The Astronomy Book

171

Lemaître’s model of a universe expanding from an initial
extremely dense concentration of mass and energy is today called
the Big Bang model of the universe. Although Lemaître described
the initial stages of the process as an “explosion,” the prevailing
view today is that expansion is a fundamental quality of space
itself and this carries galaxies away from each other,
rather than being projected by the initial
explosion into a preexisting void.

giving rise to space and time on
“a day without yesterday.” Lemaître
described the beginning of the
universe as a burst of fireworks,
comparing galaxies to the burning
embers spreading out from the
center of the blast.
The proposal initially met
with scepticism. Einstein found
it suspect but was not altogether
dismissive. In January 1933,
however, Lemaître and Einstein
traveled together to California for
a series of seminars. By this time,
Einstein (who had removed the
cosmological constant from his
general theory of relativity because
it was no longer needed) was in full
agreement with Lemaître’s theory,
calling it “the most beautiful and
satisfactory explanation of creation
to which I have ever listened.”
Lemaître’s model also provided
a solution to the long-standing
problem of Olbers’ paradox. In his

ATOMS, STARS, AND GALAXIES

model, the universe has a finite age,

and because the speed of light is

also finite, that means that only

a finite number of stars can be

observed within the given volume

of space visible from Earth. The

density of stars within this volume

is low enough that any line of sight

from Earth is unlikely to reach a star.

Refining the idea

Compressed into a tiny point,

the universe would be extremely

hot. During the 1940s, Russian-

American physicist George Gamow

and colleagues worked out details

of what might have happened

during the exceedingly hot first

few moments of a Lemaître-style

universe. The work showed that

a hot early universe, evolving

into what is observed today,

was theoretically feasible. In a

1949 radio interview, the British

astronomer Fred Hoyle coined the

term “Big Bang” for the model of

the universe Lemaître and Gamow

had been developing. Lemaître’s

hypothesis now had a name.

Lemaître’s idea about the

original size of the universe is

now considered incorrect. Today,

cosmologists believe it started from

an infinitesimally small point of

infinite density called a singularity. ■

Lemaître’s

primeval atom

Galaxies are carried

farther and farther apart

as the universe expands

TIME

A parallel exists between the

Big Bang and the Christian

notion of creation from nothing.

George Smoot

Galaxies form in

the early universe