The Science Book

250

95 PERCENT

OF THE UNIVERSE

IS MISSING

FRITZ ZWICKY (1898–1974)

IN CONTEXT

BRANCH

Physics and cosmology

BEFORE

1923 Edwin Hubble confirms

the true nature of galaxies

as independent star systems

millions of light years beyond

the Milky Way.

1929 Hubble establishes that

the universe is expanding, and

that galaxies move away from

us more rapidly the farther

away they are (the so-called

Hubble Flow).

AFTER

1950s American astronomer

George Abell compiles the first

detailed catalogue of galaxy

clusters. Subsequent studies

of galaxy clusters have

repeatedly confirmed the

existence of dark matter.

1950s–present Various

models of the Big Bang predict

that it should have generated

much more matter than that

which is currently visible.

T

he idea that the universe

might be dominated by

something other than

detectable luminous matter was

first proposed by Swiss astronomer

Fritz Zwicky. In 1922–23, Edwin

Hubble had realized that “nebulae”

were in fact distant galaxies.

A decade later, Zwicky set out to

measure the overall mass of the

Coma cluster of galaxies. He used

a mathematical model called the

Virial theorem, which allowed him

to calculate the overall mass from

the relative velocities of individual

cluster galaxies. To Zwicky’s

Expansion is

caused by dark

energy, which

accounts for

68.3 percent

of all energy.

Just 4.9 percent of the universe’s energy is accounted for

by visible matter.

This additional mass is known as

dark matter, and accounts for

26.8 percent of all energy.

The universe

is expanding

at an ever

increasing rate.

So they must have additional, hidden,

mass that would explain their rotation.

The outer regions of galaxies rotate

more quickly than their visible mass

suggests they should.