2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

Physical cosmology
Related topics
  • Astrophysics
  • General relativity
  • Particle physics
  • Quantum gravity
The deepest visible-light image of the cosmos, the Hubble Ultra Deep Field.
The deepest visible-light image of the cosmos, the Hubble Ultra Deep Field.

In strictly physical terms, the total universe is the sum 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 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

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, an 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 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.

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 behaviour 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

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. 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 centre and a diameter of about 93 billion light years. Note that many sources, including previous versions of this Wikipedia article, 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 of the universe

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

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.


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.
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.

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