Chronology of the universe
The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. The earliest stages of the universe's existence are estimated as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level. For the purposes of this summary, it is convenient to divide the chronology of the universe since it originated, into five parts, it is considered meaningless or unclear whether time existed before this chronology: Earliest stages of chronology shown below are an active area of research and based on ideas which are still speculative and subject to modification as scientific knowledge improves. The "time" column is based on extrapolation of observed metric expansion of space back in the past. For the earliest stages of chronology this extrapolation may be invalid. To give one example, eternal inflation theories propose that inflation lasts forever throughout most of the universe, making the notion of "N seconds since Big Bang" ill-defined.
The radiation temperature refers to the cosmic background radiation and is given by 2.725·, where z is the redshift. Times shorter than 10−43 seconds The Planck epoch is an era in traditional Big Bang cosmology after the event which began our known universe. During this epoch, the temperature and average energies within the universe were so high that everyday subatomic particles could not form, the four fundamental forces that shape our universe—electromagnetism, weak nuclear interaction, strong nuclear interaction—were combined and formed one fundamental force. Little is understood about physics at this temperature. Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory relies on the theory of general relativity, thought to break down for this epoch due to quantum effects. In inflationary models of cosmology, times before the end of inflation do not follow the same timeline as in traditional big bang cosmology. Models that aim to describe the universe and physics during the Planck epoch are speculative and fall under the umbrella of "New Physics".
Examples include the Hartle–Hawking initial state, string landscape, string gas cosmology, the ekpyrotic universe. Between 10−43 seconds and 10−36 seconds after the Big Bang As the universe expanded and cooled, it crossed transition temperatures at which forces separated from each other; these phase transitions can be visualised as similar to condensation and freezing phase transitions of ordinary matter. At certain temperatures/energies, water molecules change their behaviour and structure, they will behave differently. Like steam turning to water, the fields which define our universe's fundamental forces and particles completely change their behaviors and structures when the temperature/energy falls below a certain point; this is not apparent in everyday life, because it only happens at far higher temperatures than we see in our present universe. These phase transitions are believed to be caused by a phenomenon of quantum fields called "symmetry breaking". In everyday terms, as the universe cools, it becomes possible for the quantum fields that create the forces and particles around us, to settle at lower energy levels and with higher levels of stability.
In doing so, they shift how they interact. Forces and interactions arise due to these fields, so the universe can behave differently above and below a phase transition. For example, in a epoch, a side effect of one phase transition is that many particles that had no mass at all acquire a mass, a single force begins to manifest as two separate forces; the grand unification epoch began with a phase transitions of this kind, when gravitation separated from the universal combined gauge force. This caused two forces to now exist: gravity, an electrostrong interaction. There is no hard evidence yet, that such a combined force existed, but many physicists believe it did; the physics of this electrostrong interaction would be described by a so-called grand unified theory. The grand unification epoch ended with a second phase transition, as the electrostrong interaction in turn separated, began to manifest as two separate interactions, called the strong and electroweak interactions. Between 10−36 seconds and 10−32 seconds after the Big Bang Depending on how epochs are defined, the model being followed, the electroweak epoch may be considered to start before or after the inflationary epoch.
In some models it is described as including the inflationary epoch. In other models, the electroweak epoch is said to begin after the inflationary epoch ended, at 10−32 seconds. According to traditional big bang cosmology, the electroweak epoch began 10−36 seconds after the Big Bang, when the temperature of the universe was low enough for the Electronuclear Force to begin to manifest as two separate interactions, called the strong and the electroweak interactions.. The exact point where electrostrong symmetry was broken is not certain, because of the high energies of this event. Before c. 10−32 seconds after the Big BangAt this point, the early universe and rapidly expanded to at least 1078 times its previous volume. This is equivalent to a linear increase of at least 1026 times in every spatial dimension – equivalent to an object 1 nanometer (10−9 m, about half the width of a mole
In cosmology, the cosmological constant is the energy density of space, or vacuum energy, that arises in Albert Einstein's field equations of general relativity. It is associated to the concepts of dark energy and quintessence. Einstein introduced the concept in 1917 to counterbalance the effects of gravity and achieve a static universe, a notion, the accepted view at the time. Einstein abandoned the concept in 1931 after Hubble's discovery of the expanding universe. From the 1930s until the late 1990s, most physicists assumed the cosmological constant to be equal to zero; that changed with the surprising discovery in 1998 that the expansion of the universe is accelerating, implying the possibility of a positive nonzero value for the cosmological constant. Since the 1990s, studies have shown that around 68% of the mass–energy density of the universe can be attributed to so-called dark energy; the cosmological constant Λ is the simplest possible explanation for dark energy, is used in the current standard model of cosmology known as the ΛCDM model.
While dark energy is poorly understood at a fundamental level, the main required properties of dark energy are that it functions as a type of anti-gravity, it dilutes much more than matter as the universe expands, it clusters much more weakly than matter, or not at all. According to quantum field theory which underlies modern particle physics, empty space is defined by the vacuum state, a collection of quantum fields. All these quantum fields exhibit fluctuations in their ground state arising from the zero-point energy present everywhere in space; these zero-point fluctuations should act as a contribution to the cosmological constant Λ, but when calculations are performed these fluctuations give rise to an enormous vacuum energy. The discrepancy between theorized vacuum energy from QFT and observed vacuum energy from cosmology is a source of major contention, with the values predicted exceeding observation by some 120 orders of magnitude, a discrepancy, called "the worst theoretical prediction in the history of physics!".
This issue is called the cosmological constant problem and it is one of the greatest unsolved mysteries in science with many physicists believing that "the vacuum holds the key to a full understanding of nature". Einstein included the cosmological constant as a term in his field equations for general relativity because he was dissatisfied that otherwise his equations did not allow for a static universe: gravity would cause a universe, at dynamic equilibrium to contract. To counteract this possibility, Einstein added the cosmological constant. However, soon after Einstein developed his static theory, observations by Edwin Hubble indicated that the universe appears to be expanding. Einstein referred to his failure to accept the validation of his equations—when they had predicted the expansion of the universe in theory, before it was demonstrated in observation of the cosmological red shift—as his "biggest blunder". In fact, adding the cosmological constant to Einstein's equations does not lead to a static universe at equilibrium because the equilibrium is unstable: if the universe expands then the expansion releases vacuum energy, which causes yet more expansion.
A universe that contracts will continue contracting. However, the cosmological constant remained a subject of empirical interest. Empirically, the onslaught of cosmological data in the past decades suggests that our universe has a positive cosmological constant; the explanation of this small but positive value is an outstanding theoretical challenge, the so-called cosmological constant problem. Some early generalizations of Einstein's gravitational theory, known as classical unified field theories, either introduced a cosmological constant on theoretical grounds or found that it arose from the mathematics. For example, Sir Arthur Stanley Eddington claimed that the cosmological constant version of the vacuum field equation expressed the "epistemological" property that the universe is "self-gauging", Erwin Schrödinger's pure-affine theory using a simple variational principle produced the field equation with a cosmological term; the cosmological constant Λ appears in Einstein's field equation in the form R μ ν − 1 2 R g μ ν + Λ g μ ν = 8 π G c 4 T μ ν, where the Ricci tensor/scalar R and the metric tensor g describe the structure of spacetime, the stress-energy tensor T describes the energy and momentum density and flux of the matter in that point in spacetime, the universal constants G and c are conversion factors that arise from using traditional units of measurement.
When Λ is zero, this reduces to the field equation of general relativity used in the mid-20th century. When T is zero, the field equation describes empty space; the cosmological constant has the same effect as an intrinsic energy density of ρvac. In this context, it is moved onto the right-hand side of the equation, defined with a pr
General relativity is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. General relativity generalizes special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present; the relation is specified by the Einstein field equations, a system of partial differential equations. Some predictions of general relativity differ from those of classical physics concerning the passage of time, the geometry of space, the motion of bodies in free fall, the propagation of light. Examples of such differences include gravitational time dilation, gravitational lensing, the gravitational redshift of light, the gravitational time delay; the predictions of general relativity in relation to classical physics have been confirmed in all observations and experiments to date.
Although general relativity is not the only relativistic theory of gravity, it is the simplest theory, consistent with experimental data. However, unanswered questions remain, the most fundamental being how general relativity can be reconciled with the laws of quantum physics to produce a complete and self-consistent theory of quantum gravity. Einstein's theory has important astrophysical implications. For example, it implies the existence of black holes—regions of space in which space and time are distorted in such a way that nothing, not light, can escape—as an end-state for massive stars. There is ample evidence that the intense radiation emitted by certain kinds of astronomical objects is due to black holes; the bending of light by gravity can lead to the phenomenon of gravitational lensing, in which multiple images of the same distant astronomical object are visible in the sky. General relativity predicts the existence of gravitational waves, which have since been observed directly by the physics collaboration LIGO.
In addition, general relativity is the basis of current cosmological models of a expanding universe. Acknowledged as a theory of extraordinary beauty, general relativity has been described as the most beautiful of all existing physical theories. Soon after publishing the special theory of relativity in 1905, Einstein started thinking about how to incorporate gravity into his new relativistic framework. In 1907, beginning with a simple thought experiment involving an observer in free fall, he embarked on what would be an eight-year search for a relativistic theory of gravity. After numerous detours and false starts, his work culminated in the presentation to the Prussian Academy of Science in November 1915 of what are now known as the Einstein field equations; these equations specify how the geometry of space and time is influenced by whatever matter and radiation are present, form the core of Einstein's general theory of relativity. The Einstein field equations are nonlinear and difficult to solve.
Einstein used approximation methods in working out initial predictions of the theory. But as early as 1916, the astrophysicist Karl Schwarzschild found the first non-trivial exact solution to the Einstein field equations, the Schwarzschild metric; this solution laid the groundwork for the description of the final stages of gravitational collapse, the objects known today as black holes. In the same year, the first steps towards generalizing Schwarzschild's solution to electrically charged objects were taken, which resulted in the Reissner–Nordström solution, now associated with electrically charged black holes. In 1917, Einstein applied his theory to the universe as a whole, initiating the field of relativistic cosmology. In line with contemporary thinking, he assumed a static universe, adding a new parameter to his original field equations—the cosmological constant—to match that observational presumption. By 1929, the work of Hubble and others had shown that our universe is expanding; this is described by the expanding cosmological solutions found by Friedmann in 1922, which do not require a cosmological constant.
Lemaître used these solutions to formulate the earliest version of the Big Bang models, in which our universe has evolved from an hot and dense earlier state. Einstein declared the cosmological constant the biggest blunder of his life. During that period, general relativity remained something of a curiosity among physical theories, it was superior to Newtonian gravity, being consistent with special relativity and accounting for several effects unexplained by the Newtonian theory. Einstein himself had shown in 1915 how his theory explained the anomalous perihelion advance of the planet Mercury without any arbitrary parameters. A 1919 expedition led by Eddington confirmed general relativity's prediction for the deflection of starlight by the Sun during the total solar eclipse of May 29, 1919, making Einstein famous, yet the theory entered the mainstream of theoretical physics and astrophysics only with the developments between 1960 and 1975, now known as the golden age of general relativity. Physicists began to understand the concept of a black hole, to identify quasars as one of these objects' astrophysical manifestations.
More precise solar system tests confirmed the theory's predictive power, relativistic cosmology, became amenable to direct observational tests. Over the years, general relativity has acqui
In physics and mathematics, the Lorentz group is the group of all Lorentz transformations of Minkowski spacetime, the classical and quantum setting for all physical phenomena. The Lorentz group is named for the Dutch physicist Hendrik Lorentz. For example, the following laws and theories respect Lorentz symmetry: The kinematical laws of special relativity Maxwell's field equations in the theory of electromagnetism The Dirac equation in the theory of the electron The Standard model of particle physicsThe Lorentz group expresses the fundamental symmetry of space and time of all known fundamental laws of nature. In general relativity physics, in cases involving small enough regions of spacetime where gravitational variances are negligible, physical laws are Lorentz invariant in the same manner as that of special relativity physics; the Lorentz group is a subgroup of the Poincaré group—the group of all isometries of Minkowski spacetime. Lorentz transformations are isometries that leave the origin fixed.
Thus, the Lorentz group is an isotropy subgroup of the isometry group of Minkowski spacetime. For this reason, the Lorentz group is sometimes called the homogeneous Lorentz group while the Poincaré group is sometimes called the inhomogeneous Lorentz group. Lorentz transformations are examples of linear transformations. Mathematically, the Lorentz group may be described as the generalized orthogonal group O, the matrix Lie group that preserves the quadratic form ↦ t 2 − x 2 − y 2 − z 2 on R4; this quadratic form is, when put on matrix form, interpreted in physics as the metric tensor of Minkowski spacetime. The Lorentz group is a six-dimensional noncompact non-abelian real Lie group, not connected; the four connected components are not connected, but rather doubly connected. The identity component of the Lorentz group is itself a group, is called the restricted Lorentz group, is denoted SO+; the restricted Lorentz group consists of those Lorentz transformations that preserve the orientation of space and direction of time.
The restricted Lorentz group has been presented through a facility of biquaternion algebra. The restricted Lorentz group arises in other ways in pure mathematics. For example, it arises as the point symmetry group of a certain ordinary differential equation; this fact has physical significance. Because it is a Lie group, the Lorentz group O is both a group and admits a topological description as a smooth manifold; as a manifold, it has four connected components. Intuitively, this means; the four connected components can be categorized by two transformation properties its elements have: some elements are reversed under time-inverting Lorentz transformations, for example, a future-pointing timelike vector would be inverted to a past-pointing vector some elements have orientation reversed by improper Lorentz transformations, for example, certain vierbein Lorentz transformations that preserve the direction of time are called orthochronous. The subgroup of orthochronous transformations is denoted O+.
Those that preserve orientation are called proper, as linear transformations they have determinant +1. The subgroup of proper Lorentz transformations is denoted SO; the subgroup of all Lorentz transformations preserving both orientation and direction of time is called the proper, orthochronous Lorentz group or restricted Lorentz group, is denoted by SO+. The set of the four connected components can be given a group structure as the quotient group O/SO+, isomorphic to the Klein four-group; every element in O can be written as the semidirect product of a proper, orthochronous transformation and an element of the discrete group where P and T are the space inversion and time reversal operators: P = diag T = diag. Thus an arbitrary Lorentz transformation can be specified as a proper, orthochronous Lorentz transformation along with a further two bits of information, which pick out one of the four connected components; this pattern is typical of finite-dimensional Lie groups. The restricted Lorentz group is the identity component of the Lorentz group, which means that it consists of all Lorentz transformations that can be connected to the identity by a continuous curve lying in the group.
The restricted Lorentz group is a connected normal subgroup of the full Lorentz group with the same dimension, in this case with dimension six. The restricted Lorentz group is generated by ordinary spatial rotations and Lorentz boosts. Since every proper, orthochronous Lorentz transformation can be written as a product of a rotation and a boost, it takes 6 real parameters to specify an arbitrary proper orthochronous Lorentz transformation; this is one way. The set of all rotations forms a Lie subgroup isomorphic to the ordinary rotation group SO; the set of all boosts, does not form a sub
Carlo Rovelli is an Italian theoretical physicist and writer who has worked in Italy, the United States and since 2000, in France. His work is in the field of quantum gravity, where he is among the founders of the loop quantum gravity theory, he has worked in the history and philosophy of science. He collaborates with several Italian newspapers, in particular the cultural supplements of the Corriere della Sera, Il Sole 24 Ore and La Repubblica, his popular science book Seven Brief Lessons on Physics has been translated in 41 languages and has sold over a million copies worldwide. In 2019 he has been included by the Foreign Policy magazine in the list of the 100 most influential global thinkers. Carlo Rovelli was born in Verona, Italy, in 1956, he attended the Liceo Classico Scipione Maffei in Verona. In the 1970s, he participated in the student political movements in Italian universities, he was involved with the free political radio stations Radio Alice in Bologna and Radio Anguana in Verona, which he helped found.
In conjunction with his political activity, he was charged, but released, for crimes of opinion related to the book Fatti Nostri, which he co-authored with Enrico Palandri, Maurizio Torrealta, Claudio Piersanti. In 1981, Rovelli graduated with a BS/MS in Physics from the University of Bologna, in 1986 he obtained his PhD at the University of Padova, Italy. Rovelli refused military service, compulsory in Italy at the time, was therefore detained in 1987, he held postdoctoral positions at the University of Rome, at Yale University. Rovelli was on the faculty of the University of Pittsburgh from 1990 to 2000, he works in the Centre de Physique Théorique de Luminy of Aix-Marseille University. He has held the post of Affiliated Professor in the Department of History and Philosophy of Science of the University of Pittsburgh. In 1988, Carlo Rovelli, Lee Smolin, Abhay Ashtekar introduced a theory of quantum gravity called loop quantum gravity. In 1995, Rovelli and Smolin obtained a basis of states of quantum gravity, labelled by Penrose's spin networks, using this basis they were able to show that the theory predicts that area and volume are quantized.
This result indicates the existence of a discrete structure of space at small scale. In 1997, Rovelli and Michael Reisenberger introduced a "sum over surfaces" formulation of theory, which has since evolved into the covariant "spinfoam" version of loop quantum gravity. In 2008, in collaboration with Jonathan Engle and Roberto Pereira, he has introduced the spin foam vertex amplitude, the basis of the current definition of the loop quantum gravity covariant dynamics; the loop theory is today considered a candidate for a quantum theory of gravity. It finds applications in quantum cosmology, spinfoam cosmology, quantum black hole physics. In his 2004 book Quantum Gravity, Rovelli developed a formulation of classical and quantum mechanics that does not make explicit reference to the notion of time; the timeless formalism is used to describe the world in the regimes where the quantum properties of the gravitational field cannot be disregarded. This is because the quantum fluctuation of spacetime itself makes the notion of time unsuitable for writing physical laws in the conventional form of evolution laws in time.
This position led him to face the following problem: if time is not part of the fundamental theory of the world how does time emerge? In 1993, in collaboration with Alain Connes, Rovelli proposed a solution to this problem called the thermal time hypothesis. According to this hypothesis, time emerges only in a statistical context. If this is correct, the flow of time is an illusion, one deriving from the incompleteness of knowledge. In 1994, Rovelli introduced the relational interpretation of quantum mechanics, based on the idea that the quantum state of a system must always be interpreted relative to another physical system; the idea has been analyzed in particular by Bas van Fraassen and by Michel Bitbol. Among other important consequences, it provides a solution of the EPR paradox that does not violate locality. Rovelli has written a book on the Greek philosopher Anaximander, published in France, Italy, US and Brazil; the book analyses the main aspects of scientific thinking and articulates Rovelli's views on science.
Anaximander is presented in the book as a main initiator of scientific thinking. For Rovelli, science is a continuous process of exploring novel possible views of the world; the foundation of science, therefore, is not certainty but the opposite, a radical uncertainty about our own knowledge, or equivalently, an acute awareness of the extent of our ignorance. Rovelli discusses his religious views in his book on Anaximander, he argues that the conflict between rational/scientific thinking and structured religion may find periods of truce, but it is unsolvable because religions demand the acceptance of some unquestionable truths while scientific thinking is based on the continuous questioning of any truth. Thus, for Rovelli the source of the conflict is not the pretense of science to give answers—the universe, for Rovelli, is full of mystery and a source of awe and emotions—but, on the contrary, the source of the conflict is the acceptance of our ignorance at the foundation of science, which clashes with religions' pretense
A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and a mass less than that of a neutron. Protons and neutrons, each with masses of one atomic mass unit, are collectively referred to as "nucleons". One or more protons are present in the nucleus of every atom; the number of protons in the nucleus is the defining property of an element, is referred to as the atomic number. Since each element has a unique number of protons, each element has its own unique atomic number; the word proton is Greek for "first", this name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the hydrogen nucleus could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore a candidate to be a fundamental particle, hence a building block of nitrogen and all other heavier atomic nuclei. In the modern Standard Model of particle physics, protons are hadrons, like neutrons, the other nucleon, are composed of three quarks.
Although protons were considered fundamental or elementary particles, they are now known to be composed of three valence quarks: two up quarks of charge +2/3e and one down quark of charge –1/3e. The rest masses of quarks contribute only about 1% of a proton's mass, however; the remainder of a proton's mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. Because protons are not fundamental particles, they possess a physical size, though not a definite one. At sufficiently low temperatures, free protons will bind to electrons. However, the character of such bound protons does not change, they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the electron cloud of an atom; the result is a protonated atom, a chemical compound of hydrogen. In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom, chemically a free radical.
Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules, which are the most common molecular component of molecular clouds in interstellar space. Protons are composed of three valence quarks, making them baryons; the two up quarks and one down quark of a proton are held together by the strong force, mediated by gluons. A modern perspective has a proton composed of the valence quarks, the gluons, transitory pairs of sea quarks. Protons have a positive charge distribution which decays exponentially, with a mean square radius of about 0.8 fm. Protons and neutrons are both nucleons, which may be bound together by the nuclear force to form atomic nuclei; the nucleus of the most common isotope of the hydrogen atom is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.
The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815, William Prout proposed that all atoms are composed of hydrogen atoms, based on a simplistic interpretation of early values of atomic weights, disproved when more accurate values were measured. In 1886, Eugen Goldstein discovered canal rays and showed that they were positively charged particles produced from gases. However, since particles from different gases had different values of charge-to-mass ratio, they could not be identified with a single particle, unlike the negative electrons discovered by J. J. Thomson. Wilhelm Wien in 1898 identified the hydrogen ion as particle with highest charge-to-mass ratio in ionized gases. Following the discovery of the atomic nucleus by Ernest Rutherford in 1911, Antonius van den Broek proposed that the place of each element in the periodic table is equal to its nuclear charge; this was confirmed experimentally by Henry Moseley in 1913 using X-ray spectra.
In 1917, Rutherford proved that the hydrogen nucleus is present in other nuclei, a result described as the discovery of protons. Rutherford had earlier learned to produce hydrogen nuclei as a type of radiation produced as a product of the impact of alpha particles on nitrogen gas, recognize them by their unique penetration signature in air and their appearance in scintillation detectors; these experiments were begun when Rutherford had noticed that, when alpha particles were shot into air, his scintillation detectors showed the signatures of typical hydrogen nuclei as a product. After experimentation Rutherford traced the reaction to the nitrogen in air, found that when alphas were produced into pure nitrogen gas, the effect was larger. Rutherford determined that this hydrogen could have come only from the nitrogen, therefore nitrogen must contain hydrogen nuclei. One hydrogen nucleus was being knocked off by the impact of the alpha particle, producing oxygen-17 in the process; this was 14N + α → 17O + p.
(This reaction wo
A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—not particles and electromagnetic radiation such as light—can escape from inside it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole; the boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways, a black hole acts like an ideal black body. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass; this temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it impossible to observe. Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace.
The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Black holes were long considered a mathematical curiosity; the discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality. Black holes of stellar mass are expected to form when massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus. Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light.
Matter that falls onto a black hole can form an external accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbits can be used to determine the black hole's mass and location; such observations can be used to exclude possible alternatives such as neutron stars. In this way, astronomers have identified numerous stellar black hole candidates in binary systems, established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses. On 11 February 2016, the LIGO collaboration announced the first direct detection of gravitational waves, which represented the first observation of a black hole merger; as of December 2018, eleven gravitational wave events have been observed that originated from ten merging black holes. On 10 April 2019, the first direct image of a black hole and its vicinity was published, following observations made by the Event Horizon Telescope in 2017 of the supermassive black hole in Messier 87's galactic centre.
Larry Kimura, a Hawaiian language professor at the University of Hawaii at Hilo, named the hole Pōwehi—a Hawaiian phrase referring to an "embellished dark source of unending creation." The idea of a body so massive that light could not escape was proposed by astronomical pioneer and English clergyman John Michell in a letter published in November 1784. Michell's simplistic calculations assumed that such a body might have the same density as the Sun, concluded that such a body would form when a star's diameter exceeds the Sun's by a factor of 500, the surface escape velocity exceeds the usual speed of light. Michell noted that such supermassive but non-radiating bodies might be detectable through their gravitational effects on nearby visible bodies. Scholars of the time were excited by the proposal that giant but invisible stars might be hiding in plain view, but enthusiasm dampened when the wavelike nature of light became apparent in the early nineteenth century. If light were a wave rather than a "corpuscle", it became unclear what, if any, influence gravity would have on escaping light waves.
Modern relativity discredits Michell's notion of a light ray shooting directly from the surface of a supermassive star, being slowed down by the star's gravity and free-falling back to the star's surface. In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. Only a few months Karl Schwarzschild found a solution to the Einstein field equations, which describes the gravitational field of a point mass and a spherical mass. A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties; this solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates, although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was a non-physical coordinate singularity.
Arthur Eddington did however comment on the possibility of a star with mass c