Universe

The universe is the whole spacetime continuum in which we exist, together with all the energy and matter within it. Attempts to understand the universe in this sense, on the largest possible scales, are made in cosmology, a science that has grown from physics and astronomy. During the second half of the 20th century, the development of observational cosmology, also called physical cosmology, led to a split in the meaning of the word universe, between observational cosmologists and theoretical cosmologists; where the former (usually) abandon the hope of observing the whole spacetime continuum, the latter retain this hope, attempting to find the most reasonable speculations for modeling the whole of spacetime, despite the extreme difficulty in imagining any empirical constraints on these speculations and the risk of declining into metaphysics.

Multiverse

There is some speculation that multiple universes exist in a higher-level multiverse (also known as a megaverse), our universe being one of those universes (lower case). For example, matter that falls into a black hole in our universe could emerge as a "Big Bang," starting another universe. However, all such ideas are currently untestable and cannot be regarded as anything more than speculation.

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The concept of parallel universes are understood only when related to string theory.

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