Universe

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

This article discusses the universe from a physical or empirical viewpoint. For other uses, see Universe (disambiguation).

The term universe has a variety of meanings, based on the context in which it is used. In strictly physical terms, the total universe is the summation 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 usually 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

Main article: Age of the universe

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, a rather 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 × 10⁹) 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 it is big 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.

See also: Timeline of the Big Bang

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 behavior 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

Main article: Observable universe

There is no generally accepted theory making a pronouncement concerning whether the universe is indeed finite or infinite in spatial extent. For an overview of the possibilities, see Shape of the Universe.

However, the observable universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The edge of the cosmic light horizon is 15.8 billion light years distant. The present distance (comoving distance) to the edge of the observable universe is larger, due to the ever increasing rate at which the universe has been expanding; it is estimated to be about 78  billion light years (7.8 × 10¹⁰ light years, or 7.4 × 10²⁶ m). This would make the volume, of the known universe, equal to 1.9 × 10³³ cubic light years (assuming this region is perfectly spherical). As of 2006, the observable universe is thought to contain about 7 × 10²² stars, organized in about 100 billion (10¹¹) 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 Hubble Space Telescope discovered galaxies such as Abell 1835 IR1916, which are over 13 billion light years from Earth.

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; that is, they cannot affect any events that we can perceive. See also Causality (physics).

Shape of the universe

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 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

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.

Multiverse

Main article: Multiverse (science)

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.

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.

External links

• Stephen Hawking's Universe - Where do we come from? How did the universe begin? Why is the universe the way it is? How will it end?
• Richard Powell: An Atlas of the Universe - a series of 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 by Jim Holt, on various arguments for and against an infinite universe 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 with each other, the sun, and other stars