Big History: The Big Bang, Life on Earth, and the Rise of Humanity

Lecture 5: Threshold 1—Origins of Big Bang Cosmology

Galileo Galilei (1564–1642) used a new instrument, the telescope, to

show that heavenly bodies contained blemishes (such as sunspots). This

disproved Ptolemy’s claim that the heavens were a realm of perfection,

and hinted that the same fundamental laws might apply on Earth as in the

heavens. Galileo also showed that

we do not experience the Earth’s

motion through space because, like

passengers on a ship, we share in

that motion.

During the “Scienti¿ c Revolution”

of the 17th century, a new model of

the Universe was constructed. In

the Principia Mathematica, published in 1687, Isaac Newton (1642–1727)

explained the elliptical orbits of the planets as the result of a universal force

of attraction (or “gravity”) that existed between all objects, both in the

heavens and on Earth. Newton’s cosmology had no obvious limits in time

or space, suggesting that the Universe might be in¿ nitely large and in¿ nitely

old. But the idea of an in¿ nitely large Universe raised new problems. Kepler

had pointed out that if the Universe was in¿ nite it ought to be bathed in an

in¿ nite amount of heat and light, which was patently not true. The second

law of thermodynamics (see Lecture Three) suggested that the amount of

usable energy in the Universe was constantly diminishing. For 19th-century

astronomers this posed a fundamental problem, for in an in¿ nitely old

Universe there should be no usable energy left.

These problems were resolved early in the 20th century with the appearance

of an entirely new model of the Universe. In the late 1920s, American

astronomer Edwin Hubble (1889–1953) showed that the entire Universe

was expanding. His achievement rested on 19th century attempts to map the

position and movements of the stars. First, astronomers tried to estimate the

distances to the stars and the size of the Universe.

Hold your ¿ nger up at arm’s length, then move your head from side to side.

Your ¿ nger will seem to move against the background. That is “parallax.”

By measuring those movements you could, in principle, calculate the

Assuming that our part of the

Universe was typical, Hubble’s

discovery suggested that the

entire Universe was expanding.