The Science Book

245

universe must be essentially
uniform in both time and space;
but if it originated 10–20 billion
years ago, as the Big Bang theory
suggested, and evolved throughout
its history, then distant reaches of
the universe, whose radiation had
taken billions of years to reach
Earth, should appear substantially
different. (This cosmic time
machine effect, whereby we see
more distant celestial objects as
they were in the distant past, is
known as “lookback time.”) By
measuring the number of distant
galaxies emitting radiation above
a certain brightness, it should be
possible to distinguish between
the two scenarios.
The first of the Cambridge
experiments delivered a result that
seemed to support the Big Bang.
However, problems were discovered
with the radio detectors, so the
results had to be disregarded.
Later results proved more equivocal.

Traces of the Big Bang
Fortunately, the question soon
resolved itself by other means.
As early as 1948, Alpher and his
colleague Robert Herman had
predicted that the Big Bang would
have left a residual heating effect
throughout the universe. According
to the theory, when the universe
was about 380,000 years old, it
had cooled enough to become
transparent, allowing light photons
to travel freely through space for the
first time. The photons that existed
at this time had been propagating
through space ever since, growing
longer and redder as space
expanded. In 1964, Robert Dicke

and his colleagues at Princeton

University set out to build a radio

telescope that could detect this faint

signal, which they thought would

take the form of low-energy radio

waves. However, they were

ultimately beaten to the prize by

Arno Penzias and Robert Wilson,

two engineers working at the

nearby Bell Telephone Laboratories.

Penzias and Wilson had built a radio

telescope for satellite communication,

but found themselves plagued by an

unwanted background signal that

they could not eliminate. Coming

from all over the sky, it corresponded

to microwave emission from a body

at a temperature of 3.5K—just 6°F

(3.5°C) above absolute zero. When

Bell Labs contacted Dicke to ask

for help with their problem, Dicke

realized that they had found the

remnants of the Big Bang—now

known as the cosmic microwave

background radiation (CMBR).

The discovery that the CMBR

permeates the universe—a

phenomenon for which the steady

state theory had no explanation—

decided the case in favor of the Big

Bang. Subsequent measurements

have shown that the CMBR’s true

average temperature is about

2.73K, and high-precision satellite

measurements have revealed

A PARADIGM SHIFT

minute variations in the signal that

allow us to study conditions in the

universe back to 380,000 years

after the Big Bang.

Later developments

Despite being proved correct in

principle, the Big Bang theory has

undergone many transformations

since the 1960s to match it to

our growing understanding of

the universe. Among the most

significant are the introduction

of dark matter and dark energy

to the story, and the addition of a

violent growth spurt in the instant

after creation, known as Inflation.

The events that triggered the Big

Bang remain beyond our reach but

measurements of the rate of cosmic

expansion, aided by instruments

such as the Hubble Space Telescope,

now allow us to pin down the epoch

of cosmic creation with great

accuracy—the universe came into

existence 13.798 billion years ago,

give or take 0.037 billion years.

Various theories exist about the

future of the universe, but many

think that it is set to continue

expanding until it reaches a state

of thermodynamic equilibrium, or

“heat death,” in which matter has

disintegrated into cold subatomic

particles, in around 10^100 years’ time. ■

Arno Penzias and Robert Wilson
detected the background radiation by
accident. At first, they thought the
interference had been caused by bird
droppings on their radio antenna.