- Anti de Sitter space or AdSn - n-dimensional: It is a maximally symmetric Lorentzian manifold with constant negative scalar curvature. It is the Lorentzian analogue of n-dimensional hyperbolic space, just as Minkowski space and de Sitter space are the analogues of Euclidean and elliptical spaces respectively. In the language of general relativity, anti de Sitter space is a maximally symmetric, vacuum solution of Einstein's field equation with an attractive cosmological constant ? (corresponding to a negative vacuum energy density and positive pressure). In mathematics, anti de Sitter space is sometimes defined more generally as a space of arbitrary signature (p,q). Generally in physics only the case of one timelike dimension is relevant.
- Aristarchean universe: The earth rotates daily on its axis and revolves annually about the sun in a circular orbit. Sphere of fixed stars is centred about the sun.
- Aristotelian universe: In this model the earth is spherical and surrounded by concentric celestial spheres. The universe exists unchanged throughout eternity. It contains a 5th element called aether (later known as quintessence).
- Big Bang: It is the cosmological model of the universe based on the assumption that the universe has expanded into its current state from a primordial condition of enormous density and temperature. The term is also used to describe the fundamental "fireball" that erupted at or close to an initial timepoint in the history of our observed space-time.
- Big Bounce: The Big Bounce is a theorized scientific model related to the creation of the known Universe. It derives from the oscillatory universe or cyclic repetition interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe.
- Big Crunch: The Big Crunch theory is a symmetric view of the ultimate fate of the universe. Just as the Big Bang started a cosmological expansion, this theory postulates that the average density of the universe is enough to stop its expansion and begin contracting. The end result is unknown; a simple extrapolation would have all the matter and space-time in the universe collapse into a dimensionless singularity, but at these scales unknown quantum effects need to be considered. This scenario allows the Big Bang to have been immediately preceded by the Big Crunch of a preceding universe. If this occurs repeatedly, we have an oscillatory universe. The universe could then consist of an infinite sequence of finite universes, each finite universe ending with a Big Crunch that is also the Big Bang of the next universe.
- Big Freeze: The Big Freeze is a scenario under which continued expansion results in a universe that is too cold to sustain life. It could, in the absence of dark energy, occur only under a flat or hyperbolic geometry, because such geometries then are a necessary condition for a universe that expands forever. With a positive cosmological constant, it could also occur in a closed universe.
- Big Rip: Finite Lifespan: In the special case of phantom dark energy, which has even more negative pressure than a simple cosmological constant, the density of dark energy increases with time, causing the rate of acceleration to increase, leading to a steady increase in the Hubble constant. As a result, all material objects in the universe, starting with galaxies and eventually (in a finite time) all life forms, no matter how small, will disintegrate into unbound elementary particles and radiation, ripped apart by the phantom energy force and shooting apart from each other. The end state of the universe is a singularity, as the dark energy density and expansion rate becomes infinite.
- Black hole thermodynamics: It is the area of study that seeks to reconcile the laws of thermodynamics with the existence of black hole event horizons. Much as the study of the statistical mechanics of black body radiation led to the advent of the theory of quantum mechanics, the effort to understand the statistical mechanics of black holes has had a deep impact upon the understanding of quantum gravity, leading to the formulation of the holographic principle.
- Bubble theory: It is based on an infinite number of open multiverses, each with different physical constants. This Bubble universe theory fits well with the widely accepted theory of cosmic inflation. The bubble universe concept involves creation of universes from the quantum foam of a "parent universe." On very small scales, energy fluctuations may create tiny bubbles and wormholes; a tiny bubble universe may form, experience some expansion like an inflating balloon, and then contract and disappear from existence. However, if the energy fluctuation is greater than a particular critical value, a tiny bubble universe forms from the parent universe, experiences long-term expansion, and allows matter and large-scale galactic structures to form.
- Chandrasekhar limit: It is the maximum nonrotating mass which can be
supported against gravitational collapse by electron degeneracy pressure.
It is commonly given as being about 1.4 solar masses. Computed values for
the limit will vary depending on the approximations used, the nuclear composition
of the mass, and the temperature. Chandrasekhar gives a value of
?e is the average molecular weight per electron, mH is the mass of the hydrogen
atom, and is a constant connected with the solution to the Lane-Emden equation.
Numerically, this value is approximately (2/?e)2 · 2.85 ·
1030 kg, or , where is the standard solar mass. As is the Planck mass, ,
the limit is of the order of MPl3/mH2.
As white dwarf stars are supported by electron degeneracy pressure, this is an upper limit for the mass of a white dwarf. Main-sequence stars with a mass exceeding approximately 8 solar masses therefore cannot lose enough mass to form a stable white dwarf at the end of their lives, and instead form either a neutron star or black hole.
- Chaotic inflation theory or bubble universe: It is a variant model of the inflationary model of the big bang. This model, proposed by physicist Andrei Linde, postulates that our universe is one of many that grew from a multiverse consisting of vacuum that had not decayed to its ground state.
- Closed universe: If the density parameter ? > 1, then the geometry of space is closed like the surface of a sphere. The sum of the angles of a triangle exceeds 180 degrees and there are no parallel lines; all lines eventually meet. The geometry of the universe is, at least on a very large scale, elliptic. In a closed universe lacking the repulsive effect of dark energy, gravity eventually stops the expansion of the universe, after which it starts to contract until all matter in the universe collapses to a point, a final singularity termed the "Big Crunch," by analogy with Big Bang. However, if the universe has a large amount of dark energy (as suggested by recent findings), then the expansion of the universe can continue forever - even if ? > 1.
- Cosmology: It is the quantitative (usually mathematical) study of the Universe in its totality, and by extension, humanity's place in it. Though the word cosmology is recent (first used in 1730 in Christian Wolff's Cosmologia Generalis), study of the Universe has a long history involving science, philosophy, esotericism, and religion.
- Curvature of space: The definition of curvature is closely connected with non-Euclidean geometry. The theory of general relativity, which describes gravity and cosmology, deals with the "curvature of space-time"; in relativity theory space-time is a pseudo-Riemannian manifold. Once a time coordinate is defined, the three-dimensional space corresponding to a particular time is generally a curved Riemannian manifold; but since the time coordinate choice is largely arbitrary, it is the underlying space-time curvature that is physically significant. Although an arbitrarily-curved space is very complex to describe, the curvature of a space which is locally isotropic and homogeneous is described by a single Gaussian curvature, as for a surface. A positive curvature corresponds to the inverse square radius of curvature; an example is a sphere or hypersphere. An example of negatively curved space is hyperbolic geometry. A space or space-time without curvature (formally, with zero curvature) is called flat. For example, Euclidean space is an example of a flat space, and Minkowski space is an example of a flat space-time.
- Cyclic model: It refers to several cosmological models in which the universe
follows infinite, self-sustaining cycles (for example: an eternity of Big
Bang-Big crunches. In the 1930s, theoretical physicists, most notably Einstein,
considered the possibility of a cyclic model for the universe as an (everlasting)
alternative to the Big Bang. However, work by Richard C. Tolman showed that
these early attempts failed because of the entropy problem that, in statistical
mechanics, entropy only increases because of the Second law of thermodynamics.
This implies that successive cycles grow longer and larger. Extrapolating
back in time, cycles before the present one become shorter and smaller culminating
again in a Big Bang and thus not replacing it. Recently discovered dark
energy component provided new hope for a consistent cyclic cosmology. Examples
of cyclic universes:
. One new cyclic model is a brane cosmology model of the creation of the
universe. The theory describes a universe exploding into existence not just
once, but repeatedly over time.
. The Steinhardt-Turok model where two parallel orbifold planes or M-branes
collide periodically in a higher dimensional space. The visible four-dimensional
universe lies on one of these branes. The collisions correspond to a reversal
from contraction to expansion, or a big crunch followed immediately by a
big bang.
. The Baum-Frampton model makes a different technical assumption concerning
the equation of state of the dark energy which relates pressure and density
through a parameter w. It assumes w < -1 throughout a cycle, including
at present. A trillion-trillionth (or less) of a second before the would-be
Big Rip a turnaround occurs and only one causal patch is retained as our
universe.
- Dark energy: In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Assuming the existence of dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for almost three-quarters of the total mass-energy of the universe. Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. The cosmological constant is thought to arise from the vacuum energy. Scalar fields which do change in space are hard to distinguish from a cosmological constant, because the change may be extremely slow.
- Density parameter: An important parameter in fate of the universe theory is the Density parameter, Omega (?), defined as the average matter density of the universe divided by a critical value of that density. This selects one of three possible geometries depending on whether ? is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes. Hence cosmologists aimed to determine the fate of the universe by measuring ?, or equivalently the rate at which the expansion was decelerating.
- de Sitter universe (or de Sitter expansion): It is a solution to Einstein's field equations of General. It models the universe as spatially flat and neglects ordinary matter, so the dynamics of the universe are dominated by the cosmological constant, thought to correspond to dark energy.
- Einstein ring: It is the deformation of the light from a source (such as a galaxy or star) into a ring through gravitational deflection of the source's light by a lens (such as another galaxy, or a black hole). This occurs when the source, lens and observer are all aligned.
- Ekpyrotic universe, or ekpyrotic scenario: It is a cosmological model about the origin and shape of the universe. The ekpyrotic model of the universe is an alternative to the standard cosmic inflation paradigm, both of which accept that the standard big bang Lambda-CDM model of our universe is an appropriate description up to very early times. The ekpyrotic model is a precursor to, and part of the cyclic model.
- Event horizon: In general relativity, event horizon is a general term for a boundary in space-time, an area surrounding the black hole, beyond which events cannot affect an outside observer. Light emitted from inside the horizon can never reach the observer and anything that passes through the horizon from the observer's side is never seen again.
- Expansion of space (metric): It is a key part of science's current understanding of the universe, whereby space-time itself is described by a metric which changes over time in such a way that the spatial dimensions grow or stretch as the universe gets older. It explains how the universe expands in the Big Bang model, a feature of our universe supported by all cosmological experiments, astrophysics calculations, and measurements to date. The expansion of space is conceptually different from other kinds of expansions that are seen in nature. Our understanding of the "fabric of the universe" (space-time) requires that what we see normally as "space", "time", and "distance" are not absolutes, but are determined by a metric that can change. In the metric expansion of space, rather than objects in a fixed "space" moving apart into "emptiness", it is the space that contains the objects which is itself changing. It is as if without objects themselves moving, space is somehow "growing" in between them.
- Flatness problem: It is a cosmological fine-tuning problem within the Big Bang model. Along with the monopole problem and the horizon problem, it is one of the three primary motivations for the theory of an inflationary universe. The flatness problem arises because of the observation that the density of the universe today is very close to the critical density required for spatial flatness. Since the total energy density of the universe departs rapidly from the critical value over cosmic time the early universe must have had a density even closer to the critical density, leading cosmologists to question how the density of the early universe came to be fine-tuned to this 'special' value.
- Flat universe: If the average density of the universe exactly equals the critical density so that ?=1, then the geometry of the universe is flat: as in Euclidean geometry, the sum of the angles of a triangle is 180 degrees and parallel lines never meet. Absent dark energy, a flat universe expands forever but at a continually decelerating rate, with expansion asymptotically approaching a fixed rate. With dark energy, the expansion rate of the universe initially slows down, due to the effect of gravity, but eventually increases. The ultimate fate of the universe is the same as an open universe. In such a universe all of the local curvature and local geometry is flat. It is generally assumed that it is described by a Euclidean space, however there are some spatial geometries which are flat and bounded in one or more directions. The alternative two-dimensional spaces with a Euclidean metric are the cylinder and the Möbius strip, which are bounded in one direction but not the other, and the torus and Klein bottle, which are compact.
- Friedmann equations (or Friedmann expansion): They are a set of equations in cosmology that govern the expansion of space in homogeneous and isotropic models of the universe within the context of general relativity. They were first derived by Alexander Friedmann in 1922 from Einstein's field equations of gravitation for the Friedmann-Lemaître-Robertson-Walker metric and a fluid with a given mass density ? and pressure p. The equations for negative spatial curvature were given by Friedmann in 1924.
- Geocentric model: In astronomy it is the disproved theory that the Earth is at the centre of the universe and the Sun and other objects go around it. Two common observations were believed to support the idea that the Earth is in the centre of the Universe. The first is that the stars (including the Sun and planets) appear to revolve around the Earth each day, with the stars circling around the pole and those stars nearer the equator rising and setting each day and circling back to their rising point. The second is the common sense perception that the Earth is solid and stable; it is not moving but is at rest. The geocentric model remained accepted into the early modern age; from the late 16th century onward it was gradually replaced by the heliocentric model of Copernicus, Galileo and Kepler.
- Gibbons-Hawking effect: It is the statement that a temperature can be
associated to each solution of the Einstein field equations that contains
a causal horizon. The term "causal horizon" does not necessarily
refer to event horizons only, but could also stand for the horizon of the
visible universe. For example, Schwarzschild spacetime contains an event
horizon and so can be associated a temperature. In the case of Schwarzschild
spacetime this is the temperature T of a black hole of mass M, satisfying
. A second example is de Sitter space which contains a particle horizon.
In this case the temperature T is proportional to the Hubble parameter H,
i.e. .
- Heliocentrism: In astronomy it is the theory that the sun is at the centre
of the Solar System. Historically, heliocentrism is opposed to geocentrism
which places the earth at the centre. Although many early cosmologies such
as Aristotle speculated about the motion of the Earth around a stationary
Sun, it was not until the 16th century that Copernicus presented a fully
predictive mathematical model of a heliocentric system, which was later
elaborated by Kepler and defended by Galileo, becoming the centre of a major
religious dispute.
- Hyperbolic universe: This universe is described by hyperbolic geometry, and can be thought of locally as a three-dimensional analogue of an infinitely extended saddle shape. There are a great variety of hyperboli 3-manifolds, and their classification is not completely understood. Hyperbolic universes are frequently but confusingly called "open". A flat universe can also be open (as manifold): standard Euclidean space is a flat open universe. The ultimate fate of an open universe is that it will continue to expand forever, ending in a Heat Death, a Big Freeze or a Big Rip.
- Heat Death: Heat Death states that the universe goes to a state of maximum entropy in which everything is evenly distributed, and there are no gradients - which are needed to sustain information processing, one form of which is life. The Heat Death scenario is compatible with any of the three spatial models, but requires that the universe reach an eventual temperature minimum.
- Inflation (of the universe or cosmic inflation): it is the idea that the nascent universe passed through a phase of exponential expansion that was driven by a negative-pressure vacuum energy density. As a direct consequence, all of the observable universe originated in a small causally-connected region. Inflation answers the classic conundrum of the big bang cosmology: why does the universe appear flat, homogeneous and isotropic in accordance with the cosmological principle when one would expect, on the basis of the physics of the big bang, a highly curved, inhomogeneous universe? Inflation also explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the universe.
- Isotropy of the universe: Isotropy is the quality of a property which does not depend on the direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. The Big Bang theory of the evolution of the observable universe assumes that space is isotropic. It also assumes that space is homogeneous. These two assumptions together are known as the Cosmological Principle. As of now, the observations suggest that, on distance scales much larger than galaxies, galaxy clusters are "Great" features, but small compared to so-called multi-verse scenarios.
- Many-worlds interpretation or MWI: It is an interpretation of quantum mechanics. Many-worlds deny the objective reality of wavefunction collapse. Many-worlds then explain the subjective appearance of wavefunction collapse with the mechanism of quantum decoherence. Consequently, many-worlds claims this resolves all the "paradoxes" of quantum theory since every possible outcome to every event defines or exists in its own "history" or "world". In layman's terms, this means that there are an infinite number of universes and that everything that could possibly happen in our universe (but doesn't) does happen in another.
- Metric: A metric defines how a distance can be measured between two nearby points in space, in terms of the coordinates of those points. A coordinate system locates points in a space (of whatever number of dimensions) by assigning unique numbers known as coordinates, to each point. The metric is then a formula which converts coordinates of two points into distances.
- Multiverse: The multiverse hypothesis states that our universe is but one universe among infinite parallel universes, possibly with different physical laws. Whatever the ultimate fate of our universe may be, almost all parallel universes will have different fates. And while many universes may be closed, many others may be open. The multiverse as a whole may never end completely.
- Non-standard cosmology: It is any physical cosmological model of the universe proposed as an alternative to the big bang model of (standard) physical cosmology. Various scientists and researchers have disputed parts or all of the big bang due to a rejection or addition of fundamental assumptions needed to develop a theoretical model of the universe. From the 1940s to the 1960s, the astrophysical community was equally divided between supporters of the big bang theory and supporters of a rival steady state universe. It was not until advances in observational cosmology that the big bang would eventually become the dominant theory.
- Oscillatory universe: It is a cosmological model, originally derived by Alexander Friedman in 1922, investigated briefly by Einstein in 1930 and critiqued by Richard Tolman in 1934, in which the universe undergoes a series of oscillations, each beginning with a big bang and ending with a big crunch. After the big bang, the universe expands for a while before the gravitational attraction of matter causes it to collapse back in and undergo a bounce.
- Open universe: If the density parameter ?<1, the geometry of space is open, i.e., negatively curved like the surface of a saddle. The angles of a triangle sum to less than 180 degrees, and lines that do not meet are never equidistant; they have a point of least distance and otherwise grow apart. The geometry of the universe is hyperbolic. Even without dark energy, a negatively curved universe expands forever, with gravity barely slowing the rate of expansion. With dark energy, the expansion not only continues but accelerates. The ultimate fate of an open universe is either universal heat death, the "Big Freeze", or the "Big Rip," where the acceleration caused by dark energy eventually becomes so strong that it completely overwhelms the effects of the gravitational, electromagnetic and weak binding forces. Conversely, a negative cosmological constant, which would correspond to a negative energy density and positive pressure, would cause even an open universe to recollapse to a big crunch. This option is ruled out by observations, unless the universe undergoes an unexpected phase transition at some point in the future.
- Picard horn (also called the Picard topology or Picard model): It is a theoretical model for the shape of the Universe. It is a horn topology, meaning it has hyperbolic geometry.
- Poincaré homology sphere: This sphere (also known as Poincaré dodecahedral space) is a particular example of a homology sphere. It is the only homology 3-sphere (besides the 3-sphere itself) with a finite fundamental group. A simple construction of this space, which makes clear the term "dodecahedral space", begins with a dodecahedron. Each face of the dodecahedron can be identified with its opposite face by using the minimal clockwise twist to line up the faces. Glue each pair of opposite faces together using this identification. After this gluing, the result is a closed 3-manifold.The Poincaré homology sphere is a spherical 3-manifold. In 2003, an apparent periodicity in the cosmic microwave background led to the suggestion, by Jean-Pierre Luminet of the Observatoire de Paris and colleagues, that the shape of the Universe is a Poincaré sphere.
- Ptolemaic model: The universe orbits about a stationary Earth. The planets move in circular epicycles, each having a centre that moved in a larger circular orbit (called an eccentric or a deferent) around a centre-point near the Earth. The use of equants added another level of complexity and allowed astronomers to predict the positions of the planets. The most successful universe model of all time, using the criterion of longevity.
- Ptolemy model of the universe: Ptolemy wrote Almagest, the only surviving comprehensive ancient treatise on astronomy. He presented his astronomical models in tables, which could be used to compute the future or past position of the planets. The Almages also contains a star catalogue, which is an updated version of a catalogue created by Hipparchus. Its list of forty-eight constellations, but unlike the modern system it only cover the sky he could see. Ptolemy's model, like those of his predecessors, was geocentric and was almost universally accepted until Nicolaeus Copernicus introduced his heliocentric geometrical model. He saw the universe as a set of nested spheres in which he used the epicycles of his planetary model to compute the dimensions of the universe. He estimated the Sun was at an average distance of 1210 Earth radii while the radius of the sphere of the fixed stars was 20,000 times the radius of the Earth. Ptolemy presented a useful tool for astronomical calculations in his Handy Tables which tabulated all the data needed to compute the positions of the Sun, Moon and planets, the rising and setting of the stars, and eclipses of the Sun and Moon. In the Phaseis (Risings of the Fixed Stars) Ptolemy gave a star calendar or almanac based on the appearances and disappearances of stars over the course of the solar year.
- Quintessence: In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. Quintessence is a scalar field which has an equation of state (relating its pressure pq and density ?q) of pq = w?q, where w is less than -1/3. Quintessence is dynamic, and generally has a density and equation of state that varies through time and space. By contrast, a cosmological constant is static, with a fixed energy density and w = ?1.
- Repulsive force: Observations of supernovae in distant galaxies have been interpreted as consistent with a universe whose rate of expansion is accelerating. Subsequent cosmological theorizing has been designed so as to allow for this possible acceleration, nearly always by involving dark energy, which in its simplest form is just a positive cosmological constant. In general dark energy is a catch-all term for any hypothesised field with negative pressure, usually with a density that changes as the universe expands.
- Schwarzschild black hole or static black hole: It is a black hole that has no charge or angular momentum. A Schwarzschild black hole has a Schwarzschild metric, and cannot be distinguished from any other Schwarzschild black hole except by its mass.
- Singularity: Gravitational singularity, a point in space-time in which gravitational forces cause matter to have an infinite density and zero volume.
- Space-time: In physics it is any mathematical model that combines space and time into a single construct called the space-time continuum. Space-time is usually interpreted with space being three-dimensional and time playing the role of the fourth dimension. According to Euclidean space perception, the universe has three dimensions of space, and one dimension of time. By combining space and time, physicists have significantly simplified a large amount of physical theory, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels. In classical mechanics, the use of space-time over Euclidean space is optional, as time is independent of mechanical motion in three dimensions. In relativistic contexts, however, time cannot be separated from the three dimensions of space as it depends on an object's velocity relative to the speed of light.
- Spherical universe: It is a positively curved universe described by spherical geometry, and can be thought of as a three-dimensional hypersphere, or some other spherical 3-manifold. In a closed universe lacking the repulsive effect of dark energy, gravity eventually stops the expansion of the universe, after which it starts to contract until all matter in the universe collapses to a point, a final singularity termed the "Big Crunch," by analogy with Big Bang. However, if the universe has a large amount of dark energy (as suggested by recent findings), then the expansion of the universe can continue forever.
Based on analyses of the WMAP data, cosmologists during 2004-2006 focused on the Poincaré dodecahedral space (PDS), but also considered horn topologies (which are hyperbolic) to be compatible with the data.
- Standard cosmology: The twentieth century advances allowed scientists to establish the Big Bang as the leading cosmological theory, which most cosmologists now accept as the basis for their theories and observations. Physical cosmology, roughly speaking, deals with the very largest objects in the universe (galaxies, clusters and superclusters), the very earliest distinct objects to form (quasars) and the very early universe, when it was nearly homogeneous (hot big bang, cosmic inflation and the cosmic microwave background radiation).
- Time or grandfather paradox: It is a paradox of time travel. The paradox is this: suppose a man travelled back in time and killed his biological grandfather before the latter met the traveller's grandmother. As a result, one of the traveller's parents (and by extension, the traveller himself) would never have been conceived. This would imply that he could not have travelled back in time after all, which in turn implies the grandfather would still be alive, and the traveller would have been conceived, allowing him to travel back in time and kill his grandfather. Thus each possibility seems to imply its own negation, a type of logical paradox. This paradox has been used to argue that backwards time travel must be impossible. However, a number of possible ways of avoiding the paradox have been proposed, such as the idea that the timeline is fixed and unchangeable, or the idea that the time traveller will end up in a parallel timeline, while the timeline in which the traveller was born continues to exist.
- Tolman-Oppenheimer-Volkoff (TOV) limit: It is an upper bound to the mass of stars composed of neutron-degenerate matter (neutron stars). It is analogous to the Chandrasekhar limit for white dwarf stars. The limit leads to a limiting mass of approximately 0.7 solar masses. Modern estimates range from approximately 1.5 to 3.0 solar masses. The uncertainty in the value reflects the fact that the equations of state for extremely dense matter are not well-known. In a neutron star lighter than the limit, the weight of the star is supported by short-range repulsive neutron-neutron interactions mediated by the strong force and also by the quantum degeneracy pressure of neutrons. If a neutron star is heavier than the limit, it will collapse to some denser form. It could form a black hole, or change composition and be supported in some other way (for example, by quark degeneracy pressure if it becomes a quark star). A black hole formed by the collapse of an individual star must have mass exceeding the Tolman-Oppenheimer-Volkoff limit. Theory predicts that because of mass loss during stellar evolution, a black hole formed from an isolated star of solar metallicity can have mass no more than approximately 10 solar masses. A number of massive objects in X-ray binaries are thought to be stellar black holes. These black hole candidates are estimated to have masses between 3 and 20 solar masses.
- Uniform space: It is defined as a set with a uniform structure. Uniform spaces are topological spaces with additional structure which is used to define uniform properties such as completeness, uniform continuity and uniform convergence. The conceptual difference between uniform and topological structures is that in a uniform space, you can formalize certain notions of relative closeness and closeness of points. In other words, ideas like "x is closer to a than y is to b" make sense in uniform spaces.
- Zodiac: The term denotes an annual cycle of twelve stations along the ecliptic, the apparent path of the sun across the heavens through the constellations that divide the ecliptic into twelve equal zones of celestial longitude. The zodiac is the first known celestial coordinate system. There are two apparently independently created zodiacs.
Sign English name Element
Aries The Ram Fire
Taurus The Bull Earth
Gemini The Twins Air
Cancer The Crab Water
Leo The Lion Fire
Virgo The Virgin Earth
Libra The Scales Air
Scorpio The Scorpion Water
Sagittarius The Archer Fire
Capricorn The Sea-goat Earth
Aquarius The Water Carrier Air
Pisces The two fish Water