258 The Poetry of Physics and The Physics of Poetry
motion of the dots on the surface of a balloon being blown up. From the
point of view of any dot all the other dots are moving away from it with a
velocity proportional to their distance. An analogy in three dimensions
often cited to describe the expanding universe is the one in which the
galactic clusters are likened to the raisins in a raisin bread which is
rising.
An observer sitting in any cluster of galaxies will observe all the
other clusters receding away from him at exactly the same rate as his
counterpart on any other cluster. The universe appears identical from the
point of view of any cluster. The universe is, therefore, more than just
symmetrical; it is also isotropic (the same in all directions) and uniform.
One part of the universe is the same as any other part. This assertion
about the nature of the universe is known as the cosmological principle
and forms the basis of all contemporary descriptions of the universe.
This more or less completes our description of the universe, as we
presently know it. It is a universe, which displays a hierarchical structure.
Stars and their satellites, i.e. planets are embedded in galaxies, which in
turn are embedded in clusters. One of the elements we left out of our
description of the universe is the immense space between the planets in
the solar system, between the stars within a galaxy, between the galaxies
within a cluster and between the clusters within the universe. Venus, our
closest neighbor in the solar system, is never less than 40 million
kilometers away. The nearest star is 4 million light years away; the
nearest galaxy is 1 million light years away. The universe is, therefore,
basically space but not empty space.
The space between the stars and the galaxies is filled with a very
dilute gas. The density of the interstellar gas is estimated to be 10-24
grams per cubic centimeter or one atom per cubic centimeter. The
density of water is one gram per cubic centimeter or 10^24 atoms per cubic
centimeter. The interstellar gas is primarily hydrogen and helium, which
represent about 68% and 30% of the gas by weight, respectively. The
remainder of the gas consists of sodium, potassium, calcium, carbon,
oxygen, nitrogen, iron and titanium. Even though the density of the
interstellar gas is very low, there is a great deal of it because of the large
volume of the galaxy, which is approximately 10^67 cubic centimeters.
The total amount of gas is, therefore, 10-24 × 10^67 grams or 10^43 grams
which represents 10^10 solar masses or 10% of the total mass of the
galaxy. In addition to the interstellar gas there are also interstellar dust
particles whose total mass and density is approximately one percent that