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. There are at least 2 trillion galaxies in the observable universe. Assuming the Universe is isotropic, the distance to the edge of the observable universe is the same in every direction; that is, the observable universe has a spherical volume centered on the observer. Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth; the word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the speed of light itself; because no signals can travel faster than light, any object farther away from us than light could travel in the age of the Universe cannot be detected, as the signals could not have reached us yet.
Sometimes astrophysicists distinguish between the visible universe, which includes only signals emitted since recombination —and the observable universe, which includes signals since the beginning of the cosmological expansion. 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 was emitted, which represent the radius of the visible universe, is about 14.0 billion parsecs, while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs, about 2% larger. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years and its diameter about 28.5 gigaparsecs. The total mass of ordinary matter in the universe can be calculated using the critical density and the diameter of the observable universe to be about 1.5 × 1053 kg. In November 2018, astronomers reported that the extragalactic background light amounted to 4 × 1084 photons.
Since the expansion of the universe is known to accelerate and will become exponential in the future, the light emitted from all distant objects, past some time dependent on their current redshift, will never reach the Earth. In the future all observable objects will freeze in time while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years. In addition, light emitted by objects situated beyond a certain comoving distance will never reach Earth; some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth or its scientific space-based instruments, so lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so additional regions will become observable. However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light and furthermore the expansion rate appears to be accelerating due to dark energy.
Assuming dark energy remains constant, so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit would never reach the Earth. This future visibility limit is calculated at a comoving distance of 19 billion parsecs, assuming the universe will keep expanding forever, which implies the number of galaxies that we can theoretically observe in the infinite future is only larger than the number observable by a factor of 2.36. Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. An additional subtlety is that a galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its past history, but because of the universe's expansion, there may be some age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future (so, for example, we might never see what the galaxy looked like 10 billion years after the Bi
A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity with typical masses ranging from 1014–1015 solar masses. 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. One of the key features of clusters is the intracluster medium; the ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV, dependent on the total mass of the cluster. Galaxy clusters should not be confused with star clusters, such as open clusters, which are structures of stars within galaxies, or with globular clusters, which orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies; the galaxy groups and clusters can themselves cluster together to form superclusters. Notable galaxy clusters in the nearby Universe include the Virgo Cluster, Fornax Cluster, Hercules Cluster, the Coma Cluster.
A large aggregation of galaxies known as the Great Attractor, dominated by the Norma Cluster, is massive enough to affect the local expansion of the Universe. Notable galaxy clusters in the distant, high-redshift Universe include SPT-CL J0546-5345 and SPT-CL J2106-5844, the most massive galaxy clusters found in the early Universe. In the last few decades, they are found to be relevant sites of particle acceleration, a feature, discovered by observing non-thermal diffuse radio emissions, such as radio halos and radio relics. Using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have been found in many galaxy clusters. Galaxy clusters have the following properties: They contain 100 to 1,000 galaxies, hot X-ray emitting gas and large amounts of dark matter. Details are described in the "Composition" section; the distribution of the three components is the same in the cluster. They have total masses of 1014 to 1015 solar masses, they have a diameter from 2 to 10 Mpc. The spread of velocities for the individual galaxies is about 800–1000 km/s.
There are three main components of a galaxy cluster. They are tabulated below: Stars, Star clusters, Galaxy clusters, Super clusters Abell catalogue Intracluster medium List of Abell clusters
Dark Energy Survey
The Dark Energy Survey is a visible and near-infrared survey that aims to probe the dynamics of the expansion of the Universe and the growth of large-scale structure. The collaboration is composed of research institutions and universities from the United States, the United Kingdom, Germany and Switzerland; the survey uses the 4-meter Victor M. Blanco Telescope located at Cerro Tololo Inter-American Observatory in Chile, outfitted with the Dark Energy Camera; this camera allows for more sensitive images in the red part of the visible spectrum and in the near infrared, in comparison to previous instruments. DECam has one of the widest fields of view available for ground-based infrared imaging; the survey will image 5,000 square degrees of the southern sky in a footprint that overlaps with the South Pole Telescope and Stripe 82. The survey will take five years to complete, the survey footprint will nominally be covered ten times in five photometric bands. DES began in August 2013 and completed its second season in February 2015.
The Dark Energy Survey investigates the dynamics and large scale structure of the Universe using four probes: Type Ia supernovae, baryon acoustic oscillations, the number of galaxy clusters, weak gravitational lensing. Type Ia supernovae are believed to be thermonuclear explosions that occur when white dwarf stars in binary systems accrete mass from their companion stars; these events are important for the study of cosmology because they are bright, which allows astronomers to detect them at large distance. The expansion of the universe can be constrained based on observations of the luminosity distance and redshift of distant type IA supernova; the other three techniques used by the Dark Energy Survey allow scientists to understand the expansion of the universe and the evolution of the dark matter density field perturbations. These perturbations were intrinsically tied to the formation of galaxies and galaxy clusters; the standard model of cosmology assumes that quantum fluctuations of the density field of the various components that were present when our universe was young were enhanced through a rapid expansion called inflation.
Gravitational collapse enhances these initial fluctuation as baryons fall into the gravitational potential field of more dense regions of space to form galaxies. The growth rate of these dark matter halos is sensitive to the dynamics of the expansion of the Universe and DES will use this connection to probe the properties of that expansion. DECam, the new camera installed at the Victor M. Blanco Telescope by the DES collaboration, brings new observational possibilities that were not available for previous surveys, such as the Sloan Digital Sky Survey. One significant difference between previous CCD at the Victor M. Blanco Telescope and DECam is the improved quantum efficiency in the red part of the visible spectra and in the near infrared; this is a important property for the observation of distant sources, like Type IA supernovae or galaxy cluster, because the expansion of the universe shifts the photons emitted from a given source towards redder wavelengths. On the other hand, the main element used to make CCDs, becomes transparent for infrared light, this issue made the development of the DECam CCD a technological challenge.
The director of DES is Josh Frieman and the collaboration is composed of many research institutes and universities. The DES collaboration itself is divided into a number of science working groups; some of the primary working groups are: the weak lensing working group, the galaxy clusters working group, the large-scale structure working group, the supernova working group, the galaxy evolution working group, the strong lensing working group. Other science topics include simulations, photometric redshifts and Milky Way science. A large responsibility of the DES collaboration was the mechanical and optical development of the DECam; the collaboration has a website, where scientist can release new results and articles. Some of the releases in this website are open for the general public. DECam is a large camera built to replace the previous prime focus camera on the Victor M. Blanco Telescope; the camera consists of three major components: mechanics, CCDs. The mechanics of the camera consists of a filter changer with shutter.
There is an optical barrel that supports 5 corrector lenses, the largest of, 98 cm in diameter. These components are attached to the CCD focal plane, cooled to −100 °C with liquid nitrogen in order to reduce thermal noise in the CCDs; the focal plane is kept in an low vacuum of 10−6 Torr to prevent the formation of condensation on the sensors. The entire camera with lenses, CCDs weighs 4 tons; when mounted at the prime focus it will be supported with a hexapod system allowing for real time focal adjustment. The camera is outfitted with u, g, r, i, z, Y filters similar to those used in the Sloan Digital Sky Survey; this allows DES to obtain photometric redshift measurements to z≈1, using the 400 nm break for galaxies, a step-like spectral feature that occurs due to a number of absorption lines from ionized metals, light curve fitting techniques for Type Ia supernova. DECam contains five lenses acting as corrector optics to extend the telescope's field of view to a diameter of 2.2°. The scientific sensor array on DECam is an array of 62 2048×4096 pixel back-illuminated CCDs totaling 520 megapixels.
Shape of the universe
The shape of the universe is the local and global geometry of the universe. The local features of the geometry of the universe are described by its curvature, whereas the topology of the universe describes general global properties of its shape as of a continuous object; the shape of the universe is related to general relativity, which describes how spacetime is curved and bent by mass and energy. Cosmologists distinguish between the global universe; the observable universe consists of the part of the universe that can, in principle, be observed by light reaching Earth within the age of the universe. It encompasses a region of space that forms a ball centered at Earth of estimated radius 46.5 billion light-years. This does not mean. Assuming an isotropic nature, the observable universe is similar for all contemporary vantage points; the global shape of the universe can be described with three attributes: Finite or infinite Flat, open, or closed Connectivity, how the universe is put together, i.e. connected space or multiply connected.
There are certain logical connections among these properties. For example, a universe with positive curvature is finite. Although it is assumed in the literature that a flat or negatively curved universe is infinite, this need not be the case if the topology is not the trivial one; the exact shape is still a matter of debate in physical cosmology, but experimental data from various independent sources confirm that the observable universe is flat with only a 0.4% margin of error. Theorists have been trying to construct a formal mathematical model of the shape of the universe. In formal terms, this is a 3-manifold model corresponding to the spatial section of the 4-dimensional spacetime of the universe; the model most theorists use is the Friedmann–Lemaître–Robertson–Walker model. Arguments have been put forward that the observational data best fit with the conclusion that the shape of the global universe is infinite and flat, but the data are consistent with other possible shapes, such as the so-called Poincaré dodecahedral space and the Sokolov–Starobinskii space.
As stated in the introduction, there are two aspects to consider: its local geometry, which predominantly concerns the curvature of the universe the observable universe, its global geometry, which concerns the topology of the universe as a whole. The observable universe can be thought of as a sphere that extends outwards from any observation point for 46.5 billion light years, going farther back in time and more redshifted the more distant away one looks. Ideally, one can continue to look back all the way to the Big Bang. Experimental investigations show that the observable universe is close to isotropic and homogeneous. If the observable universe encompasses the entire universe, we may be able to determine the structure of the entire universe by observation. However, if the observable universe is smaller than the entire universe, our observations will be limited to only a part of the whole, we may not be able to determine its global geometry through measurement. From experiments, it is possible to construct different mathematical models of the global geometry of the entire universe all of which are consistent with current observational data and so it is unknown whether the observable universe is identical to the global universe or it is instead many orders of magnitude smaller than it.
The universe may be small in some dimensions and not in others. To test whether a given mathematical model describes the universe scientists look for the model's novel implications—what are some phenomena in the universe that we have not yet observed, but that must exist if the model is correct—and they devise experiments to test whether those phenomena occur or not. For example, if the universe is a small closed loop, one would expect to see multiple images of an object in the sky, although not images of the same age. Cosmologists work with a given space-like slice of spacetime called the comoving coordinates, the existence of a preferred set of, possible and accepted in present-day physical cosmology; the section of spacetime that can be observed is the backward light cone, while the related term Hubble volume can be used to describe either the past light cone or comoving space up to the surface of last scattering. To speak of "the shape of the universe" is ontologically naive from the point of view of special relativity alone: due to the relativity of simultaneity we cannot speak of different points in space as being "at the same point in time" nor, therefore, of "the shape of the universe at a point in time".
However, the comoving coordinates provide a strict sense to those by using the time since the Big Bang as a distinguished universal time. The c
In the field of Big Bang theory, cosmology, reionization is the process that caused the matter in the universe to reionize after the lapse of the "dark ages". Reionization is the second of two major phase transitions of gas in the universe. While the majority of baryonic matter in the universe is in the form of hydrogen and helium, reionization refers to the reionization of hydrogen, the element. It's believed that the primordial helium experienced the same phase of reionization changes, but at different points in the history of the universe; this is referred to as helium reionization. The first phase change of hydrogen in the universe was recombination, which occurred at a redshift z = 1089, due to the cooling of the universe to the point where the rate of recombination of electrons and protons to form neutral hydrogen was higher than the reionization rate; the universe was opaque before the recombination, due to the scattering of photons off free electrons, but it became transparent as more electrons and protons combined to form neutral hydrogen atoms.
While the electrons of neutral hydrogen can absorb photons of some wavelengths by rising to an excited state, a universe full of neutral hydrogen will be opaque only at those absorbed wavelengths, but transparent throughout most of the spectrum. The Dark Ages of the universe start at that point, because there were no light sources other than the redshifting cosmic background radiation; the second phase change occurred once objects started to condense in the early universe that were energetic enough to re-ionize neutral hydrogen. As these objects formed and radiated energy, the universe reverted from being neutral, to once again being an ionized plasma; this occurred between 150 one billion years after the Big Bang. At that time, matter had been diffused by the expansion of the universe, the scattering interactions of photons and electrons were much less frequent than before electron-proton recombination. Thus, the universe was full of low density ionized hydrogen and remained transparent, as is the case today.
Looking back so far in the history of the universe presents some observational challenges. There are, however, a few observational methods for studying reionization. One means of studying reionization uses the spectra of distant quasars. Quasars release an extraordinary amount of energy, meaning they are among the brightest objects in the universe; as a result, some quasars are detectable from as far back as the epoch of reionization. Quasars happen to have uniform spectral features, regardless of their position in the sky or distance from the Earth, thus it can be inferred that any major differences between quasar spectra will be caused by the interaction of their emission with atoms along the line of sight. For wavelengths of light at the energies of one of the Lyman transitions of hydrogen, the scattering cross-section is large, meaning that for low levels of neutral hydrogen in the intergalactic medium, absorption at those wavelengths is likely. For nearby objects in the universe, spectral absorption lines are sharp, as only photons with energies just sufficient to cause an atomic transition can cause that transition.
However, the distances between quasars and the telescopes which detect them are large, which means that the expansion of the universe causes light to undergo noticeable redshifting. This means that as light from the quasar travels through the IGM and is redshifted, wavelengths, below the Lyman Alpha limit are stretched, will in effect begin to fill in the Lyman absorption band; this means that instead of showing sharp spectral absorption lines, a quasar's light which has traveled through a large, spread out region of neutral hydrogen will show a Gunn-Peterson trough. The redshifting for a particular quasar provides temporal information about reionization. Since an object's redshift corresponds to the time at which it emitted the light, it is possible to determine when reionization ended. Quasars below a certain redshift do not show the Gunn-Peterson trough, while quasars emitting light prior to reionization will feature a Gunn-Peterson trough. In 2001, four quasars were detected with redshifts ranging from z = 5.82 to z = 6.28.
While the quasars above z = 6 showed a Gunn-Peterson trough, indicating that the IGM was still at least neutral, the ones below did not, meaning the hydrogen was ionized. As reionization is expected to occur over short timescales, the results suggest that the universe was approaching the end of reionization at z = 6. This, in turn, suggests that the universe must still have been entirely neutral at z > 10. The anisotropy of the cosmic microwave background on different angular scales can be used to study reionization. Photons undergo scattering when there are free electrons present, in a process known as Thomson scattering. However, as the universe expands, the density of free electrons will decrease, scattering will occur less frequently. In the period during and after reionization, but before significant expansion had occurred to sufficiently lower the electron density, the light that composes the CMB will experience observable Thomson scattering; this scattering will leave its mark on the CMB anisotropy map.
The overall effect is to erase anisotropies. While anisotropies on small scales are erased, polarization anisotropies are introduced becau
A supercluster is a large group of smaller galaxy clusters or galaxy groups. The Milky Way is part of the Local Group galaxy group, which in turn is part of the Laniakea Supercluster; this supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years. The large size and low density of superclusters means they, unlike clusters, expand with the Hubble expansion; the number of superclusters in the observable universe is estimated to be 10 million. The existence of superclusters indicates that the galaxies in the Universe are not uniformly distributed; those groups and clusters and additional isolated galaxies in turn form larger structures called superclusters. Their existence was first postulated by George Abell in his 1958 Abell catalogue of galaxy clusters, he called them clusters of clusters. Superclusters form massive structures of galaxies, called "filaments", "supercluster complexes", "walls" or "sheets", that may span between several hundred million light-years to 10 billion light-years, covering more than 5% of the observable universe.
These are the largest known structures to date. Observations of superclusters can give information about the initial condition of the universe, when these superclusters were created; the directions of the rotational axes of galaxies within superclusters may give insight and information into the early formation process of galaxies in the history of the Universe. Interspersed among superclusters are large voids of space. Superclusters are subdivided into groups of clusters called galaxy groups and clusters. Freedman, Roger. "Galaxies". Universe. New York: W. H. Freedman. ISBN 978-1-319-04238-7. Media related to Superclusters of galaxies at Wikimedia Commons Overview of local superclusters The Nearest Superclusters Universe family tree: Supercluster Superclusters - Large Scale Structures