Observable universe
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Disambiguation: See Universe for a general discussion of the Universe.
The Observable universe is a scientific term used to describe a spherical region of the Universe surrounding the Earth. It is defined such that light from this region has had time to reach the Earth since the Big Bang occurred (and hence if there was no matter in the way, and we had sufficiently good instrumentation, we could observe light coming from anywhere within this region). In reality much of the observable universe is not visible - we cannot see it because there is material along the line of sight or because no light was emitted in that region of the Universe at that time.
The observable universe thus consists of all locations that could have affected us since the Big Bang, and given the finite speed of light is certainly finite. The edge of the cosmic light horizon is 13.7 billion light years distant. The present distance (comoving distance) to the edge of the observable universe is larger, since the universe has been expanding; it is estimated to be about 78 billion light years (<math>7.4\times 10^{23}<math>km). This would make the comoving volume, of the known universe, equal to <math>1.9\times 10^{33}<math> cubic light years (assuming this region is perfectly spherical). The observable universe contains about <math>7\times 10^{22}<math> stars, organized in about 10 billion galaxies, which themselves form clusters and superclusters. The number of galaxies may be even larger, based on the Hubble Deep Field observed with the Hubble Space Telescope.
Both popular and professional research articles in cosmology often use the term "Universe" when they really mean "observable universe". This is because unobservable physical phenomena are scientifically irrelevant; that is, they cannot affect any events that we can perceive, and therefore, it is argued, effectively do not exist. See also Causality (physics). They also cannot be measured, and so hypotheses about parts of the Universe which are not observable cannot be tested.
We live in the centre of the universe that we observe, in apparent contradiction to the Copernican principle which says that the Universe is more or less uniform and it has no distinguished centre. This is simply because light does not travel infinitely fast, and we make observations of the past. As we look further and further away, we see things from epochs (times) closer and closer to the limit of time=zero of the Big bang model. And since light travels at the same speed in any direction towards us, we live at the centre of our observable universe.