Redshift is the stretching of light to longer wavelengths, making it appear shifted toward the red end of the spectrum. Astronomers use redshift to measure how fast distant galaxies are moving away from us and how much the universe has expanded while the light was traveling. This makes redshift one of the most important tools for studying the size, age, and history of the universe.
It connects a simple property of waves to some of the largest questions in astronomy.
Light from a galaxy carries a pattern of spectral lines from the atoms in its stars and gas. When those lines are observed at longer wavelengths than expected, the galaxy is redshifted. For nearby objects this can be caused by the Doppler effect, where motion away from Earth stretches the light waves.
For very distant galaxies, the main cause is cosmological redshift, where expanding space stretches the wavelength of light during its journey.
Key Facts
- Redshift means observed wavelength is longer than emitted wavelength.
- z = (lambda_observed - lambda_emitted) / lambda_emitted
- If z > 0, the light is redshifted and the source is usually moving away or light has been stretched by expansion.
- For small speeds, v approx cz, where c = 3.00 x 10^8 m/s.
- Cosmological redshift is caused by the expansion of space, not by galaxies moving through space like ordinary projectiles.
- A larger redshift usually means a more distant galaxy and a lookback time farther into the past.
Vocabulary
- Redshift
- Redshift is the increase in the wavelength of light compared with the wavelength at which it was emitted.
- Wavelength
- Wavelength is the distance from one crest of a wave to the next crest.
- Spectrum
- A spectrum is the spread of light by wavelength or color, often showing lines that reveal the atoms that produced or absorbed the light.
- Doppler Effect
- The Doppler effect is the change in observed wavelength or frequency caused by motion between a wave source and an observer.
- Cosmological Redshift
- Cosmological redshift is the stretching of light waves caused by the expansion of space as light travels across the universe.
Common Mistakes to Avoid
- Thinking redshift always means an object is moving through space away from Earth is wrong because distant galaxy redshift is mostly caused by the expansion of space itself.
- Using frequency and wavelength as if they both increase during redshift is wrong because redshift makes wavelength increase while frequency decreases.
- Forgetting to use the emitted wavelength in the denominator of z is wrong because redshift compares the wavelength change to the original wavelength.
- Assuming a redder-looking object must have a higher redshift is wrong because color can also depend on temperature, dust, and the mix of stars in the galaxy.
Practice Questions
- 1 A spectral line emitted at 500 nm is observed from a galaxy at 650 nm. Calculate the redshift z.
- 2 A nearby galaxy has redshift z = 0.020. Using v approx cz and c = 3.00 x 10^8 m/s, estimate its recession speed in m/s and km/s.
- 3 Two galaxies show the same hydrogen spectral line, but one line is shifted farther toward longer wavelengths. Explain what this suggests about the two galaxies and why redshift is useful for studying cosmic expansion.