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A Closer Look

Redshift and Hubble’s Law

s described in chapter 5, the Doppler ing toward the observer, a shift occurs in the distance. Farther-away galaxies were
Aeffect tells us that the frequency of a the spectral lines toward a higher frequency moving away from the Milky Way, but
wave depends on the relative motion of the (blueshift). If the star or galaxy is moving galaxies that are more distant are mov-
source and the observer. When the source away from the observer, a shift occurs in ing away faster than closer galaxies. This

and observer are moving toward each other, the spectral lines toward a lower frequency proportional relationship between galac-
the frequencies appear to be higher. If the (redshift). Thus, a redshift or blueshift in tic speed and distances was discovered
source and observer are moving apart, the spectral lines will tell you if a star or in 1929 by Hubble and today is know as
the frequency appears to be lower. galaxy is moving toward or away from you. Hubble’s law. The conclusion was that all
Light from a star or galaxy is changed One of the first accurate measure- the galaxies are moving away from one
by the Doppler effect, and the frequency of ments of the distance to other galaxies another, and an observer on any given
the observed spectral lines depends on the was made by Edwin Hubble at Mount galaxy would have the impression that
relative motion. The Doppler effect changes Wilson Observatory in California. When all galaxies were moving away in all di-
the frequency from what it would be if the Hubble compared the distance figures rections. In other words, the universe is
star or galaxy were motionless relative to with the observed redshifts, he found that expanding with component galaxies mov-
the observer. If the star or galaxy is mov- the recession speeds were proportional to ing farther and farther apart.

Among its many uses, the Hubble Space Telescope is used
to study young galaxies and galaxies that are on collision courses.
Based on these studies, astronomers today recognize that the dif-
ferent shapes of galaxies do not represent an evolutionary sequence.
The different shapes of galaxies are understood to be a result of
the different conditions under which the galaxies were formed.
The current model of how galaxies form is based on the
big bang theory of the creation of the universe. The big bang
theory considers the universe to have had an explosive begin-
ning. According to this theory, all matter in the universe was
located together in an arbitrarily dense state from which it be-
gan to expand, an expansion that continues today. Evidence
that supports the big bang theory comes from (1) present-day
microwave radiation from outer space, (2) current data on the
expansion of the universe, (3) the relative abundance of elements
that were altered in the core of older stars—this agrees with pre-
dictions based on analysis of the big bang, and (4) the Cosmic
Background Explorer (COBE) spacecraft, which studied diffuse
cosmic background radiation to help answer such questions as
how matter is distributed in the universe, whether the universe is
uniformly expanding, and how and when galaxies first formed.
The 2003 results from NASA’s orbiting Wilkinson Microwave
Anisotropy Probe (WMAP) produced a precision map of the
remaining cosmic microwave background from the big bang.
WMAP surveyed the entire sky for a whole year with a reso-
lution some 40 times greater than that of COBE. Analysis of
WMAP data revealed that the universe is 13.7 billion years old,
with 1 percent margin of error. The WMAP data found strong
support for the big bang and expanding universe theories. It
also revealed that the contents of the universe include 4 percent
ordinary matter, 23 percent of an unknown type of dark matter,
and 73 percent of a mysterious dark energy.
The initial evidence for the big bang theory came from
FIGURE 14.16 The Andromeda galaxy, which is believed to be Edwin Hubble and his earlier work with galaxies. Hubble had de-
similar in size, shape, and structure to the Milky Way galaxy. termined the distances to some of the galaxies that had redshifted

14-15 CHAPTER 14 The Universe 365