April 21, 2009, 8:41 pm
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Most scientists subscribe to the basic definition that the Universe refers to the physical system that consists of all of the contents of space that we can observe, or in principle, detect, at the vast scales of interstellar and intergalactic space. It also refers to all of the contents of microscopic, atomic, and sub-atomic space. These contents include both matter, and any forms of electromagnetic radiation.

A worldline through a light cone in 2D space plus a time dimension. (Source: Wikipedia.)

A worldline through a light cone in 2D space plus a time dimension. (Source: Wikipedia.)

Following the advent of general relativity, this working definition had to be extended. The reason for this is that, in relativistic cosmology, the universe had a finite origin in time which we call the Big Bang. This occurred 13.7 billion years ago, and consequently, every observer is surrounded by a space partitioned into "observable" and "unobservable." The spherical boundary that separates these two domains of 3-dimensional space is called the "Horizon," and is a general feature of all cosmological models with a finite origin in time, in the past light cone of the observer. [1]  Consequently, the Big Bang may have been the origin for space and time, but observationally there is far more 'space' beyond an observer's horizon, that will not be detectable until the universe has grown older.

Cosmologists now consider the Universe to represent the entire spacetime that emerged from the Big Bang. It represents a potentially infinite 3-dimensional space, in which all observers share the same physical laws and contents, but where some observers are unobservable to others. The uniformity of the physics in these distant, unobservable corners of 3-dimensional space is an extrapolation of the principle that, what we see within our own observable universe is statistically unremarkable, and since the preconditions emerged from the Big Bang, we assume that these same preconditions apply to all other regions of space beyond our horizon, and lead to the same kinds of matter, stars and galaxies that we see.

This basic definition for Universe has been extended by physicists in several important ways. We delineate these extensions in several articles within the "Universe:" series of articles here in the Encyclopedia of the Cosmos. For example, see Universe: Higher Dimensions.


  1. Light cone: See "What is a light cone?" - Patricia Schwarz, Caltech.  See also, Light cones in general relativity - Wikipedia.

Related EoC Articles

External Links

  • Wilkinson Microwave Anisotropy Probe (WMAP) - A NASA Explorer mission that launched June, 2001 to make fundamental measurements of cosmology – the study of the properties of our universe as a whole. WMAP has been stunningly successful, producing our new Standard Model of Cosmology. WMAP continues to collect high quality scientific data. NASA.
  • WMAPs Universe - Universe 101, NASA.
  • WMAP Images - Universe 101, NASA.
  • WMAP Image Topics - Universe 101, NASA. ("Drill-down" into the various topics where there is an interesting collection of appropriate images.)

Preview Image

"Five Year Microwave Sky" - The detailed, all-sky picture of the infant universe from three years of WMAP data. The image reveals 13.7 billion year old temperature fluctuations (shown as color differences) that correspond to the seeds that grew to become the galaxies. The signal from our Galaxy was subtracted using the multi-frequency data. This image shows a temperature range of ± 200 microKelvin.  View full-size image.  Internal Linear Combination Map, Galactic coordinates, Mollweide projection. Linear scale from -200 to 200 uK. Wilkinson Microwave Anisotropy Probe (WMAP) Science Team, NASA.


Odenwald, Sten, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Universe." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published January 12, 2008].




(2009). Universe. Retrieved from http://www.cosmosportal.org/view/article/138892


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