Redshift

Redshift, change, or shift, in the light radiated by an object, such as a star or galaxy, that indicates the object’s motion. Scientists have used redshifts to measure the velocities (speed and direction) of distant galaxies. Knowing the velocities of galaxies helps astronomers understand how the universe is changing. This knowledge allows scientists to interpret the distant past of the universe and to predict the universe’s distant future. See also Light.

Redshift only occurs when an object is moving. Another mechanism can also redden the light of astronomical objects, but it is not considered to be the same as redshift. Dust particles between stars are just the right size to scatter light with short wavelengths more than they scatter light with long wavelengths. As the light of a star passes through a cloud of dust on the light’s way to Earth, more of the long, red wavelengths get through the dust than the short, blue wavelengths do. This makes the star appear redder than it really is, but the light that reaches Earth is the true red light of the star and has not actually shifted. See also Interstellar Matter.

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

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GALAXIES, REDSHIFTS, AND DISTANCES

Redshifts of galaxies allow astronomers to measure the distance from Earth to the galaxies. Knowing the distances to galaxies gives astronomers an idea of the way the universe is expanding and provides clues to the origin, evolution, and future of the universe. The relationship between the redshift (and therefore velocity) and distance of a galaxy is called Hubble’s law, which was named after American astronomer Edwin Hubble.

Hubble’s law states that the velocity (v) of a galaxy moving away from Earth is proportional to the galaxy’s distance (d) from Earth. As distance increases, velocity also increases. The constant value that relates velocity and distance is called Hubble’s constant and is usually written as H₀ or simply H. Hubble’s law, written mathematically, is v = H₀d.

Hubble identified the relationship between velocity and distance in 1929, but the numeric value of Hubble’s constant is still uncertain. Astronomers know that it falls between 64 and 78 kilometers per second per megaparsec (between 40 and 48 miles per second per megaparsec). A parsec is a unit of length equal to 30.86 trillion km (19.18 trillion mi), and a megaparsec is 1 million parsecs. Astronomers use these units to make redshift calculations easier.

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REDSHIFT AND COSMOLOGY

Redshift and Hubble’s law are vital tools to scientists who study the structure, evolution, and age of the universe. This field is called cosmology. Redshift provides astronomers with a good idea of the general motion of matter in the universe. Observing objects that do not follow Hubble’s law enables astronomers to see the motion of individual galaxies and groups of galaxies and provides useful information about the structure of the universe.

The gravitational pull of nearby galaxies affects the motion of a galaxy. Measurements of redshift reveal that the Great Andromeda Spiral Galaxy, one of the Milky Way’s nearest neighbors, is actually moving toward the Milky Way at about 50 km/s (about 30 mi/s). Neighboring groups of galaxies also affect each other’s motion. The Milky Way and its neighbors are called the Local Group. The Local Group’s neighbor, the Virgo Cluster, is moving away at only about 80 percent of the velocity predicted by Hubble’s law. Deviations from Hubble’s law (also called the Hubble flow) provide one of the best means of calculating the total density of matter in the universe.

Astronomers can also use redshift to identify the oldest and most distant objects in the observable universe. Astronomers believe that quasars are the most distant objects in the universe, because they have some of the largest redshifts. Quasars are objects in space that strongly emit radio waves. Astronomers originally named these objects quasars, which stands for quasi-stellar (or starlike) radio source, because they appear as points of light, like stars, in photographs of the sky. When astronomers began studying quasars in radio and other wavelengths, however, they discovered that quasars are not really starlike at all. They emit far more radiation, especially radio-wavelength radiation, than stars do, and quasars have huge redshifts. Their redshifts are so large that the radiation they emit in the ultraviolet range (with wavelengths shorter than visible light) reaches Earth in the infrared range (with wavelengths longer than visible light). The redshift for some of the most distant quasars is about 5.0, meaning that the shift in wavelength is about five times greater than the wavelength itself. A quasar with a redshift of 5.0 would be between about 3000 Mpc and 6000 Mpc away from Earth—so far away that light from the quasar would take between 9 billion and 19 billion years to reach Earth. Astronomers believe that quasars may be huge black holes, or regions that are so dense that not even light can escape their gravitational pull, surrounded by swirling matter. The matter swirling around black holes is very hot and is moving very quickly. Under these conditions, matter can produce light. This may be the source of quasars’ radiation.

If Hubble’s law holds for most of the age of the universe, Hubble’s constant would give an accurate age of the universe. The universe’s age would be the inverse of the constant (1 divided by Hubble’s constant), or between 12 billion and 16 billion years. However, astronomers have evidence that Hubble’s constant probably is not really constant—that the rate of expansion of the universe has changed and will keep changing as the universe evolves. The estimated age of the universe is actually only about 14 billion years.