When astronomers analyze a galaxy spectrum, how is the known wavelength of spectral lines like hydrogen used to calculate distance?
By looking at spectral lines whose shifted position confirms the light has been stretched by cosmic expansion
To determine the degree of stretching caused by cosmic expansion (redshift), astronomers use spectroscopy, which involves measuring specific spectral lines that correspond to known elements like hydrogen or calcium. In a stationary laboratory setting, these elements produce light at precise, known wavelengths. When observing a distant galaxy, astronomers look for these characteristic spectral features; if they appear shifted toward the longer, redder wavelengths compared to their known laboratory positions, this shift directly quantifies the amount the space expanded while the light traveled. This measurement allows astronomers to calculate the specific redshift value ($z$) and infer the distance and time elapsed since emission.
