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Physical cosmology

The Big Bang
The shape of the universe in big bang theory

..... Click the link for more information. , a science that has grown from physics Physics (from Greek from φυσικός (physikos): natural, from φύσις (physis): Nature) is the study of energy and its interaction with matter (see chemistry, biology). Because of the primacy of energy in the history of the universe, because all matter must interact with energy to express its properties and engage in transformations, and because energy is the key player when matter is decomposed into its most basic parts, physics is often considered to be the fundamental science.
..... Click the link for more information. and astronomy Astronomy, which etymologically means "law of the stars", (from Greek: αστρονομία = άστρον + νόμος) is a science involving the observation and explanation of events occurring outside Earth and its atmosphere. It studies the origins, evolution, physical and chemical properties of objects that can be observed in the sky (and are outside the earth), as well as the processes involving them.
..... Click the link for more information. . During the second half of the 20th century, the development of observational cosmology Observational cosmology is the study of the structure, the evolution and the origin of the universe through observation, using instruments such as telescopes and cosmic ray detectors.

..... Click the link for more information. , also called physical cosmology Physical cosmology:

a=x

The cosmic exploration examines quasi-physical matters of an optical universe, unveiling divine fabricated time zones outer-inner dimensions in space. Highly proficient energy resources compose cosmic manifestations by exerting the astral plane coordinates. Homo sapiens optical-imagination integrates this imaginary binding point inner- psyche realities revealing outer-physical
..... Click the link for more information. , led to a split in the meaning of the word Universe, between observational cosmologists and theoretical cosmologists; where the former (usually) abandon the hope of observing the whole spacetime continuum, the latter retain this hope, attempting to find the most reasonable speculations for modelling the whole of spacetime, despite the extreme difficulty in imagining any empirical constraints on these speculations and the risk of declining into metaphysics Metaphysics is a branch of philosophy, and related to the natural sciences, like physics, psychology and the biology of the brain; and also to mysticism and religious and spiritual subjects. It is notoriously difficult to define, but for purposes of briefly introducing it to nonphilosophers, it can be identified as the study of any of the most fundamental concepts and beliefs about the basic nature of reality, on which many other concepts and beliefs rest -- concepts such as being, existence, universal, property, relation, causation, space, time, event, and many others.
..... Click the link for more information. .

The terms known universe, observable universe, or visible universe are often used to describe the part of the Universe that we can see or otherwise observe. Those who believe it is impossible to observe the whole continuum may use our universe, referring only to that knowable by human beings in particular.

Expansion and age, and the Big Bang theory

The most important result of cosmology, that the Universe is expanding, is derived from redshift

This article is about the light phenomenon. For other uses of the word "Redshift" and the phrase "Red Shift", see Red Shift

Redshift is the phenomenon that the frequency of light when observed, under certain circumstances, can be lower than the frequency of light when it was emitted at the source. This usually occurs when the source moves away from the observer,
..... Click the link for more information. observations and quantified by Hubble's Law Hubble's law is the statement in astronomy that galaxies move away from each other, and that the velocity with which they recede is proportional to their distance. It leads to the picture of an expanding universe and, by extrapolating back in time, to the Big Bang theory.

The law was first formulated by Edwin Hubble in 1929. Hubble compared the distances to nearby galaxies to their redshift, found a linear relationship, and interpreted the redshift as caused by the receding velocity. His estimate of the proportionality constant, now known as
..... Click the link for more information. . Extrapolating this expansion back in time, one approaches a gravitational singularity A gravitational singularity occurs when an astrophysical model, typically based on general relativity, predicts a point of infinite curvature. The term is closely related to the mathematical meaning of "singularity": a gravitational singularity occurs when the equations produce a mathematical singularity.

The Big Bang cosmological model of the universe contains a gravitational singularity at the start of time (t=0). At the "Big Bang Singularity," the model predicts that the density of the universe and the curvature of space-time are infinite. However, the basic Big Bang model does not include quantum effects, so its predictions are valid only shortly after the projected singularity.
..... Click the link for more information. , a rather abstract mathematical concept, which may or may not correspond to reality. This gives rise to the Big Bang In astrophysics, the term Big Bang is used both in a narrow sense to refer to the interval of time roughly 13.7 billion years ago when the photons observed in the microwave cosmic background radiation acquired their black-body form, and in a more general sense to refer to a hypothesized point in time when the observed expansion of the universe (Hubble's law) began.

In cosmology, the Big Bang theory
..... Click the link for more information. theory, the dominant model in cosmology today. The age of the Universe The age of the Universe was estimated to have happened be about 13.7 billion (13.7 × 10⁹) years, with an uncertainty of 200 million years, according to NASA's Wilkinson Microwave Anisotropy Probe project (WMAP). However this based on the fact we have assumed the underlying model we used is correct. Other methods of esimating the age of the universe give different ages.

Some recent studies, found the carbon-nitrogen-oxygen cycle to be two times slower than previously believed, leading to the conclusion that the Universe must be at least 14.7 billion years old.
..... Click the link for more information. was estimated to be about 13.7 billion The numeral billion refers to either of two different numbers.

10¹² The original meaning, since the beginnings of modern times, is "a million of a million" (= 10¹²). This system, known in French as the échelle longue ("long scale") was maintained or readopted all around the world—with the exceptions of the USA and Brazil
10
..... Click the link for more information. (13.7 × 10⁹) years, with an uncertainty of 200 million years, according to NASA The National Aeronautics and Space Administration (NASA) (established 1958) is the government agency responsible for the United States of America's space program and long-term general aerospace research. A civilian organization, it conducts (or oversees) research into both civilian and military aerospace systems.

Vision and mission

Its vision is:

To improve life here,
To extend life to there, and
To find life beyond.

..... Click the link for more information. 's Wilkinson Microwave Anisotropy Probe

The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. The satellite was launched on June 30, 2001, at 3:46 p.m. EDT at Cape Canaveral Air Force Station, Florida, USA.

The goal of WMAP is to map out minute differences in the Cosmic Microwave Background (CMB) radiation in order to help test theories of the nature of the universe. It is the successor to COBE and one of the series of medium-class explorer (MIDEX) satellites.
..... Click the link for more information. ). 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 This article is about a celestial body. For alternate meanings see galaxy (disambiguation).

Stars are almost always found in collections called galaxies, together with gas, dust, and "dark matter"; ~10-20% of a galaxy is composed of stars, gas, and dust. Galaxies are held together by gravitational attraction and the galactic components orbit a common centre. There is some evidence that black holes may exist at the centre of some, or most, galaxies. Galaxies "evolve" from protogalaxies.
..... Click the link for more information. are, the faster they move away from us. It can also be seen in the microwave background radiation When any patch of the sky is observed where no individual sources can be discerned, and the effects of interplanetary dust, and interstellar matter are taken into account, there is still radiation.

This radiation is known as Cosmic Background Radiation. The origin of this radiation depends on the region of the spectrum that we are observing. Certainly the most famous component is the Cosmic microwave background radiation. In the standard model, this is taken to be a remainder of the epoch when the universe, still hot, became transparent for the first time to radiation. The Sunyaev Zeldovic theory shows the phenomena that radiant cosmic background radiation interacting with "electron" clouds distorting the spectrum of the radiation.
..... Click the link for more information. 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 initial period of rapid inflation

Inflation is the idea—first proposed by Alan Guth (1981)—that the nascent universe passed through a phase of exponential expansion that was driven by a negative vacuum energy density (positive vacuum pressure). This expansion can be modelled by a non-zero cosmological constant. As a direct consequence of this expansion, all of the observable universe is posited to have originated in a small, initially causally-connected region. Quantum fluctuations in this microscopic region, magnified to cosmic size, then became the seeds for the growth of structure in the universe (see Galaxy Formation and Evolution).
..... Click the link for more information. following the Big Bang.

Size of Universe and observable universe

There is disagreement over whether the Universe is finite or infinite

Infinity is a theoretical value that is larger than any other value. To count to infinity is to count forever, without end. Infinity comes from the Latin "infinitus", meaning without end and usually denoted by the symbol "∞" (lemniscate). Infinite is the quality of being greater than anything. It is also used to denote the quality of being unbounded or having no limit. The Infinite is usually defined as that which has no bounds in space or time.
..... Click the link for more information. 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

The speed of light in a vacuum (denoted as , reputedly from the Latin celeritas, "speed") is exactly equal to 299,792,458 metres per second, which is approximately 300,000 kilometres per second, or 186,000 miles per second. This exact speed is a definition, not a measurement, as the metre is defined in terms of the speed of light and not vice versa. The speed of light through a medium (that is, not in vacuum) is less than due to refraction.
..... Click the link for more information. , is certainly finite. The edge of the cosmic light horizon In astronomy, the cosmic light horizon is a "horizon" which marks the edge of the visible universe. This horizon is the edge of a sphere, centered upon the Earth, which is approximately 13.7 billion light years in radius; this figure is sometimes referred to as the "lookback distance" (because astronomers, who view distant objects, are "looking back" into the history of the universe -- see: speed of light). The general homogeneity of temperatures, at the edge of the cosmic horizon, is part of the evidence for the "Big Bang".
..... Click the link for more information. is 13.7 billion light years A light year, abbreviated ly, is the distance light travels in one year: roughly 9.46 × 10¹² kilometres (or about 5.88 × 10¹² miles). More specifically, a light-year is defined as the distance that a photon would travel, in free space and infinitely far away from any gravitational or magnetic fields, in one Julian year (365.25 days of 86400 seconds each). Since the speed of light in a vacuum is 299,792,458 m/s, one light-year is approximately equal to 9.46 × 10¹⁵ m = 9.46 petametres.
..... Click the link for more information. distant. The present distance (comoving distance Thinking about the shape of the universe in the context of the standard Big Bang model is simplest using comoving coordinates.

While special relativity states that all inertial reference frames are equivalent, i.e. that there is no favoured set of space-time coordinates, this is only a local theory.

General relativity is also a local theory, but it is used to constrain the local properties of a Riemannian manifold, which itself is global.
..... Click the link for more information. ) to the edge of the observable universe is larger, since the universe has been expanding; it is estimated to be about 78 billion The numeral billion refers to either of two different numbers.

10¹² The original meaning, since the beginnings of modern times, is "a million of a million" (= 10¹²). This system, known in French as the échelle longue ("long scale") was maintained or readopted all around the world—with the exceptions of the USA and Brazil
10
..... Click the link for more information. light years (7.4 × 10²³ km). This would make the comoving volume Volume (also called capacity) is a quantification of how much space an object occupies. The SI unit for volume is the cubic metre (American spelling meter).

The volume of a solid object is a numerical value given to describe the three-dimensional concept of how much space it occupies. One-dimensional objects (such as lines) and two-dimensional objects (such as squares) are assigned zero volume in three-dimensional space.
..... Click the link for more information. , 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 10¹⁰ 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 reader should be warned that 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. Thus the term our...

We live in the centre 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 centre. 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=zero of the Big bang model. And since light travels at the same speed in any direction towards us, we live at the centre of our observable universe.

Shape of the Universe

An important open question of cosmology is the shape of the universe.

Firstly, whether or not the Universe is 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 (nearly) flat, with local wrinkles where massive objects distort spacetime, just as a lake is (nearly) flat. This opinion was strengthened by the latest data from WMAP, looking at "acoustic oscillations" in the cosmic background radiation temperature variations.

Secondly, whether or not 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. This can be understood using a two-dimensional analogy: a sphere has no edge, but nonetheless has a finite area (4πR²). It is a two-dimensional surface with constant curvature in a third dimension. A three-dimensional equivalent is the unbounded "spherical space" discovered by Bernhard Riemann, which has a finite volume (2π²R³). In it, all three dimensions are constantly curved in a fourth. (Other possibilities include a similar "elliptical space", and a "cylindrical space", where, in conflict with ordinary geometry, the two ends of the cylinder are joined together, but without bending the cylinder. These, also, are two-dimensional spaces with finite areas; innumerable others exist. However, the sphere has the unique and, perhaps, more aesthetically pleasing property that all points on it are geometrically similar.) If the universe is indeed unbounded yet 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 after traveling a distance equal to the "circumference" of the universe (which is impossible to our current understanding of the Universe, as its size is much greater than the size of the observable universe).

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 be smaller than currently accounted. Although this possibility has not been ruled out, the results of latest cosmic microwave background (CMB) research make this 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 gravitionally slowed 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 the whole of eternity, see accelerating universe. 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, our Universe being one of those universes (lower case). 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 so cannot be regarded as anything more than speculation.

Other terms

Different words have been used throughout history to denote "all of space", including the equivalents in various languages of "heavens", "cosmos" and "world".

Although words like world and its equivalents in other language 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").

References

Einstein, Albert (1952). Relativity: The Special and the General Theory (Fifteenth Edition). ISBN 0-517-88441-0
Alastair Hewitt (2004). A Cosmological Model Utilizing Causal-temporal Mobile Automaton (poster presentation, part 2). http://encyclopedia.thefreedictionary.com/#

External links

Richard Powell: An Atlas of the Universe, http://encyclopedia.thefreedictionary.com/# A series of images at various scales, with explanations.
Age of the Universe at Space.Com
My So-Called Universe by Jim Holt, on various arguments for and against an infinite Universe and parallel universes.
Parallel Universes by Max Tegmark.

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References in classic literature:	More
On earth, here on this earth" (Pierre pointed to the fields), "there is no truth, all is false and evil; but in the universe, in the whole universe there is a kingdom of truth, and we who are now the children of earth are- eternally- children of the whole universe. War and Peace by Tolstoy, Leo View in context
Man, with his brain, can penetrate the intoxicating show of things and look upon the universe brazen with indifference toward him and his dreams. John Barleycorn by London, Jack View in context
think that all the universe is straining towards the obscure significance of your pictures. Middlemarch by Eliot, George View in context

Some articles mentioning "Universe":

Captain Universe
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Deterministic universe

Einstein's radius of the universe
Einstein's universe
Elders of the Universe
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Everything
Expanded Universe
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Mr. Universe

New Universe
Omega point
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Topology of the universe

Ultimate fate of the Universe
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