bei48482_FM

14 Chapter One

proportional to distance, which suggests that the entire universe is expanding (Fig. 1.8).

This proportionality is called Hubble’s law.

The expansion apparently began about 13 billion years ago when a very small, in-

tensely hot mass of primeval matter exploded, an event usually called the Big Bang.

As described in Chap. 13, the matter soon turned into the electrons, protons, and neu-

trons of which the present universe is composed. Individual aggregates that formed

during the expansion became the galaxies of today. Present data suggest that the current

expansion will continue forever.

Example 1.3

A distant galaxy in the constellation Hydra is receding from the earth at 6.12  107 m/s. By

how much is a green spectral line of wavelength 500 nm (1 nm  10 ^9 m) emitted by this

galaxy shifted toward the red end of the spectrum?

(b)

(a)

Approximate distance, light-years

Recession speed, km/s

01234 ✕ 109

2

4

6

8 ✕ 104

Figure 1.8(a) Graph of recession speed versus distance for distant galaxies. The speed of recession

averages about 21 km/s per million light-years. (b) Two-dimensional analogy of the expanding uni-

verse. As the balloon is inflated, the spots on it become farther apart. A bug on the balloon would

find that the farther away a spot is from its location, the faster the spot seems to be moving away;

this is true no matter where the bug is. In the case of the universe, the more distant a galaxy is from

us, the faster it is moving away, which means that the universe is expanding uniformly.

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