homework encyclopedia

26 Space

The Universe is everything that exists –

space and all the stars, planets, and

other matter it contains. Astronomers

believe that a huge explosion called

the . BIG BANG created the Universe.

WHAT IS THE UNIVERSE MADE OF?

Between groups of galaxies, most of the Universe

seems empty, but it is full of a mysterious dark energy

and radiation, such as light and radio waves. The stars,

nebulas, and planets in galaxies are made of ordinary

matter, but galaxies are also surrounded by vast

amounts of invisible. DARK MATTER. Four basic

forces control the Universe – electromagnetism,

the weak force, the strong force, and gravity.

HOW BIG IS THE UNIVERSE?

The Universe is bigger than we can ever see

or imagine. Astronomers can now spot objects

more than 13 billion (13 thousand million)

light years away from Earth. They lie an

incredible 120,000,000,000,000,000,000,000

km (75 billion trillion miles) away.

Astronomers believe that a giant explosion,

the Big Bang, created the Universe about

14 billion years ago. Before that, there

was nothing – no matter, no space, and

no time. The Universe began expanding

at the Big Bang, and it is expanding still.

CAN DARK MATTER SLOW THE UNIVERSE DOWN?

Until recently, some scientists believed that the gravity

of dark and ordinary matter was slowing down the

expansion of the Universe. New evidence shows that

a mysterious dark energy is working against gravity,

and in fact making the Universe expand even faster.

HOW DO WE KNOW ABOUT THE BIG BANG?

The Universe is expanding, so it was smaller in the

past, and there must have been a moment when it

began as something tiny. Scientists have also found

that the background temperature of space matches

their calculations for the dying heat of the Big Bang.

WHAT HAPPENED AFTER THE BIG BANG?

In a fraction of a second, the newborn Universe grew

from the size of an atom to a searingly hot fireball

bigger than a galaxy. As it spread out and cooled, it

formed a thick soup of tiny particles of matter. It took

another 300,000 years for the first atoms to appear.

We only know dark matter exists because

its gravity pulls on stars and galaxies, and

bends light rays. No one has discovered

what it is made of. There is 10 times more

dark matter than ordinary matter.

1 EXPANDING THE UNIVERSE

Astronomers believe that the whole Universe is expanding

because almost all the galaxies we can see are rushing away

from each other. Imagine the Universe as a balloon, with galaxies on

the surface. As the balloon is blown up, the galaxies move further apart.

UNIVERSE

Universe

4 GRAVITATIONAL LENSING

The gravity of this galaxy cluster

acts like a lens. It bends light

from distant galaxies, stretching

it into curved shapes and broken

rings. Most of the gravity comes

from invisible dark matter.

1 BIG BANG RIPPLES

Minute ripples in the background

temperature of space (above)

show how stars and galaxies could

have begun to form from matter

created in the Big Bang (top).

Light from a galaxy

10 billion light years away is

bent into a curve by gravity

This galaxy cluster’s gravity

is distorting the image of the

galaxies behind it

DARK MATTER

BIG BANG

FIND OUT MORE. Energy 166 • Forces 164 • Light 178–179 • Matter 156 • Space 10 • Stars 24–25 • Time 158

.

Universe