1.
Physical cosmology
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Physical cosmology is the study of the largest-scale structures and dynamics of the Universe and is concerned with fundamental questions about its origin, structure, evolution, and ultimate fate. These advances made it possible to speculate about the origin of the universe, a few researchers still advocate a handful of alternative cosmologies, however, most cosmologists agree that the Big Bang theory explains the observations better. Cosmology draws heavily on the work of many areas of research in theoretical. Areas relevant to include particle physics experiments and theory, theoretical and observational astrophysics, general relativity, quantum mechanics. Modern cosmology developed along tandem tracks of theory and observation, in 1916, Albert Einstein published his theory of general relativity, which provided a unified description of gravity as a geometric property of space and time. At the time, Einstein believed in a universe. This is because masses distributed throughout the universe gravitationally attract, however, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale. Einstein published his first paper on relativistic cosmology in 1917, in which he added this cosmological constant to his equations in order to force them to model a static universe. However, this so-called Einstein model is unstable to small perturbations—it will eventually start to expand or contract, the Einstein model describes a static universe, space is finite and unbounded. It was later realized that Einsteins model was just one of a set of possibilities, all of which were consistent with general relativity. The cosmological solutions of general relativity were found by Alexander Friedmann in the early 1920s and his equations describe the Friedmann–Lemaître–Robertson–Walker universe, which may expand or contract, and whose geometry may be open, flat, or closed. In the 1910s, Vesto Slipher interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth, however, it is difficult to determine the distance to astronomical objects. One way is to compare the size of an object to its angular size. Another method is to measure the brightness of an object and assume an intrinsic luminosity, due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own Milky Way, nor did they speculate about the cosmological implications. In 1929, Edwin Hubble provided a basis for Lemaîtres theory. Hubble showed that the nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance and he interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance. Given the cosmological principle, Hubbles law suggested that the universe was expanding, two primary explanations were proposed for the expansion
2.
Big Bang
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The Big Bang theory is the prevailing cosmological model for the universe from the earliest known periods through its subsequent large-scale evolution. If the known laws of physics are extrapolated to the highest density regime, detailed measurements of the expansion rate of the universe place this moment at approximately 13.8 billion years ago, which is thus considered the age of the universe. After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, giant clouds of these primordial elements later coalesced through gravity in halos of dark matter, eventually forming the stars and galaxies visible today. Since Georges Lemaître first noted in 1927 that a universe could be traced back in time to an originating single point. More recently, measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, the known physical laws of nature can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature. American astronomer Edwin Hubble observed that the distances to faraway galaxies were strongly correlated with their redshifts, assuming the Copernican principle, the only remaining interpretation is that all observable regions of the universe are receding from all others. Since we know that the distance between galaxies increases today, it must mean that in the past galaxies were closer together, the continuous expansion of the universe implies that the universe was denser and hotter in the past. Large particle accelerators can replicate the conditions that prevailed after the early moments of the universe, resulting in confirmation, however, these accelerators can only probe so far into high energy regimes. Consequently, the state of the universe in the earliest instants of the Big Bang expansion is still poorly understood, the first subatomic particles to be formed included protons, neutrons, and electrons. Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed, the majority of atoms produced by the Big Bang were hydrogen, along with helium and traces of lithium. Giant clouds of primordial elements later coalesced through gravity to form stars and galaxies. The framework for the Big Bang model relies on Albert Einsteins theory of relativity and on simplifying assumptions such as homogeneity. The governing equations were formulated by Alexander Friedmann, and similar solutions were worked on by Willem de Sitter, extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity indicates that general relativity is not a description of the laws of physics in this regime. How closely models based on general relativity alone can be used to extrapolate toward the singularity is debated—certainly no closer than the end of the Planck epoch. This primordial singularity is itself called the Big Bang, but the term can also refer to a more generic early hot. The agreement of independent measurements of this age supports the model that describes in detail the characteristics of the universe. The earliest phases of the Big Bang are subject to much speculation, in the most common models the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures and was very rapidly expanding and cooling
3.
Universe
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The Universe is all of time and space and its contents. It includes planets, moons, minor planets, stars, galaxies, the contents of intergalactic space, the size of the entire Universe is unknown. The earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric, 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 gravitation, Sir Isaac Newton built upon Copernicuss work as well as observations by Tycho Brahe. Further observational improvements led to the realization that our Solar System is located in the Milky Way galaxy and it is assumed that galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. Discoveries in the early 20th century have suggested that the Universe had a beginning, the majority of mass in the Universe appears to exist in an unknown form called dark matter. The Big Bang theory is the prevailing cosmological description of the development of the Universe, under this theory, space and time emerged together 13. 799±0.021 billion years ago with a fixed amount of energy and matter that has become less dense as the Universe has expanded. After the initial expansion, the Universe cooled, allowing the first subatomic particles to form, giant clouds later merged through gravity to form galaxies, stars, and everything else seen today. Some physicists have suggested various multiverse hypotheses, in which the Universe might be one among many universes that likewise exist, the Universe can be defined as everything that exists, everything that has existed, and everything that will exist. According to our current understanding, the Universe consists of spacetime, forms of energy, the Universe encompasses all of life, all of history, and some philosophers and scientists suggest that it even encompasses ideas such as mathematics and logic. 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 later Latin authors in many of the same senses as the modern English word is used. Another synonym was ὁ κόσμος ho kósmos, synonyms are also found in Latin authors and survive in modern languages, e. g. the German words Das All, Weltall, and Natur for Universe. The same synonyms are found in English, such as everything, the cosmos, the world, the prevailing model for the evolution of the Universe is the Big Bang theory. The Big Bang model states that the earliest state of the Universe was extremely hot and dense, the model is based on general relativity and on simplifying assumptions such as homogeneity and isotropy of space. The Big Bang model accounts for such as the correlation of distance and redshift of galaxies, the ratio of the number of hydrogen to helium atoms. The initial hot, dense state is called the Planck epoch, after the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these epochs encompassed less than 10 seconds of time following the Big Bang, the observed abundance of the elements can be explained by combining the overall expansion of space with nuclear and atomic physics. As the Universe expands, the density of electromagnetic radiation decreases more quickly than does that of matter because the energy of a photon decreases with its wavelength
4.
Chronology of the universe
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The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. The metric expansion of space is estimated to have begun 13.8 billion years ago, the time since the Big Bang is also known as cosmic time. The solar system formed at about 4.6 billion years ago, the far future, after cessation of stellar formation, with various scenarios for the ultimate fate of the universe. Little is understood about physics at this temperature, different hypotheses propose different scenarios, in inflationary cosmology, times before the end of inflation does not follow the traditional big bang timeline. Models attempting to formulate processes of the Planck epoch are speculative proposals for New Physics, examples include the Hartle–Hawking initial state, string landscape, string gas cosmology, and the ekpyrotic universe. Between 10−43 second and 10−36 second after the Big Bang As the universe expanded and cooled and these can be regarded as phase transitions much like condensation and freezing phase transitions of ordinary matter. The grand unification epoch began when gravitation separated from the gauge forces, the non-gravitational physics of this epoch would be described by a so-called grand unified theory. The grand unification epoch ended when the GUT forces further separate into the strong, while decelerating expansion would magnify deviations from homogeneity, making the universe more chaotic, accelerating expansion would make the universe more homogeneous. Inflation ended when the field decayed into ordinary particles in a process called reheating. The time of reheating is usually quoted as a time after the Big Bang, according to the simplest inflationary models, inflation ended at a temperature corresponding to roughly 10−32 second after the Big Bang. As explained above, this does not imply that the inflationary era lasted less than 10−32 second, in fact, in order to explain the observed homogeneity of the universe, the duration must be longer than 10−32 second. In inflationary cosmology, the earliest meaningful time after the Big Bang is the time of the end of inflation, in inflationary cosmology, the electroweak epoch began when the inflationary epoch ended, at roughly 10−32 seconds. There is currently insufficient observational evidence to explain why the universe contains far more baryons than antibaryons, a candidate explanation for this phenomenon must allow the Sakharov conditions to be satisfied at some time after the end of cosmological inflation. While particle physics suggests asymmetries under which conditions are met. After cosmic inflation ends, the universe is filled with a quark–gluon plasma, from this point onwards the physics of the early universe is better understood, and the energies involved in the Quark epoch are directly amenable to experiment. If supersymmetry is a property of our universe, then it must be broken at an energy that is no lower than 1 TeV, the electroweak symmetry scale. The masses of particles and their superpartners would then no longer be equal, between 10−6 second and 1 second after the Big Bang The quark–gluon plasma that composes the universe cools until hadrons, including baryons such as protons and neutrons, can form. At approximately 1 second after the Big Bang neutrinos decouple and begin traveling freely through space and this cosmic neutrino background, while unlikely to ever be observed in detail since the neutrino energies are very low, is analogous to the cosmic microwave background that was emitted much later
5.
Planck epoch
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Planck units have significance for theoretical physics since they simplify several recurring algebraic expressions of physical law by nondimensionalization. They are relevant in research on unified theories such as quantum gravity and this occurs at energy 7009195465531414000♠1. 22×1019 GeV, at times 6956538999999999999♠5. 39×10−44 s and length 6965162000000000000♠1. 62×10−35 m. All systems of measurement feature base units, in the International System of Units, for example, the base unit of length is the metre. In the system of Planck units, the Planck base unit of length is simply as the Planck length, the base unit of time is the Planck time. For example, Newtons law of gravitation, F = G m 1 m 2 r 2 = m 1 m 2 r 2 can be expressed as F F P =2. In order for this last equation to be valid, F, m1, m2, and r are understood to be the dimensionless numerical values of these quantities measured in terms of Planck units. This is why Planck units or any use of natural units should be employed with care, referring to G = c =1, Paul S. Wesson wrote that. Physically it represents a loss of information and can lead to confusion, key, L = length, M = mass, T = time, Q = electric charge, Θ = temperature. As can be seen above, the attractive force of two bodies of 1 Planck mass each, set apart by 1 Planck length is 1 Planck force. Likewise, the distance traveled by light during 1 Planck time is 1 Planck length, yet relative to other units of measurement such as SI, the values of the Planck units are only known approximately. This is mostly due to uncertainty in the value of the gravitational constant G, hence the value of c is now exact by definition, and contributes no uncertainty to the SI equivalents of the Planck units. The same is true of the value of the vacuum permittivity ε0, due to the definition of ampere which sets the vacuum permeability μ0 to 4π × 10−7 H/m and the fact that μ0ε0 = 1/c2. The numerical value of the reduced Planck constant ħ has been determined experimentally to 44 parts per billion, G appears in the definition of almost every Planck unit in Tables 2 and 3. Some physicists argue that communication with extraterrestrial intelligence would have to such a system of units in order to be understood. Unlike the metre and second, which exist as units in the SI system for historical reasons. Natural units help physicists to reframe questions, Frank Wilczek puts it succinctly, We see that the question is not, Why is gravity so feeble. But rather, Why is the protons mass so small, for in natural units, the strength of gravity simply is what it is, a primary quantity, while the protons mass is the tiny number. From the point of view of Planck units, this is comparing apples to oranges, because mass and electric charge are incommensurable quantities
6.
Cosmic microwave background
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The cosmic microwave background is the electromagnetic radiation left over from the time of recombination in Big Bang cosmology. In older literature, the CMB is also known as cosmic microwave background radiation or relic radiation. The CMB is a cosmic radiation that is fundamental to observational cosmology because it is the oldest light in the universe. With a traditional optical telescope, the space between stars and galaxies is completely dark, however, a sufficiently sensitive radio telescope shows a faint background glow, almost isotropic, that is not associated with any star, galaxy, or other object. This glow is strongest in the region of the radio spectrum. The discovery of CMB is landmark evidence of the Big Bang origin of the universe, when the universe was young, before the formation of stars and planets, it was denser, much hotter, and filled with a uniform glow from a white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler, when the universe cooled enough, protons and electrons combined to form neutral hydrogen atoms. These atoms could no longer absorb the radiation, and so the universe became transparent instead of being an opaque fog. This is the source of the alternative term relic radiation, precise measurements of the CMB are critical to cosmology, since any proposed model of the universe must explain this radiation. The CMB has a black body spectrum at a temperature of 2. 72548±0.00057 K. The spectral radiance dEν/dν peaks at 160.23 GHz, in the range of frequencies. In particular, the radiance at different angles of observation in the sky contains small anisotropies, or irregularities. This is an active field of study, with scientists seeking both better data and better interpretations of the initial conditions of expansion. Although many different processes might produce the form of a black body spectrum. As a result, most cosmologists consider the Big Bang model of the universe to be the best explanation for the CMB. The high degree of uniformity throughout the universe and its faint but measured anisotropy lend strong support for the Big Bang model in general. Moreover, the fluctuations are coherent on angular scales that are larger than the apparent cosmological horizon at recombination, either such coherence is acausally fine-tuned, or cosmic inflation occurred. The cosmic microwave radiation is an emission of uniform, black body thermal energy coming from all parts of the sky
7.
Hubble's law
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Hubbles law is considered the first observational basis for the expansion of the universe and today serves as one of the pieces of evidence most often cited in support of the Big Bang model. The motion of objects due solely to this expansion is known as the Hubble flow. Two years later Edwin Hubble confirmed the existence of that law, Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917. The law is expressed by the equation v = H0D. The SI unit of H0 is s−1 but it is most frequently quoted in /Mpc, the reciprocal of H0 is the Hubble time. Applying the most general principles to the nature of the universe yielded a solution that conflicted with the then-prevailing notion of a static universe. In 1922, Alexander Friedmann derived his Friedmann equations from Einsteins field equations, the parameter used by Friedmann is known today as the scale factor which can be considered as a scale invariant form of the proportionality constant of Hubbles law. Georges Lemaître independently found a solution in 1927. The Friedmann equations are derived by inserting the metric for a homogeneous and isotropic universe into Einsteins field equations for a fluid with a given density and this idea of an expanding spacetime would eventually lead to the Big Bang and Steady State theories of cosmology. In 1927, two years before Hubble published his own article, the Belgian priest and astronomer Georges Lemaître was the first to publish research deriving what is now known as Hubbles Law. According to the Canadian astronomer Sidney van den Bergh, The 1927 discovery of the expansion of the Universe by Lemaitre was published in French in a low-impact journal. In the 1931 high-impact English translation of this article a critical equation was changed by omitting reference to what is now known as the Hubble constant and it is now known that the alterations in the translated paper were carried out by Lemaitre himself. Before the advent of modern cosmology, there was talk about the size. In 1920, the famous Shapley-Curtis debate took place between Harlow Shapley and Heber D. Curtis over this issue, Shapley argued for a small universe the size of the Milky Way galaxy and Curtis argued that the Universe was much larger. The issue was resolved in the decade with Hubbles improved observations. Edwin Hubble did most of his professional astronomical observing work at Mount Wilson Observatory and his observations of Cepheid variable stars in spiral nebulae enabled him to calculate the distances to these objects. Surprisingly, these objects were discovered to be at distances which placed them well outside the Milky Way and they continued to be called nebulae and it was only gradually that the term galaxies took over. The parameters that appear in Hubble’s law, velocities and distances, are not directly measured, in reality we determine, say, a supernova brightness, which provides information about its distance, and the redshift z = ∆λ/λ of its spectrum of radiation
8.
Redshift
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In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. Some redshifts are an example of the Doppler effect, familiar in the change of apparent pitches of sirens, a redshift occurs whenever a light source moves away from an observer. Finally, gravitational redshift is an effect observed in electromagnetic radiation moving out of gravitational fields. However, redshift is a common term and sometimes blueshift is referred to as negative redshift. Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns, Redshifts are also seen in the spectroscopic observations of astronomical objects. Its value is represented by the letter z, a special relativistic redshift formula can be used to calculate the redshift of a nearby object when spacetime is flat. However, in contexts, such as black holes and Big Bang cosmology. Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws, the history of the subject began with the development in the 19th century of wave mechanics and the exploration of phenomena associated with the Doppler effect. The effect is named after Christian Doppler, who offered the first known physical explanation for the phenomenon in 1842, the hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot in 1845. Doppler correctly predicted that the phenomenon should apply to all waves, before this was verified, however, it was found that stellar colors were primarily due to a stars temperature, not motion. Only later was Doppler vindicated by verified redshift observations, the first Doppler redshift was described by French physicist Hippolyte Fizeau in 1848, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The effect is called the Doppler–Fizeau effect. In 1868, British astronomer William Huggins was the first to determine the velocity of a moving away from the Earth by this method. In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In 1887, Vogel and Scheiner discovered the annual Doppler effect, in 1901, Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors. The word does not appear unhyphenated until about 1934 by Willem de Sitter, perhaps indicating that up to point its German equivalent. Beginning with observations in 1912, Vesto Slipher discovered that most spiral galaxies, Slipher first reports on his measurement in the inaugural volume of the Lowell Observatory Bulletin. Three years later, he wrote a review in the journal Popular Astronomy, Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable positive velocities
9.
Metric expansion of space
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The metric expansion of space is the increase of the distance between two distant parts of the universe with time. It is an intrinsic expansion whereby the scale of space itself changes, metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the Friedmann-Lemaître-Robertson-Walker metric and is a generic property of the universe we inhabit. However, the model is only on large scales. At smaller scales matter has become bound together under the influence of gravitational attraction, the source of this acceleration is currently unknown. Physicists have postulated the existence of energy, appearing as a cosmological constant in the simplest gravitational models as a way to explain the acceleration. According to the simplest extrapolation of the cosmological model, this acceleration becomes more dominant into the future. The definition of distance used here is the summation or integration of local comoving distances, all done at constant local proper time. For example, galaxies that are more than the Hubble radius, approximately 4.5 gigaparsecs or 14.7 billion light-years, visibility of these objects depends on the exact expansion history of the universe. Because of the rate of expansion, it is also possible for a distance between two objects to be greater than the value calculated by multiplying the speed of light by the age of the universe. These details are a frequent source of confusion among amateurs and even professional physicists, in June 2016, NASA and ESA scientists reported that the universe was found to be expanding 5% to 9% faster than thought earlier, based on studies using the Hubble Space Telescope. To understand the expansion of the universe, it is helpful to discuss briefly what a metric is. A metric defines how a distance can be measured between two points in space, in terms of the coordinate system. Coordinate systems locate points in a space by assigning unique positions on a grid, known as coordinates, the metric is then a formula which describes how displacement through the space of interest can be translated into distances. For example, consider the measurement of distance between two places on the surface of the Earth and this is a simple, familiar example of spherical geometry. Because the surface of the Earth is two-dimensional, points on the surface of the Earth can be specified by two coordinates — for example, the latitude and longitude, specification of a metric requires that one first specify the coordinates used. In our simple example of the surface of the Earth, we could choose any kind of coordinate system we wish, for example latitude and longitude, in general, such shortest-distance paths are called geodesics. In Euclidean geometry, the geodesic is a line, while in non-Euclidean geometry such as on the Earths surface. Indeed, even the great circle path is always longer than the Euclidean straight line path which passes through the interior of the Earth
10.
Future of an expanding universe
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Observations suggest that the expansion of the universe will continue forever. If so, then a popular theory is that the universe will cool as it expands, for this reason, this future scenario is popularly called the Heat Death. Redshift will stretch ancient, incoming photons to undetectably long wavelengths, stars are expected to form normally for 1012 to 1014 years, but eventually the supply of gas needed for star formation will be exhausted. As existing stars run out of fuel and cease to shine, ultimately, if the universe reaches a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe. Infinite expansion does not determine the spatial curvature of the universe and it can be open, flat, although if it is closed, sufficient dark energy must be present to counteract the gravitational forces. Open and flat universes will expand forever even in the absence of dark energy, in this case, the universe should continue to expand at an accelerating rate. The acceleration of the expansion has also been confirmed by observations of distant supernovae. It is possible that the energy equation of state could change again resulting in an event that would have consequences which are extremely difficult to parametrize or predict. In the 1970s, the future of a universe was studied by the astrophysicist Jamal Islam. Then, in their 1999 book The Five Ages of the Universe, the first, the Primordial Era, is the time in the past just after the Big Bang when stars had not yet formed. The second, the Stelliferous Era, includes the present day and all of the stars and it is the time during which stars form from collapsing clouds of gas. In the subsequent Degenerate Era, the stars will have burnt out, leaving all stellar-mass objects as stellar remnants—white dwarfs, neutron stars, and black holes. In the Black Hole Era, white dwarfs, neutron stars, finally, in the Dark Era, even black holes have disappeared, leaving only a dilute gas of photons and leptons. This future history and the timeline below assume the expansion of the universe. From 106 years to 1014 years after the Big Bang The observable universe is currently 1. 38×1010 years old and this time is in the Stelliferous Era. About 155 million years after the Big Bang, the first star formed, since then, stars have formed by the collapse of small, dense core regions in large, cold molecular clouds of hydrogen gas. At first, this produces a protostar, which is hot, after the protostar contracts for a while, its center will become hot enough to fuse hydrogen and its lifetime as a star will properly begin. Stars of very low mass will eventually exhaust all their hydrogen and then become helium white dwarfs
11.
Heat death of the universe
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Heat death does not imply any particular absolute temperature, it only requires that temperature differences or other processes may no longer be exploited to perform work. In the language of physics, this is when the universe reaches thermodynamic equilibrium, 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 infers that if the universe lasts for a sufficient time, in Baillys view, all planets have an internal heat and are now at some particular stage of cooling. Jupiter, for instance, is too hot for life to arise there for thousands of years. The final state, in view, is described as one of equilibrium in which all motion ceases. Thomson’s views were elaborated on more definitively over the next decade by Hermann von Helmholtz. The idea of death of the universe derives from discussion of the application of the first two laws of thermodynamics to universal processes. The ideas in this paper, in relation to their application to the age of the sun, the three of them were said to have exchanged ideas on this subject. In a key paragraph, Thomson wrote, The result would inevitably be a state of universal rest and death, if the universe were finite, in a hypothesized open or flat universe that continues expanding indefinitely, a heat death is expected to occur. If the cosmological constant is zero, the universe will approach absolute zero temperature over a long timescale. Therefore, the universe is not in equilibrium and objects can do physical work. The decay time for a black hole of roughly 1 galaxy-mass due to Hawking radiation is on the order of 10100 years. Speculatively, it is possible that the universe may enter a second inflationary epoch, or, assuming that the current vacuum state is a false vacuum and it is also possible that entropy production will cease and the universe will reach heat death. Possibly another universe could be created by quantum fluctuations or quantum tunneling in roughly 10101056 years. Over an infinite time, there would be a spontaneous entropy decrease via the Poincaré recurrence theorem, thermal fluctuations, max Planck wrote that the phrase entropy of the universe has no meaning because it admits of no accurate definition. According to Tisza, If an isolated system is not in equilibrium, buchdahl writes of the entirely unjustifiable assumption that the universe can be treated as a closed thermodynamic system. There is no universally accepted notion of entropy for systems out of equilibrium, in the opinion of Čápek and Sheehan, no known formulation applies to all possible thermodynamic regimes. In Landsbergs opinion, The third misconception is that thermodynamics, and in particular and these questions have a certain fascination, but the answers are speculations, and lie beyond the scope of this book
12.
Big Crunch
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Sudden singularities and crunch or rip singularities at late times occur only for hypothetical matter with implausible physical properties. If the universes expansion speed does not exceed the escape velocity, if entropy continues to increase in the contracting phase, the contraction would appear very different from the time reversal of the expansion. While the early universe was highly uniform, a universe would become increasingly clumped. Eventually all matter would collapse into black holes, which would then coalesce producing a black hole or Big Crunch singularity. For a contracting Universe similar to ours in composition its expected that superclusters would merge among themselves followed by galaxy clusters and later galaxies. Conversely, if the density of the universe is less than the density, the universe will continue to expand. This scenario would result in the Big Freeze, where the universe cools as it expands, another scenario results in a flat universe which occurs when the critical density is just right. In this state the universe would always be slowing down, although, it is now understood that the critical density has been measured and determined to be a flat universe. Recent experimental evidence has led to speculation that the expansion of the universe is not being slowed down by gravity but rather accelerating
13.
Lambda-CDM model
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The ΛCDM model can be extended by adding cosmological inflation, quintessence and other elements that are current areas of speculation and research in cosmology. Some alternative models challenge the assumptions of the ΛCDM model, examples of these are modified Newtonian dynamics, modified gravity and theories of large-scale variations in the matter density of the universe. Both effects are attributed to a Doppler shift in electromagnetic radiation as it travels across expanding space, although this expansion increases the distance between objects that are not under shared gravitational influence, it does not increase the size of the objects in space. A cosmological constant has negative pressure, p = − ρ c 2, the fraction of the total energy density of our universe that is dark energy, Ω Λ, is currently estimated to be 69.2 ±1. 2% based on Planck satellite data. Cold dark matter is a form of matter introduced in order to account for gravitational effects observed in very large-scale structures that cannot be accounted for by the quantity of observed matter, dark matter is described as being cold, non-baryonic, dissipationless, and collisionless. The dark matter component is currently estimated to constitute about 26. 8% of the density of the universe. The remaining 4. 9% comprises all ordinary matter observed as atoms, chemical elements, gas and plasma, also, the energy density includes a very small fraction in cosmic microwave background radiation, and not more than 0. 5% in relic neutrinos. Although very small today, these were more important in the distant past. The model includes a single originating event, the Big Bang and this was immediately followed by an exponential expansion of space by a scale multiplier of 1027 or more, known as cosmic inflation. Cosmic inflation also addresses the problem in the CMB, indeed. The model uses the FLRW metric, the Friedmann equations and the equations of state to describe the observable universe from right after the inflationary epoch to present. The scale factor is related to the redshift z of the light emitted at time t e m by 1 a =1 + z. The expansion rate is described by the time-dependent Hubble parameter, H, defined as H ≡ a ˙ a, where a ˙ is the time-derivative of the scale factor. A critical density ρ c r i t is the present-day density, if the cosmological constant were actually zero, the critical density would also be the dividing line between eventual recollapse of the universe to a Big Crunch, or unlimited expansion. Since the densities of various species scale as different powers of a, e. g. a −3 for matter etc, the various Ω parameters add up to 1 by construction.01 or t >10 Myr. Solving for a =1 gives the present age of the universe t 0 in terms of the other parameters. It follows that the transition from decelerating to accelerating expansion occurred when a =1 /3 which evaluates to a ~0.6 or z ~0.66 for the Planck best-fit parameters. The discovery of the Cosmic Microwave Background in 1965 confirmed a key prediction of the Big Bang cosmology, from that point on, it was generally accepted that the universe started in a hot, dense state and has been expanding over time
14.
Baryon
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A baryon is a composite subatomic particle made up of three quarks. Baryons and mesons belong to the family of particles, which are the quark-based particles. The name baryon comes from the Greek word for heavy, because, at the time of their naming, as quark-based particles, baryons participate in the strong interaction, whereas leptons, which are not quark-based, do not. The most familiar baryons are the protons and neutrons that make up most of the mass of the matter in the universe. Each baryon has a corresponding antiparticle where quarks are replaced by their corresponding antiquarks, for example, a proton is made of two up quarks and one down quark, and its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark. This is in contrast to the bosons, which do not obey the exclusion principle, Baryons, along with mesons, are hadrons, meaning they are particles composed of quarks. Quarks have baryon numbers of B = 1/3 and antiquarks have baryon number of B = −1/3, the term baryon usually refers to triquarks—baryons made of three quarks. Other exotic baryons have been proposed, such as made of four quarks and one antiquark. The particle physics community as a whole did not view their existence as likely in 2006, however, in July 2015, the LHCb experiment observed two resonances consistent with pentaquark states in the Λ0 b → J/ψK−p decay, with a combined statistical significance of 15σ. In theory, heptaquarks, nonaquarks, etc. could also exist, nearly all matter that may be encountered or experienced in everyday life is baryonic matter, which includes atoms of any sort, and provides those with the property of mass. Non-baryonic matter, as implied by the name, is any sort of matter that is not composed primarily of baryons and this might include neutrinos and free electrons, dark matter, such as supersymmetric particles, axions, and black holes. The very existence of baryons is also a significant issue in cosmology, the process by which baryons came to outnumber their antiparticles is called baryogenesis. Some grand unified theories of physics also predict that a single proton can decay, changing the baryon number by one, however. The excess of baryons over antibaryons in the present universe is thought to be due to non-conservation of baryon number in the early universe. The concept of isospin was first proposed by Werner Heisenberg in 1932 to explain the similarities between protons and neutrons under the strong interaction, although they had different electric charges, their masses were so similar that physicists believed they were the same particle. The different electric charges were explained as being the result of some unknown excitation similar to spin and this unknown excitation was later dubbed isospin by Eugene Wigner in 1937. This belief lasted until Murray Gell-Mann proposed the model in 1964. The success of the model is now understood to be the result of the similar masses of the u and d quarks
15.
Energy
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In physics, energy is the property that must be transferred to an object in order to perform work on – or to heat – the object, and can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton. Mass and energy are closely related, for example, with a sensitive enough scale, one could measure an increase in mass after heating an object. Living organisms require available energy to stay alive, such as the humans get from food. Civilisation gets the energy it needs from energy resources such as fuels, nuclear fuel. The processes of Earths climate and ecosystem are driven by the radiant energy Earth receives from the sun, the total energy of a system can be subdivided and classified in various ways. It may also be convenient to distinguish gravitational energy, thermal energy, several types of energy, electric energy. Many of these overlap, for instance, thermal energy usually consists partly of kinetic. Some types of energy are a mix of both potential and kinetic energy. An example is energy which is the sum of kinetic. Whenever physical scientists discover that a phenomenon appears to violate the law of energy conservation. Heat and work are special cases in that they are not properties of systems, in general we cannot measure how much heat or work are present in an object, but rather only how much energy is transferred among objects in certain ways during the occurrence of a given process. Heat and work are measured as positive or negative depending on which side of the transfer we view them from, the distinctions between different kinds of energy is not always clear-cut. In contrast to the definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness. The modern analog of this property, kinetic energy, differs from vis viva only by a factor of two, in 1807, Thomas Young was possibly the first to use the term energy instead of vis viva, in its modern sense. Gustave-Gaspard Coriolis described kinetic energy in 1829 in its modern sense, the law of conservation of energy was also first postulated in the early 19th century, and applies to any isolated system. It was argued for years whether heat was a physical substance, dubbed the caloric, or merely a physical quantity. In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat and these developments led to the theory of conservation of energy, formalized largely by William Thomson as the field of thermodynamics
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Radiation
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In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. Radiation is often categorized as either ionizing or non-ionizing depending on the energy of the radiated particles, Ionizing radiation carries more than 10 eV, which is enough to ionize atoms and molecules, and break chemical bonds. This is an important distinction due to the difference in harmfulness to living organisms. A common source of ionizing radiation is radioactive materials that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, gamma rays, X-rays and the higher energy range of ultraviolet light constitute the ionizing part of the electromagnetic spectrum. The lower-energy, longer-wavelength part of the spectrum including visible light, infrared light, microwaves and this type of radiation only damages cells if the intensity is high enough to cause excessive heating. Ultraviolet radiation has some features of both ionizing and non-ionizing radiation and these properties derive from ultraviolets power to alter chemical bonds, even without having quite enough energy to ionize atoms. The word radiation arises from the phenomenon of waves radiating from a source and this aspect leads to a system of measurements and physical units that are applicable to all types of radiation. This law does not apply close to a source of radiation or for focused beams. Radiation with sufficiently high energy can ionize atoms, that is to say it can knock electrons off atoms, ionization occurs when an electron is stripped from an electron shell of the atom, which leaves the atom with a net positive charge. Because living cells and, more importantly, the DNA in those cells can be damaged by this ionization, thus ionizing radiation is somewhat artificially separated from particle radiation and electromagnetic radiation, simply due to its great potential for biological damage. While an individual cell is made of trillions of atoms, only a fraction of those will be ionized at low to moderate radiation powers. If the source of the radiation is a radioactive material or a nuclear process such as fission or fusion. Particle radiation is subatomic particles accelerated to relativistic speeds by nuclear reactions, because of their momenta they are quite capable of knocking out electrons and ionizing materials, but since most have an electrical charge, they dont have the penetrating power of ionizing radiation. The exception is neutron particles, see below, there are several different kinds of these particles, but the majority are alpha particles, beta particles, neutrons, and protons. Roughly speaking, photons and particles with energies above about 10 electron volts are ionizing, particle radiation from radioactive material or cosmic rays almost invariably carries enough energy to be ionizing. The radiation is invisible and not directly detectable by human senses, as a result, in some cases, it may lead to secondary emission of visible light upon its interaction with matter, as in the case of Cherenkov radiation and radio-luminescence. Ionizing radiation has many uses in medicine, research and construction. Ultraviolet, of wavelengths from 10 nm to 125 nm, ionizes air molecules, causing it to be absorbed by air
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Dark energy
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In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe. Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate. Assuming that the model of cosmology is correct, the best current measurements indicate that dark energy contributes 68. 3% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary matter contribute 26. 8% and 4. 9%, respectively, the density of dark energy is very low, much less than the density of ordinary matter or dark matter within galaxies. However, it comes to dominate the mass–energy of the universe because it is uniform across space, contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to the zero-point radiation of space i. e. the vacuum energy, scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow. High-precision measurements of the expansion of the universe are required to understand how the rate changes over time. In general relativity, the evolution of the rate is estimated from the curvature of the universe. Measuring the equation of state for energy is one of the biggest efforts in observational cosmology today. The equilibrium is unstable, if the universe expands slightly, then the expansion releases vacuum energy, likewise, a universe which contracts slightly will continue contracting. These sorts of disturbances are inevitable, due to the distribution of matter throughout the universe. Further, observations made by Edwin Hubble in 1929 showed that the universe appears to be expanding, einstein reportedly referred to his failure to predict the idea of a dynamic universe, in contrast to a static universe, as his greatest blunder. Alan Guth and Alexei Starobinsky proposed in 1980 that a negative pressure field, similar in concept to dark energy, inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the Big Bang. Such expansion is a feature of most current models of the Big Bang. It is unclear what relation, if any, exists between energy and inflation. Even after inflationary models became accepted, the constant was thought to be irrelevant to the current universe. Nearly all inflation models predict that the density of the universe should be very close to the critical density. During the 1980s, most cosmological research focused on models with critical density in matter only, usually 95% cold dark matter, then in 2001, the 2dF Galaxy Redshift Survey gave strong evidence that the matter density is around 30% of critical
18.
Dark matter
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Dark matter is an unidentified type of matter distinct from dark energy, baryonic matter, and neutrinos whose existence would explain a number of otherwise puzzling astronomical observations. The name refers to the fact that it does not emit or interact with radiation, such as light. The standard model of cosmology indicates that the total mass–energy of the universe contains 4. 9% ordinary matter,26. 8% dark matter and 68. 3% dark energy. Thus, dark matter constitutes 84. 5% of total mass, the most widely accepted hypothesis on the form for dark matter is that it is composed of weakly interacting massive particles that interact only through gravity and the weak force. All these lines of evidence suggest that galaxies, galaxy clusters, and the universe as a whole contain far more matter than that which is observable via electromagnetic signals. Many experiments to detect proposed dark matter particles through non-gravitational means are under way, however, no dark matter particle has been conclusively identified. ”In 1906 Henri Poincaré in the “The Milky Way and Theory of Gases” used dark matter, ” or “matière obscure” in French in discussing Kelvins work. The first to suggest the existence of matter was Dutch astronomer Jacobus Kapteyn in 1922. Fellow Dutchman and radio astronomy pioneer Jan Oort also hypothesized the existence of matter in 1932. In 1933, Swiss astrophysicist Fritz Zwicky, who studied galactic clusters while working at the California Institute of Technology, Zwicky applied the virial theorem to the Coma galaxy cluster and obtained evidence of unseen mass that he called dunkle Materie dark matter. Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and he estimated that the cluster had about 400 times more mass than was visually observable. The gravity effect of the galaxies was far too small for such fast orbits. Based on these conclusions, Zwicky inferred that some unseen matter provided the mass and this was the first formal inference about the existence of dark matter. However, Zwicky did correctly infer that the bulk of the matter was dark, the first robust indications that the mass to light ratio was anything other than unity came from measurements of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve for the Andromeda nebula and he attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to missing matter. Vera Rubin and Kent Ford in the 1960s–1970s provided further strong evidence, Rubin worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy. This result was confirmed in 1978, an influential paper presented Rubins results in 1980. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle, the search for this particle, by a variety of means, is one of the major efforts in particle physics. In cosmology, the CMB is explained as relic radiation which has travelled freely since the era of recombination, the CMB’s anisotropies are explained as the result of small primordial density fluctuations, and subsequent acoustic oscillations in the photon-baryon plasma whose restoring force is gravity
19.
Cold dark matter
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It is believed that approximately 84. 54% of matter in the Universe is dark matter, with only a small fraction being the ordinary baryonic matter that composes stars, planets and living organisms. An influential review article in 1984 by Blumenthal, Sandra Moore Faber, Primack, predictions of the cold dark matter paradigm are in general agreement with astronomical observations. Dark matter is detected through its interactions with ordinary matter. As such, it is difficult to determine what the constituents of cold dark matter are. The candidates fall roughly into three categories, Axions are very light particles with a type of self-interaction that makes them a suitable CDM candidate. Axions have the advantage that their existence solves the Strong CP problem in QCD. MACHOs or Massive Compact Halo Objects are large, condensed objects such as holes, neutron stars, white dwarfs, very faint stars. The search for these consists of using gravitational lensing to see the effect of these objects on background galaxies, most experts believe that the constraints from those searches rule out MACHOs as a viable dark matter candidate. WIMPs, Dark matter is composed of Weakly Interacting Massive Particles, there is no currently known particle with the required properties, but many extensions of the standard model of particle physics predict such particles. The search for WIMPs involves attempts at direct detection by highly sensitive detectors, WIMPs are generally regarded as the most promising dark matter candidates. The missing satellites problem, cold dark matter simulations predict much larger numbers of small galaxies than are observed around galaxies like the Milky Way. Some of these problems have proposed solutions but it unclear whether they can be solved without abandoning the CDM paradigm. Fuzzy cold dark matter Meta-cold dark matter Dark matter Hot dark matter Warm dark matter Self-interacting dark matter Lambda-CDM model Modified Newtonian dynamics Bertone, particle Dark Matter, Observations, Models and Searches
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Warm dark matter
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The most common WDM candidates are sterile neutrinos and gravitinos. The WIMPs, when produced non-thermally could be candidates for dark matter. In general, however the thermally produced WIMPs are cold dark matter candidates and these particles are considered inert because they only have suppressed interactions with the Z boson. Sterile neutrinos with masses of a few keV are possible candidates for keVins, at temperatures below the electroweak scale their only interactions with standard model particles are weak interactions due to their mixing with ordinary neutrinos. Due to the smallness of the angle they are not overproduced because they freeze out before reaching thermal equilibrium. Their properties are consistent with astrophysical bounds coming from structure formation, in February 2014, different analyses have extracted from the spectrum of X-ray emissions observed by XMM-Newton, a monochromatic signal around 3.5 keV. This signal is coming from different galaxy clusters and several scenarios of dark matter can justify such a line. We can cite, for example, a 3.5 keV candidate annihilating into 2 photons, or a 7 keV dark matter particle decaying into a photon, IOP Science, Journal of Cosmology and Astroparticle Physics,1208016. The first star formation in WDM Universe W. B. Lin, D. H. Huang, X. Zhang, R. Brandenberger, Non-Thermal Production of WIMPs, particle Dark Matter, Observations, Models and Searches
21.
Hot dark matter
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Hot dark matter is a theoretical form of dark matter which consists of particles that travel with ultrarelativistic velocities. Dark matter is matter that does not interact with, and therefore cannot be detected by, electromagnetic radiation and it is postulated to exist to explain how clusters and superclusters of galaxies formed after the Big Bang. Data from galaxy rotation curves indicate that around 90% of the mass of a galaxy cannot be seen and it can only be detected by its gravitational effect. Hot dark matter cannot explain how individual galaxies formed from the Big Bang, due to theory, in order to explain small scale structure in the Universe, it is necessary to invoke cold dark matter or warm dark matter. Hot dark matter as the explanation of dark matter is no longer viable, therefore. An example of a hot dark matter particle is the neutrino, neutrinos have very small masses, and do not take part in two of the four fundamental forces, the electromagnetic interaction and the strong interaction. They theoretically interact by the interaction, and gravity, but due to the feeble strength of these forces. A number of projects, such as the Super-Kamiokande neutrino observatory, in Gifu, lambda-CDM model Modified Newtonian Dynamics Bertone, Gianfranco. Particle Dark Matter, Observations, Models and Searches, hot dark matter by Berkeley Dark Matter
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Dark radiation
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Dark radiation is a postulated type of radiation that mediates interactions of dark matter. By analogy to the way photons mediate electromagnetic interactions between particles in the Standard Model, dark radiation is proposed to mediate interactions between matter particles. Similar to dark matter particles, the hypothetical dark radiation does not interact with Standard Model particles, there has been no notable evidence for the existence of such radiation, but since baryonic matter contains multiple interacting particle types, it is reasonable to suppose that dark matter does also. This extra degree of freedom could arise from having a non-trivial amount of radiation in the universe. One possible candidate for dark radiation is the sterile neutrino
23.
Shape of the universe
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The shape of the universe is the local and global geometry of the Universe, in terms of both curvature and topology. The shape of the universe is related to general relativity which describes how spacetime is curved and bent by mass, cosmologists distinguish between the observable universe and the global universe. The observable universe consists of the part of the universe that can, in principle, simply connected space or multiply connected. There are certain logical connections among these properties, for example, a universe with positive curvature is necessarily finite. Although it is assumed in the literature that a flat or negatively curved universe is infinite. Theorists have been trying to construct a mathematical model of the shape of the universe. In formal terms, this is a 3-manifold model corresponding to the section of the 4-dimensional space-time of the universe. The model most theorists currently use is the Friedmann–Lemaître–Robertson–Walker model, ideally, one can continue to look back all the way to the Big Bang, in practice, however, the farthest away one can look is the cosmic microwave background, as anything past that was opaque. Experimental investigations show that the universe is very close to isotropic. If the observable universe encompasses the entire universe, we may be able to determine the structure of the entire universe by observation. The universe may be small in dimensions and not in others. For example, if the universe is a closed loop, one would expect to see multiple images of an object in the sky. The curvature of space is a description of length relationships in spatial coordinates. In mathematics, any geometry has three possible curvatures, so the geometry of the universe has the three possible curvatures. Flat Positively curved Negatively curved An example of a flat curvature would be any Euclidean geometry, curved geometries are in the domain of Non-Euclidean geometry. An example of a curved surface would be the surface of a sphere such as the Earth. A triangle drawn from the equator to a pole will result in at least two angles being 90°, making the sum of the 3 angles greater than 180°, an example of a negative curved surface would be the shape of a saddle or mountain pass. A triangle drawn on a saddle shape will result in the sum of the angles adding up to less than 180° due to the curving away as the triangle moves away from the center
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Reionization
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In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the dark ages, and is the second of two major phase transitions of gas in the universe. As the majority of matter is in the form of hydrogen. The primordial helium in the universe experienced the same changes, but at different points in the history of the universe. The Dark Ages of the start at that point, because there were no light sources other than the gradually redshifting cosmic background radiation. The second phase change occurred once objects started to condense in the universe that were energetic enough to re-ionize neutral hydrogen. As these objects formed and radiated energy, the universe reverted from being neutral and this occurred between 150 million and one billion years after the Big Bang. Thus, a full of low density ionized hydrogen will remain transparent. 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 also happen to have relatively uniform spectral features, regardless of their position in the sky or distance from the Earth. Thus it can be inferred that any differences between quasar spectra will be caused by the interaction of their emission with atoms along the line of sight. For nearby objects in the universe, spectral lines are very sharp. 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 instead of showing sharp spectral lines, a quasars light which has traveled through a large. The redshifting for a particular quasar provides temporal information about reionization, since an objects 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, 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 partly neutral, as reionization is expected to occur over relatively 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 almost entirely neutral at z >10, the anisotropy of the cosmic microwave background on different angular scales can also be used to study reionization
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Structure formation
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In physical cosmology, structure formation refers to the formation of galaxies, galaxy clusters and larger structures from small early density fluctuations. The universe, as is now known from observations of the microwave background radiation, began in a hot, dense. Structure formation attempts to model how these structures formed by gravitational instability of small early density ripples, understanding the processes of galaxy formation is a major topic of modern cosmology research, both via observations such as the Hubble Ultra-Deep Field and via large computer simulations. Cosmic inflation also would have amplified minute quantum fluctuations into slight density ripples of overdensity and underdensity, structures smaller than the horizon remained essentially frozen due to radiation domination impeding growth. As the universe expanded, the density of radiation drops faster than matter, after this all dark matter ripples could grow freely, forming seeds into which the baryons could later fall. The size of the universe at this epoch forms a turnover in the power spectrum which can be measured in large redshift surveys. The universe was dominated by radiation for most of this stage, and due to the heat and radiation. In this hot and dense situation, the radiation could not travel far before Thomson scattering off an electron, the universe was very hot and dense, but expanding rapidly and therefore cooling. Finally, at a less than 400,000 years after the bang, it become cool enough for the protons to capture negatively charged electrons. Several remarkable space-based missions, have detected very slight variations in the density and these variations were subtle, and the CMB appears very nearly uniformly the same in every direction. The very early universe is still a poorly understood epoch, from the viewpoint of fundamental physics, the prevailing theory, cosmic inflation, does a good job explaining the observed flatness, homogeneity and isotropy of the universe, as well as the absence of exotic relic particles. Another prediction borne out by observation is that tiny perturbations in the universe seed the later formation of structure. These fluctuations, while they form the foundation for all structure and these fluctuations are critical, because they provide the seeds from which the largest structures can grow and eventually collapse to form galaxies and stars. COBE provided the first detection of the fluctuations in the cosmic microwave background radiation in the 1990s. These perturbations are thought to have a specific character, they form a Gaussian random field whose covariance function is diagonal. Another important property of the perturbations, that they are adiabatic, is predicted by cosmic inflation and has been confirmed by observations. Some theories, such as strings, have largely been refuted by increasingly precise data. An important concept in structure formation is the notion of the Hubble radius, often called simply the horizon, since the universe is continually expanding, its energy density is continually decreasing
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Galaxy formation and evolution
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Galaxy formation is hypothesized to occur, from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. Because of the inability to conduct experiments in space, the only way to “test” theories. Explanations for how galaxies formed and evolved must be able to predict the properties and types of galaxies. Edwin Hubble created the first galaxy classification scheme known as the Hubble tuning-fork diagram and it partitioned galaxies into ellipticals, normal spirals, barred spirals, and irregulars. These groups divide into blue star-forming galaxies that are more like spiral types, spiral galaxies are quite thin, dense, and rotate relatively fast, while the stars in elliptical galaxies have randomly-oriented orbits. The majority of mass in galaxies is made up of matter, a substance which is not directly observable. The majority of giant galaxies contain a black hole in their centers. The black hole mass is tied to the host galaxy bulge or spheroid mass, metallicity has a positive correlation with the absolute magnitude of a galaxy. This is not the case instead the tuning fork diagram shows an evolution from simple to complex with no temporal connotations intended, astronomers now believe that disk galaxies likely formed first, then evolved into elliptical galaxies through galaxy mergers. The earliest stage in the evolution of galaxies is the formation, when a galaxy forms, it has a disk shape and is called a spiral galaxy due to spiral-like arm structures located on the disk. There are different theories on how these disk-like distributions of stars develop from a cloud of matter, however, at present, olin Eggen, Donald Lynden-Bell, and Allan Sandage in 1962, proposed a theory that disk galaxies form through a monolithic collapse of a large gas cloud. The distribution of matter in the universe was in clumps that consisted mostly of dark matter. These clumps interacted gravitationally, putting tidal torques on each other that acted to give them some angular momentum, as the baryonic matter cooled, it dissipated some energy and contracted toward the center. With angular momentum conserved, the matter near the center speeds up its rotation, then, like a spinning ball of pizza dough, the matter forms into a tight disk. Once the disk cools, the gas is not gravitationally stable and it breaks, and these smaller clouds of gas form stars. Since the dark matter does not dissipate as it only interacts gravitationally, observations show that there are stars located outside the disk, which does not quite fit the pizza dough model. It was first proposed by Leonard Searle and Robert Zinn that galaxies form by the coalescence of smaller progenitors, known as a top-down formation scenario, this theory is quite simple yet no longer widely accepted. More recent theories include the clustering of dark matter halos in the bottom-up process and this still results in disk-like distributions of baryonic matter with dark matter forming the halo for all the same reasons as in the top-down theory
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Observable universe
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There are at least two trillion galaxies in the observable universe, containing more stars than all the grains of sand on planet Earth. Assuming the universe is isotropic, the distance to the edge of the universe is roughly the same in every direction. That is, the universe is a spherical volume centered on the observer. Every location in the Universe has its own universe, which may or may not overlap with the one centered on Earth. The word observable used in this sense does not depend on modern technology actually permits detection of radiation from an object in this region. It simply indicates that it is possible in principle for light or other signals from the object to reach an observer on Earth, in practice, we can see light only from as far back as the time of photon decoupling in the recombination epoch. That is when particles were first able to emit photons that were not quickly re-absorbed by other particles, before then, the Universe was filled with a plasma that was opaque to photons. The detection of gravitational waves indicates there is now a possibility of detecting signals from before the recombination epoch. The surface of last scattering is the collection of points in space at the distance that photons from the time of photon decoupling just reach us today. These are the photons we detect today as cosmic microwave background radiation, however, with future technology, it may be possible to observe the still older relic neutrino background, or even more distant events via gravitational waves. It is estimated that the diameter of the universe is about 28.5 gigaparsecs. The total mass of matter in the universe can be calculated using the critical density. Some parts of the Universe are too far away for the light emitted since the Big Bang to have had time to reach Earth. In the future, light from distant galaxies will have had time to travel. This fact can be used to define a type of cosmic event horizon whose distance from the Earth changes over time, both popular and professional research articles in cosmology often use the term universe to mean observable universe. It is plausible that the galaxies within our observable universe represent only a fraction of the galaxies in the Universe. If the Universe is finite but unbounded, it is 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 and 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
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Galaxy filament
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In physical cosmology, galaxy filaments are the largest known structures in the universe. They are massive, thread-like formations, with a length of 50 to 80 megaparsecs h−1 that form the boundaries between large voids in the universe. Filaments consist of gravitationally bound galaxies, parts wherein many galaxies are very close to one another are called superclusters. In the standard model of the evolution of the universe, galactic filaments form along and it is thought that this dark matter dictates the structure of the Universe on the grandest of scales. Dark matter gravitationally attracts baryonic matter, and it is this matter that astronomers see forming long. Discovery of structures larger than superclusters began in the late-1980s, in 1987, astronomer R. Brent Tully of the University of Hawaiis Institute of Astronomy identified what he called the Pisces–Cetus Supercluster Complex. In 1989, the CfA2 Great Wall was discovered, followed by the Sloan Great Wall in 2003, filament subtype of filaments have roughly similar major and minor axes in cross-section, along the lengthwise axis. A short filament, detected by identifying an alignment of star-forming galaxies, in the neighborhood of the Milky Way and the Local Group was proposed by Adi Zitrin and Noah Brosch. The reality of this filament, and the identification of a similar, the galaxy wall subtype of filaments have a significantly greater major axis than minor axis in cross-section, along the lengthwise axis. A wall was proposed to be the embodiment of the Great Attractor. It is sometimes referred to as the Great Attractor Wall or Norma Wall and this suggestion was superseded by the proposal of a supercluster, Laniakea, that would encompass the Great Attractor, Virgo Supercluster, Hydra-Centaurus Superclusters. A wall was proposed in 2000 to lie at z=1.47 in the vicinity of radio galaxy B3 0003+387, a wall was proposed in 2000 to lie at z=0.559 in the northern Hubble Deep Field. Large quasar groups are some of the largest structures known and they are theorized to be protohyperclusters/proto-supercluster-complexes/galaxy filament precursors. Kevin A. Pimbblet, Pulling Out Threads from the Cosmic Tapestry, pictures of the filamentary network The Universe Within One Billion Light Years with List of Nearby Superclusters
29.
Supercluster
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A supercluster is a large group of smaller galaxy clusters or galaxy groups, which is among the largest-known structures of the cosmos. The Milky Way is part of the Local Group galaxy cluster and this supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years. The number of superclusters in the universe is estimated to be 10 million. Galaxies are grouped into clusters instead of being dispersed randomly, clusters of galaxies, in turn, are grouped together to form superclusters. Typically, superclusters contain dozens of individual clusters throughout an area of space about 150 million light-years across, unlike clusters, most superclusters are not bound together by gravity. The component clusters are generally shifting away from each due to the Hubble flow. The Milky Way galaxy falls within the Local Group, which is a poor, poor clusters may contain only a few dozen galaxies, as compared to rich clusters with hundreds or even thousands. The Local Group is near the Local Supercluster, which has a diameter of 100 million light-years, the Local Supercluster contains a total of about 1015 times the mass of the Sun. The biggest cluster in the universe is called the Great Attractor. Its gravity is so strong that the Local Supercluster, including the Milky Way, is moving in a direction towards it at a rate of several hundred kilometers per second, the biggest supercluster outside of the local universe is the Perseus-Pegasus Filament. It contains the Perseus supercluster and it spans about a billion light-years, research has tried to understand the way superclusters are arranged in space. Maps are used to display the positions of 1.6 million galaxies, three-dimensional maps are used to further understand the positions of these superclusters. To map them three-dimensionally, the position of the galaxy in the sky as well as the galaxys redshift are used for calculation, the galaxys redshift is used with Hubbles law to determine its position in three-dimensional space. It was discovered from those maps that superclusters of galaxies are not spread uniformly across the universe, maps reveal huge voids where there are extremely few galaxies. Some dim galaxies or hydrogen clouds can be found in some voids, the voids themselves are often spherical but the superclusters are not. They can range from being 100 million to 400 million light-years in diameter, the pattern of sheets and voids contains information about how galaxy clusters formed in the early universe. Those groups and clusters and additional isolated galaxies in turn form even larger structures called superclusters and their existence was first postulated by George Abell in his 1958 Abell catalogue of galaxy clusters. He called them second-order clusters, or clusters of clusters and these are the largest known structures to date
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Galaxy cluster
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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 that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with star clusters, such as clusters, which are structures of stars within galaxies, or with globular clusters. Small aggregates of galaxies are referred to as groups of galaxies rather than clusters of galaxies, the groups and clusters can themselves cluster together to form superclusters. Notable galaxy clusters in the relatively nearby Universe include the Virgo Cluster, Fornax Cluster, Hercules Cluster, a very 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, using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters. Galaxy clusters typically have the properties, They contain 100 to 1,000 galaxies, hot X-ray- emitting gas. Details are described in the Composition section, the distribution of the three components is approximately the same in the cluster. They have total masses of 1014 to 1015 solar masses and they typically have a diameter from 2 to 10 Mpc. The spread of velocities for the galaxies is about 800–1000 km/s. There are three components of a galaxy cluster. They are tabulated below, Stars, Star clusters, Galaxies, Galaxy clusters, Super clusters Abell catalogue Intracluster medium List of Abell clusters
. Bibcode:2017LRR....20....2R. doi:10.1007/s41114-017-0004-1.