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ORIGINS OF THE UNIVERSE
reflect
The universe includes everything that exists: all
matter, energy, space, and time. But how did the
universe come into existence? People in every
culture throughout history have attempted to
answer this question. Scientists are no exception.
How do you think scientists attempt to study and
answer questions about the universe’s beginnings?
What kinds of theories do you think scientists have
proposed? What kinds of evidence might support
these theories?
Has the universe always existed,
or did it have a beginning?
The Big Bang Theory
It may be difficult to imagine the origins of the universe. After all, if the universe came
into existence, it must have not existed once. Where did it and its components come
from? Though scientists do not know the answer to this last question, they have found
solid evidence that the universe had a beginning. The Big Bang theory is the most widely
accepted theory to explain the origin of the universe. According to this theory, the universe
began as a single, tiny point—smaller than an atom—called a singularity. This singularity
was infinitely hot and infinitely dense—it contained all the matter and energy currently in
the universe. The Big Bang was the moment when all of this matter and energy suddenly
expanded out from this singularity. The universe has been expanding ever since.
The laws of physics, including
gravity: the force
gravity, came into effect during
that attracts one
the Big Bang. In the first seconds
object with mass to
after the Big Bang, gravity caused
another
matter to come together to form
the particles from which all
components in the universe are made: protons, neutrons, and
electrons. During these seconds, the universe was too hot for
these particles to form atoms. Also, the clouds of subatomic
particles—these protons, neutrons, and electrons—were so
dense that no light could shine through them.
As the universe cooled
after the Big Bang, atoms
of more complex elements
formed. This nitrogen atom
contains 7 protons (+),
7 electrons (–), and
7 neutrons (black).
About three seconds after the Big
Bang, the universe had cooled
enough for protons and neutrons
to come together to form simple
nuclei.
nucleus: the center
of an atom
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ORIGINS OF THE UNIVERSE
After about 300,000 years, electrons joined with these nuclei to form atoms of the simplest
elements. Hydrogen, which contains only one proton per atom, was the first element to
form. Helium, which contains two protons per atom, formed next. To this day, hydrogen and
helium remain the most common elements in the universe. As atoms continued to form,
the dense cloud of subatomic particles began to clear. For the first time, light could shine
through the universe.
look out!
Prior to the discovery of the Big Bang, many scientists thought the universe had
always existed—and will always exist—in an unchanging state. This was called the
steady state theory. The steady state theory was disproved in the 1920s by a scientist
named Edwin Hubble. By analyzing light from distant stars, Hubble discovered the
universe is actually expanding. This discovery led to the Big Bang theory. Scientists
reasoned that if the universe is expanding, it must once have been compressed
into a much smaller, denser space. Later discoveries have continued to support the
Big Bang theory.
what do you think?
Scientists cannot directly observe the beginning of the universe. How, then, do you think
scientists can study the Big Bang? How can scientists know when the Big Bang happened?
Take a moment to answer these questions for yourself, and then read on to learn!
The Expanding Universe and Redshift
The fact that the universe is expanding was one of the most important early
pieces of evidence for the Big Bang theory. To understand how scientists know the
universe is expanding, you need to
know a bit about light waves. When stars
give off light, it travels in waves through
space. Some waves are longer; we see
longer waves as red light. Some waves
are shorter; we see shorter waves as
blue light.
As the object moves away from the man
and toward the woman, it gives off light
waves (numbered 1–4 in this diagram).
Each observer experiences these light
waves differently.
As a star moves away from Earth, light
waves from the star that reach Earth seem
to have stretched. As a result, the star
appears redder. This is called redshift. In
contrast, as a star moves toward Earth,
light waves from the star that reaches
Earth seem to have compressed. As a
result, the star appears bluer. This is
called blueshift.
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ORIGINS OF THE UNIVERSE
The stars and galaxies that Edwin Hubble observed experienced redshift. In other words,
they were moving away from Earth. If galaxies are moving away from Earth (and each
other), the universe cannot exist in a steady state. Hubble ultimately concluded the
universe must be expanding. He also discovered galaxies farther from Earth experience
greater redshift. In other words, the farther a galaxy is from Earth, the faster it is moving
away from Earth. Not only is the universe expanding, Hubble concluded that it must be
expanding more quickly all the time!
Scientists can now measure the intensity of a galaxy’s redshift and use this value to
calculate the galaxy’s distance from Earth and the speed at which it is moving. Scientists
still have not developed a way to measure the current size of the universe.
The Age of the Universe
Visible light is a form of electromagnetic radiation: energy that travels through space in
wavelike patterns at high speeds. In fact, this energy is the fastest thing in the universe.
Electromagnetic radiation travels at a constant rate of about 300,000 km/s. (We call this
the speed of light.) Because the universe is so vast and light moves so quickly, light gives
scientists a useful tool for measuring distances in space. As you will see, scientists can also
use light to measure the universe’s age.
A light-year is the distance light travels in one year: about 9.5 trillion km. If a star is
4 light-years from Earth, light from that star takes 4 years to reach Earth. If a star is
4 million light-years from Earth, light from that star takes 4 million years to reach Earth.
This means scientists are looking into the past when they observe electromagnetic
radiation in space. Scientists can estimate the age of the universe by observing the
universe’s oldest stars and the intensity of their redshift. Scientists can use these
observations to calculate the rate of the expansion of the universe, and work backward
to determine when the expansion began. Using this method, scientists have estimated
that the universe is about 13.7 billion years old. In other words, the Big Bang began
13.7 billion years ago.
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ORIGINS OF THE UNIVERSE
Getting Technical: Remote Sensing Instruments
Much of what we know about the nature and origin of the universe
comes from the study of visible light through light-sensing telescopes.
However, all objects in the universe emit forms of electromagnetic
radiation other than visible light. Scientists can use remote sensing
instruments—special telescopes and other tools—to detect these
invisible forms of electromagnetic radiation and study the objects that
emit them.
Remote sensing instruments are essential for studying the far
reaches of the universe. Many objects are too distant or dim to detect
with visible light. They give off low-energy forms of electromagnetic
radiation: radio waves, microwaves, and infrared radiation. Scientists
can use large radio telescopes, similar in appearance to large satellite
dishes, to detect radio waves from space. These radio telescopes are
located on Earth’s surface, but they allow us to study distant stars that
would otherwise be invisible.
Scientists
can use radio
telescopes
like this one to
observe distant
objects in the
universe.
Other remote sensing instruments are launched into space. This
allows scientists to observe the objects in the universe without interference from manmade
lights on Earth’s surface or particles in Earth’s atmosphere. One such instrument is the
Wilkinson Microwave Anisotropy Probe (WMAP). WMAP is a satellite launched in 2001
to examine cosmic microwave background (CMB) radiation, which is electromagnetic
radiation left over from the initial stages of the Big Bang.
The massive explosion that started the Big Bang
gave off tremendous amounts of energy. With
the help of WMAP, scientists have been able to
observe this leftover radiation throughout the
universe. Having a clear picture of CMB radiation
allows scientists to map out matter in the universe.
CMB radiation also provides evidence for the Big
Bang theory. Nearly all of the CMB radiation is the
same temperature; this indicates the radiation was
released at the same time. Some small temperature
variations do exist, however, indicating different
amounts of matter in different parts of the universe.
Areas where the CMB radiation indicates more
matter was created in the early universe correspond
to areas with large clusters of galaxies in the
modern universe.
The Wilkinson Microwave
Anisotropy Probe (WMAP) is
used to detect cosmic microwave
background (CMB) radiation.
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ORIGINS OF THE UNIVERSE
What do you know?
The following image was created by WMAP. It is an image of the cosmic microwave
background (CMB) radiation left over from the Big Bang. The different colors represent
differences in temperature, which correspond to different densities of matter. (Red and
yellow areas are hotter, and dark blue areas are cooler.) Take a few moments to study this
image. Circle the areas in the image that you think indicate areas containing more matter.
Box the areas in the image that you think indicate areas containing less matter. Notice
that most of the image is the same color, meaning most of the CMB radiation is the same
temperature. Write a paragraph explaining in your own words how this observation provides
evidence to support the Big Bang theory.
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ORIGINS OF THE UNIVERSE
connecting with your child
Visualizing the Big Bang
To help your child visualize the Big Bang,
try doing a simple demonstration together.
Go outside and gather a handful of soil
and clump it into a tight ball. Let your
child feel the density of the soil when it is
compressed. Then, have your child throw
the ball of soil down onto the sidewalk. You
can put down a white piece of paper first if a
sidewalk is not available or will not provide
enough visual contrast.
Take a look at the soil together. Discuss how
all of the soil expanded from a single point
upon impact, and take a look at where the
soil ended up after this “explosion.” Some
of the soil will still be in a large clump in
the middle, while some will have spread
out at some distance. This can represent
how matter expands at different speeds
throughout the universe. The soil that is
farther from the point of impact traveled
more quickly than the soil that stayed
in the middle. However, everything can
still be traced back to that central point.
This is similar to the way all matter in the
universe expanded from a single point.
The matter that is farther away is moving
faster, but it can still be traced back to
the original singularity. This is a simplified
demonstration, but it may help your child
visualize the expansion of the universe.
Here are some questions to discuss with
your child:
• How does the density of the soil change
from its original state compared to its
current state after the expansion?
• Why is some of the soil farther away
than the rest of it?
• How does this demonstration relate to
what you have learned about the Big
Bang theory and the expansion of the
universe?
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