The Universe comprises everything perceived to exist physically, the entirety of space and time, and all forms of matter and energy. 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.
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