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Universe

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

          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


   The deepest visible-light image of the cosmos, the Hubble Ultra Deep
   Field.
   Enlarge
   The deepest visible-light image of the cosmos, the Hubble Ultra Deep
   Field.

   In strictly physical terms, the total universe is the sum of all matter
   that exists and the space in which all events occur or could occur. 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. Extrapolating this expansion back in time,
   one approaches a gravitational singularity, an 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, according to current
   information provided by NASA's WMAP (Wilkinson Microwave Anisotropy
   Probe), is estimated to be about 13.7 billion (13.7 × 10^9) years, with
   a margin of error of about 1 % (± 200 million years). Other methods of
   estimating the age of the universe give different ages with a range
   from 11 billion to 20 billion. 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. 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.

   In the 1977 book The First Three Minutes, Nobel Prize-winner Steven
   Weinberg laid out the physics of what happened just moments after the
   Big Bang. As with most things in physics, that certainly wasn't the end
   of the story, as attested by the update and reissue of The First Three
   Minutes in 1993.

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. New experiments at the Relativistic
   Heavy Ion Collider in Brookhaven National Laboratory have provided
   physicists with a glimpse through this curtain of high energy, so they
   can directly observe the sorts of behaviour that might have been taking
   place in this time frame.

   At these energies, the quarks that comprise protons and neutrons were
   not yet joined together, and a dense, superhot mix of quarks and
   gluons, with some electrons thrown in, was all that could exist in the
   microseconds before it cooled enough to form into the sort of matter
   particles we observe today.

First galaxies

   Fast forwarding to after the existence of matter, more information is
   coming in on the formation of galaxies. It is believed that the
   earliest galaxies were tiny "dwarf galaxies" that released so much
   radiation they stripped gas atoms of their electrons. This gas, in
   turn, heated up and expanded, and thus was able to obtain the mass
   needed to form the larger galaxies that we know today.

   Current telescopes are just now beginning to have the capacity to
   observe the galaxies from this distant time. Studying the light from
   quasars, they observe how it passes through the intervening gas clouds.
   The ionization of these gas clouds is determined by the number of
   nearby bright galaxies, and if such galaxies are spread around, the
   ionization level should be constant. It turns out that in galaxies from
   the period after cosmic reionization there are large fluctuations in
   this ionization level. The evidence seems to confirm the pre-ionization
   galaxies were less common and that the post-ionization galaxies have
   100 times the mass of the dwarf galaxies.

   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. It may be
   trillions of light years across, or even infinite in size. 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 centre 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. whether the rules
   of Euclidean geometry are valid on the largest scales, is unknown.
   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. This
   opinion was strengthened by the latest data from WMAP, looking at
   "acoustic oscillations" in the cosmic microwave background radiation
   temperature variations.

   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). This can be
   understood using a two-dimensional analogy: the surface of a sphere has
   no edge, but nonetheless has a finite area. It is a two-dimensional
   surface with constant curvature in a third dimension. The 3-sphere is a
   three-dimensional equivalent in which all three dimensions are
   constantly curved in a fourth.

   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. (Think of a house of mirrors.)
   If so, the actual number of physically distinct stars and galaxies
   would be smaller than currently accounted. Although this possibility
   has not been ruled out, the results of the latest cosmic microwave
   background research make this appear very unlikely.

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. However, all such ideas are currently untestable and cannot
   be regarded as anything more than speculation. 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.

Other terms

   Colorized version of the Flammarion woodcut. The original was published
   in Paris in 1888.
   Enlarge
   Colorized version of the Flammarion woodcut. The original was published
   in Paris in 1888.

   Different words have been used throughout history to denote "all of
   space", including the equivalents and variants in various languages of
   "heavens," " cosmos," and "world." Macrocosm has also been used to this
   effect, although it is more specifically defined as a system that
   reflects in large scale one, some, or all of its component systems or
   parts. (Similarly, a microcosm is a system that reflects in small scale
   a much larger system of which it is a part.)

   Although words like world and its equivalents in other languages now
   almost always refer to the planet Earth, they previously referred to
   everything that exists — see Copernicus, for example — and still
   sometimes do (as in "the whole wide world"). Some languages use the
   word for "world" as part of the word for "outer space", e.g. in the
   German word "Weltraum" Albert Einstein (1952). Relativity: The Special
   and the General Theory (Fifteenth Edition), ISBN 0-517-88441-0.
   Retrieved from " http://en.wikipedia.org/wiki/Universe"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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   Documentation License. See also our Disclaimer.
