The term Universe refers to everything within the Cosmos – planets, stars, galaxies, light and even the vacuum between these entities.

Astronomers often talk about “the observable Universe” which is essentially what we can see – the solar system, local stars a few light years/parsecs away, to nearby galaxies some millions of light years/parsecs away to the edge of the visible Universe some 13.6 billion light years distant. Modern astronomy has revealed a great deal about the observable Universe which is generally accepted to have begun via the hot big bang some 13.6 billion years ago.

The Universe then expanded and cooled, allowing the formation of atoms prior to the first stars being formed which re-ionised the Universe. At this “epoch of reionisation” galaxy formation was well underway. Since that time the Universe has been evolving through the merging of proto-galaxies to form larger and larger galaxies, like the Milky Way. Matter in the universe appears to be dominated by an as yet unknown form of dark matter.

Observations of distant type Ia supernovae, combined with high spatial observations of the cosmic background radiation suggest that the Universe has a density close to, if not exactly equal to critical density required to make spacetime flat. As well the universe appears to be undergoing a period of accelerated expansion, driven by dark energy either as a cosmological constant or variable vacuum energy called quintessence.

The large-scale structure of the Universe can be described as filamentary, and cosmological N-body simulations produced by theoretical astronomers agree well with the observed structure as revealed by surveys such as the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey.

“Other Universes” refers to Universes which may exist but have no effect on us. Philosophers and relativists ponder whether other Universes might exist, e.g. multiverses, and if black holes might lead to them.

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