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spectra. From this expansive redshift, it was known that these How old is the universe? As mentioned earlier, astronomical
galaxies were moving away from the Milky Way. Hubble found a and physical “clocks” indicate that the universe formed in a “big
relationship between the distance to a galaxy and the velocity bang” some 13.7 billion years ago, expanding as an intense and
with which it was moving away. He found the velocity to be brilliant explosion from a primeval fireball with a temperature of
12
directly proportional to the distance; that is, the greater the some 10 kelvins. This estimate of the age is based on precise
distance to a galaxy, the greater the velocity. This means that measurements of the rate at which galaxies are moving apart, an
a galaxy twice as far from the Milky Way as a second galaxy expansion that started with the big bang. Astronomers used data
is moving away from the Milky Way at twice the speed as the on the expansion to back-calculate the age, much like running a
second galaxy. Since this relationship was seen in all directions, it movie backward, to arrive at the age estimate. At first, this tech-
meant that the universe is expanding uniformly. The same effect nique found that the universe was about 18 to 20 billion years old.
would be viewed from any particular galaxy; that is, all the other But then astronomers found that the universe is not expanding at
galaxies are moving away with a velocity proportional to the dis- a constant rate. Instead, the separation of galaxies is actually ac-
tance to the other galaxies. This points to a common beginning, celerating, pushed by a mysterious force known as “dark energy.”
a time when all matter in the universe was together. By adding in calculations for this poorly understood force, the
estimate of 13 to 14 billion years was developed.
Other age-estimating techniques agree with this ballpark
Myths, Mistakes, & Misunderstandings age. Studies of white dwarfs, for example, established their
rate of cooling. By looking at the very faintest and oldest white
Hello, Enterprise? dwarfs with the Hubble Space Telescope, astronomers were
able to use the cooling rate to estimate the age of the uni-
It is a myth that spaceships could have instant two-way commu- verse. The result found the dimmest of the white dwarfs to be
nication. Radio waves travel at the speed of light, but distances in
space are huge. The table below gives the approximate time re- about 13 billion years old, plus or minus about one-half bil-
quired for a radio signal to travel from Earth to the planet or place. lion years. More recently, data from the orbiting WMAP space-
Two-way communication would require twice the time listed. craft has produced a precision map of the cosmic microwave
background that shows the universe to be 13.7 billion years old.
Mars 4.3 min An age of 13.7 billion years may not be the final answer for the
age of the universe, but when three independent measurement
Jupiter 35 min
techniques agree closely, the answer becomes more believable.
Saturn 1.2 h
An often used analogy for the movement of galaxies after
Uranus 2.6 h
the big bang is a loaf of rising raisin bread. Consider galaxies as
Neptune 4 h
raisins in a loaf of raisin bread dough as it rises (Figure 14.17). As
Pluto 5.3 h
the dough expands, it carries along the raisins, which are moved
Across Milky Way galaxy 100,000 yr
farther and farther apart by the expansion. If you were on a rai-
To Andromeda galaxy 2,000,000 yr
sin, you would see all the other raisins moving away from you,
and the speed of this movement would be directly proportional

32 cm D
2 cm
B A

1 cm 16 cm 10 cm
B D
A
C
5 cm
C 1 cm 16 cm

5 cm

30 cm 60 cm
A Raisin bread dough B Raisin bread dough after rising
before rising
FIGURE 14.17 As the universe (here represented by a loaf of raisin bread) expands, the expansion carries galaxies (represented by the
raisins) away from one another at speeds that are proportional to their distances from one another. It doesn’t matter within which galaxy an
astronomer resides; the other galaxies all appear to be moving away.

366 CHAPTER 14 The Universe 14-16