The universe is a vast expanse of space which contains all of the matter and energy in existence. The universe contains all of the galaxies, stars, and planets. The exact size of the universe is unknown. Scientists believe the universe is still expanding outward. They believe this outward expansion is the result of a violent, powerful explosion that occurred about 13.7 billion years ago. This explosion is known as the Big Bang. By looking at an object's electromagnetic spectrum, scientists can determine if an object is moving away from Earth or towards Earth. When distant objects, such as quasars, are viewed from Earth, their spectrum is shifted towards red. Whenever there is a shift in a spectrum, it is called a Doppler Shift. If the shift is toward red, the light given off by the object is in longer wavelengths. When an object moves away from Earth, the light that it is giving off is seen in longer wavelengths. When an object moves toward Earth, the light that it is giving off is seen in shorter wavelengths. This causes a shift in the object's spectrum towards violet. The amount of shift in an object's spectrum is determined by how fast the object is moving. All of the distant galaxies have tremendous red shifts. Based on these data, scientists believe the universe is still expanding outward.
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