The observable universe is about 28 billion parsecs (91 billion light-years) in diameter at the present time.[2] The size of the whole Universe is not known and may be infinite.[14] Observations and the development of physical theories have led to inferences about the composition and evolution of the Universe.
Throughout recorded history, cosmologies, cosmogonies, and scientific models have been proposed to explain observations of the Universe. The earliest quantitative geocentric models were developed by ancient Greek philosophers and Indian philosophers.[15][16] Over the centuries, more precise astronomical observations led to Nicolaus Copernicus's heliocentric model of the Solar System and Johannes Kepler's improvement on that model with elliptical orbits, which was eventually explained by Isaac Newton's theory of gravity. Further observational improvements led to the realization that the Solar System is located in a galaxy composed of billions of stars, the Milky Way. It was subsequently discovered that our galaxy is just one of many. Observations of the distribution of these galaxies and their spectral lines have led to many of the theories of modern physical cosmology. The discovery in the early 20th century that galaxies are systematically redshifted suggested that the Universe is expanding, and the discovery of the cosmic microwave background radiation suggested that the Universe had a beginning.[17] Finally, observations in the late 1990s indicated the rate of the expansion of the Universe is increasing.[18]
The Big Bang theory is the prevailing cosmological model describing the development of the Universe. Space and time were created in the Big Bang, and these were imbued with a fixed amount of energy and matter; as space expands, the density of that matter and energy decreases. After the initial expansion, the Universe cooled sufficiently to allow the formation first of subatomic particles and later of simple atoms. Giant clouds of these primordial elements later coalesced through gravity to form stars. Assuming that the prevailing model is correct, the age of the Universe is measured to be 13.798±0.037 billion years.[1][19]
There are many competing hypotheses about the ultimate fate of the Universe. Physicists and philosophers remain unsure about what, if anything, preceded the Big Bang. Many refuse to speculate, doubting that any information from any such prior state could ever be accessible. There are various multiverse hypotheses, in which some physicists have suggested that the Universe might be one among many universes that likewise exist.[20][21]
Contents
Definition
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Etymology
The word universe derives from the Old French word univers, which in turn derives from the Latin word universum.[28] The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.[29] The Latin word derives from the poetic contraction unvorsum — first used by Lucretius in Book IV (line 262) of his De rerum natura (On the Nature of Things) — which connects un, uni (the combining form of unus, or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning "something rotated, rolled, changed").[29]An alternative interpretation of unvorsum is "everything rotated as one" or "everything rotated by one". In this sense, it may be considered a translation of an earlier Greek word for the Universe, περιφορά, (periforá, "circumambulation"), originally used to describe a course of a meal, the food being carried around the circle of dinner guests.[30] This Greek word refers to celestial spheres, an early Greek model of the Universe. Regarding Plato's Metaphor of the Sun, Aristotle suggests that the rotation of the sphere of fixed stars inspired by the prime mover,[31] motivates, in turn, terrestrial change via the Sun. Astronomical and physical measurements, such as the Foucault pendulum, demonstrate that the Earth rotates on its axis.
Synonyms
A term for the Universe in ancient Greece was τὸ πᾶν (tò pán, The All, Pan). Related terms were matter, (τὸ ὅλον, tò hólon, see also Hyle, lit. wood) and place (τὸ κενόν, tò kenón).[32][33] Other synonyms for the Universe among the ancient Greek philosophers included κόσμος (cosmos) and φύσις (meaning nature, Greek physis, from which we derive the word physics).[34] The same synonyms are found in Latin authors (totum, mundus, natura)[35] and survive in modern languages, e.g., the German words Das All, Weltall, and Natur for Universe. The same synonyms are found in English, such as everything (as in the theory of everything), the cosmos (as in cosmology), the world (as in the many-worlds interpretation), and nature (as in natural laws or natural philosophy).[36]Chronology and the Big Bang
Main articles: Big Bang and Chronology of the Universe
| Timeline of the Universe |
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The initial hot, dense state is called the Planck epoch, a brief period extending from time zero to one Planck time unit of approximately 10−43 seconds. During the Planck epoch, all types of matter and all types of energy were concentrated into a dense state, where gravitation is believed to have been as strong as the other fundamental forces, and all the forces may have been unified. Since the Planck epoch, the Universe has been expanding to its present form, possibly with a very brief period (less than 10−32 seconds) of cosmic inflation which caused the Universe to reach a much larger size almost instantaneously.[39] Several independent measurements support this theoretical expansion.[citation needed]
After the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these epochs encompassd less than 10 seconds of time following the Big Bang. Other observations [clarification needed] can be explained by combining the overall expansion of space with nuclear and atomic physics. As the Universe expands, the energy density of electromagnetic radiation decreases more quickly than does that of matter because the energy of a photon decreases with its wavelength. As the Universe expanded and cooled, elementary particles associated stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then associated into atomic nuclei. At this stage, matter in the Universe was mainly a hot, dense plasma of negatively charged electrons, neutral neutrinos and positive nuclei. This era, called the photon epoch, lasted about 380 thousand years. Nuclear reactions among nuclei led to the present abundances of lighter nuclei, particularly hydrogen, deuterium, and helium through a process known as Big Bang nucleosynthesis.
Eventually, at time known as recombination, electrons and nuclei formed stable atoms, which are transparent to most wavelengths of radiation. With photons decoupled from matter, the Universe entered the matter-dominated era. Light from this era could now travel freely, and it can still be seen in the Universe as the cosmic microwave background (CMB). After around 100 million years, the first stars formed; these were likely very massive, luminous, and responsible for the reionization of the Universe. Having no elements heavier than lithium, these stars also produced the first heavy elements through stellar nucleosynthesis.[40] The Universe also contains a mysterious energy called dark energy; the energy density of dark energy does not change over time. After about 9.8 billion years, the Universe had expanded sufficiently so that the density of matter was less than the density of dark energy, marking the beginning of the present dark-energy-dominated era.[41] In this era, the expansion of the Universe is accelerating due to dark energy.
Properties
The spacetime of the Universe is usually interpreted from a Euclidean perspective, with space as consisting of three dimensions, and time as consisting of one dimension, the "fourth dimension". By combining space and time into a single manifold called Minkowski space, physicists have simplified a large number of physical theories, as well as described in a more uniform way the workings of the Universe at both the supergalactic and subatomic levels.The four dimensions of spacetime consist of events that are not absolutely defined spatially and temporally, but rather are known relative to the motion of an observer. Minkowski space first approximates the Universe without gravity; the pseudo-Riemannian manifolds of general relativity describe spacetime with matter and gravity. Some areas of theoretical physics, such as string theory, postulate the existence of additional dimensions.
Of the four fundamental interactions, gravitation is dominant at cosmological length scales; that is, the other three forces play a negligible role in determining structures at the level of galaxies and larger-scale structures. Gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on cosmological length scales. The remaining two interactions, the weak and strong nuclear forces, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.
The Universe appears to have much more matter than antimatter, an asymmetry possibly related to the observations of CP violation.[42] The Universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological length scales. The Universe also appears to have neither net momentum nor angular momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the Universe were finite.[43]
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Shape
The three possible options of the shape of the Universe.
Main article: Shape of the Universe
The shape of the Universe is related to general relativity, which describes how spacetime is curved and bent by mass and energy. The curvature, topology or geometry of the Universe includes both local geometry in the observable universe and global geometry, which is possibly measurable. Investigations include, which 3-manifold corresponds to the spatial section in comoving coordinates of the four-dimensional spacetime of the Universe.{unclear} Cosmologists often work with a given space-like slice of spacetime called the comoving coordinates. In terms of observation, the section of spacetime that can be observed is the backward light cone, being the time it takes to reach a given observer within the cosmological horizon (particle horizon).The particle horizon, the light horizon, or the cosmic light horizon, is the maximum distance from which particles can have traveled to the observer in the age of the Universe. An horizon represents the boundary between the observable and the unobservable region of the Universe.[44][45] The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model.
Observational data suggest the cosmological topological of the Universe is infinite in extent with finite age, supported by the so-called Friedmann–Lemaître–Robertson–Walker (FLRW) models.[46][47][48][49] These FLRW models of space are consistent with the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck maps of cosmic background radiation, thus supporting inflationary models and the standard model of cosmology, describing a flat, homogeneous universe dominated by dark matter and dark energy.[1][50]
Size and Regions
See also: Observable universe and Observational cosmology
According to a restrictive definition, the Universe is everything within our connected spacetime that could have a chance to interact with us and vice versa.[51] According to the general theory of relativity, some regions of space may never interact with ours even in the lifetime of the Universe due to the finite speed of light and the ongoing expansion of space.
For example, radio messages sent from Earth may never reach some
regions of space, even if the Universe were to exist forever: space may
expand faster than light can traverse it.[52]
Since we cannot observe space beyond the limitations of light, or any
electromagnetic radiation, it is unknown whether the size of the
Universe is finite or infinite.[14][53][54]Distant regions of space are taken to exist and be part of reality as much as we are, yet we can never interact with them. The spatial region within which we can affect and be affected is the observable universe. The observable Universe depends on the location of the observer. By traveling, an observer can come into contact with a greater region of spacetime than an observer who remains still. Nevertheless, even the most rapid traveler will not be able to interact with all of space. Typically, the observable Universe is taken to mean the Universe observable from our vantage point in the Milky Way Galaxy.
The proper distance – the distance as would be measured at a specific time, including the present – between Earth and the edge of the observable universe is 46 billion light-years (14×109 pc), making the diameter of the observable universe about 91 billion light-years (28×109 pc). The distance the light from the edge of the observable universe has travelled is very close to the age of the Universe times the speed of light, 13.8 billion light-years (4.2×109 pc), but this does not represent the distance at any given time because the edge of the universe and the Earth have moved since further apart.[55] For comparison, the diameter of a typical galaxy is 30,000 light-years, and the typical distance between two neighboring galaxies is 3 million light-years.[56] As an example, the Milky Way Galaxy is roughly 100,000 light years in diameter,[57] and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light years away.[58]
Age and expansion
Main articles: Age of the universe and Metric expansion of space
Astronomers calculate the age of the Universe by assuming that the Lambda-CDM model
accurately describes the evolution of the Universe from a very uniform,
hot, dense primordial state to its present state and measuring the
cosmological parameters which constitute the model. This model is well
understood theoretically and supported by recent high-precision astronomical observations such as WMAP and Planck. Commonly, the set of observations fitted includes the cosmic microwave background anisotropy, the brightness/redshift relation for Type Ia supernovae, and large-scale galaxy clustering including the baryon acoustic oscillation feature. Other observations, such as the Hubble constant, the abundance of galaxy clusters, weak gravitational lensing
and globular cluster ages, are generally consistent with these,
providing a check of the model, but are less accurately measured at
present. With the prior
that the Lambda-CDM model is correct and the values of the parameters
measured by various experiments including Planck, the best value of the
age of the Universe as of 2015 is 13.798 ± 0.037 billion years.[1]Over time, the Universe and its contents have evolved; for example, the relative population of quasars and galaxies has changed and space itself has expanded. This expansion accounts for how it is that scientists on Earth can observe the light from a galaxy 30 billion light years away, even if that light has traveled for only 13 billion years; the very space between them has expanded, and that is one of the tools used to calculate the age of the Universe. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; the photons emitted have been stretched to longer wavelengths and lower frequency during their journey. The rate of this spatial expansion is accelerating, based on studies of Type Ia supernovae.
The more matter there is in the Universe, the stronger will be the gravitational pull among the matter. If the Universe were too dense then it would re-collapse into singularity. However, if the Universe contained too little matter then the expansion is accelerated greatly, thereby leaving no time for planets and planetary systems to form. After the Big Bang, the universe is continuously expanding. The rate of expansion is affected by the gravity among the matter present. Surprisingly, our universe has just the right mass density of about 5 protons per cubic meter which has allowed it to expand for last 13.8 billion years, giving time to form the universe as we see it today.[59]
There are dynamical forces acting on the particles in the Universe which affect the expansion rate. It was earlier expected that the Hubble Constant would be decreasing as time went on due to the influence of gravitational interactions in the Universe, and thus there is an additional observable quantity in the Universe called the deceleration parameter which cosmologists expected to be directly related to the matter density of the Universe. Surprisingly, the deceleration parameter was measured by two different groups to be less than zero (actually, consistent with −1) which implied that today Hubble's Constant is increasing as time goes on. Some cosmologists have whimsically called the effect associated with the "accelerating universe" the "cosmic jerk".[60] The 2011 Nobel Prize in Physics was given for the discovery of this phenomenon.[61]
Spacetime
Main articles: Spacetime and World line
See also: Lorentz transformation
Spacetimes are the arenas in which all physical events take place—an
event is a point in spacetime specified by its time and place. For
example, the motion of planets around the sun may be described in a particular type of spacetime, or the motion of light around a rotating star
may be described in another type of spacetime. The basic elements of
spacetime are events. In any given spacetime, an event is a unique
position at a unique time. Because events are spacetime points, an
example of an event in classical relativistic physics is 
,
the location of an elementary (point-like) particle at a particular
time. A spacetime itself can be viewed as the union of all events in the
same way that a line is the union of all of its points, formally
organized into a manifold, a space which can be described at small scales using coordinate systems.The Universe appears to be a smooth spacetime continuum consisting of three spatial dimensions and one temporal (time) dimension. In spacetime, the displacement four-vector ΔR is given by the space displacement vector Δr and the time difference Δt between the events. The spacetime interval, also called invariant interval, is the interval between the two events, s2
On the average, space is observed to be very nearly flat (close to zero curvature), meaning that Euclidean geometry is empirically true with high accuracy throughout most of the Universe.[62] Spacetime also appears to have a simply connected topology, in analogy with a sphere, at least on the length-scale of the observable Universe. However, present observations cannot exclude the possibilities that the Universe has more dimensions and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or toroidal topologies of two-dimensional spaces.[47][63]
Contents
The formation of clusters and large-scale filaments in the Cold Dark Matter model with dark energy.
The frames show the evolution of structures in a 43 million parsecs (or
140 million light years) box from redshift of 30 to the present epoch
(upper left z=30 to lower right z=0).
A map of the Superclusters and voids nearest to Earth
The observable matter of the Universe is also isotropic on large scales, meaning that no direction of observation appears to be different from any other.[73] The Universe is also bathed in a highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.725 kelvin.[74] The hypothesis that the large-scale Universe is homogeneous and isotropic is known as the cosmological principle,[75] which is supported by astronomical observations.
Dark energy
Main article: Dark energy
The nature of the energy responsible for the accelerating expansion
of the Universe is unknown. It is given the name "dark energy", and it
is hypothesized to permeate all of space.[64] On a mass–energy equivalence basis, the density of dark energy (6.91 × 10−27 kg/m3)
is much less than the density of ordinary matter or dark matter within
galaxies. However, in the present dark-energy era, it dominates the
mass–energy of the universe because it is uniform across space.[76]Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously,[77] and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy. Scalar fields having only a slight amont of spatial inhomogeneity can be difficult to distinguish from a cosmological constant.
According to the Planck mission team, and based on the standard model of cosmology (lambda-CDM model), on a mass–energy equivalence basis, the observable universe contains 26.8% dark matter, 68.3% dark energy (for a total of 95.1%) and 4.9% ordinary matter.[78]
Dark matter
Main article: Dark matter
Dark matter is a hypothetical kind of matter
that cannot be seen with telescopes, but which accounts for most of the
matter in the Universe. The existence and properties of dark matter are
inferred from its gravitational effects on visible matter, radiation,
and the large-scale structure of the Universe. Other than neutrinos, a form of hot dark matter, dark matter has not been detected directly, making it one of the greatest mysteries in modern astrophysics. Dark matter neither emits nor absorbs light or any other electromagnetic radiation
at any significant level. Dark matter is estimated to constitute 26.8%
of the total mass–energy and 84.5% of the total matter in the Universe.[79][6]Ordinary Matter
Main article: Matter
The remaining 4.9% of the mass–energy of the Universe is composed of matter made up of ordinary matter, that is atoms, ions, electrons and the objects they form. This matter includes stars, which produce nearly all of the light we see from galaxies, as well as interstellar gas in the interstellar and intergalactic media, planets, and all the objects from everyday life that we can bump into, touch or squeeze.[80]Ordinary matter commonly exists in four states (or phases): solid, liquid and gas, and plasma. However, advances in experimental techniques have revealed other previously theoretical phases, such as Bose–Einstein condensates and fermionic condensates.
More fundamentally, matter is composed of two types of elementary particles: quarks and leptons.[81] For example, the neutron is formed of two down quarks and one up quark, while the proton is formed of two up quarks and one down quark. Atoms are made up of several neutrons and protons (two types of baryons), which have several electrons (a type of lepton) orbiting them. Because most of the mass of the atom is concentrated in the atomic nucleus, which is made up of baryons, astronomers often use the term baryonic matter to describe ordinary matter, although a small fraction of this baryonic matter is also composed of electrons. A focus on an elementary-particle view of matter also leads to new phases of matter, such as the quark–gluon plasma.[82]
The primordial protons and neutrons themselves were formed from the quark–gluon plasma during the Big Bang as it cooled below two trillion degrees. A few minutes afterwards, starting with only protons and neutrons, nuclei up to lithium and beryllium were formed, but the abundances of other elements dropped sharply with growing atomic mass. Some boron may have been formed at this time, but the process stopped before significant carbon could be formed. This process, known as Big Bang nucleosynthesis, essentially shut down after about 20 minutes, due to drops in temperature and density as the universe continued to expand. The subsequent nucleosynthesis of heavier elements required the extreme temperatures and pressures found within stars and supernovas, in the processes of stellar nucleosynthesis and supernova nucleosynthesis.
Particles
A diagram summarizing the tree-level interactions between elementary
particles described in the Standard Model. Vertices (darkened circles)
represent types of particles, and edges (blue arcs) connecting them
represent interactions that can take place. The organization of the
diagram is as follows: the top row of vertices (leptons and quarks) are
the matter particles; the second row of vertices (photon, W/Z, gluons)
are the force mediating particles; and the bottom row is the Higgs
boson. The elementary particles from which the Universe is constructed. Six leptons and six quarks comprise most of the matter; for example, the protons and neutrons of atomic nuclei are composed of quarks; the ubiquitous electron is a lepton. These particles interact via the gauge bosons shown in the middle row, each corresponding to a particular type of gauge symmetry. The Higgs boson is believed to confer mass on the particles with which it is connected. The graviton, a supposed gauge boson for gravity, is not shown.
Six of the particles in the Standard Model are quarks (shown in purple). Each of the first three columns forms a generation of matter.
Main article: Particle physics
The ordinary matter in the Universe consists of particles described by particle physics. An elementary particle or fundamental particle is a particle whose substructure
(domain of the bigger structure which shares the similar
characteristics of the domain) is unknown, thus it is unknown whether it
is composed of other particles.[83] Known elementary particles include the fundamental fermions (quarks, leptons, antiquarks, and antileptons), which generally are "matter particles" and "antimatter particles", as well as the fundamental bosons (gauge bosons and Higgs boson), which generally are "force particles" that mediate interactions among fermions.[83] A particle containing two or more elementary particles is a composite particle.The Standard Model of particle physics suggests a universal set of physical laws and physical constants.[84] It concerns the electromagnetic, weak, and strong nuclear interactions, as well as classifying all the subatomic particles known. It was developed throughout the latter half of the 20th century as a collaborative effort of scientists around the world.[85] The current formulation was finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, discoveries of the top quark (1995), the tau neutrino (2000), and more recently the Higgs boson (2013), have given further credence to the Standard Model. Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a "theory of almost everything".
Each particle generation is divided into two leptons and two quarks, both of which are fermions. Bosons are the other fundamental class of elementary particles. Additionally, there are hypothetical particles.
Hadrons
Main article: Hadron
A hadron is a composite particle made of quarks bound state by the strong force (in a similar way as molecules are held together by the electromagnetic force). From approximately 10−6 seconds after the Big Bang, when the temperature of the universe had fallen sufficiently to allow the quarks from the preceding quark epoch to bind together into hadrons, the mass of the universe was dominated by hadrons. This period is known as the hadron epoch.
Initially the temperature was high enough to allow the formation of
hadron/anti-hadron pairs, which kept matter and antimatter in thermal equilibrium.
However, as the temperature of the Universe continued to fall,
hadron/anti-hadron pairs were no longer produced. Most of the hadrons
and anti-hadrons were then eliminated in annihilation
reactions, leaving a small residual of hadrons. The elimination of
anti-hadrons was completed by one second after the Big Bang, when the
following lepton epoch began.Hadrons are categorized into two families: baryons (such as protons and neutrons) made of three quarks and mesons (such as pions) made of one quark and one antiquark. Of the hadrons, protons are stable, and neutrons are bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions; free neutrons decay with a half-life of about 611 seconds. Experimentally, hadron physics is studied by colliding protons or nuclei of heavy elements such as lead, and detecting the debris in the produced particle showers.
Leptons
Main article: Lepton
A lepton is an elementary, half-integer spin (spin 1⁄2) particle that does not undergo strong interactions but is subject to the Pauli exclusion principle; no two leptons of the same species can be in exactly the same state at the same time.[86] Fermions differ from bosons, which obey Bose–Einstein statistics. The best known of all leptons is the electron, which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). The two leptons may be classified into one with electric charge
−1 (electron-like) and one neutral (neutrino). Thus electrons are
stable and the most common charged lepton in the Universe, whereas muons
and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).[87][88] Charged leptons can combine with other particles to form various composite particles such as atoms and positronium,
while neutrinos rarely interact with anything, and are consequently
rarely observed. Neutrinos of all generations stream throughout the
Universe but rarely interact with normal matter.[89]The lepton epoch was the period in the evolution of the early Universe in which the leptons dominated the mass of the Universe. It started roughly 1 second after the Big Bang, after the majority of hadrons and anti-hadrons annihilated each other at the end of the hadron epoch. During the lepton epoch the temperature of the Universe was still high enough to create lepton/anti-lepton pairs, so leptons and anti-leptons were in thermal equilibrium. Approximately 10 seconds after the Big Bang, the temperature of the Universe had fallen to the point where lepton/anti-lepton pairs were no longer created.[90] Most leptons and anti-leptons were then eliminated in annihilation reactions, leaving a small residue of leptons. The mass of the Universe was then dominated by photons as it entered the following photon epoch.
Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation (best described at present by general relativity), electroweak (includes electromagnetism), and the weak interaction. During the quark epoch, the electroweak force split into the electromagnetic and weak forces. Interactions can be described by renormalized quantum field theory, and are mediated by gauge bosons that correspond to a particular type of gauge symmetry. A renormalized quantum field theory of general relativity has not yet been achieved.
Bosons
See also: Higgs boson
Bosons may be either elementary, like photons, or composite, like mesons. Bosons are characterized by Bose–Einstein statistics and all have integer spins. While most bosons are composite particles, and these are important in superfluidity and other applications of Bose–Einstein condensates, in the Standard Model there are five bosons which are elementary: photons, which carry the electromagnetic interaction; W and Z bosons, which carry the weak interaction; gluons, which carry the strong interaction, and the scalar boson Higgs Boson.[91] Additionally, the graviton
is a hypothetical elementary particle not incorporated in the Standard
Model. If it exists, a graviton must be a boson, and could conceivably
be a gauge boson.The Higgs boson or Higgs particle is an elementary particle in the Standard Model. Observations of the particle allows scientists to explore the Higgs field[92][93]—a fundamental field of crucial importance to particle physics theory, that unlike the more familiar electromagnetic field cannot be "turned off", but instead takes a non-zero constant value almost everywhere.[93] The presence of this field, now believed to be confirmed, explains why some fundamental particles have mass even though the symmetries controlling their interactions should require them to be massless, and also answers several other long-standing puzzles in physics, such as the reason the weak force has a much shorter range than the electromagnetic force. The Higgs field can be detected through its excitations (i.e. Higgs particles), but these are extremely hard to produce and detect.
Photons
Main article: Photon epoch
See also: Photino
A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation. It is the force carrier for the electromagnetic force, even when static via virtual photons. The effects of this force are easily observable at the microscopic and at the macroscopic level because the photon has zero rest mass; this allows long distance interactions. Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, exhibiting properties of waves and of particles. For example, a single photon may be refracted by a lens or exhibit wave interference with itself but also act as a particle giving a definite result when its position is measured.In the Standard Model, the photon is one of four gauge bosons in the electroweak interaction; the other three are denoted W+, W− and Z0 and are responsible for the weak interaction. Unlike the photon, these gauge bosons have mass, owing to a mechanism that breaks their SU(2) gauge symmetry.
The photon epoch started after most leptons and anti-leptons were annihilated at the end of the lepton epoch, about 10 seconds after the Big Bang. Atomic nuclei were created in the process of nucleosynthesis which occurred during the first few minutes of the photon epoch. For the remainder of the photon epoch the universe contained a hot dense plasma of nuclei, electrons and photons. About 380,000 years after the Big Bang the temperature of the universe fell to the point where nuclei could combine with electrons to create neutral atoms. As a result, photons no longer interacted frequently with matter, the universe became transparent and the cosmic microwave background radiation was created and then structure formation took place.
In the Standard Model, photons and other elementary particles are described as a necessary consequence of physical laws having a certain symmetry at every point in spacetime. The intrinsic properties of particles, such as electric charge, mass and spin, are determined by the properties of this gauge symmetry.
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Geometry
High-precision test of general relativity by the Cassini space probe (artist's impression): radio signals sent between the Earth and the probe (green wave) are delayed by the warping of space and time (blue lines) due to the Sun's mass.
The geometry of 4-dimensional space is much more complex than that of 3-dimensional space, due to the extra degree of freedom (time, t). The set of points in Euclidean 4-space having the same distance R from a fixed point P0 forms a hypersurface known as a 3-sphere (see also Hypersphere). The hypervolume of the enclosed space can be calculated with:
Theories of physics
See also: Supersymmetry
Special relativity
Main articles: Introduction to special relativity and Special relativity
Special relativity is the generally accepted physical theory regarding the relationship between space and time. It is based on two postulates: (1) that the laws of physics are invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference); and (2) that the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source. It was originally proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".[95] The inconsistency of Newtonian mechanics with Maxwell’s equations of electromagnetism and the inability to discover Earth's motion through a luminiferous aether
led to the development of special relativity, which corrects mechanics
to handle situations involving motions nearing the speed of light. As of
today, special relativity is the most accurate model of motion at any
speed. Even so, Newtonian mechanics is still useful (due to its
simplicity and high accuracy) as an approximation at small velocities
relative to the speed of light.Special relativity implies a wide range of consequences, which have been experimentally verified,[96] including length contraction, time dilation, relativistic mass, mass–energy equivalence, a universal speed limit, and relativity of simultaneity. It has replaced the conventional notion of an absolute universal time with the notion of a time that is dependent on reference frame and spatial position. Rather than an invariant time interval between two events, there is an invariant spacetime interval. Combined with other laws of physics, the two postulates of special relativity predict the equivalence of mass and energy, as expressed in the mass–energy equivalence formula E = mc2, where c is the speed of light in vacuum.[97][98]
A defining feature of special relativity is the replacement of the Galilean transformations of Newtonian mechanics with the Lorentz transformations. Time and space cannot be defined separately from each other. Rather space and time are interwoven into a single continuum known as spacetime. Events that occur at the same time for one observer could occur at different times for another.
General relativity
Main articles: Introduction to general relativity, General relativity and Einstein's field equations
General relativity is the geometric theory of gravitation published by Albert Einstein in 1915[99] and the current description of gravitation in modern physics. It is the basis of current cosmological models of a consistently expanding universe. 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 significantly from those of classical physics, especially concerning the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light. Examples of such differences include gravitational time dilation, gravitational lensing, the gravitational redshift of light, and the gravitational time delay. The predictions of general relativity 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 that is consistent with experiments and observations. 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.
Solving Einstein's field equations
See also: Big Bang and Ultimate fate of the Universe
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This section does not cite any references or sources. (June 2015) |
The solutions for R(t) depend on k and Λ, but some qualitative features of such solutions are general. First and most importantly, the length scale R of the Universe can remain constant only if the Universe is perfectly isotropic with positive curvature (k=1) and has one precise value of density everywhere, as first noted by Albert Einstein. However, this equilibrium is unstable and because the Universe is known to be inhomogeneous on smaller scales, R must change, according to general relativity. When R changes, all the spatial distances in the Universe change in tandem; there is an overall expansion or contraction of space itself. This accounts for the observation that galaxies appear to be flying apart; the space between them is stretching. The stretching of space also accounts for the apparent paradox that two galaxies can be 40 billion light years apart, although they started from the same point 13.8 billion years ago[101] and never moved faster than the speed of light.
Second, all solutions suggest that there was a gravitational singularity in the past, when R goes to zero and matter and energy became infinitely dense. It may seem that this conclusion is uncertain because it is based on the questionable assumptions of perfect homogeneity and isotropy (the cosmological principle) and that only the gravitational interaction is significant. However, the Penrose–Hawking singularity theorems show that a singularity should exist for very general conditions. Hence, according to Einstein's field equations, R grew rapidly from an unimaginably hot, dense state that existed immediately following this singularity (when R had a small, finite value); this is the essence of the Big Bang model of the Universe. A common misconception is that the Big Bang model predicts that matter and energy exploded from a single point in space and time; that is false. Rather, space itself was created in the Big Bang and imbued with a fixed amount of energy and matter distributed uniformly throughout; as space expands (i.e., as R(t) increases), the density of that matter and energy decreases.
Third, the curvature index k determines the sign of the mean spatial curvature of spacetime averaged over length scales greater than a billion light years. If k=1, the curvature is positive and the Universe has a finite volume. Such universes are often visualized as a three-dimensional sphere S3 embedded in a four-dimensional space. Conversely, if k is zero or negative, the Universe may have infinite volume, depending on its overall topology. It may seem counter-intuitive that an infinite and yet infinitely dense Universe could be created in a single instant at the Big Bang when R=0, but exactly that is predicted mathematically when k does not equal 1. For comparison, an infinite plane has zero curvature but infinite area, whereas an infinite cylinder is finite in one direction and a torus is finite in both. A toroidal Universe could behave like a normal Universe with periodic boundary conditions, as seen in "wrap-around" video games such as Asteroids; a traveler crossing an outer "boundary" of space going outwards would reappear instantly at another point on the boundary moving inwards.
The ultimate fate of the Universe is still unknown, because it depends critically on the curvature index k and the cosmological constant Λ. If the Universe is sufficiently dense, k equals +1, meaning that its average curvature throughout is positive and the Universe will eventually recollapse in a Big Crunch, possibly starting a new Universe in a Big Bounce. Conversely, if the Universe is insufficiently dense, k equals 0 or −1 and the Universe will expand forever, cooling off and eventually becoming inhospitable for all life, as the stars die and all matter coalesces into black holes (the Big Freeze and the heat death of the Universe). As noted above, recent data suggests that the expansion speed of the Universe is not decreasing as originally expected, but increasing; if this continues indefinitely, the Universe will eventually rip itself to shreds (the Big Rip). Observationally, the Universe appears to have an overall density that is very close to the critical value between recollapse and eternal expansion; more careful astronomical observations are needed to resolve the question.
Multiverse hypothesis
Main articles: Multiverse, Many-worlds interpretation, Bubble universe theory and Parallel universe (fiction)
Depiction of a multiverse of seven "bubble" universes, which are separate spacetime continua, each having different physical laws, physical constants, and perhaps even different numbers of dimensions or topologies.
See also: Eternal inflation
Some speculative theories have proposed that our Universe is but one of a set of disconnected universes, collectively denoted as the multiverse, challenging or enhancing more limited definitions of the Universe.[20][102] Scientific multiverse models are distinct from concepts such as alternate planes of consciousness and simulated reality.Max Tegmark developed a four-part classification scheme for the different types of multiverses that scientists have suggested in various problem domains. An example of such a model is the chaotic inflation model of the early universe.[103] Another is the many-worlds interpretation of quantum mechanics. Parallel worlds are generated in a manner similar to quantum superposition and decoherence, with all states of the wave function being realized in separate worlds. Effectively, the multiverse evolves as a universal wavefunction. If the Big Bang that created our multiverse created an ensemble of multiverses, the wave function of the ensemble would be entangled in this sense.
The least controversial category of multiverse in Tegmark's scheme is Level I, which describes distant spacetime events "in our own universe", but suggests that statistical analysis exploiting the anthropic principle provides an opportunity to test multiverse theories in some cases. If space is infinite, or sufficiently large and uniform, identical instances of the history of Earth's entire Hubble volume occur every so often, simply by chance. Tegmark calculated our nearest so-called doppelgänger, is 1010115 meters away from us (a double exponential function larger than a googolplex).[104][105] In principle, it would be impossible to scientifically verify an identical Hubble volume. However, it does follow as a fairly straightforward consequence from otherwise unrelated scientific observations and theories.
It is possible to conceive of disconnected spacetimes, each existing but unable to interact with one another.[104][106] An easily visualized metaphor is a group of separate soap bubbles, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle.[107] According to one common terminology, each "soap bubble" of spacetime is denoted as a universe, whereas our particular spacetime is denoted as the Universe,[20] just as we call our moon the Moon. The entire collection of these separate spacetimes is denoted as the multiverse.[20] With this terminology, different Universes are not causally connected to each other.[20] In principle, the other unconnected Universes may have different dimensionalities and topologies of spacetime, different forms of matter and energy, and different physical laws and physical constants, although such possibilities are purely speculative.[20] Others consider each of several bubbles created as part of chaotic inflation to be separate Universes, though in this model these universes all share a causal origin.[20]
Fine-tuned Universe
Main article: Fine-tuned Universe
The fine-tuned Universe is the proposition that the conditions that allow life in the Universe can only occur when certain universal fundamental physical constants
lie within a very narrow range, so that if any of several fundamental
constants were only slightly different, the Universe would be unlikely
to be conducive to the establishment and development of matter, astronomical structures, elemental diversity, or life as it is understood.[108] The proposition is discussed among philosophers, scientists, theologians, and proponents and detractors of creationism.Historical development
See also: Cosmology, Timeline of cosmology, Nicolaus Copernicus § Copernican system and Philosophiæ Naturalis Principia Mathematica § Beginnings of the Scientific Revolution
Historically, there have been many ideas of the cosmos (cosmologies)
and its origin (cosmogonies). Theories of an impersonal Universe
governed by physical laws were first proposed by the Greeks and Indians.[16]
Over the centuries, improvements in astronomical observations and
theories of motion and gravitation led to ever more accurate
descriptions of the Universe. The modern era of cosmology began with Albert Einstein's 1915 general theory of relativity,
which made it possible to quantitatively predict the origin, evolution,
and conclusion of the Universe as a whole. Most modern, accepted
theories of cosmology are based on general relativity and, more
specifically, the predicted Big Bang.[109]Mythologies
Many cultures have stories describing the origin of the world and universe. Cultures generally regard these stories as having some truth.[110] There are however many differing beliefs in how these stories apply amongst those believing in a supernatural origin, ranging from a god directly creating the Universe as it is now to a god just setting the "wheels in motion" (for example via mechanisms such as the big bang and evolution).[citation needed]Creation stories may be roughly grouped into common types. In one type of story, the world is born from a world egg; such stories include the Finnish epic poem Kalevala, the Chinese story of Pangu or the Indian Brahmanda Purana. In related stories, the Universe is created by a single entity emanating or producing something by him- or herself, as in the Tibetan Buddhism concept of Adi-Buddha, the ancient Greek story of Gaia (Mother Earth), the Aztec goddess Coatlicue myth, the ancient Egyptian god Atum story, and the Judeo-Christian Genesis creation narrative in which the Abrahamic God created the Universe. In another type of story, the Universe is created from the union of male and female deities, as in the Maori story of Rangi and Papa. In other stories, the Universe is created by crafting it from pre-existing materials, such as the corpse of a dead god — as from Tiamat in the Babylonian epic Enuma Elish or from the giant Ymir in Norse mythology – or from chaotic materials, as in Izanagi and Izanami in Japanese mythology. In other stories, the Universe emanates from fundamental principles, such as Brahman and Prakrti, the creation myth of the Serers,[111] or the yin and yang of the Tao.
Philosophical models
Further information: Cosmology
See also: Pre-Socratic philosophy, Physics (Aristotle), Hindu cosmology, Islamic cosmology and Philosophy of space and time
The pre-Socratic Greek philosophers and Indian philosophers developed some of the earliest philosophical concepts of the Universe.[16][112]
The earliest Greek philosophers noted that appearances can be
deceiving, and sought to understand the underlying reality behind the
appearances. In particular, they noted the ability of matter to change
forms (e.g., ice to water to steam) and several philosophers proposed
that all the physical materials in the world are different forms of a
single primordial material, or arche. The first to do so was Thales, who proposed this material is water. Thales' student, Anaximander, proposed that everything came from the limitless apeiron. Anaximenes proposed air on account of its perceived attractive and repulsive qualities that cause the arche to condense or dissociate into different forms. Anaxagoras proposed the principle of Nous (Mind). Heraclitus proposed fire (and spoke of logos). Empedocles proposed the elements: earth, water, air and fire. His four-element model became very popular. Like Pythagoras, Plato believed that all things were composed of number, with Empedocles' elements taking the form of the Platonic solids. Democritus, and later philosophers—most notably Leucippus—proposed that the Universe is composed of indivisible atoms moving through void (vacuum). Aristotle did not believe that was feasible because air, like water, offers resistance to motion. Air will immediately rush in to fill a void, and moreover, without resistance, it would do so indefinitely fast.Although Heraclitus argued for eternal change, his contemporary Parmenides made the radical suggestion that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature. Parmenides denoted this reality as τὸ ἐν (The One). Parmenides' idea seemed implausible to many Greeks, but his student Zeno of Elea challenged them with several famous paradoxes. Aristotle responded to these paradoxes by developing the notion of a potential countable infinity, as well as the infinitely divisible continuum. Unlike the eternal and unchanging cycles of time, he believed the world is bounded by the celestial spheres and that cumulative stellar magnitude is only finitely multiplicative.
The Indian philosopher Kanada, founder of the Vaisheshika school, developed a notion of atomism and proposed that light and heat were varieties of the same substance.[113] In the 5th century AD, the Buddhist atomist philosopher Dignāga proposed atoms to be point-sized, durationless, and made of energy. They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.[114]
The notion of temporal finitism was inspired by the doctrine of creation shared by the three Abrahamic religions: Judaism, Christianity and Islam. The Christian philosopher, John Philoponus, presented the philosophical arguments against the ancient Greek notion of an infinite past and future. Philoponus' arguments against an infinite past were used by the early Muslim philosopher, Al-Kindi (Alkindus); the Jewish philosopher, Saadia Gaon (Saadia ben Joseph); and the Muslim theologian, Al-Ghazali (Algazel).
Astronomical concepts
Main articles: History of astronomy and Timeline of astronomy
Aristarchus's 3rd century BCE calculations on the relative sizes of from left the Sun, Earth and Moon, from a 10th-century AD Greek copy
Later Greek philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the Universe based more profoundly on empirical evidence. The first coherent model was proposed by Eudoxus of Cnidos. According to Aristotle's physical interpretation of the model, celestial spheres eternally rotate with uniform motion around a stationary Earth. Normal matter is entirely contained within the terrestrial sphere.
De Mundo (composed before 250 BC or between 350 and 200 BC), stated, Five elements, situated in spheres in five regions, the less being in each case surrounded by the greater — namely, earth surrounded by water, water by air, air by fire, and fire by ether — make up the whole Universe.[115]
This model was also refined by Callippus and after concentric spheres were abandoned, it was brought into nearly perfect agreement with astronomical observations by Ptolemy. The success of such a model is largely due to the mathematical fact that any function (such as the position of a planet) can be decomposed into a set of circular functions (the Fourier modes). Other Greek scientists, such as the Pythagorean philosopher Philolaus, postulated (according to Stobaeus account) that at the center of the Universe was a "central fire" around which the Earth, Sun, Moon and Planets revolved in uniform circular motion.[116]
The Greek astronomer Aristarchus of Samos was the first known individual to propose a heliocentric model of the Universe. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes Aristarchus' heliocentric model. Archimedes wrote: (translated into English):
"You, King Gelon, are aware the Universe is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the Universe is many times greater than the Universe just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface"
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