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In Big Bang cosmology, the 'observable universe' (also called the 'Hubble Volume') is the region of space bounded by a sphere, centered on the observer, that is small 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

The universe versus the observable universe

Hubble limit

Size

Misconceptions

Matter content

References

See also

External links

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 findings suggest, however, that the observable universe is precisely the entire universe, which is exceedingly unlikely in that it would imply that the Earth is exactly at the center of the universe. It is likely that the galaxies within our visible universe represent only a minuscule fraction of the galaxies in the universe.
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 ^[2] claims to establish a lower bound of 24 gigaparsecs (78 billion^[3] light-years) on the diameter of the universe, based on matching-circle analysis of the WMAP data.

Hubble limit

The Hubble Limit^[4] is a concept in physical cosmology that is related to the Big Bang Theory. It refers to the limit where objects receding from the observer are receding at the speed of light. It is named after the astronomer Edwin Hubble, who was the first to discover that objects on a galactic scale are moving away from us. In the aftermath of the Big Bang everything in the universe is flying apart, and due to the fact that the speed of light is constant, farther objects appear to be receding at a higher velocity. Eventually an object will appear to have a velocity which is the speed of light, and an object at this point is known to be at the Hubble Limit.

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' Misconceptions about the Big Bang . Since the visible universe is a perfect sphere and space is roughly flat, this size corresponds to a comoving volume of about

imes
pi
imes
mathrm{R}^3 =
4.20
imes 10^{32} ext{ ly}^3

or 3.56×10⁸⁰ 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. See comoving coordinates.

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. While it is commonly understood that nothing travels faster than light, it is a common misconception that the radius of the observable universe must therefore amount to only 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 is highly curved at cosmological scales by virtue of the Hubble expansion (though ''space'' is roughly flat). Distances obtained as the speed of light times a cosmological time interval have no direct physical significance. ^[5]

★ ''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^{[6] [7] [8]}. For an analysis of this claim and the paper that prompted it, see ^[9].

★ ''27 billion light-years''. This is a diameter obtained from the (incorrect) radius of 13.7 billion light-years.

★ ''78 billion light-years''. This is a lower bound (not an estimate) for the size of the whole universe (not the observable universe). If the universe is smaller than the observable universe, then light has had time to circumnavigate it since the big bang, producing multiple images of distant objects in the sky. Cornish et al looked for such an effect at scales of up to 24 gigaparsecs (78 billion light years) and failed to find it. 24 gigaparsecs is simply the upper limit of the search space of this study; it has no physical significance.

★ ''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^{[10] [11] [12]}.

★ ''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.

Matter content

The observable universe contains about 3 to 7 × 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 $3\; H^2/8\; pi\; G$, which works out to be 1×10⁻²⁶ kg/m³ or about 5 atoms of hydrogen/m³. It is believed that only 4 percent of the critical density is in the form of normal atoms, so this leaves 0.2 hydrogen atoms/m³. Multiplying this by the volume of the visible universe, you get about 7×10⁷⁹ hydrogen atoms.
#A typical star has a mass of about 2×10³⁰ kg, which is about 1×10⁵⁷ atoms of hydrogen per star. A typical galaxy has about 400 billion stars so that means each galaxy has 1×10⁵⁷ × 4×10¹¹ = 4×10⁶⁸ hydrogen atoms. There are possibly 80 billion galaxies in the Universe, so that means that there are about 4×10⁶⁸ × 8×10¹⁰ = 3×10⁷⁹ hydrogen atoms in the observable Universe. But this is definitely a lower limit calculation, and ignores many possible atom sources. ^[13]

References

1. "Large Scale Structure in the Local Universe: The 2MASS Galaxy Catalog", Jarrett, T.H. 2004, PASA, 21, 396
2. 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
3. "billion" means thousand million in this article rather than million million
4. [1]
5. Ned Wright, "Why the Light Travel Time Distance should not be used in Press Releases".
6. http://www.space.com/scienceastronomy/060807_mm_huble_revise.html
7. http://space.newscientist.com/article/dn9676-big-bang-pushed-back-two-billion-years.html
8. http://worldnetdaily.com/news/article.asp?ARTICLE_ID=51395
9. Edward L. Wright, "An Older but Larger Universe?".
10. http://www.space.com/scienceastronomy/mystery_monday_040524.html
11. http://www.msnbc.msn.com/id/5051818/
12. http://news.bbc.co.uk/2/hi/science/nature/3753115.stm
13. Matthew Champion, "Re: How many atoms make up the universe?", 1998

See also

★ Mass of the observable universe (for slightly different estimates)

★ Particle horizon

★ Causality (physics)

★ Observation

★ Universe

★ Nine Million Bicycles#Controversy (for an example of popular use of a mistaken estimate)

External links

★ Cosmology FAQ

★ Hubble, VLT and Spitzer Capture Galaxy Formation in the Early Universe

★ Star Survey reaches 70 sextillion