Redshift is the stretching of light to longer wavelengths, which makes light from distant galaxies appear redder than when it was emitted. In astronomy, this is one of the strongest clues that space itself is expanding. By measuring how much galaxy light is shifted, scientists can estimate how fast that galaxy is receding from us.
Redshift connects observations in a telescope to the large-scale history of the universe.
Cosmological redshift happens because the wavelength of light expands as the universe expands during the light's journey. This is different from a galaxy simply moving through space, since the space between galaxies is also growing. Hubble's law shows that more distant galaxies generally recede faster, creating a pattern expected in an expanding universe.
Together with the cosmic microwave background and galaxy surveys, redshift measurements support the Big Bang model.
Understanding Physics: Redshift and the Expanding Universe
Astronomers measure redshift with a spectrograph, an instrument that separates incoming light into its component colours. Atoms and molecules make dark or bright spectral lines at very specific wavelengths. Hydrogen, calcium, oxygen, and other elements each leave a recognisable pattern.
Laboratory measurements provide the reference pattern. When the same set of lines from a galaxy appears displaced toward the red end of the spectrum, the amount of displacement can be measured accurately. Using many lines matters because a single feature might be misidentified or affected by dust, detector noise, or light from Earth’s atmosphere.
The size of the shift tells astronomers about the change in the universe during the photon’s journey. A redshift value of one means the observed wavelength is twice the emitted wavelength. This implies that the universe was about half its present size when that light was released.
At very large redshifts, it is misleading to treat the result as an ordinary speed measurement using the simple Doppler rule. Relativity gives the better description.
The distance between very remote galaxies can grow faster than light because it is space between them that grows. This does not mean that either galaxy travels through its local space faster than light.
Redshift alone does not produce an exact distance for every nearby galaxy. Galaxies have local motions caused by gravity. Our Milky Way, for example, is moving relative to nearby galaxies, while galaxies in a cluster orbit within the cluster’s shared gravity.
These motions can add to or subtract from the expansion signal. For nearby objects, astronomers often compare redshift with independent distance methods.
These include Cepheid variable stars and certain supernovae with known brightness patterns. Farther away, where the general expansion is much larger than local motions, redshift becomes a more useful guide to cosmic distance.
Looking at high redshift galaxies is a form of looking back in time. Their light may have travelled for billions of years before entering a telescope. Astronomers can therefore sort galaxies into different eras and study how stars, galaxies, and clusters developed.
Redshift surveys map millions of galaxies in three dimensions, revealing large filaments and emptier regions in the cosmic web. Measurements of distant supernovae showed that the expansion rate has changed over time and is now accelerating.
When learning this topic, keep wavelength, distance, travel time, and recession motion separate. They are connected, but they are not interchangeable quantities.
Key Facts
- Redshift means observed wavelength is longer than emitted wavelength.
- Redshift is defined by z = (λobserved - λemitted) / λemitted.
- For small speeds, Doppler redshift is approximately v / c = Δλ / λ.
- Hubble's law is v = H0 d, where v is recession speed, H0 is the Hubble constant, and d is distance.
- A larger redshift usually means the galaxy is farther away and its light has traveled longer.
- Cosmological redshift is caused by the expansion of space stretching the wavelength of light.
Vocabulary
- Redshift
- Redshift is the increase in the wavelength of light, often seen when light from a distant galaxy is stretched toward the red end of the spectrum.
- Cosmological redshift
- Cosmological redshift is the redshift caused by the expansion of the universe while light travels through space.
- Hubble's law
- Hubble's law states that a galaxy's recession speed is proportional to its distance from us.
- Recession speed
- Recession speed is the rate at which a distant galaxy appears to move away because the space between galaxies is expanding.
- Hubble constant
- The Hubble constant is the proportionality value in Hubble's law that relates galaxy distance to recession speed.
Common Mistakes to Avoid
- Treating all redshift as ordinary Doppler motion is wrong because cosmological redshift comes from the expansion of space, not only motion through space.
- Thinking galaxies expand along with the universe is wrong because tightly bound systems like galaxies, solar systems, and atoms are held together by gravity or other forces.
- Using Hubble's law for nearby objects like planets is wrong because the law describes the large-scale motion of distant galaxies, not local gravitational systems.
- Assuming redshift directly gives distance without a model is wrong because converting redshift to distance depends on the expansion history of the universe, especially at large redshift.
Practice Questions
- 1 A spectral line emitted at 500 nm is observed from a galaxy at 550 nm. Calculate the redshift z.
- 2 Using H0 = 70 km/s/Mpc, estimate the recession speed of a galaxy 200 Mpc away using Hubble's law.
- 3 Explain why a raisin bread model, where raisins move apart as the dough rises, is a useful analogy for the expanding universe, and identify one way the analogy is limited.