Observable universe

large-scale structure of the universelarge-scale structurelarge-scale structure of the cosmosclusters of galaxiesvisible universecosmic weblarge scale structureLarge scale structure of the universeEnd of Greatnesscosmological horizon
The observable universe is a spherical region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion.wikipedia
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Universe

physical worldThe Universeuniverses
The observable universe is a spherical region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. Sky surveys and mappings of the various wavelength bands of electromagnetic radiation (in particular 21-cm emission) have yielded much information on the content and character of the universe's structure.
While the spatial size of the entire Universe is unknown, it is possible to measure the size of the observable universe, which is currently estimated to be 93 billion light-years in diameter.

Astronomical object

celestial bodiescelestial bodycelestial object
The observable universe is a spherical region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion.
An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe.

Galaxy

galaxiesgalacticgalactic nuclei
There are at least 2 trillion galaxies in the observable universe. Stars are organized into galaxies, which in turn form galaxy groups, galaxy clusters, superclusters, sheets, walls and filaments, which are separated by immense voids, creating a vast foam-like structure sometimes called the "cosmic web".
Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion (2e11) to a suggested two trillion (2e12) or more and, overall, as many as an estimated 1e24 stars (more stars than all the grains of sand on planet Earth).

Big Bang

Big Bang theoryThe Big Bangbig-bang
Sometimes astrophysicists distinguish between the visible universe, which includes only signals emitted since recombination (when hydrogen atoms were formed from protons and electrons and photons were emitted)—and the observable universe, which includes signals since the beginning of the cosmological expansion (the Big Bang in traditional physical cosmology, the end of the inflationary epoch in modern cosmology).
The Big Bang theory is a cosmological model for the observable universe from the earliest known periods through its subsequent large-scale evolution.

Age of the universe

13.8 billion years ago13.8 billion yearsage
Because no signals can travel faster than light, any object farther away from us than light could travel in the age of the universe (estimated around 13.799 billion years ) simply cannot be detected, as the signals could not have reached us yet.
Though the universe might in theory have a longer history, the International Astronomical Union presently use "age of the universe" to mean the duration of the Lambda-CDM expansion, or equivalently the elapsed time since the Big Bang in the current observable universe.

Dark energy

energyvacuum energycauses an acceleration in the expansion
However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light (special relativity prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding; see uses of the proper distance for a discussion) and furthermore the expansion rate appears to be accelerating due to dark energy.
Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe.

Inflation (cosmology)

cosmic inflationinflationcosmological inflation
According to the theory of cosmic inflation initially introduced by its founder, Alan Guth (and by D. Kazanas ), if it is assumed that inflation began about 10 −37 seconds after the Big Bang, then with the plausible assumption that the size of the universe before the inflation occurred was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least 3 times the radius of the observable universe.
It explains the origin of the large-scale structure of the cosmos.

Shape of the universe

flatflat universeopen universe
Assuming that space is roughly flat (in the sense of being a Euclidean space), this size corresponds to a comoving volume of about 1.22 Gpc 3 (4.22 Gly 3 or 3.57 m3).
Cosmologists distinguish between the observable universe and the global universe.

Physical cosmology

cosmologycosmologicalcosmologist
Sometimes astrophysicists distinguish between the visible universe, which includes only signals emitted since recombination (when hydrogen atoms were formed from protons and electrons and photons were emitted)—and the observable universe, which includes signals since the beginning of the cosmological expansion (the Big Bang in traditional physical cosmology, the end of the inflationary epoch in modern cosmology).
Finally, the epoch of structure formation began, when matter started to aggregate into the first stars and quasars, and ultimately galaxies, clusters of galaxies and superclusters formed.

Cosmic microwave background

cosmic microwave background radiationCMBmicrowave background radiation
According to calculations, the current comoving distance—proper distance, which takes into account that the universe has expanded since the light was emitted—to particles from which the cosmic microwave background radiation (CMBR) was emitted, which represents the radius of the visible universe, is about 14.0 billion parsecs (about 45.7 billion light-years), while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years), about 2% larger.
They have been measured in detail, and match what would be expected if small thermal variations, generated by quantum fluctuations of matter in a very tiny space, had expanded to the size of the observable universe we see today.

Matter

corporealsubstancematerial
The observable universe is a spherical region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion.
Ordinary matter, in the quarks and leptons definition, constitutes about 4% of the energy of the observable universe.

Star

starsstellarmassive star
Stars are organized into galaxies, which in turn form galaxy groups, galaxy clusters, superclusters, sheets, walls and filaments, which are separated by immense voids, creating a vast foam-like structure sometimes called the "cosmic web".
The observable Universe contains an estimated 1e24 stars, but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.

Event horizon

event horizonshorizoncosmic event horizon
This fact can be used to define a type of cosmic event horizon whose distance from the Earth changes over time.
In cosmology, the event horizon of the observable universe is the largest comoving distance from which light emitted now can ever reach the observer in the future.

Supercluster

superclustersgalaxy superclusterList of superclusters
Stars are organized into galaxies, which in turn form galaxy groups, galaxy clusters, superclusters, sheets, walls and filaments, which are separated by immense voids, creating a vast foam-like structure sometimes called the "cosmic web". The organization of structure appears to follow as a hierarchical model with organization up to the scale of superclusters and filaments.
The number of superclusters in the observable universe is estimated to be 10 million.

Accelerating expansion of the universe

accelerating universecosmic accelerationaccelerating
However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light (special relativity prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding; see uses of the proper distance for a discussion) and furthermore the expansion rate appears to be accelerating due to dark energy.
We can also look at the growth of large-scale structure, and find that the observed values of the cosmological parameters are best described by models which include an accelerating expansion.

Redshift survey

galaxy surveygalaxy surveysSky surveys
Sky surveys and mappings of the various wavelength bands of electromagnetic radiation (in particular 21-cm emission) have yielded much information on the content and character of the universe's structure. Based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the "Great Wall", a sheet of galaxies more than 500 million light-years long and 200 million light-years wide, but only 15 million light-years thick.
These observations are used to measure detailed statistical properties of the large-scale structure of the universe.

Sloan Great Wall

In April 2003, another large-scale structure was discovered, the Sloan Great Wall.
The Sloan Great Wall (SGW) is a cosmic structure formed by a giant wall of galaxies (a galaxy filament).

Galaxy cluster

galaxy clusterscluster of galaxiescluster
Stars are organized into galaxies, which in turn form galaxy groups, galaxy clusters, superclusters, sheets, walls and filaments, which are separated by immense voids, creating a vast foam-like structure sometimes called the "cosmic web".
They are the largest known gravitationally bound structures in the universe and were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered.

Void (astronomy)

voidvoidscosmic void
Stars are organized into galaxies, which in turn form galaxy groups, galaxy clusters, superclusters, sheets, walls and filaments, which are separated by immense voids, creating a vast foam-like structure sometimes called the "cosmic web".
They have less than one tenth of the average density of matter abundance that is considered typical for the observable universe.

Astrophysics

astrophysicistastrophysicaltheoretical astrophysics
The organization of structure arguably begins at the stellar level, though most cosmologists rarely address astrophysics on that scale.
Topics also studied by theoretical astrophysicists include Solar System formation and evolution; stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity, special relativity, quantum and physical cosmology, including string cosmology and astroparticle physics.

CfA2 Great Wall

Great WallGreatGreat Wall (astronomy)
Based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the "Great Wall", a sheet of galaxies more than 500 million light-years long and 200 million light-years wide, but only 15 million light-years thick.
It is one of the largest known superstructures in the observable universe.

Friedmann–Lemaître–Robertson–Walker metric

Friedmann–Lemaître–Robertson–WalkerFLRWRobertson–Walker metric
To estimate the distance to that matter at the time the light was emitted, we may first note that according to the Friedmann–Lemaître–Robertson–Walker metric, which is used to model the expanding universe, if at the present time we receive light with a redshift of z, then the scale factor at the time the light was originally emitted is given by
Most cosmologists agree that the observable universe is well approximated by an almost FLRW model, i.e., a model which follows the FLRW metric apart from primordial density fluctuations.

Margaret Geller

Margaret J. Geller
Based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the "Great Wall", a sheet of galaxies more than 500 million light-years long and 200 million light-years wide, but only 15 million light-years thick.
In the 1980s, she made pioneering maps of the large-scale structure of the universe, which led to the discovery of the Great Wall.

Light-year

light yearlight yearsMly
Based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the "Great Wall", a sheet of galaxies more than 500 million light-years long and 200 million light-years wide, but only 15 million light-years thick. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years and its diameter about 28.5 gigaparsecs (93 billion light-years, 8.8e26 m).

Huge-LQG

On January 11, 2013, another large quasar group, the Huge-LQG, was discovered, which was measured to be four billion light-years across, the largest known structure in the universe at that time.
At its discovery, it was identified as the largest and the most massive known structure in the observable universe, though it has been superseded by the Hercules-Corona Borealis Great Wall at 10 billion light-years.