Universe

"Universe" is a word derived from the Old French univers, which in turn comes from the Latin roots unus ("one") and versus (a form of vertere, "to turn"). Based on observations of the observable universe, 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 currently-accepted theory of the universe's formation is the Big Bang model, which describes the expansion of space-time from a gravitational singularity. The universe underwent a rapid period of cosmic inflation that flattened out nearly all initial irregularities. Thereafter the universe expanded and became steadily cooler and less dense. Minor variations in the distribution of mass resulted in fractal segregation into features that are found in the current universe; such as clusters of galaxies.

Observable portion

Main article: Observable universe

Theoretical and observational cosmologists vary in their usage of the term Universe to mean either this whole system or just a part of this system.^[1]

As used by observational cosmologists, the Universe (upper case "U") most frequently refers to the finite part of space-time which is directly observable by making observations using telescopes and other detectors and using the methods of theoretical and empirical physics for studying the basic components of the Universe and their interactions. 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 frequently use the term the Universe to mean either the observable part of space, the observable part of space-time or the entire space-time.^{[citation needed]}

A majority of cosmologists believe that the observable universe is an extremely tiny part of the "whole" (theoretical) Universe and that it is impossible to observe the whole of comoving space. It is presently unknown whether or not this is correct, since 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, but the question remains under debate.^[2][3] If a version of the cosmic inflation scenario is correct, then there is no known way to determine whether the (theoretical) universe is finite or infinite, in which case the observable Universe is just a tiny speck of the (theoretical) universe.

Theory

Theoretical cosmologists study models of the whole of space-time which is connected together, and search for models which are consistent with physical cosmologists' model of space-time on the scale of the observable universe.^{[citation needed]} Their models are speculative but use the methods of theoretical physics. These models are usually referred to using the term universe (lower case "u"). Sometimes theorists use the Universe (upper case "U") to refer to the whole of the specific space-time in which we live.^[4]

Multiverse

Main article: Multiverse (science)

Some theorists extend their model of "all of space-time" beyond a single connected space-time to a set of disconnected space-times, or multiverse. In order to clarify the 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.^[4]

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.

Physicist David Deutsch suggests that a multiverse is a consequence of the many-worlds interpretation, which he considers to be the best alternative explanation to the Copenhagen explanations of Quantum theory first presented by Niels Bohr, over half a century ago.

Evolution

Formation

Main articles: Age of the universe and Big Bang

The most important result of physical cosmology—that the universe is expanding—is derived from redshift observations and quantified by Hubble's Law. That is, astronomers observe that there is a direct relationship between the distance to a remote object (such as a galaxy) and the velocity with which it is receding. 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; an abstract mathematical concept that may or may not correspond to reality. This idea gave rise to the Big Bang theory, the dominant model in cosmology today.

During the earliest era of the big bang theory, the universe is believed to have formed a hot, dense plasma. As expansion proceeded, the temperature steadily dropped until a point was reached when atoms could form. At about this time the background energy (in the form of photons) became decoupled from the matter, and was free to travel through space. The left-over energy continued to cool as the universe expanded, and today it forms the cosmic microwave background radiation. 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.

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 (1.37 × 10¹⁰) 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.^[5] Most of the estimates cluster in the 13–15 billion year range.^[6][7]

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.

See also: Timeline of the Big Bang

Pre-matter soup

Until recently, the first hundredth of a second after the Big Bang 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 behavior that might have been taking place in this time frame.^[8]

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.^[9]

Protogalaxies

Main article: Protogalaxy

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.^{[10] [11]}

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.

Ultimate fate

Main article: Ultimate 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.

Composition

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. The primary constituents appear 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.^[12] 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.^[13]

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 other elements.^[14] A small portion of these elements were in the form of the isotopes deuterium (²H), helium-3 (³He) and lithium (⁷Li).^[15] 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.^[16]

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.^[17] The current density of background neutrinos is about 150 per cubic centimetre.^[18]

See also: Abundance of the chemical elements

Physical structure

Size

Main article: 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^[19] 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.^[20] 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.

Shape

Main articles: Shape of the universe and Large-scale structure of the cosmos

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.[1]

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.^[21]

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.^[22] 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.^[23] 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.^[24]

Other terms

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".^[25]

See also

Notes and references

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