Redshift Blueshift

Scientists know that light travels as wave packets of vibrating electromagnetic energy. The packets are the quantum particles of light, called photons. A photon’s rate of vibration, called frequency, corresponds to wavelength and color. We perceive higher frequency, shorter electromagnetic waves as blue; and lower frequency, longer waves as red. Light appears stretched to longer wavelengths by a source that is moving away from us. Sound waves also show this effect. A person standing beside a road hears a drop in frequency, or pitch, of the sound caused by a passing car. Superheated atoms of stellar atmospheres emit light in characteristic frequency patterns. Astronomers use spectroscopes to detect these patterns, a kind of “fingerprint” of an atom. They match the light from moving stars to atomic spectra made in stationary laboratories. The difference, called redshift or blueshift, reveals the velocity of distant stars and galaxies. All the distant galaxies are redshifted. They are moving away from us, and those furthest move the fastest. Astronomers consider this important observation direct evidence that the universe is expanding.