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
The Universe is all of space and time and their contents, including planets, stars and all other forms of matter and energy. While the spatial size of the entire Universe is unknown, it is possible to measure the size of the observable universe, estimated to be 93 billion light years in diameter. In various multiverse hypotheses, a universe is one of many causally disconnected constituent parts of a larger multiverse, which itself comprises all of space and time and its contents; the earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center of the Universe. Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Isaac Newton built upon Copernicus' work as well as observations by Tycho Brahe and Johannes Kepler's laws of planetary motion. Further observational improvements led to the realization that the Sun is one of hundreds of billions of stars in the Milky Way, one of at least hundreds of billions of galaxies in the Universe.
Many of the stars in our galaxy have planets. At the largest scale galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. At smaller scales, galaxies are distributed in clusters and superclusters which form immense filaments and voids in space, creating a vast foam-like structure. Discoveries in the early 20th century have suggested that the Universe had a beginning and that space has been expanding since and is still expanding at an increasing rate; the Big Bang theory is the prevailing cosmological description of the development of the Universe. Under this theory and time emerged together 13.799±0.021 billion years ago and the energy and matter present have become less dense as the Universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10−32 seconds, the separation of the four known fundamental forces, the Universe cooled and continued to expand, allowing the first subatomic particles and simple atoms to form.
Dark matter gathered forming a foam-like structure of filaments and voids under the influence of gravity. Giant clouds of hydrogen and helium were drawn to the places where dark matter was most dense, forming the first galaxies and everything else seen today, it is possible to see objects that are now further away than 13.799 billion light-years because space itself has expanded, it is still expanding today. This means that objects which are now up to 46.5 billion light-years away can still be seen in their distant past, because in the past when their light was emitted, they were much closer to the Earth. From studying the movement of galaxies, it has been discovered that the universe contains much more matter than is accounted for by visible objects; this unseen matter is known as dark matter. The ΛCDM model is the most accepted model of our universe, it suggests that about 69.2%±1.2% of the mass and energy in the universe is a cosmological constant, responsible for the current expansion of space, about 25.8%±1.1% is dark matter.
Ordinary matter is therefore only 4.9% of the physical universe. Stars and visible gas clouds only form about 6% of ordinary matter, or about 0.3% of the entire universe. There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will be accessible; some physicists have suggested various multiverse hypotheses, in which our universe might be one among many universes that exist. The physical Universe is defined as all of their contents; such contents comprise all of energy in its various forms, including electromagnetic radiation and matter, therefore planets, stars and the contents of intergalactic space. The Universe includes the physical laws that influence energy and matter, such as conservation laws, classical mechanics, relativity; the Universe is defined as "the totality of existence", or everything that exists, everything that has existed, everything that will exist.
In fact, some philosophers and scientists support the inclusion of ideas and abstract concepts – such as mathematics and logic – in the definition of the Universe. The word universe may refer to concepts such as the cosmos, the world, nature; the word universe derives from the Old French word univers, which in turn derives from the Latin word universum. The Latin word was used by Cicero and Latin authors in many of the same senses as the modern English word is used. A term for "universe" among the ancient Greek philosophers from Pythagoras onwards was τὸ πᾶν, tò pân, defined as all matter and all space, τὸ ὅλον, tò hólon, which did not include the void. Another synonym was ho kósmos. Synonyms are found in Latin authors and survive in modern languages, e.g. the German words Das All and Natur for Universe. The same synonyms are found in English, such as everything, the cosmos, the world (as in the many-worlds interpr
A planetary system is a set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. Speaking, systems with one or more planets constitute a planetary system, although such systems may consist of bodies such as dwarf planets, natural satellites, comets and circumstellar disks; the Sun together with its planetary system, which includes Earth, is known as the Solar System. The term exoplanetary system is sometimes used in reference to other planetary systems; as of 1 April 2019, there are 4,023 confirmed planets in 3,005 systems, with 656 systems having more than one planet. Debris disks are known to be common, though other objects are more difficult to observe. Of particular interest to astrobiology is the habitable zone of planetary systems where planets could have surface liquid water, thus the capacity to harbor Earth-like life. Heliocentrism was opposed to geocentrism; the notion of a heliocentric Solar System, with the Sun at the center, is first suggested in the Vedic literature of ancient India, which refer to the Sun as the "centre of spheres".
Some interpret Aryabhatta's writings in Āryabhaṭīya as implicitly heliocentric. The idea was first proposed in Western philosophy and Greek astronomy as early as the 3rd century BC by Aristarchus of Samos, but received no support from most other ancient astronomers. De revolutionibus orbium coelestium by Nicolaus Copernicus, published in 1543, was the first mathematically predictive heliocentric model of a planetary system. 17th-century successors Galileo Galilei, Johannes Kepler, Isaac Newton developed an understanding of physics which led to the gradual acceptance of the idea that the Earth moves round the Sun and that the planets are governed by the same physical laws that governed the Earth. In the 16th century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are accompanied by planets, he was burned at the stake for his ideas by the Roman Inquisition.
In the 18th century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."His theories gained traction through the 19th and 20th centuries despite a lack of supporting evidence. Long before their confirmation by astronomers, conjecture on the nature of planetary systems had been a focus of the search for extraterrestrial intelligence and has been a prevalent theme in fiction science fiction; the first confirmed detection of an exoplanet was in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of exoplanets of a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi; the frequency of detections has increased since particularly through advancements in methods of detecting extrasolar planets and dedicated planet finding programs such as the Kepler mission.
Planetary systems come from protoplanetary disks that form around stars as part of the process of star formation. During formation of a system much material is gravitationally scattered into far-flung orbits and some planets are ejected from the system becoming rogue planets. Planets orbiting pulsars have been discovered. Pulsars are the remnants of the supernova explosions of high-mass stars, but a planetary system that existed before the supernova would be destroyed. Planets would either evaporate, be pushed off of their orbits by the masses of gas from the exploding star, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star, or in some cases the supernova would kick the pulsar itself out of the system at high velocity so any planets that had survived the explosion would be left behind as free-floating objects. Planets found around pulsars may have formed as a result of pre-existing stellar companions that were entirely evaporated by the supernova blast, leaving behind planet-sized bodies.
Alternatively, planets may form in an accretion disk of fallback matter surrounding a pulsar. Fallback disks of matter that failed to escape orbit during a supernova may form planets around black holes; as stars evolve and turn into red giants, asymptotic giant branch stars, planetary nebulae they engulf the inner planets, evaporating or evaporating them depending on how massive they are. As the star loses mass, planets that are not engulfed move further out from the star. If an evolved star is in a binary or multiple system the mass it loses can transfer to another star, creating new protoplanetary disks and second- and third-generation planets which may differ in composition from the original planets which may be affected by the mass transfer. Planets in evolved binary systems, Hagai B. Perets, 13 Jan 2011 Can Planets survive Stellar Evolution?, Eva Villaver, Mario Livio, Feb 2007 The Orbital Evolution of Gas Giant Planets around Giant Stars, Eva Villaver, Mario Livio, 13 Oct 2009 On the survival of brown dwarfs and planets engulfed by their giant host star, Jean-Claude Passy, Mordecai-Mark Mac Low, Orsola De Marco, 2 Oct 2012 Foretellings of Ragnarök: World-engulfing Asymptotic Giants and the Inheritance of White Dwarfs, Alexander James Mustill, Eva Villaver, 5 Dec 2012 The Solar System consists of an
The Local Group is the galaxy group that includes the Milky Way. Its has a total diameter of 3 Mpc, a total mass of the order of 2×1012 solar masses, it consists of two clusters of galaxies in a "dumbbell" shape, the Milky Way and its satellites on one hand, the Andromeda Galaxy and its satellites on the other. The two clusters are separated by about 0.8 Mpc and move towards one another with a velocity of 123 km/h. The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster; the total number of galaxies in the Local Group is unknown but known to exceed 54, most of them being dwarf galaxies. The two largest members, the Andromeda Galaxy and the Milky Way, are both spiral galaxies with masses of about 1012 solar masses each, each have their own system of satellite galaxies: The Andromeda Galaxy's satellite system consists of Messier 32, Messier 110, NGC 147, NGC 185, Andromeda I, And II, And III, And V, And VI, And VII, And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies.
The Milky Way's satellite galaxies system comprises Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf Galaxy, Ursa Minor Dwarf Galaxy, Draco Dwarf Galaxy, Carina Dwarf Galaxy, Sextans Dwarf Galaxy, Sculptor Dwarf Galaxy, Fornax Dwarf Galaxy, Leo I, Leo II, Ursa Major I Dwarf Galaxy and Ursa Major II Dwarf Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies. The Triangulum Galaxy is the third largest member of the Local Group, at about 5×1010 M☉, the third spiral galaxy, it may not be a companion to the Andromeda Galaxy. Pisces Dwarf Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either; the membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy, is uncertain due to extreme distances from the center of the Local Group. The other members of the group are gravitationally secluded from these large subgroups: IC 10, IC 1613, Phoenix Dwarf Galaxy, Leo A, Tucana Dwarf Galaxy, Cetus Dwarf Galaxy, Pegasus Dwarf Irregular Galaxy, Wolf–Lundmark–Melotte, Aquarius Dwarf Galaxy, Sagittarius Dwarf Irregular Galaxy.
The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae. There, he described it as "a typical small group of nebulae, isolated in the general field" and delineated, by decreasing luminosity, its members to be M31, Milky Way, M33, Large Magellanic Cloud, Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147, he identified IC 10 as a possible part of Local Group. By 2003, the number of known Local Group members had increased from his initial 12 to 36. Smith's Cloud, a high-velocity cloud, between 32,000 and 49,000 light years from Earth and 8,000 light years from the disk of the Milky Way galaxy HVC 127-41-330, a high-velocity cloud, 2.3 million light-years from Earth Monoceros Ring, a ring of stars around the Milky Way, proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy Galaxy cluster List of nearest galaxies List of galaxy clusters IC 342/Maffei Group, the group of galaxies nearest to the Local Group Local Supercluster List of Andromeda's satellite galaxies List of Milky Way's satellite galaxies The Local Group of Galaxies, SEDS Messier pages A Survey of the Resolved Stellar Content of Nearby Galaxies Currently Forming Stars, Lowell Observatory van den Bergh, Sidney.
"Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific. 112: 529–536. ArXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. Doi:10.1086/316548
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
Heat death of the universe
The heat death of the universe known as the Big Chill or Big Freeze, is an idea of an ultimate fate of the universe in which the universe has evolved to a state of no thermodynamic free energy and therefore can no longer sustain processes that increase entropy. Heat death does not imply any particular absolute temperature. In the language of physics, this is. If the topology of the universe is open or flat, or if dark energy is a positive cosmological constant, the universe will continue expanding forever, a heat death is expected to occur, with the universe cooling to approach equilibrium at a low temperature after a long time period; the hypothesis of heat death stems from the ideas of William Thomson, 1st Baron Kelvin, who in the 1850s took the theory of heat as mechanical energy loss in nature and extrapolated it to larger processes on a universal scale. The idea of heat death stems from the second law of thermodynamics, of which one version states that entropy tends to increase in an isolated system.
From this, the hypothesis implies that if the universe lasts for a sufficient time, it will asymptotically approach a state where all energy is evenly distributed. In other words, according to this hypothesis, there is a tendency in nature to the dissipation of mechanical energy into thermal energy; the conjecture that all bodies in the universe cool off becoming too cold to support life, seems to have been first put forward by the French astronomer Jean Sylvain Bailly in 1777 in his writings on the history of astronomy and in the ensuing correspondence with Voltaire. In Bailly's view, all planets are now at some particular stage of cooling. Jupiter, for instance, is still too hot for life to arise there for thousands of years, while the Moon is too cold; the final state, in this view, is described as one of "equilibrium". The idea of heat death as a consequence of the laws of thermodynamics, was first proposed in loose terms beginning in 1851 by William Thomson, who theorized further on the mechanical energy loss views of Sadi Carnot, James Joule, Rudolf Clausius.
Thomson's views were elaborated on more definitively over the next decade by Hermann von Helmholtz and William Rankine. The idea of heat death of the universe derives from discussion of the application of the first two laws of thermodynamics to universal processes. In 1851, William Thomson outlined the view, as based on recent experiments on the dynamical theory of heat: "heat is not a substance, but a dynamical form of mechanical effect, we perceive that there must be an equivalence between mechanical work and heat, as between cause and effect." In 1852, Thomson published On a Universal Tendency in Nature to the Dissipation of Mechanical Energy, in which he outlined the rudiments of the second law of thermodynamics summarized by the view that mechanical motion and the energy used to create that motion will tend to dissipate or run down. The ideas in this paper, in relation to their application to the age of the Sun and the dynamics of the universal operation, attracted the likes of William Rankine and Hermann von Helmholtz.
The three of them were said to have exchanged ideas on this subject. In 1862, Thomson published "On the age of the Sun’s heat", an article in which he reiterated his fundamental beliefs in the indestructibility of energy and the universal dissipation of energy, leading to diffusion of heat, cessation of useful motion, exhaustion of potential energy through the material universe, while clarifying his view of the consequences for the universe as a whole. In a key paragraph, Thomson wrote: The result would be a state of universal rest and death, if the universe were finite and left to obey existing laws, but it is impossible to conceive a limit to the extent of matter in the universe. In the years to follow both Thomson's 1852 and the 1865 papers and Rankine both credited Thomson with the idea, but read further into his papers by publishing views stating that Thomson argued that the universe will end in a "heat death" which will be the "end of all physical phenomena". Proposals about the final state of the universe depend on the assumptions made about its ultimate fate, these assumptions have varied over the late 20th century and early 21st century.
In a hypothesized "open" or "flat" universe that continues expanding indefinitely, either a heat death or a Big Rip is expected to occur. If the cosmological constant is zero, the universe will approach absolute zero temperature over a long timescale. However, if the cosmological constant is positive, as appears to be the case in recent observations, the temperature will asymptote to a non-zero positive value, the universe will approach a state of maximum entropy. If a Big Rip does not happen long before that, the "heat death" situation could be avoided if there is a method or mechanism to regenerate hydrogen atoms from radiation