Universe
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The Universe comprises everything we perceive to physically exist, the entirety of space and time, all forms of matter and energy. However, the term Universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world, or Nature.
The word Universe is usually defined as encompassing everything. However, using an alternative definition, some cosmologists have speculated that the "Universe" composed of expanding space-as-we-know-it, is just one of many disconnected "universes", which are collectively called the multiverse. For example, in the many-worlds hypothesis, new "universes" are spawned with every quantum measurement. These universes are usually thought to be completely disconnected from our own and therefore impossible to detect experimentally. Observations of older parts of the universe (which are far away) suggest that the Universe has been governed by the same physical laws and constants throughout most of its extent and history. However, in bubble universe theory, there may be an infinite variety of "universes" created in various ways, and perhaps each with different physical constants.
Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the Universe. The earliest quantitative geocentric models were developed by the ancient Greeks,[citation needed] who proposed that the Universe possesses infinite space and has existed eternally, but contains a single set of concentric spheres of finite size – corresponding to the fixed stars, the Sun and various planets – rotating about a spherical but unmoving Earth. Over the centuries, more precise observations and improved theories of gravity led to Copernicus's heliocentric model and the Newtonian model of the Solar System, respectively. Further improvements in astronomy led to the realization that the Solar System is embedded in a galaxy composed of millions of stars, the Milky Way, and that other galaxies exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology. Discovery of the red shift and cosmic microwave background radiation revealed that the Universe is expanding and apparently had a beginning.
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