Reading the light of galaxies
When starlight is spread into a spectrum, it shows sharp lines at wavelengths set by the atoms that produced them. Hydrogen, for example, always emits its H-alpha line at 656.3 nm. Because these patterns are like fingerprints, astronomers can recognise them in the light of a distant galaxy, and measure exactly how far the whole pattern has shifted.
What redshift means
In the light of almost every distant galaxy, every line appears at a longer wavelength than in the laboratory: it is shifted towards the red end of the spectrum, a redshift. Redshift is written z, where1 + z = observed wavelength ÷ emitted wavelength. Every line in the spectrum is multiplied by the same factor. For large redshifts, visible lines move right out of the visible range into the infrared, and ultraviolet lines move into the visible.
Expanding space
In the 1920s, Georges Lemaître and Edwin Hubble found that the farther away a galaxy is, the larger its redshift. The explanation is that the universe is expanding. The distances between widely separated galaxies grow over time, not because the galaxies are flying through space away from a central point, but because space itself is stretching, everywhere at once. A common picture is raisins in a rising loaf: every raisin moves away from every other, and none of them is at the centre. Galaxies, stars, planets and atoms are held together by gravity and other forces, so they do not expand.
Stretching during the journey
Light from a distant galaxy takes millions or billions of years to reach us. While it travels, the space it crosses keeps expanding, and its waves are stretched along with it. When it finally arrives, its wavelength has grown by exactly the same factor as the universe:1 + z = (size of the universe now) ÷ (size when the light left). Light that left when the universe was half its present size arrives with z = 1, its wavelengths doubled. More distant galaxies show bigger redshifts because their light has been travelling longer while the universe expanded more.
Hubble’s law
For nearby galaxies, the rate at which a galaxy recedes is proportional to its distance:v = H₀ × d. The Hubble constant, H₀, is about 70 km/s per megaparsec: a galaxy 3.26 million light-years farther away recedes about 70 km/s faster. At large distances the relationship curves, because the expansion rate has changed over cosmic history: it slowed while matter dominated and has been speeding up for the last few billion years because of dark energy. The graph below the simulator shows this curve.
Expansion is not ordinary motion
A galaxy moving through space would also show a redshift, because of the Doppler effect. But that shift is fixed at the moment the light is emitted. Cosmological redshift builds up continuously during the light’s whole journey. The difference matters: very distant galaxies are now receding at more than the speed of light. This does not break relativity, which limits how fast things move through space, not how fast the space between distant objects can grow.
Formulas
1 + z = λ_observed ÷ λ_emitted = a(now) ÷ a(then)recession rate v = H₀ × d (Hubble–Lemaître law)for small z only: z ≈ v ÷ cHere a is the scale factor, the size of the universe relative to today (a = 1 now).
What redshift has revealed
- The Big Bang: running the expansion backwards shows the universe was once hot and dense, about 13.8 billion years ago.
- The cosmic microwave background: light released about 380,000 years after the Big Bang has been stretched about 1,100 times, from visible and infrared light into microwaves.
- Dark energy: in 1998, distant exploding stars revealed that the expansion is speeding up.
- The earliest galaxies: JWST observes in infrared because the light of the most distant galaxies, at redshifts above 10, has been stretched out of the visible.
Common misconceptions
- “The Big Bang was an explosion at a point in space.” It happened everywhere; there is no centre.
- “Everything expands, including us.” Bound objects such as galaxies and atoms do not.
- “Redshift is just galaxies speeding through space.” For distant galaxies it is the stretching of space during the light’s travel.
- “Nothing can recede faster than light.” Distant galaxies do, because space between us is expanding.
- “The galaxy is now as far away as its light travel time suggests.” It is much farther, because space kept expanding.
What the model simplifies
The expansion history is a standard flat universe containing matter and dark energy, with H₀ = 70 km/s/Mpc and 30% matter and 70% dark energy, ignoring radiation. It gives an age of about 13.5 billion years, slightly less than the 13.8 billion years from detailed measurements, and its distances and times are approximate. Distances are present-day (comoving) distances. The picture is two-dimensional and not to scale: the galaxies are drawn hugely enlarged, and the wave’s on-screen length only shows how much it has been stretched. Real galaxies also have small motions of their own through space, which add a little Doppler shift. The comparison mode uses the special-relativity Doppler formula for a galaxy moving through non-expanding space, chosen to give the same final redshift.
Key takeaways
- Redshift is the stretching of light to longer wavelengths: 1 + z = λ_observed ÷ λ_emitted.
- Cosmological redshift happens because space expands while the light travels; 1 + z equals how much the universe has grown since.
- More distant galaxies have larger redshifts; nearby galaxies follow Hubble’s law, v = H₀ × d.
- Expansion has no centre and does not stretch bound objects like galaxies or atoms.
- Expansion is not motion through space, so distant galaxies can recede faster than light.