The Astronomy Book

176

published in a journal called
Proceedings of the National
Academy of Science in 1929. It
contained a straight-line graph
that plotted 46 galaxies from
near to far against their redshifts.
Although there was a considerable
scatter, Hubble managed to fit a
straight line through the majority.
The graph shows that, with the
exception of the nearest galaxies,
Andromeda and Triangulum,
which are encroaching on the Milky
Way, all other galaxies are receding.
What is more, the farther away they
are, the faster their movement.

Toward an interpretation
If, from Earth’s perspective within
the universe, all galaxies are seen
flying away, then the potential
explanations are that (a) Earth
lies at the center of the universe;
or (b) the universe itself originated
from a single point and is
expanding as a whole.
Objectivity—a kind of
foundational law in science—
requires that there is no reason

to assume that Earth occupies

a unique position. Instead, the

light from distant nebulae showed

that the universe was not static.

Many astronomers quickly reached

the conclusion that this was due

to the expansion of the universe,

although Hubble never stated

this explicitly.

In reality, Vesto Slipher had

indicated the trend in 1919, four

years before Hubble made his

observations, and Georges Lemaître

BEYOND THE MILKY WAY

had proposed the expansion of the

universe from a “primeval atom”

in 1927. However, Hubble’s result

provided a simple link between

his redshift-measured velocities

and distance, and with it, the

convincing proof that the scientific

community needed. “Hubble’s

law,” stating that the redshift of

galaxies is proportional to their

distance from Earth, was accepted

almost unanimously.

Einstein’s blunder

The revelation that the universe

might be expanding made news all

over the world—not least for the fact

that it directly contradicted a theory

of Albert Einstein’s. Einstein saw

that gravity could eventually cause

the universe to collapse under its

own weight, so he used a value he

called the cosmological constant—

a kind of negative pressure—to

prevent this from occurring in the

field equations of general relativity.

He abandoned the idea in the wake

of Hubble’s discovery.

Einstein and others assumed

that the observed velocities were

Doppler effects caused by the

galaxies’ speed of recession, but

there were some dissenting voices.

Swiss astronomer Fritz Zwicky

suggested that the redshift might

be due to “tired light” reaching

Earth—caused by the interaction

of photons with the intervening

matter. Hubble himself found it

hard to believe that the velocities

indicated by the redshifts were

actually real, and was happy to use

them solely as distance indicators.

In fact, the velocities of galaxies

observed by Hubble are due to

the expansion of spacetime itself.

K-factor

Hubble showed how fast spacetime

is expanding by plotting a straight-

line graph—which he grandly

Here, the galaxies of the universe are imagined as dots stuck to
an inflating balloon (for visualization, the dimensions are reduced
from the three of space to the two of the balloon’s surface). As the
balloon doubles in diameter, the distances
between dots also double. The farther
away a dot is from another dot, the
farther, and so more quickly, it moves.
C has moved 2 cm from A, but B
has moved just 1 cm from A.

2 cm

1 cm

4 cm

2 cm

Equipped with his five

senses, man explores the

universe around him and calls

the adventure Science.

Edwin Hubble

A

B

C

A

B

C

Spacetime between

Galaxies galaxies expands