Threshold 1—Origins of Big Bang Cosmology ................................

LECTURE

As far as we know, all societies have asked this question: How did

everything begin? And as far as we know also, many societies have

offered deep, rich, and powerful answers to that question.

H

ow did everything begin? We have seen that evidence is fundamental
to modern scienti¿ c knowledge, so this lecture describes the slow
accumulation, over many centuries, of the evidence that provided
the foundations for modern answers to this most fundamental question.

We focus on the Western Christian tradition of knowledge within which the
modern answers emerged. Christian cosmology was dominated by a theology
that saw God as the creator of the Universe, coupled with an Earth-centered
model of the Universe created by a Roman/Egyptian astronomer, Ptolemy of
Alexandria (c. 90–168 C.E.).

Though alternative, Sun-centered models of the Universe also existed,
Ptolemy’s model succeeded partly because it was based on rigorous
astronomical observation and partly because it was backed by the church.
The Earth rested at the center of Ptolemy’s Universe. Around it were several
transparent crystalline spheres. They carried the Sun, stars, and planets,
and inhabited a perfect, heavenly realm that contrasted with the imperfect
“sub-lunar” realm of the Earth. Ptolemy’s model could explain most of the
movements of heavenly bodies, and also the fact that the Earth appeared to
be still. It also enjoyed the authority of the Catholic Church.

In the 16th century, it was challenged by new evidence and new theories.
Polish-born astronomer Nicolaus Copernicus (1473–1543) showed that some
problems of the Ptolemaic system (including the “retrograde” motions of the
planets: their apparent reversal of direction in each orbit) could be resolved
if we put the Sun, rather than the Earth, at the center of the Universe. A
German astronomer and mathematician, Johannes Kepler (1571–1630),
showed that the planets move in elliptical orbits, not in the perfect circles
required by Ptolemy’s system.