Observable Universe

Information about Observable Universe

This article or section may contain inappropriate or misinterpreted which do not the text.
Please help [ improve this article] by checking for inaccuracies. This article has been tagged since October 2007. (, talk, get involved!)

"Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The image is derived from the 2MASS Extended Source Catalog (XSC)—more than 1.5 million galaxies, and the Point Source Catalog (PSC)--nearly 0.5 billion Milky Way stars. The galaxies are color coded by 'redshift' obtained from the UGC, CfA, Tully NBGC, LCRS, 2dF, 6dFGS, and SDSS surveys (and from various observations compiled by the NASA Extragalactic Database), or photo-metrically deduced from the K band (2.2 um). Blue are the nearest sources (z < 0.01); green are at moderate distances (0.01 < z < 0.04) and red are the most distant sources that 2MASS resolves (0.04 < z < 0.1). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center)." ^[1] Graphic by Thomas Jarret (IPAC)

For a general discussion of the universe, see .

In Big Bang cosmology, the observable universe 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.

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 by direct experimentation about any part of the universe that is causally disconnected from us, although many credible theories, such as cosmic inflation require a universe much larger than the observable universe. No evidence exists to suggest that the boundary of the observable universe corresponds precisely to the physical boundary of the universe (if such a boundary exists); this is exceedingly unlikely in that it would imply that the Earth is exactly at the center of the universe, in violation of the cosmological principle. 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 may actually be 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.

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^[4]. Since the visible universe is a perfect sphere and space is roughly flat, this size corresponds to a comoving volume of about

$\text{[math]}$

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 $\text{[math]}$ , 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]

Mass of the observable universe

The mass of the observable universe can be estimated based on either density or size.^[14]

Estimation based on the measured mass density

Estimates of its density are obtained by studying fluctuations in cosmic microwave background radiation, superclusters, and Big Bang nucleosynthesis. These yield a density estimate of $\text{[math]}$ . Estimates of the size of the observable universe vary, but a size estimate of $\text{[math]}$ light years yields a mass estimate of $\text{[math]}$ .^[15]

Estimation based on the measured stellar density

Another way to calculate the mass of the observable universe is to assume a mean solar mass and to multiply that by an estimate of the number of stars in the observable universe. The estimate of the number of stars in the universe is in turn derived from the volume of the observable universe ( $\text{[math]}$ ) and a stellar density calculated from observations by the Hubble Space Telescope ( $\text{[math]}$ ) yielding an estimate of the number of stars in the observable universe of $\text{[math]}$ . Assuming the mass of Sol ( $\text{[math]}$ ) as the mean solar mass (on the basis that the large population of dwarf stars balances out the population of stars whose mass is greater than Sol) and rounding the estimate of the number of stars up to $\text{[math]}$ yields a mass of the observable universe as $\text{[math]}$ .^[16]

Hoyle calculates the mass of an observable steady-state universe using the formula $\text{[math]}$ , or $\text{[math]}$ .^[17]

External links

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. ^ Lineweaver, Charles; Tamara M. Davis (2005). Misconceptions about the Big Bang. Scientific American. Retrieved on 2007-03-05.
5. ^ Ned Wright, "Why the Light Travel Time Distance should not be used in Press Releases".
6. ^ [1]
7. ^ [2]
8. ^ [3]
9. ^ Edward L. Wright, "An Older but Larger Universe?".
10. ^ [4]
11. ^ [5]
12. ^ [6]
13. ^ Matthew Champion, "Re: How many atoms make up the universe?", 1998
14. ^ Mass of the Universe
15. ^ Jagadheep D. Pandian (June 2002). What is the mass of the Universe?. Curious About Astronomy.
16. ^ . "On the expansion of the universe" (PDF). NASA Glenn Research Centre.
17. ^ Helge Kragh (1999-02-22). Cosmology and Controversy: The Historical Development of Two Theories of the Universe. Princeton University Press, 212. ISBN 0-691-00546-X.

• • [ e] Earth's location in space
Earth Solar System Local Interstellar Cloud Local Bubble Orion Arm Milky Way Local Group Virgo Supercluster Observable universe Universe

Big Bang is the cosmological model of the universe whose primary assertion is that the universe has expanded into its current state from a primordial condition of enormous density and temperature.
..... Click the link for more information.

EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001. Their greatest hit, their debut single "time after time", peaked at #13 in the Oricon singles chart.
..... Click the link for more information.

Editing of this page by unregistered or newly registered users is currently disabled due to vandalism.
If you are prevented from editing this page, and you wish to make a change, please discuss changes on the talk page, request unprotection, log in, or .
..... Click the link for more information.

Radiation as used in physics, is energy in the form of waves or moving subatomic particles. Radiation can be classified as ionizing or non-ionizing radiation, depending on its effect on atomic matter.
..... Click the link for more information.

Information is the result of processing, gathering, manipulating and organizing data in a way that adds to the knowledge of the receiver. In other words, it is the context in which data is taken.
..... Click the link for more information.

In physics, a gravitational wave is a fluctuation in the curvature of spacetime which propagates as a wave, traveling outward from a moving object or system of objects. Gravitational radiation is the energy transported by these waves.
..... Click the link for more information.

Physical cosmology, as a branch of astronomy, is the study of the large-scale structure of the universe and is concerned with fundamental questions about its formation and evolution. Cosmology involves itself with studying the motions of the celestial bodies and the first cause.
..... Click the link for more information.

The age of the universe, in physics, is the time elapsed between the Big Bang and the present day. Current observations suggest that this is about 13.7 billion years, with an uncertainty of about +/-200 million years.
..... Click the link for more information.

Blue Shift refers to a shortening of a transmitted signal's wavelength, and/or an increase in its frequency. The name comes from the fact that the shorter-wavelength end of the optical spectrum is the blue (or violet) end, hence, when visible light is compacted in
..... Click the link for more information.

In standard cosmology, 'comoving' distance or 'proper distance' is one of several distance measures used by cosmologists to define distances between objects.

Comoving coordinates

..... Click the link for more information.

cosmic microwave background radiation (most often abbreviated CMB but occasionally CMBR, CBR or MBR, also referred to as relic radiation) is a form of electromagnetic radiation discovered in 1965 that fills the entire universe ^[1].
..... Click the link for more information.

In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. ^[1]
..... Click the link for more information.

In astrophysics and cosmology, dark matter is hypothetical matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter.
..... Click the link for more information.

Friedmann-Lemaître-Robertson-Walker (FLRW) metric is an exact solution of the Einstein field equations of general relativity; it describes a , isotropic expanding or contracting universe.
..... Click the link for more information.

The Friedmann equations are a set of equations in cosmology that govern the expansion of space in homogeneous and isotropic models of the universe within the context of general relativity.
..... Click the link for more information.

formation of galaxies is still one of the most active research areas in astrophysics; and, to some extent, this is also true for galaxy evolution. Some ideas, however, have gained wide acceptance.
..... Click the link for more information.

Hubble's law is a statement in physical cosmology which states that the redshift in light coming from distant galaxies is proportional to their distance. The law was first formulated by Edwin Hubble and Milton Humason in 1929^[1] after nearly a decade of observations.
..... Click the link for more information.

In physical cosmology, cosmic inflation is the idea that the nascent universe passed through a phase of exponential expansion that was driven by a negative-pressure vacuum energy density.
..... Click the link for more information.

large-scale structure refers to the characterization of observable distributions of matter and light on the largest scales (typically on the order of billions of light-years).
..... Click the link for more information.

ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter. It is frequently referred to as the concordance model of big bang cosmology, since it attempts to explain cosmic microwave background observations, as well as large scale structure
..... Click the link for more information.

The metric expansion of space is a key part of science's current understanding of the universe, whereby spacetime itself is described by a metric which changes over time in such a way that the spatial dimensions appear to grow or stretch as the universe gets older.
..... Click the link for more information.

In physical cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than those of H-1 (i.e. the normal, light isotope of hydrogen, whose nuclei consist of a single proton each) during the early phases of the
..... Click the link for more information.

redshift occurs when the electromagnetic radiation, usually visible light, that is emitted from or reflected off an object is shifted toward the (less energetic) red end of the electromagnetic spectrum.
..... Click the link for more information.

The shape of the Universe is an informal name for a subject of investigation within physical cosmology. Cosmologists and astronomers describe the geometry of the universe which includes both local geometry and global geometry.
..... Click the link for more information.

Structure formation refers to a fundamental problem in physical cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.7 Gyr ago.
..... Click the link for more information.

Physical cosmology

Age of the universe
Big Bang
Blueshift
Comoving distance
Cosmic microwave background
Dark energy
Dark matter
FLRW metric
Friedmann equations
Galaxy formation
Hubble's law
Inflation

..... Click the link for more information.

For a timeline of the cosmos (or universe), see .

This timeline of cosmological theories and discoveries is a chronological catalog of the evolution of humankind's understanding of the cosmos over the last two-plus millennia.
..... Click the link for more information.

The ultimate fate of the universe is a topic in physical cosmology. Many possible fates are predicted by rival scientific theories, including futures of both finite and infinite duration.
..... Click the link for more information.

The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. Based on observations of the portion of the Universe that is observable, physicists attempt to describe the whole of space-time, including all matter and
..... Click the link for more information.

This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.

Herod_Archelaus

iPedia Free Information Center