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Universe

Alternate uses: See Universe (disambiguation)

In the first half of the 20th century, the name Universe was used to mean the whole spacetime continuum in which we exist, together with all the energy and matter within it. Attempts to understand the Universe in this sense, on the largest possible scales, are made in cosmology, a science that has grown from physics and astronomy. During the second half of the 20th century, the development of observational cosmology, also called physical cosmology, 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.

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.

Table of contents

1 Expansion and age, and the Big Bang theory 1.1 Size of Universe and observable universe 2 Shape of the Universe 3 Fate of the Universe 4 Multiverse 5 Other terms 6 See also 7 References 8 External links

Expansion and age, and the Big Bang theory

The most important result of cosmology, 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 time=zero of the Big Bang was estimated to have happened about 13.7 billion (13.7 × 10⁹) years ago, with an uncertainty of 200 million years, according to NASA's Wilkinson Microwave Anisotropy Probe project (WMAP). Some recent studies, however, found the carbon-nitrogen-oxygen (CNO) 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.

The actual age of the universe can be calculated directly from the temperature of the Cosmic Microwave Background Radiation (CMB). Using Planck units, the age of the universe is simply the inverse square of its temperature. This gives an age of 15.556 billion years, with an uncertainty of only 24 million years, thanks in part to the accurate CMB temperature measurement made by the Cosmic Background Explorer (COBE).

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 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 initial period of rapid inflation following the Big Bang.

Size of Universe and observable universe

There is disagreement over whether the Universe is finite or infinite 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, is certainly finite. The edge of the cosmic light horizon is 13.7 billion light years distant. The present distance (comoving distance) to the edge of the observable universe is larger, since the universe has been expanding; it is estimated to be about 78 billion light years (7.4 × 10²³ km). This would make the comoving volume, 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 it 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. For example, matter that falls into a black hole in this 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").

When speculating about a multiverse, one often thinks of it as consisting of many "universes" (lower case), our Universe being one of them.

See also

• Cosmos
• Parallel universe

• 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://www.wolframscience.com/conference/2004/presentations/material/ahewitt-2.pdf

• Richard Powell: An Atlas of the Universe, http://www.anzwers.org/free/universe/index.html. 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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