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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