Observable universe
From Wikipedia, the free encyclopedia
- See universe for a general discussion of the universe.
The observable universe is a term used in Big Bang cosmology to describe a ball-shaped region of space surrounding the observer that is close enough that we might observe objects in it, i.e. there has been sufficient time for light emitted by an object to arrive at the observer. Every position has its own observable universe which may or may not overlap with the one centered around the Earth.
The word observable used in this sense has nothing to do with whether modern technology actually permits us to detect radiation from an object in this region. It simply means that it is possible for light or other radiation from the object to reach an observer on earth. In practice, we can only observe objects as far as the surface of last scattering, when the universe became transparent. However, it may be possible to infer information from before this time through the detection of gravitational waves.
Contents |
[edit] The universe versus the observable universe
Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe". This can be justified on the grounds that we can never know anything about any part of the universe that is causally disconnected from us. No one believes, however, that the observable universe is precisely the entire universe; that would imply that the Earth is exactly at the center of the universe, violating a fundamental assumption of astronomy (and indeed all of science). It is likely that the galaxies within our visible universe represent only a minuscule fraction of the galaxies in the universe. The total number of galaxies may even be infinite.
It is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies are actually duplicate images of nearby galaxies, formed by light that has circumnavigated the universe. It is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different. A 2004 paper [1] claims to establish a lower bound of 24 gigaparsecs (78 billion[2] light-years) on the diameter of the universe, based on matching-circle analysis of the WMAP data.
[edit] Size
The comoving distance from the Earth to the edge of the visible universe is about 46.5 billion light-years in any direction; this is the comoving radius of the visible universe. It is sometimes quoted as a diameter of 92-94 billion light-years. Since the visible universe is a perfect sphere and space is roughly flat, this size corresponds to a comoving volume of about 4/3 π R3 = 4 × 1032 cubic light-years or 3.4 × 1080 cubic meters.
The figures quoted above are distances now (in cosmological time), not distances at the time the light was emitted. For example, the cosmic microwave background radiation that we see right now was emitted about 13.7 billion years ago by matter that has, in the intervening time, condensed into galaxies. Those galaxies are now about 46 billion light-years from us, but at the time the light was emitted, that matter was only about 40 million light-years away from the matter that would eventually become the Earth.
[edit] Misconceptions
Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these are listed below.
- 13.7 billion light-years. The age of the universe is about 13.7 billion years, and nothing travels faster than light; does it not follow that the radius of the observable universe must be 13.7 billion light-years? This reasoning might make sense if we lived in the flat spacetime of special relativity, but in the real universe spacetime (not space!) is highly curved at cosmological scales, and light does not move rectilinearly. Distances obtained as the speed of light times a cosmological time interval have no direct physical significance. [3]
- 15.8 billion light-years. This is obtained in the same way as the 13.7 billion light-year figure, but starting from an incorrect age of the universe which was reported in the popular press in mid-2006 (e.g. [1] [2] [3]). For an analysis of this claim and the paper that prompted it, see [4].
- 27 billion light-years. This is a diameter obtained from the (incorrect) radius of 13.7 billion light-years.
- 78 billion light-years. This figure, as mentioned above, is a lower bound on the size of the whole universe, and has nothing to do with the size of the visible universe.
- 156 billion light-years. This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius. Since 78 billion light-years is already a diameter (or rather a circumference), the doubled figure is meaningless even in its original context. This figure was very widely reported (e.g. [4] [5] [6]).
- 180 billion light-years. This estimate accompanied the age estimate of 15.8 billion years in some sources; it was obtained by incorrectly adding 15% to the incorrect figure of 156 billion light-years.
[edit] Matter content
The observable universe contains about 3 to 5 × 1022 stars, organized in around 80 billion galaxies, which themselves form clusters and superclusters.
Two back-of-envelope calculations give the number of atoms in the observable universe to be around 1080.
- The critical density of the universe is 3 H2 / 8 π G, which works out to be 1×10−29 grams/cubic centimeter or about 5×10−6 atoms of hydrogen/cc. It is believed that only 4 percent of the critical density is in the form of normal atoms, so this leaves 2×10−7 hydrogen atoms/cc. Multiplying this by the volume of the visible universe, you get about 7 × 1079 hydrogen atoms.
- A typical star weighs about 2×1033 grams, which is about 1×1057 atoms of hydrogen per star. A typical galaxy has about 400 thousand million stars so that means each galaxy has 1×1057 × 4×1011 = 4×1068 hydrogen atoms. There are possibly 80 thousand million galaxies in the Universe, so that means that there are about 4×1068 × 8×1010 = 3×1079 hydrogen atoms in the Universe. But this is definitely a lower limit calculation, and ignores many possible atom sources. [5]
[edit] See also
- Mass of the observable universe (for slightly different estimates)
- Particle horizon
- Causality (physics)
- Observation
- Universe
[edit] References
- ^ Neil J. Cornish, David N. Spergel, Glenn D. Starkman, and Eiichiro Komatsu, Constraining the Topology of the Universe. Phys. Rev. Lett. 92, 201302 (2004). astro-ph/0310233
- ^ "billion" means thousand million in this article rather than million million
- ^ Edward L. Wright, "Why the Light Travel Time Distance should not be used in Press Releases".
- ^ Edward L. Wright, "An Older but Larger Universe?".
- ^ Matthew Champion, "Re: How many atoms make up the universe?", 1998
