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89-405: Reionization is the second of two major phase transitions of gas in the universe (the first is recombination ). While the majority of baryonic matter in the universe is in the form of hydrogen and helium , reionization usually refers strictly to the reionization of hydrogen , the element. It is believed that the primordial helium also experienced a similar reionization phase change, but at

178-463: A miscibility gap . Separation into multiple phases can occur via spinodal decomposition , in which a single phase is cooled and separates into two different compositions. Non-equilibrium mixtures can occur, such as in supersaturation . Other phase changes include: Phase transitions occur when the thermodynamic free energy of a system is non-analytic for some choice of thermodynamic variables (cf. phases ). This condition generally stems from

267-428: A phase transition (or phase change ) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of matter : solid , liquid , and gas , and in rare cases, plasma . A phase of a thermodynamic system and the states of matter have uniform physical properties . During a phase transition of a given medium, certain properties of

356-472: A class of active galactic nuclei (AGN), were considered a good candidate source because they are highly efficient at converting mass to energy , and emit a great deal of light above the threshold for ionizing hydrogen. It is unknown, however, how many quasars existed prior to reionization. Only the brightest of quasars present during reionization can be detected, which means there is no direct information about dimmer quasars that existed. However, by looking at

445-557: A compelling source. They are more efficient and effective ionizers than Population II stars, as they emit more ionizing photons, and are capable of reionizing hydrogen on their own in some reionization models with reasonable initial mass functions . As a consequence, Population III stars are currently considered the most likely energy source to initiate the reionization of the universe, though other sources are likely to have taken over and driven reionization to completion. In June 2015, astronomers reported evidence for Population III stars in

534-428: A discontinuous change in density, which is the (inverse of the) first derivative of the free energy with respect to pressure. Second-order phase transitions are continuous in the first derivative (the order parameter , which is the first derivative of the free energy with respect to the external field, is continuous across the transition) but exhibit discontinuity in a second derivative of the free energy. These include

623-415: A finite discontinuity of the heat capacity at the transition temperature, and the two-dimensional Ising model has a logarithmic divergence. However, these systems are limiting cases and an exception to the rule. Real phase transitions exhibit power-law behavior. Several other critical exponents, β , γ , δ , ν , and η , are defined, examining the power law behavior of a measurable physical quantity near

712-494: A first-order transition. That is, the transformation is completed over a finite range of temperatures, but phenomena like supercooling and superheating survive and hysteresis is observed on thermal cycling. Second-order phase transition s are also called "continuous phase transitions" . They are characterized by a divergent susceptibility, an infinite correlation length , and a power law decay of correlations near criticality . Examples of second-order phase transitions are

801-459: A later epoch in the history of the universe. This is usually called "helium reionization". The first phase change of hydrogen in the universe was recombination , which occurred at a redshift z  = 1089 (379,000 years after the Big Bang), 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

890-415: A liquid is called a eutectoid transformation. A peritectic transformation, in which a two-component single-phase solid is heated and transforms into a solid phase and a liquid phase. A peritectoid reaction is a peritectoid reaction, except involving only solid phases. A monotectic reaction consists of change from a liquid and to a combination of a solid and a second liquid, where the two liquids display

979-425: A majority of intergalactic gas was ionized prior to z=7. Lyman alpha emission can be used in other ways to further probe reionization. Theory suggests that reionization was patchy, meaning that the clustering of Lyman alpha selected samples should be strongly enhanced during the middle phases of reionization. Moreover, specific ionized regions can be pinpointed by identifying groups of Lyman alpha emitters. Even with

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1068-452: A model for displacive phase transformations . Order-disorder transitions such as in alpha- titanium aluminides . As with states of matter, there is also a metastable to equilibrium phase transformation for structural phase transitions. A metastable polymorph which forms rapidly due to lower surface energy will transform to an equilibrium phase given sufficient thermal input to overcome an energetic barrier. Phase transitions can also describe

1157-405: A phase transition system; it normally ranges between zero in one phase (usually above the critical point) and nonzero in the other. At the critical point, the order parameter susceptibility will usually diverge. An example of an order parameter is the net magnetization in a ferromagnetic system undergoing a phase transition. For liquid/gas transitions, the order parameter is the difference of

1246-416: A phase transition. The resulting state is metastable , i.e., less stable than the phase to which the transition would have occurred, but not unstable either. This occurs in superheating and supercooling , for example. Metastable states do not appear on usual phase diagrams. Phase transitions can also occur when a solid changes to a different structure without changing its chemical makeup. In elements, this

1335-425: 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 (closer in space and time) do not show

1424-564: A relatively sudden change at the glass transition temperature which enables accurate detection using differential scanning calorimetry measurements. Lev Landau gave a phenomenological theory of second-order phase transitions. Apart from isolated, simple phase transitions, there exist transition lines as well as multicritical points , when varying external parameters like the magnetic field or composition. Several transitions are known as infinite-order phase transitions . They are continuous but break no symmetries . The most famous example

1513-404: A special combination of pressure and temperature, known as the critical point , at which the transition between liquid and gas becomes a second-order transition. Near the critical point, the fluid is sufficiently hot and compressed that the distinction between the liquid and gaseous phases is almost non-existent. This is associated with the phenomenon of critical opalescence , a milky appearance of

1602-401: A substance transforms between one of the four states of matter to another. At the phase transition point for a substance, for instance the boiling point , the two phases involved - liquid and vapor , have identical free energies and therefore are equally likely to exist. Below the boiling point, the liquid is the more stable state of the two, whereas above the boiling point the gaseous form is

1691-482: A system near a phase transition depend only on a small number of features, such as dimensionality and symmetry, and are insensitive to the underlying microscopic properties of the system. Again, the divergence of the correlation length is the essential point. There are also other critical phenomena; e.g., besides static functions there is also critical dynamics . As a consequence, at a phase transition one may observe critical slowing down or speeding up . Connected to

1780-403: A temperature span where solid and liquid coexist in equilibrium. This is often the case in solid solutions , where the two components are isostructural. There are also a number of phase transitions involving three phases: a eutectic transformation, in which a two-component single-phase liquid is cooled and transforms into two solid phases. The same process, but beginning with a solid instead of

1869-476: A universe full of neutral hydrogen will be relatively opaque only at those few wavelengths. The remaining light could travel freely and become the cosmic microwave background radiation . The only other light at this point would be provided by those excited hydrogen atoms, marking the beginning of an era called the Dark Ages of the universe. The second phase change occurred once gas clouds started to condense in

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1958-408: A variety of first-order magnetic transitions. These include colossal-magnetoresistance manganite materials, magnetocaloric materials, magnetic shape memory materials, and other materials. The interesting feature of these observations of T g falling within the temperature range over which the transition occurs is that the first-order magnetic transition is influenced by magnetic field, just like

2047-467: Is an interesting possibility that the transition is arrested when it is partial and incomplete. Extending these ideas to first-order magnetic transitions being arrested at low temperatures, resulted in the observation of incomplete magnetic transitions, with two magnetic phases coexisting, down to the lowest temperature. First reported in the case of a ferromagnetic to anti-ferromagnetic transition, such persistent phase coexistence has now been reported across

2136-404: Is known as allotropy , whereas in compounds it is known as polymorphism . The change from one crystal structure to another, from a crystalline solid to an amorphous solid , or from one amorphous structure to another ( polyamorphs ) are all examples of solid to solid phase transitions. The martensitic transformation occurs as one of the many phase transformations in carbon steel and stands as

2225-434: Is not continuously provided to keep them apart. Altogether, the critical parameter for any source considered can be summarized as its "emission rate of hydrogen-ionizing photons per unit cosmological volume." With these constraints, it is expected that quasars and first generation stars and galaxies were the main sources of energy. Dwarf galaxies are currently considered to be the primary source of ionizing photons during

2314-407: Is perhaps the exponent describing the divergence of the thermal correlation length by approaching the transition. For instance, let us examine the behavior of the heat capacity near such a transition. We vary the temperature T of the system while keeping all the other thermodynamic variables fixed and find that the transition occurs at some critical temperature T c . When T is near T c ,

2403-484: Is provided by the Ising Model Phase transitions involving solutions and mixtures are more complicated than transitions involving a single compound. While chemically pure compounds exhibit a single temperature melting point between solid and liquid phases, mixtures can either have a single melting point, known as congruent melting , or they have different liquidus and solidus temperatures resulting in

2492-412: Is required, which corresponds to photons with a wavelength of 91.2 nm or shorter. This is in the ultraviolet part of the electromagnetic spectrum , which means that the primary candidates are all sources which produce a significant amount of energy in the ultraviolet and above. How numerous the source is must also be considered, as well as the longevity, as protons and electrons will recombine if energy

2581-496: Is the Kosterlitz–Thouless transition in the two-dimensional XY model . Many quantum phase transitions , e.g., in two-dimensional electron gases , belong to this class. The liquid–glass transition is observed in many polymers and other liquids that can be supercooled far below the melting point of the crystalline phase. This is atypical in several respects. It is not a transition between thermodynamic ground states: it

2670-449: Is the behavior of liquid helium at the lambda transition from a normal state to the superfluid state, for which experiments have found α = −0.013 ± 0.003. At least one experiment was performed in the zero-gravity conditions of an orbiting satellite to minimize pressure differences in the sample. This experimental value of α agrees with theoretical predictions based on variational perturbation theory . For 0 < α < 1,

2759-522: Is the n=2 to n=1 transition of neutral hydrogen, and can be produced copiously by galaxies with young stars. Moreover, Lyman alpha photons interact strongly with neutral hydrogen in intergalactic gas through resonant scattering, wherein neutral atoms in the ground (n=1) state absorb Lyman alpha photons and almost immediately re-emit them in a random direction. This obscures Lyman alpha emission from galaxies that are embedded in neutral gas. Thus, experiments to find galaxies by their Lyman alpha light can indicate

Reionization - Misplaced Pages Continue

2848-500: Is widely believed that the true ground state is always crystalline. Glass is a quenched disorder state, and its entropy, density, and so on, depend on the thermal history. Therefore, the glass transition is primarily a dynamic phenomenon: on cooling a liquid, internal degrees of freedom successively fall out of equilibrium. Some theoretical methods predict an underlying phase transition in the hypothetical limit of infinitely long relaxation times. No direct experimental evidence supports

2937-417: Is τ, the "optical depth to reionization," or alternatively, z re , the redshift of reionization, assuming it was an instantaneous event. While this is unlikely to be physical, since reionization was very likely not instantaneous, z re provides an estimate of the mean redshift of reionization. Lyman alpha light from galaxies offers a complementary tool set to study reionization.  The Lyman alpha line

3026-464: The Cosmos Redshift 7 galaxy at z = 6.60 . Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it. Phase transition In physics , chemistry , and other related fields like biology,

3115-429: The ferromagnetic transition, superconducting transition (for a Type-I superconductor the phase transition is second-order at zero external field and for a Type-II superconductor the phase transition is second-order for both normal-state–mixed-state and mixed-state–superconducting-state transitions) and the superfluid transition. In contrast to viscosity, thermal expansion and heat capacity of amorphous materials show

3204-504: The spectra of distant quasars . Quasars release an extraordinary amount of energy, being among the brightest objects in the universe. As a result, some quasars are detectable from as long ago 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 major differences between quasar spectra will be caused by

3293-544: The CMB anisotropies observed, and comparing with what they would look like had reionization not taken place, the electron column density at the time of reionization can be determined. With this, the age of the universe when reionization occurred can then be calculated. The Wilkinson Microwave Anisotropy Probe allowed that comparison to be made. The initial observations, released in 2003, suggested that reionization took place from 30 >  z  > 11. This redshift range

3382-522: The Ehrenfest classification scheme, there could in principle be third, fourth, and higher-order phase transitions. For example, the Gross–Witten–Wadia phase transition in 2-d lattice quantum chromodynamics is a third-order phase transition. The Curie points of many ferromagnetics is also a third-order transition, as shown by their specific heat having a sudden change in slope. In practice, only

3471-468: The Ehrenfest classification was the exact solution of the Ising model , discovered in 1944 by Lars Onsager . The exact specific heat differed from the earlier mean-field approximations, which had predicted that it has a simple discontinuity at critical temperature. Instead, the exact specific heat had a logarithmic divergence at the critical temperature. In the following decades, the Ehrenfest classification

3560-524: The Epoch of Reionization. With new observations from JWST , populations of LCEs are now being studied at cosmological redshifts greater than 6, allowing for the first time a detailed and direct assessment of the origins of cosmic Reionization. Combining these large samples of galaxies with new constraints on the UV luminosity function indicates that dwarf galaxies overwhelmingly contribute to Reionization. Quasars ,

3649-532: The GPs are considered excellent low-redshift analogs of high-redshift Lyman-alpha and LyC emitters (LAEs and LCEs, respectively). At that time, only two other LCEs were known: Haro 11 and Tololo-1247-232 . Finding local LyC emitters has thus become crucial to the theories about the early universe and the epoch of reionization. Subsequently, motivated, a series of surveys have been conducted using Hubble Space Telescope 's Cosmic Origins Spectrograph ( HST /COS) to measure

Reionization - Misplaced Pages Continue

3738-649: The Gunn-Peterson trough (though they may show the Lyman-alpha forest ), while quasars emitting light prior to reionization will feature a Gunn-Peterson trough. In 2001, four quasars were detected by the Sloan Digital Sky Survey 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

3827-634: The IGM Spin Temperature (MIST), the Dark Ages Radio Explorer (DARE) mission, and the Large-Aperture Experiment to Detect the Dark Ages (LEDA). While observations have come in which narrow the window during which the epoch of reionization could have taken place, it is still uncertain which objects provided the photons that reionized the IGM. To ionize neutral hydrogen, an energy larger than 13.6 eV

3916-595: The LyC directly. These efforts culminated in the Low-redshift Lyman Continuum Survey, a large HST /COS program which nearly tripled the number of direct measurements of the LyC from dwarf galaxies. To date, at least 50 LCEs have been confirmed using HST /COS with LyC escape fractions anywhere from ≈ 0 to 88%. The results from the Low-redshift Lyman Continuum Survey have provided the empirical foundation necessary to identify and understand LCEs at

4005-531: The Lyman-alpha and Lyman-beta forests suggest that reionization potentially extends later than z  = 6. The anisotropy of the cosmic microwave background on different angular scales can also 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, and scattering will occur less frequently. In

4094-480: The behavior of the thermodynamic free energy as a function of other thermodynamic variables. Under this scheme, phase transitions were labeled by the lowest derivative of the free energy that is discontinuous at the transition. First-order phase transitions exhibit a discontinuity in the first derivative of the free energy with respect to some thermodynamic variable. The various solid/liquid/gas transitions are classified as first-order transitions because they involve

4183-495: The change between different kinds of magnetic ordering . The most well-known is the transition between the ferromagnetic and paramagnetic phases of magnetic materials, which occurs at what is called the Curie point . Another example is the transition between differently ordered, commensurate or incommensurate , magnetic structures, such as in cerium antimonide . A simplified but highly useful model of magnetic phase transitions

4272-399: The chemical composition of the fluid. More impressively, but understandably from above, they are an exact match for the critical exponents of the ferromagnetic phase transition in uniaxial magnets. Such systems are said to be in the same universality class. Universality is a prediction of the renormalization group theory of phase transitions, which states that the thermodynamic properties of

4361-622: The densities. From a theoretical perspective, order parameters arise from symmetry breaking. When this happens, one needs to introduce one or more extra variables to describe the state of the system. For example, in the ferromagnetic phase, one must provide the net magnetization , whose direction was spontaneously chosen when the system cooled below the Curie point . However, note that order parameters can also be defined for non-symmetry-breaking transitions. Some phase transitions, such as superconducting and ferromagnetic, can have order parameters for more than one degree of freedom. In such phases,

4450-618: The development of order in the universe, as is illustrated by the work of Eric Chaisson and David Layzer . See also relational order theories and order and disorder . Continuous phase transitions are easier to study than first-order transitions due to the absence of latent heat , and they have been discovered to have many interesting properties. The phenomena associated with continuous phase transitions are called critical phenomena, due to their association with critical points. Continuous phase transitions can be characterized by parameters known as critical exponents . The most important one

4539-571: 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 which had been below the Lyman Alpha limit are stretched, and will in effect begin to fill in the Lyman absorption band. This means that instead of showing sharp spectral absorption lines,

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4628-464: The early universe that were energetic enough to re-ionize neutral hydrogen. As these objects formed and radiated energy, the universe reverted from being composed of neutral atoms, to once again being an ionized plasma . This occurred between 150 million and one billion years after the Big Bang (at a redshift 20 >  z  > 6). At that time, however, matter had been diffused by

4717-680: The early structures that formed. Observations from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) points to a signal from this era, although follow-up observations will be needed to confirm it. Several other projects hope to make headway in this area in the near future, such as the Precision Array for Probing the Epoch of Reionization (PAPER), Low Frequency Array (LOFAR), Murchison Widefield Array (MWA), Giant Metrewave Radio Telescope (GMRT), Mapper of

4806-675: The epoch of reionization. For most scenarios, this would require the log-slope of the UV galaxy luminosity function , often denoted α, to be steeper than it is today, approaching α = -2. With the advent of the James Webb Space Telescope (JWST), constraints on the UV luminosity function at the Epoch of Reionization have become commonplace, allowing for better constraints on the faint, low-mass population of galaxies. In 2014, two separate studies identified two Green Pea galaxies (GPs) to be likely Lyman Continuum (LyC)-emitting candidates. Compact dwarf star-forming galaxies like

4895-469: The equilibrium crystal phase. This happens if the cooling rate is faster than a critical cooling rate, and is attributed to the molecular motions becoming so slow that the molecules cannot rearrange into the crystal positions. This slowing down happens below a glass-formation temperature T g , which may depend on the applied pressure. If the first-order freezing transition occurs over a range of temperatures, and T g falls within this range, then there

4984-422: The existence of these transitions. A disorder-broadened first-order transition occurs over a finite range of temperatures where the fraction of the low-temperature equilibrium phase grows from zero to one (100%) as the temperature is lowered. This continuous variation of the coexisting fractions with temperature raised interesting possibilities. On cooling, some liquids vitrify into a glass rather than transform to

5073-472: The expansion of the universe, and 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

5162-403: The ferromagnetic phase transition in materials such as iron, where the magnetization , which is the first derivative of the free energy with respect to the applied magnetic field strength, increases continuously from zero as the temperature is lowered below the Curie temperature . The magnetic susceptibility , the second derivative of the free energy with the field, changes discontinuously. Under

5251-466: The first- and second-order phase transitions are typically observed. The second-order phase transition was for a while controversial, as it seems to require two sheets of the Gibbs free energy to osculate exactly, which is so unlikely as to never occur in practice. Cornelis Gorter replied the criticism by pointing out that the Gibbs free energy surface might have two sheets on one side, but only one sheet on

5340-491: The heat capacity C typically has a power law behavior: The heat capacity of amorphous materials has such a behaviour near the glass transition temperature where the universal critical exponent α = 0.59 A similar behavior, but with the exponent ν instead of α , applies for the correlation length. The exponent ν is positive. This is different with α . Its actual value depends on the type of phase transition we are considering. The critical exponents are not necessarily

5429-406: The heat capacity diverges at the transition temperature (though, since α < 1, the enthalpy stays finite). An example of such behavior is the 3D ferromagnetic phase transition. In the three-dimensional Ising model for uniaxial magnets, detailed theoretical studies have yielded the exponent α ≈ +0.110. Some model systems do not obey a power-law behavior. For example, mean field theory predicts

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5518-587: 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 even for low levels of neutral hydrogen in the intergalactic medium (IGM), absorption at those wavelengths is highly likely. For nearby objects in the universe, spectral absorption lines are very sharp, as only photons with energies just sufficient to cause an atomic transition can cause that transition. However,

5607-541: The interactions of a large number of particles in a system, and does not appear in systems that are small. Phase transitions can occur for non-thermodynamic systems, where temperature is not a parameter. Examples include: quantum phase transitions , dynamic phase transitions, and topological (structural) phase transitions. In these types of systems other parameters take the place of temperature. For instance, connection probability replaces temperature for percolating networks. Paul Ehrenfest classified phase transitions based on

5696-462: The ionization state of the surrounding gas.  An average density of galaxies with detectable Lyman alpha emission means the surrounding gas must be ionized; while an absence of detectable Lyman alpha sources may indicate neutral regions.  A closely related class of experiments measures the Lyman alpha line strength in samples of galaxies identified by other methods (primarily Lyman break galaxy searches). The earliest application of this method

5785-402: The ionizing background is dominated by low-luminosity AGNs can the quasar luminosity function provide enough ionizing photons." Population III stars were the earliest stars, which had no elements more massive than hydrogen or helium . During Big Bang nucleosynthesis , the only elements that formed aside from hydrogen and helium were trace amounts of lithium . Yet quasar spectra have revealed

5874-406: The lattice points of the crystal lattice). Typically, the high-temperature phase contains more symmetries than the low-temperature phase due to spontaneous symmetry breaking , with the exception of certain accidental symmetries (e.g. the formation of heavy virtual particles , which only occurs at low temperatures). An order parameter is a measure of the degree of order across the boundaries in

5963-419: The liquid due to density fluctuations at all possible wavelengths (including those of visible light). Phase transitions often involve a symmetry breaking process. For instance, the cooling of a fluid into a crystalline solid breaks continuous translation symmetry : each point in the fluid has the same properties, but each point in a crystal does not have the same properties (unless the points are chosen from

6052-401: The magnetic fields and temperature differences from the critical value. Phase transitions play many important roles in biological systems. Examples include the lipid bilayer formation, the coil-globule transition in the process of protein folding and DNA melting , liquid crystal-like transitions in the process of DNA condensation , and cooperative ligand binding to DNA and proteins with

6141-413: The medium change as a result of the change of external conditions, such as temperature or pressure . This can be a discontinuous change; for example, a liquid may become gas upon heating to its boiling point , resulting in an abrupt change in volume. The identification of the external conditions at which a transformation occurs defines the phase transition point. Phase transitions commonly refer to when

6230-415: The more easily observed quasars in the nearby universe, and assuming that the luminosity function (number of quasars as a function of luminosity ) during reionization will be approximately the same as it is today, it is possible to make estimates of the quasar populations at earlier times. Such studies have found that quasars do not exist in high enough numbers to reionize the IGM alone, saying that "only if

6319-414: The more stable. Common transitions between the solid, liquid, and gaseous phases of a single component, due to the effects of temperature and/or pressure are identified in the following table: For a single component, the most stable phase at different temperatures and pressures can be shown on a phase diagram . Such a diagram usually depicts states in equilibrium. A phase transition usually occurs when

6408-409: The order parameter may take the form of a complex number, a vector, or even a tensor, the magnitude of which goes to zero at the phase transition. There also exist dual descriptions of phase transitions in terms of disorder parameters. These indicate the presence of line-like excitations such as vortex - or defect lines. Symmetry-breaking phase transitions play an important role in cosmology . As

6497-399: The other side, creating a forked appearance. ( pp. 146--150) The Ehrenfest classification implicitly allows for continuous phase transformations, where the bonding character of a material changes, but there is no discontinuity in any free energy derivative. An example of this occurs at the supercritical liquid–gas boundaries . The first example of a phase transition which did not fit into

6586-569: 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, introducing secondary anisotropies (anisotropies introduced after recombination). The overall effect is to erase anisotropies that occur on smaller scales. While anisotropies on small scales are erased, polarization anisotropies are actually introduced because of reionization. By looking at

6675-434: The phase transition. Exponents are related by scaling relations, such as It can be shown that there are only two independent exponents, e.g. ν and η . It is a remarkable fact that phase transitions arising in different systems often possess the same set of critical exponents. This phenomenon is known as universality . For example, the critical exponents at the liquid–gas critical point have been found to be independent of

6764-530: The presence of heavy elements in the intergalactic medium at an early era. Supernova explosions produce such heavy elements, so hot, large, Population III stars which will form supernovae are a possible mechanism for reionization. While they have not been directly observed, they are consistent according to models using numerical simulation and current observations. A gravitationally lensed galaxy also provides indirect evidence of Population III stars. Even without direct observations of Population III stars, they are

6853-409: The pressure or temperature changes and the system crosses from one region to another, like water turning from liquid to solid as soon as the temperature drops below the freezing point . In exception to the usual case, it is sometimes possible to change the state of a system diabatically (as opposed to adiabatically ) in such a way that it can be brought past a phase transition point without undergoing

6942-425: The previous phenomenon is also the phenomenon of enhanced fluctuations before the phase transition, as a consequence of lower degree of stability of the initial phase of the system. The large static universality classes of a continuous phase transition split into smaller dynamic universality classes. In addition to the critical exponents, there are also universal relations for certain static or dynamic functions of

7031-450: The quasar data roughly in agreement with the CMB anisotropy data, there are still a number of questions, especially concerning the energy sources of reionization and the effects on, and role of, structure formation during reionization. The 21-cm line in hydrogen is potentially a means of studying this period, as well as the "dark ages" that preceded reionization. The 21-cm line occurs in neutral hydrogen, due to differences in energy between

7120-423: The re ionization rate. The universe was opaque before the recombination, due to the scattering of photons of all wavelengths off free electrons (and free protons, to a significantly lesser extent), but it became increasingly 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 ,

7209-430: The same above and below the critical temperature. When a continuous symmetry is explicitly broken down to a discrete symmetry by irrelevant (in the renormalization group sense) anisotropies, then some exponents (such as γ {\displaystyle \gamma } , the exponent of the susceptibility) are not identical. For −1 < α < 0, the heat capacity has a "kink" at the transition temperature. This

7298-499: The spin triplet and spin singlet states of the electron and proton. This transition is forbidden , meaning it occurs extremely rarely. The transition is also highly temperature dependent, meaning that as objects form in the "dark ages" and emit Lyman-alpha photons that are absorbed and re-emitted by surrounding neutral hydrogen, it will produce a 21-cm line signal in that hydrogen through Wouthuysen-Field coupling . By studying 21-cm line emission, it will be possible to learn more about

7387-454: The structural transition is influenced by pressure. The relative ease with which magnetic fields can be controlled, in contrast to pressure, raises the possibility that one can study the interplay between T g and T c in an exhaustive way. Phase coexistence across first-order magnetic transitions will then enable the resolution of outstanding issues in understanding glasses. In any system containing liquid and gaseous phases, there exists

7476-455: The temperature of the system will stay constant as heat is added: the system is in a "mixed-phase regime" in which some parts of the system have completed the transition and others have not. Familiar examples are the melting of ice or the boiling of water (the water does not instantly turn into vapor , but forms a turbulent mixture of liquid water and vapor bubbles). Yoseph Imry and Michael Wortis showed that quenched disorder can broaden

7565-624: The universe expanded and cooled, the vacuum underwent a series of symmetry-breaking phase transitions. For example, the electroweak transition broke the SU(2)×U(1) symmetry of the electroweak field into the U(1) symmetry of the present-day electromagnetic field . This transition is important to explain the asymmetry between the amount of matter and antimatter in the present-day universe, according to electroweak baryogenesis theory. Progressive phase transitions in an expanding universe are implicated in

7654-458: Was in 2004, when the tension between late neutral gas indicated by quasar spectra and early reionization suggested by CMB results was strong.  The detection of Lyman alpha galaxies at redshift z=6.5 demonstrated that the intergalactic gas was already predominantly ionized at an earlier time than the quasar spectra suggested.  Subsequent applications of the method suggested some residual neutral gas as recently as z=6.5, but still indicate that

7743-429: Was in clear disagreement with the results from studying quasar spectra. However, the three year WMAP data returned a different result, with reionization beginning at z  = 11 and the universe ionized by z  = 7. This is in much better agreement with the quasar data. Results in 2018 from Planck mission, yield an instantaneous reionization redshift of z = 7.68 ± 0.79. The parameter usually quoted here

7832-451: Was replaced by a simplified classification scheme that is able to incorporate such transitions. In the modern classification scheme, phase transitions are divided into two broad categories, named similarly to the Ehrenfest classes: First-order phase transitions are those that involve a latent heat . During such a transition, a system either absorbs or releases a fixed (and typically large) amount of energy per volume. During this process,

7921-462: Was still at least partly neutral, the ones below did not, meaning the hydrogen was ionized. 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. On the other hand, long absorption troughs persisting down to z < 5.5 in

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