Quantum mechanics, including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles. Classical physics, the physics existing before quantum mechanics, describes nature at ordinary scale. Most theories in classical physics can be derived from quantum mechanics as an approximation valid at large scale. Quantum mechanics differs from classical physics in that energy, angular momentum and other quantities of a bound system are restricted to discrete values. Quantum mechanics arose from theories to explain observations which could not be reconciled with classical physics, such as Max Planck's solution in 1900 to the black-body radiation problem, from the correspondence between energy and frequency in Albert Einstein's 1905 paper which explained the photoelectric effect. Early quantum theory was profoundly re-conceived in the mid-1920s by Erwin Schrödinger, Werner Heisenberg, Max Born and others; the modern theory is formulated in various specially developed mathematical formalisms.
In one of them, a mathematical function, the wave function, provides information about the probability amplitude of position and other physical properties of a particle. Important applications of quantum theory include quantum chemistry, quantum optics, quantum computing, superconducting magnets, light-emitting diodes, the laser, the transistor and semiconductors such as the microprocessor and research imaging such as magnetic resonance imaging and electron microscopy. Explanations for many biological and physical phenomena are rooted in the nature of the chemical bond, most notably the macro-molecule DNA. Scientific inquiry into the wave nature of light began in the 17th and 18th centuries, when scientists such as Robert Hooke, Christiaan Huygens and Leonhard Euler proposed a wave theory of light based on experimental observations. In 1803, Thomas Young, an English polymath, performed the famous double-slit experiment that he described in a paper titled On the nature of light and colours.
This experiment played a major role in the general acceptance of the wave theory of light. In 1838, Michael Faraday discovered cathode rays; these studies were followed by the 1859 statement of the black-body radiation problem by Gustav Kirchhoff, the 1877 suggestion by Ludwig Boltzmann that the energy states of a physical system can be discrete, the 1900 quantum hypothesis of Max Planck. Planck's hypothesis that energy is radiated and absorbed in discrete "quanta" matched the observed patterns of black-body radiation. In 1896, Wilhelm Wien empirically determined a distribution law of black-body radiation, known as Wien's law in his honor. Ludwig Boltzmann independently arrived at this result by considerations of Maxwell's equations. However, it underestimated the radiance at low frequencies. Planck corrected this model using Boltzmann's statistical interpretation of thermodynamics and proposed what is now called Planck's law, which led to the development of quantum mechanics. Following Max Planck's solution in 1900 to the black-body radiation problem, Albert Einstein offered a quantum-based theory to explain the photoelectric effect.
Around 1900–1910, the atomic theory and the corpuscular theory of light first came to be accepted as scientific fact. Among the first to study quantum phenomena in nature were Arthur Compton, C. V. Raman, Pieter Zeeman, each of whom has a quantum effect named after him. Robert Andrews Millikan studied the photoelectric effect experimentally, Albert Einstein developed a theory for it. At the same time, Ernest Rutherford experimentally discovered the nuclear model of the atom, for which Niels Bohr developed his theory of the atomic structure, confirmed by the experiments of Henry Moseley. In 1913, Peter Debye extended Niels Bohr's theory of atomic structure, introducing elliptical orbits, a concept introduced by Arnold Sommerfeld; this phase is known as old quantum theory. According to Planck, each energy element is proportional to its frequency: E = h ν, where h is Planck's constant. Planck cautiously insisted that this was an aspect of the processes of absorption and emission of radiation and had nothing to do with the physical reality of the radiation itself.
In fact, he considered his quantum hypothesis a mathematical trick to get the right answer rather than a sizable discovery. However, in 1905 Albert Einstein interpreted Planck's quantum hypothesis realistically and used it to explain the photoelectric effect, in which shining light on certain materials can eject electrons from the material, he won the 1921 Nobel Prize in Physics for this work. Einstein further developed this idea to show that an electromagnetic wave such as light could be described as a particle, with a discrete quantum of energy, dependent on its frequency; the foundations of quantum mechanics were established during the first half of the 20th century by Max Planck, Niels Bohr, Werner Heisenberg, Louis de Broglie, Arthur Compton, Albert Einstein, Erwin Schrödinger, Max Born, John von Neumann, Paul Dirac, Enrico Fermi, Wolfgang Pauli, Max von Laue, Freeman Dyson, David Hilbert, Wi
Stephen William Hawking was an English theoretical physicist and author, director of research at the Centre for Theoretical Cosmology at the University of Cambridge at the time of his death. He was the Lucasian Professor of Mathematics at the University of Cambridge between 1979 and 2009, his scientific works included a collaboration with Roger Penrose on gravitational singularity theorems in the framework of general relativity and the theoretical prediction that black holes emit radiation called Hawking radiation. Hawking was the first to set out a theory of cosmology explained by a union of the general theory of relativity and quantum mechanics, he was a vigorous supporter of the many-worlds interpretation of quantum mechanics. Hawking achieved commercial success with several works of popular science in which he discusses his own theories and cosmology in general, his book A Brief History of Time appeared on the British Sunday Times best-seller list for a record-breaking 237 weeks. Hawking was a Fellow of the Royal Society, a lifetime member of the Pontifical Academy of Sciences, a recipient of the Presidential Medal of Freedom, the highest civilian award in the United States.
In 2002, Hawking was ranked number 25 in the BBC's poll of the 100 Greatest Britons. In 1963, Hawking was diagnosed with an early-onset slow-progressing form of motor neurone disease that paralysed him over the decades. After the loss of his speech, he was still able to communicate through a speech-generating device through use of a hand-held switch, by using a single cheek muscle, he died on 14 March 2018 after living with the disease for more than 50 years. Hawking was born on 8 January 1942 in Oxford to Isobel Eileen Hawking. Hawking's mother was born into a family of doctors in Scotland, his wealthy paternal great-grandfather, from Yorkshire, over-extended himself buying farm land and went bankrupt in the great agricultural depression during the early 20th century. His paternal great-grandmother saved the family from financial ruin by opening a school in their home. Despite their families' financial constraints, both parents attended the University of Oxford, where Frank read medicine and Isobel read Philosophy and Economics.
Isobel worked as a secretary for a medical research institute, Frank was a medical researcher. Hawking had two younger sisters and Mary, an adopted brother, Edward Frank David. In 1950, when Hawking's father became head of the division of parasitology at the National Institute for Medical Research, the family moved to St Albans, Hertfordshire. In St Albans, the family was considered intelligent and somewhat eccentric, they lived a frugal existence in a large and poorly maintained house and travelled in a converted London taxicab. During one of Hawking's father's frequent absences working in Africa, the rest of the family spent four months in Majorca visiting his mother's friend Beryl and her husband, the poet Robert Graves. Hawking began his schooling at the Byron House School in London, he blamed its "progressive methods" for his failure to learn to read while at the school. In St Albans, the eight-year-old Hawking attended St Albans High School for Girls for a few months. At that time, younger boys could attend one of the houses.
Hawking attended two independent schools, first Radlett School and from September 1952, St Albans School, after passing the eleven-plus a year early. The family placed a high value on education. Hawking's father wanted his son to attend the well-regarded Westminster School, but the 13-year-old Hawking was ill on the day of the scholarship examination, his family could not afford the school fees without the financial aid of a scholarship, so Hawking remained at St Albans. A positive consequence was that Hawking remained close to a group of friends with whom he enjoyed board games, the manufacture of fireworks, model aeroplanes and boats, long discussions about Christianity and extrasensory perception. From 1958 on, with the help of the mathematics teacher Dikran Tahta, they built a computer from clock parts, an old telephone switchboard and other recycled components. Although known at school as "Einstein", Hawking was not successful academically. With time, he began to show considerable aptitude for scientific subjects and, inspired by Tahta, decided to read mathematics at university.
Hawking's father advised him to study medicine, concerned that there were few jobs for mathematics graduates. He wanted his son to attend University College, his own alma mater; as it was not possible to read mathematics there at the time, Hawking decided to study physics and chemistry. Despite his headmaster's advice to wait until the next year, Hawking was awarded a scholarship after taking the examinations in March 1959. Hawking began his university education at University College, Oxford, in October 1959 at the age of 17. For the first 18 months, he was bored and lonely – he found the academic work "ridiculously easy", his physics tutor, Robert Berman said, "It was only necessary for him to know that something could be done, he could do it without looking to see how other people did it." A change occurred during his second and third year when, according to Berman, Hawking made more of an effort "to be one of the boys". He developed into a popular and witty college member, interested in classical music and science fiction.
Part of the transformation resulted from his decision to join the college boat club, the Univ
Quantum field theory
In theoretical physics, quantum field theory is a theoretical framework that combines classical field theory, special relativity, quantum mechanics and is used to construct physical models of subatomic particles and quasiparticles. QFT treats particles as excited states of their underlying fields, which are—in a sense—more fundamental than the basic particles. Interactions between particles are described by interaction terms in the Lagrangian involving their corresponding fields; each interaction can be visually represented by Feynman diagrams, which are formal computational tools, in the process of relativistic perturbation theory. As a successful theoretical framework today, quantum field theory emerged from the work of generations of theoretical physicists spanning much of the 20th century, its development began in the 1920s with the description of interactions between light and electrons, culminating in the first quantum field theory — quantum electrodynamics. A major theoretical obstacle soon followed with the appearance and persistence of various infinities in perturbative calculations, a problem only resolved in the 1950s with the invention of the renormalization procedure.
A second major barrier came with QFT's apparent inability to describe the weak and strong interactions, to the point where some theorists called for the abandonment of the field theoretic approach. The development of gauge theory and the completion of the Standard Model in the 1970s led to a renaissance of quantum field theory. Quantum field theory is the result of the combination of classical field theory, quantum mechanics, special relativity. A brief overview of these theoretical precursors is in order; the earliest successful classical field theory is one that emerged from Newton's law of universal gravitation, despite the complete absence of the concept of fields from his 1687 treatise Philosophiæ Naturalis Principia Mathematica. The force of gravity as described by Newton is an "action at a distance" — its effects on faraway objects are instantaneous, no matter the distance. In an exchange of letters with Richard Bentley, Newton stated that "it is inconceivable that inanimate brute matter should, without the mediation of something else, not material, operate upon and affect other matter without mutual contact."
It was not until the 18th century that mathematical physicists discovered a convenient description of gravity based on fields — a numerical quantity assigned to every point in space indicating the action of gravity on any particle at that point. However, this was considered a mathematical trick. Fields began to take on an existence of their own with the development of electromagnetism in the 19th century. Michael Faraday coined the English term "field" in 1845, he introduced fields as properties of space having physical effects. He argued against "action at a distance", proposed that interactions between objects occur via space-filling "lines of force"; this description of fields remains to this day. The theory of classical electromagnetism was completed in 1862 with Maxwell's equations, which described the relationship between the electric field, the magnetic field, electric current, electric charge. Maxwell's equations implied the existence of electromagnetic waves, a phenomenon whereby electric and magnetic fields propagate from one spatial point to another at a finite speed, which turns out to be the speed of light.
Action-at-a-distance was thus conclusively refuted. Despite the enormous success of classical electromagnetism, it was unable to account for the discrete lines in atomic spectra, nor for the distribution of blackbody radiation in different wavelengths. Max Planck's study of blackbody radiation marked the beginning of quantum mechanics, he treated atoms, which absorb and emit electromagnetic radiation, as tiny oscillators with the crucial property that their energies can only take on a series of discrete, rather than continuous, values. These are known as quantum harmonic oscillators; this process of restricting energies to discrete values is called quantization. Building on this idea, Albert Einstein proposed in 1905 an explanation for the photoelectric effect, that light is composed of individual packets of energy called photons; this implied that the electromagnetic radiation, while being waves in the classical electromagnetic field exists in the form of particles. In 1913, Niels Bohr introduced the Bohr model of atomic structure, wherein electrons within atoms can only take on a series of discrete, rather than continuous, energies.
This is another example of quantization. The Bohr model explained the discrete nature of atomic spectral lines. In 1924, Louis de Broglie proposed the hypothesis of wave-particle duality, that microscopic particles exhibit both wave-like and particle-like properties under different circumstances. Uniting these scattered ideas, a coherent discipline, quantum mechanics, was formulated between 1925 and 1926, with important contributions from de Broglie, Werner Heisenberg, Max Born, Erwin Schrödinger, Paul Dirac, Wolfgang Pauli.:22-23In the same year as his paper on the photoelectric effect, Einstein published his theory of special relativity, built on Maxwell's electromagnetism. New rules, called Lorentz transformation, were given for the way time and space coordinates of an event change under changes in the observer's velocity, the distinction between time and space was blurred.:19 It was proposed that all physical laws must be the same for observers at different velocities, i.e. that physical laws be invariant under Lorentz transformations.
Two difficulties remained. Observationally, the Schrödinger equation underlying q
A complex number is a number that can be expressed in the form a + bi, where a and b are real numbers, i is a solution of the equation x2 = −1. Because no real number satisfies this equation, i is called an imaginary number. For the complex number a + bi, a is called the real part, b is called the imaginary part. Despite the historical nomenclature "imaginary", complex numbers are regarded in the mathematical sciences as just as "real" as the real numbers, are fundamental in many aspects of the scientific description of the natural world. Complex numbers allow solutions to certain equations. For example, the equation 2 = − 9 has no real solution, since the square of a real number cannot be negative. Complex numbers provide a solution to this problem; the idea is to extend the real numbers with an indeterminate i, taken to satisfy the relation i2 = −1, so that solutions to equations like the preceding one can be found. In this case the solutions are −1 + 3i and −1 − 3i, as can be verified using the fact that i2 = −1: 2 = 2 = = 9 = − 9, 2 = 2 = 2 = 9 = − 9.
According to the fundamental theorem of algebra, all polynomial equations with real or complex coefficients in a single variable have a solution in complex numbers. In contrast, some polynomial equations with real coefficients have no solution in real numbers; the 16th century Italian mathematician Gerolamo Cardano is credited with introducing complex numbers in his attempts to find solutions to cubic equations. Formally, the complex number system can be defined as the algebraic extension of the ordinary real numbers by an imaginary number i; this means that complex numbers can be added and multiplied, as polynomials in the variable i, with the rule i2 = −1 imposed. Furthermore, complex numbers can be divided by nonzero complex numbers. Overall, the complex number system is a field. Geometrically, complex numbers extend the concept of the one-dimensional number line to the two-dimensional complex plane by using the horizontal axis for the real part and the vertical axis for the imaginary part.
The complex number a + bi can be identified with the point in the complex plane. A complex number whose real part is zero is said to be purely imaginary. A complex number whose imaginary part is zero can be viewed as a real number. Complex numbers can be represented in polar form, which associates each complex number with its distance from the origin and with a particular angle known as the argument of this complex number; the geometric identification of the complex numbers with the complex plane, a Euclidean plane, makes their structure as a real 2-dimensional vector space evident. Real and imaginary parts of a complex number may be taken as components of a vector with respect to the canonical standard basis; the addition of complex numbers is thus depicted as the usual component-wise addition of vectors. However, the complex numbers allow for a richer algebraic structure, comprising additional operations, that are not available in a vector space. Based on the concept of real numbers, a complex number is a number of the form a + bi, where a and b are real numbers and i is an indeterminate satisfying i2 = −1.
For example, 2 + 3i is a complex number. This way, a complex number is defined as a polynomial with real coefficients in the single indeterminate i, for which the relation i2 + 1 = 0 is imposed. Based on this definition, complex numbers can be added and multiplied, using the addition and multiplication for polynomials; the relation i2 + 1 = 0 induces the equalities i4k = 1, i4k+1 = i, i4k+2 = −1, i4k+3 = −i, which hold for all integers k. The real number a is called the real part of the complex number a + bi. To emphasize, the imaginary part does not include a factor i and b, not bi, is the imaginary part. Formally, the complex numbers are defined as the quotient ring of the polynomia
Cosmology is a branch of astronomy concerned with the studies of the origin and evolution of the universe, from the Big Bang to today and on into the future. It is the scientific study of the origin and eventual fate of the universe. Physical cosmology is the scientific study of the universe's origin, its large-scale structures and dynamics, its ultimate fate, as well as the laws of science that govern these areas; the term cosmology was first used in English in 1656 in Thomas Blount's Glossographia, in 1731 taken up in Latin by German philosopher Christian Wolff, in Cosmologia Generalis. Religious or mythological cosmology is a body of beliefs based on mythological and esoteric literature and traditions of creation myths and eschatology. Physical cosmology is studied by scientists, such as astronomers and physicists, as well as philosophers, such as metaphysicians, philosophers of physics, philosophers of space and time; because of this shared scope with philosophy, theories in physical cosmology may include both scientific and non-scientific propositions, may depend upon assumptions that cannot be tested.
Cosmology differs from astronomy in that the former is concerned with the Universe as a whole while the latter deals with individual celestial objects. Modern physical cosmology is dominated by the Big Bang theory, which attempts to bring together observational astronomy and particle physics. Theoretical astrophysicist David N. Spergel has described cosmology as a "historical science" because "when we look out in space, we look back in time" due to the finite nature of the speed of light. Physics and astrophysics have played a central role in shaping the understanding of the universe through scientific observation and experiment. Physical cosmology was shaped through both mathematics and observation in an analysis of the whole universe; the universe is understood to have begun with the Big Bang, followed instantaneously by cosmic inflation. Cosmogony studies the origin of the Universe, cosmography maps the features of the Universe. In Diderot's Encyclopédie, cosmology is broken down into uranology, aerology and hydrology.
Metaphysical cosmology has been described as the placing of humans in the universe in relationship to all other entities. This is exemplified by Marcus Aurelius's observation that a man's place in that relationship: "He who does not know what the world is does not know where he is, he who does not know for what purpose the world exists, does not know who he is, nor what the world is." Physical cosmology is the branch of physics and astrophysics that deals with the study of the physical origins and evolution of the Universe. It includes the study of the nature of the Universe on a large scale. In its earliest form, it was, the study of the heavens. Greek philosophers Aristarchus of Samos and Ptolemy proposed different cosmological theories; the geocentric Ptolemaic system was the prevailing theory until the 16th century when Nicolaus Copernicus, subsequently Johannes Kepler and Galileo Galilei, proposed a heliocentric system. This is one of the most famous examples of epistemological rupture in physical cosmology.
Isaac Newton's Principia Mathematica, published in 1687, was the first description of the law of universal gravitation. It provided a physical mechanism for Kepler's laws and allowed the anomalies in previous systems, caused by gravitational interaction between the planets, to be resolved. A fundamental difference between Newton's cosmology and those preceding it was the Copernican principle—that the bodies on earth obey the same physical laws as all the celestial bodies; this was a crucial philosophical advance in physical cosmology. Modern scientific cosmology is considered to have begun in 1917 with Albert Einstein's publication of his final modification of general relativity in the paper "Cosmological Considerations of the General Theory of Relativity". General relativity prompted cosmogonists such as Willem de Sitter, Karl Schwarzschild, Arthur Eddington to explore its astronomical ramifications, which enhanced the ability of astronomers to study distant objects. Physicists unchanging. In 1922 Alexander Friedmann introduced the idea of an expanding universe that contained moving matter.
Around the same time the Great Debate took place, with early cosmologists such as Heber Curtis and Ernst Öpik determining that some nebulae seen in telescopes were separate galaxies far distant from our own. In parallel to this dynamic approach to cosmology, one long-standing debate about the structure of the cosmos was coming to a climax. Mount Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the Milky Way star system only; this difference of ideas came to a climax with the organization of the Great Debate on 26 April 1920 at the meeting of the U. S. National Academy of Sciences in Washington, D. C; the debate was resolved when Edwin Hubble detected Cepheid Variables in the Andromeda galaxy in 1923 and 1924. Their distance established spiral nebulae well beyond the edge of the Milky Way. S
The Big Bang theory is the prevailing cosmological model for the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from a high-density and high-temperature state, offers a comprehensive explanation for a broad range of phenomena, including the abundance of light elements, the cosmic microwave background, large scale structure and Hubble's law. If the observed conditions are extrapolated backwards in time using the known laws of physics, the prediction is that just before a period of high density there was a singularity, associated with the Big Bang. Physicists are undecided whether this means the universe began from a singularity, or that current knowledge is insufficient to describe the universe at that time. Detailed measurements of the expansion rate of the universe place the Big Bang at around 13.8 billion years ago, thus considered the age of the universe. After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, simple atoms.
Giant clouds of these primordial elements coalesced through gravity forming early stars and galaxies, the descendants of which are visible today. Astronomers observe the gravitational effects of dark matter surrounding galaxies. Though most of the mass in the universe seems to be in the form of dark matter, Big Bang theory and various observations seem to indicate that it is not made out of conventional baryonic matter but it is unclear what it is made out of. Since Georges Lemaître first noted in 1927 that an expanding universe could be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion; the scientific community was once divided between supporters of two different theories, the Big Bang and the Steady State theory, but a wide range of empirical evidence has favored the Big Bang, now universally accepted. In 1929, from analysis of galactic redshifts, Edwin Hubble concluded that galaxies are drifting apart. In 1964, the cosmic microwave background radiation was discovered, crucial evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the universe before it was discovered.
More measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy's existence. The known physical laws of nature can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature. In 1922, Russian mathematician Alexander Friedmann proposed on theoretical grounds that the universe is expanding, rederived independently and observationally confirmed soon afterwards by Belgian astronomer and Catholic priest Georges Lemaître in 1927 Lemaître proposed what became known as the "Big Bang theory" of the creation of the universe calling it the "hypothesis of the primeval atom".: in his paper Annales de la Société Scientifique de Bruxelles under the title "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques", he presented his new idea that the universe is expanding and provided the first observational estimation of what is known as the Hubble constant.
What will be known as the "Big Bang theory" of the origin of the universe, he called his "hypothesis of the primeval atom" or the "Cosmic Egg". American astronomer Edwin Hubble observed that the distances to faraway galaxies were correlated with their redshifts; this was interpreted to mean that all distant galaxies and clusters are receding away from our vantage point with an apparent velocity proportional to their distance: that is, the farther they are, the faster they move away from us, regardless of direction. Assuming the Copernican principle, the only remaining interpretation is that all observable regions of the universe are receding from all others. Since we know that the distance between galaxies increases today, it must mean that in the past galaxies were closer together; the continuous expansion of the universe implies that the universe was denser and hotter in the past. Large particle accelerators can replicate the conditions that prevailed after the early moments of the universe, resulting in confirmation and refinement of the details of the Big Bang model.
However, these accelerators can only probe so far into high energy regimes. The state of the universe in the earliest instants of the Big Bang expansion is still poorly understood and an area of open investigation and speculation; the first subatomic particles to be formed included protons and electrons. Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed; the majority of atoms produced by the Big Bang were hydrogen, along with helium and traces of lithium. Giant clouds of these primordial elements coalesced through gravity to form stars and galaxies, the heavier elements were synthesized either within stars or during supernovae; the Big Bang theory offers a comprehensive explanation for a broad range of observed phenomena
The Universe is all of space and time and their contents, including planets, stars and all other forms of matter and energy. While the spatial size of the entire Universe is unknown, it is possible to measure the size of the observable universe, estimated to be 93 billion light years in diameter. In various multiverse hypotheses, a universe is one of many causally disconnected constituent parts of a larger multiverse, which itself comprises all of space and time and its contents; the earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center of the Universe. Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Isaac Newton built upon Copernicus' work as well as observations by Tycho Brahe and Johannes Kepler's laws of planetary motion. Further observational improvements led to the realization that the Sun is one of hundreds of billions of stars in the Milky Way, one of at least hundreds of billions of galaxies in the Universe.
Many of the stars in our galaxy have planets. At the largest scale galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. At smaller scales, galaxies are distributed in clusters and superclusters which form immense filaments and voids in space, creating a vast foam-like structure. Discoveries in the early 20th century have suggested that the Universe had a beginning and that space has been expanding since and is still expanding at an increasing rate; the Big Bang theory is the prevailing cosmological description of the development of the Universe. Under this theory and time emerged together 13.799±0.021 billion years ago and the energy and matter present have become less dense as the Universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10−32 seconds, the separation of the four known fundamental forces, the Universe cooled and continued to expand, allowing the first subatomic particles and simple atoms to form.
Dark matter gathered forming a foam-like structure of filaments and voids under the influence of gravity. Giant clouds of hydrogen and helium were drawn to the places where dark matter was most dense, forming the first galaxies and everything else seen today, it is possible to see objects that are now further away than 13.799 billion light-years because space itself has expanded, it is still expanding today. This means that objects which are now up to 46.5 billion light-years away can still be seen in their distant past, because in the past when their light was emitted, they were much closer to the Earth. From studying the movement of galaxies, it has been discovered that the universe contains much more matter than is accounted for by visible objects; this unseen matter is known as dark matter. The ΛCDM model is the most accepted model of our universe, it suggests that about 69.2%±1.2% of the mass and energy in the universe is a cosmological constant, responsible for the current expansion of space, about 25.8%±1.1% is dark matter.
Ordinary matter is therefore only 4.9% of the physical universe. Stars and visible gas clouds only form about 6% of ordinary matter, or about 0.3% of the entire universe. There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will be accessible; some physicists have suggested various multiverse hypotheses, in which our universe might be one among many universes that exist. The physical Universe is defined as all of their contents; such contents comprise all of energy in its various forms, including electromagnetic radiation and matter, therefore planets, stars and the contents of intergalactic space. The Universe includes the physical laws that influence energy and matter, such as conservation laws, classical mechanics, relativity; the Universe is defined as "the totality of existence", or everything that exists, everything that has existed, everything that will exist.
In fact, some philosophers and scientists support the inclusion of ideas and abstract concepts – such as mathematics and logic – in the definition of the Universe. The word universe may refer to concepts such as the cosmos, the world, nature; the word universe derives from the Old French word univers, which in turn derives from the Latin word universum. The Latin word was used by Cicero and Latin authors in many of the same senses as the modern English word is used. A term for "universe" among the ancient Greek philosophers from Pythagoras onwards was τὸ πᾶν, tò pân, defined as all matter and all space, τὸ ὅλον, tò hólon, which did not include the void. Another synonym was ho kósmos. Synonyms are found in Latin authors and survive in modern languages, e.g. the German words Das All and Natur for Universe. The same synonyms are found in English, such as everything, the cosmos, the world (as in the many-worlds interpr