Deep Space
Cosmological Redshift: How Old Light Reaches Webb
Calculate a simple redshift and distinguish stretched wavelengths, light travel time, and distance today.
Prepared with AI assistance using the primary sources linked below. How Astroboat works
Older light can arrive at a longer wavelength
Light takes time to travel, so a distant galaxy is seen as it was when its light left, not as it is now. While the light travels through an expanding universe, its wavelength stretches. Light that began in visible or ultraviolet wavelengths can arrive in the infrared. That is one reason Webb's infrared instruments are useful for studying early galaxies.
Reference: NASA Hubble: Cosmological Redshift; NASA Webb: Early Universe
Compare the measured line with its rest wavelength
Suppose a known spectral feature is emitted at 400 nanometres and is measured at 1,200 nanometres. Its wavelength is three times longer. Redshift is the observed wavelength divided by the rest wavelength, minus one, so z equals 2 in this illustrative example. Redshift is a ratio, not a number of years or kilometres. Astronomers look for matching features rather than guessing the rest wavelength from color alone.
Reference: NASA Hubble: Cosmological Redshift
A travel time and a present-day distance are different
Lookback time is how long the light has been traveling. Redshift describes the change in wavelength, and translating it into lookback time requires a model for the expansion history. Distance has several definitions in cosmology because the universe changes during the journey. When a headline gives a distance or an age, check which quantity it means; do not multiply a large redshift by the speed of light and call it the galaxy's velocity.
Reference: NASA Hubble: Cosmological Redshift; NASA Webb: Early Universe
Ask whether the estimate comes from a spectrum or colors
Broad-band colors can suggest a redshift, but dust and other effects can mimic some of the signatures. A spectrum can test the estimate by matching identified features at a consistent shift. When reading about an early-galaxy candidate, look for the measurement method and stated uncertainty. 'Earlier in cosmic history' is an inference from those measurements and a model, not a date stamped on the image.
Reference: NASA Webb: Early Universe