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.

Expansion and age, and the Big Bang theory

The most important result of cosmology, that the universe is expanding, is derived from redshift observations and quantified by Hubble's Law. Extrapolating this expansion back in time, one approaches a gravitational singularity, a rather abstract mathematical concept, which may or may not correspond to reality. This gives rise to the Big Bang theory, the dominant model in cosmology today. The age of the Universe was estimated to be about 13.7 billion (13.7 × 109) years, with a margin of error of about 1 percent (± 200 million years), according to NASA's Wilkinson Microwave Anisotropy Probe (WMAP). However, this is based on the assumption that the underlying model used for data analysis is correct. Other methods of estimating the age of the universe give different ages.

Related Topics:
Redshift - Hubble's Law - Gravitational singularity - Big Bang - Theory - Age of the Universe - Billion - Margin of error - NASA - Wilkinson Microwave Anisotropy Probe

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A fundamental aspect of the Big Bang can be seen today in the observation that the farther away from us galaxies are, the faster they move away from us. It can also be seen in the cosmic microwave background radiation which is the much-attenuated radiation that originated soon after the Big Bang. This background radiation is remarkably uniform in all directions, which cosmologists have attempted to explain by an initial period of rapid inflation following the Big Bang.

Related Topics:
Galaxies - Cosmic microwave background radiation - Inflation

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Size of universe and observable universe

There is disagreement over whether the Universe is finite or infinite in spatial extent and volume.

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However, the observable universe, consisting of all locations that could have affected us since the Big Bang 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

Related Topics:
Speed of light - Cosmic light horizon - Light year - Comoving distance - Billion

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(7.8 × 1010 light years, or 7.4 × 1023 km).

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This would make the comoving volume, of the known universe, equal to 1.9 × 1033 cubic light years (assuming this region is perfectly spherical). The observable universe contains about 7 × 1022 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.

Related Topics:
Volume - Spherical - Star - Galaxies - Clusters - Supercluster - Hubble Deep Field - Hubble Space Telescope

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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).

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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.

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