The deepest visible-light image of the cosmos. Hubble Ultra Deep Field. Image Credit: NASA
, S. Beckwith (STScI) and the HUDF team.
In the first half of the 20th century, the word 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.
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 age of the Universe was estimated to be about 13.7 billion () years, with an uncertainty of 200 million years, according to NASA's Wilkinson Microwave Anisotropy Probe project (WMAP). 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 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 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 (km). This would make the comoving volume, of the known universe, equal to cubic light years (assuming this region is perfectly spherical). The observable universe contains about stars, organized in about 10 billion 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.
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, and therefore, it is argued, effectively do not exist. See also Causality (physics).
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.
But some of the objects outside of the observable universe can, in principle, be observed indirectly. For example, it is theoretically possible to meet an observer located near the end of our observable universe, who in his past has observed some galaxies that left our observable universe because of expansion.
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: the surface of 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 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 gravitationally 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.
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 (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 cannot be regarded as anything more than speculation.
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 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").