Universe: Encyclopedia II - Universe - Expansion and age and the Big Bang theory

Universe - Expansion and age and the Big Bang theory

The most important result of cosmology, the understanding 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 from the time of the Big Bang, 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 WMAP (Wilkinson Microwave Anisotropy Probe). 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.

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 early period of inflationary expansion following the Big Bang.

Universe - Size of Universe and observable Universe

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

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 (4.19 gpc) 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 (7.8 × 10¹⁰ light years, or 7.4 × 10²³ km). 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 100 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. The Hubble Space Telescope discovered galaxies such as Abell 1835 IR1916, which are over 13 billion light years from Earth.

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. See also Causality (physics).

We live in the center 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 center. 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, which equals zero, according to the Big Bang model. Since light travels at the same speed in any direction towards us, it is reasonable to suggest that we live at the center of our observable Universe.

Adapted from the Wikipedia article "Expansion and age and the Big Bang theory", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki