II.

What Is Redshift?

Light is made up of waves, and redshift is a change, caused by the object’s motion, in the wavelength of light radiated by an object. Redshifts occur because of a phenomenon scientists call the Doppler effect. The Doppler effect occurs when a wave-emitting object moves toward or away from an observer, and the observer sees or hears the waves differently than he or she would if the object were stationary relative to the observer. If a light-emitting object is moving away from an observer, each wave of light leaves the object from a point slightly farther away from the observer than the previous wave did. Therefore, the distance between waves (called the wavelength) that the observer sees is longer than it would be if the object were stationary. Austrian physicist Christian Johann Doppler described this effect in sound waves in the mid-1800s, and it became known as the Doppler effect for all types of waves.

In visible light, red light has the longest wavelength, and violet light the shortest. The light of an object moving away from an observer is shifted toward a longer wavelength, or toward the color red. The light from an object moving toward an observer is shifted toward the color violet. Astronomers most often use the Doppler effect to measure the velocity of galaxies, which are almost all moving away from Earth, so their light is shifted toward the color red. This is why astronomers call the effect redshift.

Astronomers can study redshift by separating an object’s light into its different colors. This technique is called spectroscopy and is similar to the way water vapor in the atmosphere separates the whitish light of the Sun into its different colors in a rainbow. Once the light of an object has been separated into its colors, scientists call the resulting rainbowlike display a spectrum. Chemical elements present in a light-emitting object produce bright and dark lines called emission and absorption lines on the object’s spectrum. These lines appear because atoms of different elements can only emit and absorb light at certain wavelengths. Spectroscopy helps astronomers learn about the chemical elements that make up an object, and, through the study of redshifts, the movement of the object.

When the light of a star or galaxy is redshifted, its entire spectrum is shifted by the same amount. Astronomers need some sort of marker to tell how far the light has shifted. Emission and absorption lines in the spectrum, created by the elements that make up the star, serve as markers. Certain elements occur in almost every astronomical object and provide handy reference points for measuring redshift. For instance, astronomers know that hydrogen is present in most stars and that it forms a characteristic pattern—which includes absorption lines at certain wavelengths—in the spectrum of an object that isn’t moving with respect to Earth. If this same pattern appears but is shifted toward the red end of the spectrum, scientists know the object is moving away from Earth.

Astronomers begin measuring redshifts by determining how much a chosen reference point, such as an emission or absorption line, has shifted. They define redshift as the amount the line has shifted divided by the wavelength of the original reference point (the place in the spectrum where the line should appear). This number (often abbreviated z) is equal to the velocity (v) of the object divided by the speed of light (c), so the mathematical formula for redshift is z = v/c. The speed of light is 300,000 km/s (190,000 mi/s). If the redshift of a star is 0.0001, the velocity of the star would be 0.01 percent of the speed of light, or about 30 km/s (19 mi/s).

If the velocity of an object is close to the speed of light, the equation for redshift, z = v/c, is no longer as simple, because the rules of relativity apply. German-born American physicist Albert Einstein developed the special theory of relativity in 1905 to explain how objects behave when their speeds are near the speed of light. The formula for redshift for objects with relativistic speeds is