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
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WHAT IS REDSHIFT?
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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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III
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GALAXIES,
REDSHIFTS, AND DISTANCES
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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.
IV
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REDSHIFT AND
COSMOLOGY
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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.
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