. It is space moving through the source. There is a difference. Doppler redshift happens when a galaxy moves through a static space — like a car driving away from you, stretching the sound waves. Cosmological redshift happens when the space itself grows, carrying the galaxy and the light along. The photon does not know it is being stretched. It simply arrives at our detector with a longer wavelength than when it was born.
The redshift parameter $z$ is defined as the fractional change in wavelength: $z = (\lambda_{observed} - \lambda_{emitted}) / \lambda_{emitted}$. If a spectral line that was emitted at 500 nanometers arrives at 1000 nanometers, the redshift is $z = 1$. A galaxy at $z = 3$ is emitting light that has been stretched to four times its original wavelength — ultraviolet visible light that we detect as infrared.
Redshift and distance
Cosmological redshift is a distance indicator. The farther away a galaxy is, the more its light has been stretched, the higher its redshift. This is why redshift has become one of the primary tools of observational cosmology. We do not need to know a galaxy's intrinsic brightness or its exact distance — we measure its spectrum, find the redshift, and we have a coordinate in cosmic history.
The relationship between redshift and distance is not simple at large values. At low redshift ($z < 0.1$), redshift is approximately proportional to distance, and Hubble's Law applies directly. At higher redshift, you need the full cosmological model — the density parameters, the expansion history, the curvature of space — to convert redshift into a distance. But even then, redshift is the one quantity you can measure directly. Everything else is inferred.
Seeing the universe's growth record
Every observation of a distant galaxy carries a redshift stamp. And by collecting thousands of them, we can reconstruct the expansion history of the universe. Redshift surveys — mapping millions of galaxies across the sky — have revealed the cosmic web: filaments of galaxies separated by vast voids. The distribution of redshifts tells us how structure has grown over time, how gravity pulled matter into clusters and superclusters, and how the expansion of space has worked against that pull.
Type Ia supernovae — the standard candles of the cosmos — were observed at high redshift in the late 1990s, and the results changed everything. They were dimmer than expected. They were farther away than they should have been if the universe had been expanding at a constant rate. Something had changed — the expansion was accelerating. The discovery led to the concept of dark energy, a repulsive component of the universe that now dominates its dynamics. And it was discovered through redshift.
The limits of redshift
Redshift is powerful but not omniscient. Peculiar velocities — the motion of galaxies relative to the Hubble flow — contaminate the signal at low redshift. A galaxy falling into a cluster may have its redshift shifted by its own motion, independent of cosmic expansion. Gravitational redshift — photons losing energy as they climb out of a gravitational well — adds another small contribution. At very high redshift, the expansion of the universe stretches light so far into the infrared that our telescopes struggle to detect it.
But the fundamental simplicity remains. The universe expands. Light stretches. And by reading the stretched light, we can reconstruct the story of every epoch — from the first galaxies to the present day. Redshift is not just a measurement. It is a time machine. Every spectrum is a message from the past, carrying information that has survived the stretching of billions of years of cosmic expansion.