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.
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.