The Astronomy Book

300 DARK ENERGY

T

he Big Bang theory has

at its heart a simple idea—

the universe started out

very small and then expanded.

In 1998, two teams of scientists

discovered that the expansion of

the universe is itself speeding up.

This discovery revealed that what

astronomers can directly detect

makes up just 5 percent of the total

mass and energy in the universe.

Invisible dark matter makes up

another 24 percent, while the

rest is a mysterious phenomenon,

known simply as dark energy.

In 2011, three Americans, Saul

Perlmutter, Brian Schmidt, and

Adam Riess, won the Nobel Prize

for Physics for this discovery.

Expanding space

The year after Georges Lemaître’s

paper hypothesized the Big Bang,

Edwin Hubble found proof of the

expanding universe when he

showed that galaxies were moving

away from Earth—and the ones

that were farther away were

moving faster. These were not

simply objects blasting away from

each other through space; this was

space itself growing in size and

moving the matter with it. The

IN CONTEXT

KEY ASTRONOMERS

Saul Perlmutter (1959 –)

Brian Schmidt (1967–)

Adam Riess (1969 –)

BEFORE

1917 Albert Einstein adds a

cosmological constant to his

field calculations to keep the

universe static.

1927 Georges Lemaître

suggests that the universe

could be dynamic, not static.

1928 Edwin Hubble finds

evidence of cosmic expansion.

1948 Fred Hoyle, Hermann

Bondi, and Thomas Gold

propose the steady-state theory

of the expanding universe.

AFTER

2013 The Dark Energy Survey

begins to map the universe.

2016 The Hubble Space

Telescope shows that cosmic

acceleration is 9 percent faster

than originally measured.

galaxies are not only moving away

from Earth; they are expanding

away from everywhere all at once.

Better picture

Subsequent observations helped

to tell the history of the expanding

universe. The 1964 discovery of

the cosmic microwave background

(CMB), a cold glow left over

from the Big Bang, showed

that the universe has been

expanding for approximately

13.8 billion years. Surveys of

The expansion of the universe is

assumed to be slowing down due

to the force of gravity.

This acceleration must be due to a previously unknown

force that works against gravity, called dark energy.

It’s everywhere, really. It’s

between the galaxies. It is in

this room. We believe that

everywhere that you have

space, empty space, you

cannot avoid having some

of this dark energy.

Adam Riess

However, when measured, cosmic expansion

is found to be accelerating.

Measuring this deceleration should reveal

the ultimate fate of the universe.