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This is a picture of the Big Bang. The Big Bang was an explosion that began the Universe.
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The History of the Universe

Astronomers have tried to explain what we see in the universe around us. The simplest explanation for everything we know about the universe is called the Big Bang theory. This theory says that in the beginning, everything in the universe was all in one place. Something unknown caused the universe to begin growing. The universe began growing very quickly, and the universe is still growing even today.

At the beginning, when all the matter in the universe was contained in a very tiny volume, it was very hot. As the universe started to grow, it cooled off very quickly. When the universe was about 500,000 years old, it had cooled enough that it was possible for the atoms that make up stars and galaxies to begin to form. Energy released when the universe was small and dense, the cosmic microwave background radiation, still fills the universe today. By studying this energy, astronomers can learn what the universe was like when stars and galaxies were first able to form.

So how old is the universe? Astronomers argue about the exact age of the universe for many reasons. But they do agree that it is somewhere between 10 and 20 billion years old.

Last modified May 6, 2008 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA