Redshift means an object is moving away from you. When light from a star or galaxy is redshifted, its wavelengths have been stretched longer, pushing the light toward the red end of the spectrum. The opposite, blueshift, means an object is moving toward you. Together, these two shifts tell astronomers the direction and speed of nearly everything in the observable universe.
Why “Red” Means “Away”
Light travels in waves, and the color you see depends on the wavelength. Red light has longer wavelengths, while blue and violet light have shorter ones. When a light source moves away from you, each successive wave crest has a little farther to travel before it reaches your eyes, so the waves get stretched out. Longer waves mean the light shifts toward the red end of the spectrum. If the source moves toward you instead, the waves get compressed, shifting the light toward the blue end.
This is the same principle behind the changing pitch of a passing ambulance siren. As the ambulance approaches, sound waves compress and the pitch rises. As it drives away, the waves stretch and the pitch drops. Light behaves the same way, just with color instead of pitch.
How Astronomers Measure It
Every chemical element absorbs and emits light at very specific wavelengths, creating a unique fingerprint pattern in the spectrum. Hydrogen, for example, always produces the same set of lines. When astronomers split the light from a distant galaxy into its spectrum, they look for these known patterns. If the entire pattern is shifted toward longer (redder) wavelengths compared to where it appears in a lab on Earth, the galaxy is redshifted.
The amount of shift is expressed as a number called z. The formula is straightforward: z equals the observed wavelength minus the original wavelength, divided by the original wavelength. A z of 0 means no shift at all. A z of 1 means the light’s wavelength has doubled. The higher the z value, the faster the object is receding and, in cosmology, the farther away it is.
Three Different Causes of Redshift
Not all redshift comes from the same mechanism. The most familiar type is the Doppler redshift, caused by an object physically moving through space away from you. A star drifting away from Earth within our own galaxy, for instance, will show a small Doppler redshift.
The second type, cosmological redshift, is responsible for almost all the redshift astronomers observe in distant galaxies. It isn’t caused by galaxies flying through space like rockets. Instead, the fabric of space itself is expanding, and as it does, light waves traveling through that space get stretched along with it. The farther away a galaxy is, the more its light has been stretched during its journey, and the higher its redshift. Edwin Hubble first noticed this pattern in the 1920s: more distant galaxies consistently showed greater redshifts, which became the key evidence that the universe is expanding.
The third type is gravitational redshift. A strong gravitational field, like the one near a black hole or a very dense star, slows the passage of time. Light climbing out of that gravity well loses energy, and since it must always travel at the speed of light, the only way it can lose energy is by stretching to a longer wavelength. This type of redshift has nothing to do with motion at all. It depends entirely on the difference in gravitational strength between where the light was emitted and where it’s received.
Blueshift: The Opposite Direction
When an object moves toward us, its light waves compress to shorter wavelengths, shifting toward the blue end of the spectrum. This is called blueshift. In an expanding universe, blueshift is relatively rare because most galaxies are moving away from us. The galaxies that do show blueshift are nearby ones whose local motion through space (pulled by gravity) is strong enough to override the overall expansion. The Andromeda Galaxy is the most famous example: it’s blueshifted because it’s on a collision course with the Milky Way, approaching at roughly 110 kilometers per second.
What the Highest Redshifts Tell Us
The most extreme redshifts correspond to the most distant, oldest objects we can see. The James Webb Space Telescope identified a galaxy called JADES-GS-z14-0 at a redshift of 14.32, meaning its light has been stretched more than fifteenfold since it was emitted. That light left the galaxy only about 290 million years after the Big Bang, making it the most distant known galaxy as of its confirmation in 2024.
At redshifts above roughly z = 1, the math reveals something counterintuitive: the recession velocity of a galaxy actually exceeds the speed of light. This doesn’t violate relativity because the galaxy isn’t moving through space faster than light. Space itself is expanding between us and the galaxy, and there’s no speed limit on that expansion. The light still reaches us because it was emitted when the galaxy was much closer, and it has been gradually stretched during the billions of years it spent crossing the growing distance.
For objects within our own gravitationally bound neighborhood, called the Local Group, cosmological redshift doesn’t apply. These nearby galaxies, including Andromeda and the Milky Way’s satellite galaxies, are held together by gravity and don’t participate in the universal expansion. Their redshifts or blueshifts are purely from local motion.