universe

In modern astronomy, everything that is in the cosmos and that can affect us by means of physical forces. The definition excludes anything that is in principle undetectable physically, such as regions of spacetime that were, are, or will be irreversibly cut off from our own spacetime.

Physical cosmology

Age of the universe Big Bang Comoving distance Cosmic microwave background Dark energy Dark matter FLRW metric Friedmann equations Galaxy formation Hubble's law Inflation Large-scale structure Lambda-CDM model Metric expansion of space Nucleosynthesis Observable universe Redshift Shape of the universe Structure formation Timeline of the Big Bang Timeline of cosmology Ultimate fate of the universe Universe

Related topics

Astrophysics General relativity Particle physics Quantum gravity

This box: view • talk • edit The part of the universe that can be seen or otherwise observed to have occurred is called the known universe, observable universe, or visible universe. Because cosmic inflation removes vast parts of the total universe from our observable horizon, most cosmologists accept that it is impossible to observe the whole continuum and may use the expression our universe, referring to only that which is knowable by human beings in particular. In cosmological terms, the universe is thought to be a finite or infinite space-time continuum in which all matter and energy exist. Some scientists hypothesize that the universe may be part of a system of many other universes, known as the multiverse.

Expansion and age, and the Big Bang theory

The most important result of physical cosmology, the understanding that the universe is expanding, is derived from redshift observations and quantified by Hubble's Law. The age of the universe from the time of the Big Bang, according to current information provided by NASA's WMAP (Wilkinson Microwave Anisotropy Probe), is estimated to be about 13.7 billion (13.7 × 10 Most of the estimates cluster in the 13-15 billion year range.

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.

See also: Timeline of the Big Bang

Pre-matter soup

Until recently, the first hundredth of a second was a bit of a mystery, leaving Weinberg and others unable to describe exactly what the universe would have been like.

First galaxies

Fast forwarding to after the existence of matter, more information is coming in on the formation of galaxies.

Current telescopes are just now beginning to have the capacity to observe the galaxies from this distant time.

The next generation of telescopes should be able to see the dwarf galaxies directly, which will help resolve the problem that many astronomical predictions in galaxy formation theory predict more nearby small galaxies.

Size of the universe and observable universe

Very little is known about the size of the universe. A 2003 paper claims to establish a lower bound of 24 gigaparsecs (78 billion light years) on the size of the universe, but there is no reason to believe that this bound is anywhere near tight. See Shape of the Universe for more information.

The observable (or visible) universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The comoving distance to the edge of the visible universe is about 46.5 billion light years in all directions from the earth; thus the visible universe may be thought of as a perfect sphere with the earth at its center and a diameter of about 93 billion light years. Note that many sources, including previous versions of this Wikipedia article, have reported a wide variety of incorrect figures for the size of the visible universe, ranging from 13.7 to 180 billion light years. See Observable universe for a list of incorrect figures published in the popular press with explanations of each.

Shape of the universe

An important open question of cosmology is the shape of the universe. Mathematically, which 3-manifold represents best the spatial part of the universe?

Firstly, whether the universe is spatially flat, i.e. Currently, most cosmologists believe that the observable universe is very nearly spatially flat, with local wrinkles where massive objects distort spacetime, just as the surface of a lake is nearly flat.

Secondly, whether the universe is multiply connected, is unknown. The universe has no spatial boundary according to the standard Big Bang model, but nevertheless may be spatially finite (compact).

If the universe is indeed spatially finite, as described, then traveling in a "straight" line, in any given direction, would theoretically cause one to eventually arrive back at the starting point.

Strictly speaking, we should call the stars and galaxies "views" of stars and galaxies, since it is possible that the universe is multiply-connected and sufficiently small (and of an appropriate, perhaps complex, shape) that we can see once or several times around it in various, and perhaps all, directions.

Fate of the universe

Depending on the average density of matter and energy in the universe, it will either keep on expanding forever or it will be gravitationally slowed down and will eventually collapse back on itself in a "Big Crunch". Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the universe seems to be accelerating and will accelerate for eternity (see accelerating universe). Other ideas of the fate of our universe include the Big Rip, the Big Freeze, and Heat death of the universe theory. For a more detailed discussion of other theories, see the ultimate fate of the universe.

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. For example, matter that falls into a black hole in our universe could emerge as a Big Bang, starting another universe. The concept of parallel universes is understood only when related to string theory. String theorist Michio Kaku offered several explanations to possible parallel universe phenomena. (2005) "Age of the Universe" ^ http://www.sciencemag.org/cgi/content/abstract/299/5603/65?ijkey=3D7y0Qonz=GO7ig.&keytype=3Dref&siteid=3Dsci ^ Neil J.