169
See also: Gravitational theory 66–73 ■ The theory of relativity 146–53 ■
Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77 ■ The primeval atom 196–97
cannot be infinite in both time and
space, as otherwise the stars shining
from every direction would make
the whole sky bright. His argument
was restated in 1823 by German
astronomer Wilhelm Olbers and
became known as Olbers’ paradox.
Despite this problem, Isaac Newton
stated that the universe was static
(not getting any bigger or smaller)
and infinite in time and space, with
its matter distributed more or less
uniformly over a large scale. At the
end of the 19th century, this was
still the prevailing view, and one
that Einstein himself initially held.
An unchanging universe?
Einstein’s general theory of
relativity explains how gravity
works at the largest scales. He
realized that it could be used to
test whether the Newtonian model
of the universe could exist long-
term without becoming unstable,
and to explore which other types
of universe might be feasible. The
exact relationship between mass,
space, and time was explained
in a series of 10 complex equations.
These were called Einstein’s field
equations. Einstein found an
initial solution to his equations
that suggested the universe is
contracting. Since he could not
believe this, he introduced a “fix”—
an expansion-inducing factor called
the cosmological constant—to
balance the inward pull of gravity.
This allowed for a static universe.
In 1922, Russian mathematician
Alexander Friedmann attempted
to find solutions to Einstein’s field
equations. Starting with the
assumption that the universe
is homogenous (made of more or
less the same material everywhere)
and spread out evenly in every
direction, he found several solutions.
These allowed for models in which
the universe could be expanding,
contracting, or static. Friedmann
was probably the first person to
use the expression “expanding ❯❯ATOMS, STARS, AND GALAXIES
Georges Lemaître
Georges Lemaître was born
in 1894 in Charleroi, Belgium.
Following distinguished
service in World War I, in 1920
he was awarded a doctoral
degree in engineering. He
subsequently entered a
seminary, where, in his leisure
time, he studied mathematics
and science.
After his ordination in 1923,
Lemaître studied mathematics
and solar physics at Cambridge
University, studying under
Arthur Eddington. In 1927,
he was appointed professor of
astrophysics at the University
of Leuven, Belgium, and
published his first major paper
on the expanding universe. In
1931, Lemaître put forward his
theory of the primeval atom in
a report in the journal Nature,
and his fame soon spread.
He died in 1966, shortly after
learning of the discovery of
cosmic microwave background
radiation, which provided
evidence for the Big Bang.Key works1931 The Beginning of the
World from the Point of View
of Quantum Theory
1946 The Primeval Atom
HypothesisWhat an observer would seeOlbers’ paradox is the argument
that, if the universe is infinite, not
expanding, has always existed, and
everywhere contains roughly the same
density of stars, then any sight line
from Earth must end at the surface
of a star. The night sky should be
uniformly bright, but this contradicts
the observed darkness of the night.