Redshift is the stretching of light to longer wavelengths, shifting it toward the red end of the visible spectrum. It happens when the source of light is moving away from you, or when space itself is expanding between you and the source. Astronomers use redshift to measure how fast objects are receding, how far away galaxies are, and how the universe has evolved over billions of years.
How Redshift Works
Light travels in waves, and those waves have a measurable wavelength. Shorter wavelengths look blue or violet to our eyes; longer wavelengths look red. When something stretches those waves out, the light shifts toward the red end of the spectrum. That shift is redshift.
Think of a siren on an ambulance driving away from you. The sound waves get stretched out, making the pitch drop lower. Light behaves the same way. When a galaxy moves away from Earth, or when the space between us and that galaxy expands, the light waves arriving here are longer than they were when they left. Colors that started as ultraviolet or blue arrive looking redder, or even shift entirely out of the visible range into infrared.
Astronomers express redshift as a number called “z.” The formula is simple: z equals the difference between the observed wavelength and 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 number, the more the light has been stretched, and the farther away or faster-moving the source is.
Three Types of Redshift
Doppler Redshift
This is the most intuitive type. When an object physically moves away from an observer, its light waves get stretched. The faster the object recedes, the greater the stretch. This works the same way in reverse: an object approaching you compresses its light to shorter, bluer wavelengths. That opposite effect is called blueshift.
Cosmological Redshift
This is the big one in modern astronomy. The universe is expanding, and that expansion stretches the fabric of space itself. Light traveling through expanding space gets stretched along with it, even if neither the source galaxy nor Earth is moving in any particular direction. The longer light has been traveling, the more stretching it accumulates. A photon that left a distant galaxy billions of years ago has been riding through expanding space the entire time, arriving at our telescopes with a dramatically longer wavelength than it started with.
Cosmological redshift looks similar to Doppler redshift in practice, but the mechanism is different. It’s not that galaxies are flying through space away from us like shrapnel from an explosion. Instead, the space between galaxies is growing. A common analogy is raisins in a loaf of bread as it bakes: each raisin moves farther from every other raisin, not because the raisins are crawling through the dough, but because the dough itself is expanding.
Gravitational Redshift
Massive objects warp space and time, and light climbing out of a deep gravity well loses energy in the process. Losing energy means its frequency drops and its wavelength increases, producing a redshift. Light leaving the surface of the Sun, for example, is very slightly redshifted by the time it reaches Earth. Near a black hole, the effect is extreme. Photons get redder as they climb up through gravity, and bluer as they fall down into it.
How Astronomers Measure It
Every chemical element absorbs light at specific, known wavelengths. Hydrogen, for instance, absorbs at a characteristic set of wavelengths that shows up as dark lines in a spectrum. When astronomers split the light from a distant galaxy into its component wavelengths using a spectrometer, they see those same dark absorption lines, but shifted from their expected positions. By measuring exactly how far the lines have moved, they can calculate a precise z value.
This technique works because the pattern of lines is like a fingerprint. You can recognize hydrogen’s signature even when every line has been shifted to a longer wavelength. The amount of shift tells you the redshift, and the redshift tells you how fast the galaxy is receding or how much space has expanded since the light was emitted.
Redshift and the Expanding Universe
In the late 1920s, Edwin Hubble observed that nearly every galaxy he measured was redshifted, and that more distant galaxies had greater redshifts. This led to what’s now called the Hubble-Lemaître Law: a galaxy’s recession velocity is proportional to its distance from us. Double the distance, and the galaxy is moving away twice as fast. This was the first direct evidence that the universe is expanding.
The relationship is expressed as v = H₀ × d, where v is the galaxy’s recession velocity, d is its distance, and H₀ is a constant called the Hubble constant. This simple equation allows astronomers to estimate the distance to a galaxy just by measuring its redshift. Large surveys like the Sloan Digital Sky Survey have used this method to map the three-dimensional positions of millions of galaxies and quasars, building the most detailed picture we have of the universe’s large-scale structure.
What About Blueshift?
Not everything in the universe is redshifted. Objects close enough to us can be gravitationally pulled toward the Milky Way faster than the expansion of space pushes them away. The Andromeda Galaxy is the most famous example. It has a blueshift of z = -0.00042, meaning it’s approaching us at about 125 kilometers per second. In roughly four to five billion years, it will collide and merge with our galaxy.
Blueshifts only show up for nearby objects, typically within about 15 million light-years. Beyond that distance, the expansion of the universe dominates, and virtually everything is redshifted. If the universe were contracting instead of expanding, we would see blueshifts everywhere instead.
The Most Distant Objects Ever Seen
Redshift gives astronomers a way to peer back in time. Because light takes time to travel, a galaxy with a high redshift is not just far away; we’re seeing it as it existed in the distant past. The higher the z value, the earlier in the universe’s history we’re looking.
The cosmic microwave background, the faint glow left over from the early universe, has a redshift of about 1,100. That light was emitted roughly 380,000 years after the Big Bang, when the universe first cooled enough for atoms to form and light to travel freely. Its temperature today is just 2.726 Kelvin (about -270°C), stretched and cooled by the expansion of space over 13.8 billion years.
For individual galaxies, the James Webb Space Telescope has pushed the record to remarkable distances. In May 2024, NASA announced that a galaxy called JADES-GS-z14-0 had been confirmed at a redshift of 14.32, making it the most distant known galaxy. Its light has been traveling for over 13.5 billion years, meaning we’re seeing it as it existed just a few hundred million years after the Big Bang. The previous record holder, JADES-GS-z13-0, had a redshift of 13.2. Each new detection pushes our view closer to the very beginning of galaxy formation.
Why Redshift Matters
Redshift is one of the most important measurements in all of astronomy. It confirmed that the universe is expanding, which led directly to the Big Bang theory. It provides a distance scale for the cosmos, letting astronomers map structures spanning billions of light-years. It acts as a time machine, allowing telescopes to study galaxies as they looked billions of years ago. And in the late 1990s, careful redshift measurements of distant supernovae revealed that the expansion of the universe is accelerating, driven by something still not fully understood, commonly called dark energy.
Every time you see a news headline about a “most distant galaxy” or “earliest stars ever found,” redshift is the measurement behind it. It transforms a simple change in the color of light into a tool for understanding the size, age, and fate of the universe.