Universe

The universe is the summation of all particles and energy that exist and the space-time in which all events occur. The study of the physical universe is the domain of cosmology.

There are more than one hundred billion (1011) galaxies in the universe, each containing hundreds of billions of stars, with each star containing about 1057 atoms of hydrogen.

Etymology
The word "universe" is derived from Old French univers, from Latin universum, which combines uni- ("one") with versus ("turn"), meaning "turned into one". However, 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". (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.)

Models of universes
The generally accepted scientific theory that describes the origin and evolution of the universe is Big Bang Theory. This cosmic explosion, by current estimates, happened between 13-15 billion years ago. 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.

Evolution
According to redshift observations, and Hubble's Law, the universe is expanding. Conversely, if this expansion has continued over the entire age of the universe, then in the past, these distant, receding objects must once have been closer together. By extrapolating this expansion back in time, one approaches a gravitational singularity where everything in the universe was compressed into an infinitesimal point. This idea gave rise to the Big Bang Theory, which describes the expansion of space from an extremely hot and dense state of unknown characteristics.

Age
Examination of small variations in the microwave background radiation provides information about the nature of the universe, including the age and composition. 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 years, with a margin of error of about 1 % (± 200 million years). Other methods of estimation give different ages ranging from 11 billion to 20 billion. Most of the estimates cluster in the 13–15 billion year range.

Composition
The currently observable universe appears to have a mass-energy density of 9.9 × 10-30 grams per cubic centimeter, assuming the universe is flat. 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. 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.

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 (4He) (24% of total mass) and trace amounts of the isotopes deuterium (2H), helium-3 (3He) and lithium (7Li). 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.

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 K. The neutrino background is not observable with present-day technology, but is theorized to have a density of about 150 neutrinos per cubic centimeter.

Size
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.

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.

Shape
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.

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 was 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.