The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. Based on observations of the portion of the universe that is observable, physicists attempt to describe the whole of space-time, including all matter and energy and events which occur, as a single system corresponding to a mathematical model.
The generally accepted scientific theory which describes the origin and evolution of the universe is Big Bang cosmology, which describes the expansion of space from an extremely hot and dense state of unknown characteristics. The universe underwent a rapid period of cosmic inflation that flattened out nearly all initial irregularities in the energy density; thereafter the universe expanded and became steadily cooler and less dense. Minor variations in the distribution of mass resulted in hierarchical segregation of the features that are found in the current universe; such as clusters and superclusters of galaxies. There are more than one hundred billion galaxies in the universe, To see the Universe in a Grain of Taranaki Sand, Swinburne University each containing hundreds of billions of stars, with each star containing about 10x57th power of atoms of hydrogen.
Name of our universe
As used by observational cosmologists, the Universe most frequently refers to the finite part of space-time. The Universe is directly observable by making observations using telescopes and other detectors, and by using the methods of theoretical and empirical physics for studying its components. Physical cosmologists assume that the observable part of (comoving) space (also called our universe) corresponds to a part of a model of the whole of space, and usually not to the whole space. They use the term the Universe ambiguously to mean either the observable part of space, the observable part of space-time, or the entire space-time.
In order to clarify terminology, George Ellis, U. Kirchner and W.R. Stoeger recommend using the term the Universe for the theoretical model of all of the connected space-time in which we live, universe domain for the observable universe or a similar part of the same space-time, universe for a general space-time (either our own Universe or another one disconnected from our own), multiverse for a set of disconnected space-times, and multi-domain universe to refer to a model of the whole of a single connected space-time in the sense of chaotic inflation models.
A majority of cosmologists believe that the observable universe is an extremely tiny part of the whole universe and that it is impossible to observe the whole of comoving space. It is presently unknown if this is correct, and remains under debate. According to studies of the shape of the Universe, it is possible that the observable universe is of nearly the same size as the whole of space.
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. Additional discoveries and refinements of theories prompted him to update and reissue that book in 1993.
Others suggest that the universe had no beginning, because time goes in a loop. However, any such ideas are at best hypothetical and much more research is needed before anything can be concluded for certain.
Until recently, the first hundredth of a second after the Big Bang was a mystery, leaving Weinberg and others unable to describe exactly what the universe would have been like during this period. 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 behavior that might have been taking place in this time frame., Heavy Ion Collisions, Brookhaven National Laboratory
At these energies, the quarks that comprise protons and neutrons(ups and downs) 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. Thomas Ludlam, Larry McLerran, October 2003  What Have We Learned From the Relativistic Heavy Ion Collider? Physics Today
Moving forward 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. Ken Tan  New 'Hobbit' Galaxies Discovered Around Milky Way space.com  Dwarf Spheroidal Galaxies, The Uppsala Astronomical Observatory 
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.
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.
The currently observable universe appears to have a geometrically flat space-time containing the equivalent mass-energy density of 9.9 × 10-30 grams per cubic centimetre. This mass-energy appears to consist of 73% dark energy, 23% cold dark matter and 4% atoms. Thus the density of atoms is on the order of a single hydrogen nucleus (or atom) for every four cubic meters of volume. What is the Universe Made Of? NASA WMAP. The exact nature of dark energy and cold dark matter remain a mystery.
During the early phases of the big bang, equal amounts of matter and antimatter were formed. However, through a CP-violation, physical processes resulted in an asymmetry in the amount of matter as compared to anti-matter. This asymmetry explains the amount of residual matter found in the universe today, as nearly all the matter and anti-matter would otherwise have annihilated each other when they came into contact  Antimatter, Particle Physics and Astronomy Research Council
Prior to the formation of the first stars, the chemical composition of the Universe consisted primarily of hydrogen (75% of total mass), with a lesser amount of helium-4 (24% of total mass) and trace amounts of the isotopes deuteriumand lithium  Big Bang Nucleosynthesis UCLA, M. Harwit, M. Spaans Chemical Composition of the Early Universe, The Astrophysical Journal  Subsequently the interstellar medium within galaxies has been steadily enriched by heavier elements. These are introduced as a result of supernova explosions, stellar winds and the expulsion of the outer envelope of evolved stars. Kobulnicky, E. D. Skillman, Chemical Composition of the Early Universe, Bulletin of the American Astronomical Society 
The big bang left behind a background flux of photons and neutrinos. The temperature of the background radiation has steadily decreased as the universe expands, and now primarily consists of microwave energy equivalent to a temperature of 2.725  Tests of the Big Bang: The CMB, NASA WMAP The neutrino background is not observable with present-day technology, but is theorized to have a density of about 150 neutrinos per cubic centimetre., Background neutrinos, Institute of Physics Publishing
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, Neil J. Cornish, David N. Spergel, Glenn D. Starkman, and Eiichiro Komatsu, Constraining the Topology of the Universe. astro-ph/0310233 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 "no, this is not a misspelling of "right" 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. | title = Misconceptions about the Big Bang | publisher = Scientific American | accessdate = 2007-03-05 Note that many sources 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.
An important open question of cosmology is the shape of the universe. Mathematically, which 3-manifold best represents 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. shape
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 were compact and without boundary, it would be possible after traveling a sufficient distance to arrive back where one began. Hence, the light from stars and galaxies could pass through the observable universe more than once. If the universe were multiply-connected and sufficiently small (and of an appropriate, perhaps complex, shape) then conceivably one might be able to see once or several times around it in some (or all) directions. Although this possibility has not been ruled out, the results of the latest cosmic microwave background research make this appear very unlikely.
Homogeneity and isotropy
While there is considerable fractalized structure at the local level (arranged in a hierarchy of clustering), on the highest orders of distance the universe is very homogeneous. On these scales the density of the universe is very uniform, and there is no preferred direction or significant asymmetry to the universe. This homogeneity is a requirement of the Friedmann-Lemaître-Robertson-Walker metric employed in modern cosmological models.Large-scale homogeneity of the Universe measured by the microwave background 
The question of anisotropy in the early universe was significantly answered by the Wilkinson Microwave Anisotropy Probe, which looked for fluctuations in the microwave background intensity., New Three Year Results on the Oldest Light in the Universe, NASA WMAP. The measurements of this anisotropy have provided useful information and constraints about the evolution of the universe.
To the limit of the observing power of astronomical instruments, objects radiate and absorb energy according to the same physical laws as they do within our own galaxy , The Composition of Stars, Astronomy Notes. Based on this, it is believed that the same physical laws and constants are universally applicable throughout the observable universe. No confirmed evidence has yet been found to show that physical constants have varied since the big bang, and the possible variation is becoming well constrained.Have physical constants changed with time?, Astrophysics
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.Studies of the Universe other than Earth, usually fall into the terms of the various "Astronomy" Sciences; also called Astrosciences.
- Anthropic principle
- Big Bang
- Big Crunch
- Big Freeze
- Dyson's eternal intelligence
- Esoteric cosmology
- False vacuum
- Final anthropic principle
- Fine-tuned universe
- Gaia hypothesis
- Heat death of the universe
- Hindu Cycle Of The Universe
- Kardashev scale
- Multiverse (religion)
- Omega point
- Origin of life
- Rare Earth hypothesis
- Religious cosmology
- Shape of the universe
- Ultimate fate of the universe
- World view
- Stephen Hawking's Universe - why is the universe the way it is?
- Richard Powell: An Atlas of the Universe - images at various scales, with explanations.
- Cosmos - an "illustrated dimensional journey from microcosmos to macrocosmos"
- Age of the Universe at Space.Com
- My So-Called Universe arguments for and against an infinite and parallel universes
- Parallel Universes by Max Tegmark
- Logarithmic Maps of the Universe
- Seti@Home - the Search for Extraterrestrial Intelligence
- Universe - Space Information Centre by Exploreuniverse.com
- Number of Galaxies in the Universe
- Size of the Universe at Space.Com
- Illustration comparing the sizes of the planets, the sun, and other stars
- Cosmology FAQ