Redshift is one of the strongest clues that the universe is expanding. When astronomers study light from distant galaxies, they find that familiar spectral lines are shifted toward longer, redder wavelengths. This matters because the shift is not just a change in color, but a measurement of how much space has stretched while the light traveled to us.
The farther away a galaxy is, the more its light is usually redshifted.
Key Facts
- Redshift is defined by z = (observed wavelength - rest wavelength) / rest wavelength.
- A positive redshift means light has been stretched to longer wavelengths.
- For small redshifts, recession speed is approximately v = cz, where c is the speed of light.
- Hubble's law states v = H0d, where v is recession speed, H0 is the Hubble constant, and d is distance.
- Cosmological redshift happens because space itself expands while light is traveling.
- Greater redshift usually means the light came from farther away and from an earlier time in the universe.
Vocabulary
- Redshift
- Redshift is the increase in the wavelength of light from an object, often seen when a galaxy is moving away or when space expands.
- Spectral line
- A spectral line is a specific wavelength of light emitted or absorbed by an atom or molecule, acting like a fingerprint for identifying elements.
- Cosmological redshift
- Cosmological redshift is the stretching of light caused by the expansion of space between the source and the observer.
- Hubble's law
- Hubble's law is the relationship that more distant galaxies recede faster, written as v = H0d.
- Lookback time
- Lookback time is how far into the past we are seeing an object because its light took time to reach Earth.
Common Mistakes to Avoid
- Confusing redshift with a galaxy simply turning red is wrong because redshift means specific wavelengths and spectral lines are stretched, not that the whole galaxy changes paint-like color.
- Using v = cz for every redshift is wrong because that approximation works best only for small redshifts and breaks down for very distant galaxies where relativity and cosmic expansion matter.
- Thinking galaxies are flying through preexisting empty space is incomplete because cosmological redshift mainly comes from the stretching of space itself between distant galaxies.
- Forgetting to compare observed wavelength to rest wavelength is wrong because redshift is measured from a known spectral line, not from the observed wavelength alone.
Practice Questions
- 1 A hydrogen spectral line has a rest wavelength of 656 nm but is observed from a galaxy at 722 nm. Calculate the redshift z.
- 2 A galaxy has redshift z = 0.03. Using c = 300,000 km/s and the small-redshift approximation v = cz, estimate its recession speed.
- 3 Two galaxies have the same type of hydrogen spectral line, but Galaxy A's line is shifted slightly toward red while Galaxy B's line is shifted much farther toward red. Explain which galaxy is likely farther away and what this says about the expansion of the universe.