The question “How close is the nearest planet to Earth?” is not a question in modern cosmology. Even a question on a larger scale, such as “What is the distance from the Milky Way to its nearest galaxy neighbor?” is not considered to be a question in cosmology. Cosmology deals with the average properties of our observable universe—the ones you obtain once you smooth out, over the range that our most powerful telescopes can reach. Even though galaxies tend to reside in small groups or in rich clusters, both held together by the force of gravity, once we sample large enough volumes, the universe appears to be very homogeneous and isotropic. In other words, there is no privileged position in the universe, and things look the same in all directions. Statistically speaking, any cosmic cube with a side of five hundred million light-years or larger would look roughly the same in terms of its contents, irrespective of its location in the universe. (One light-year is the distance light travels in one year, or about six trillion miles.) This broad-brush homogeneity becomes increasingly more accurate the larger the scale, up to the “horizon” of our telescopes. Cosmology deals with precisely those questions that would yield the same answer independent of the galaxy we happen to be in or the direction in which we happen to point our telescope.

Einstein had introduced the assumption of the large-scale homogeneity and isotropy of space in 1917, but this simplifying conjecture was elevated to the status of a fundamental principle in a paper published in 1933 by the English astrophysicist Edward Arthur Milne. Milne called his principle the “extended principle of relativity,” requiring that “not only the laws of nature but also the events occurring in nature, the world itself, must appear the same to all observers, wherever they be.” Today the stipulation of homogeneity and isotropy is known as the cosmological principle (a name coined by the German astronomer Erwin Finlay-Freundlich), and the most powerful direct evidence for its validity comes from observations of the “afterglow of creation”: the cosmic microwave background radiation. This radiation is a relic of the primeval hot, dense, and opaque fireball. It comes from all directions and is isotropic to better than one part in ten thousand. (In the words of astronomer Bob Kirshner: “much smoother than a baby’s bottom.)” Large-scale galaxy surveys also indicate a high degree of homogeneity. In all surveys that encompass a large enough slice of the cosmos to constitute a “fair sample,” even the most conspicuous structural features are dwarfed and smoothed out.

Since the cosmological principle has proven to be so effective when applied to different positions in space, it was only natural to wonder whether it could be extended to apply to time as well. That is, could one argue that the universe is unchanging in its large-scale appearance as well as in its physical laws? This was the big question raised by Hoyle, Bondi, and Gold in 1948. Amusingly enough, the illustrious trio may have been inspired to ask this question by a British horror film called Dead of Night. (Figure 24 shows the original poster of the film.) Here is how Hoyle himself described the sequence of events:

Figure 24