Further observational improvements led to the realization that the Sun is one of a few hundred billion stars in theMilky Way, which is one of a few hundred billion galaxies in the observable universe. Many of the stars in a galaxyhave 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 inclusters andsuperclusters which form immensefilaments andvoids in space, creating a vast foam-like structure.[14] Discoveries in the early 20th century have suggested that the universe had a beginning and has been expanding since then.[15]
According to the Big Bang theory, the energy and matter initially present have become less dense as the universe expanded. After an initial accelerated expansion called theinflationary epoch at around 10−32 seconds, and the separation of the four knownfundamental forces, the universe gradually cooled and continued to expand, allowing the firstsubatomic particles and simpleatoms to form. Giant clouds ofhydrogen andhelium were gradually drawn to the places where matter was mostdense, forming the first galaxies, stars, and everything else seen today.
From studying the effects ofgravity on both matter and light, it has been discovered that the universe contains much morematter than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter is known asdark matter.[16] In the widely acceptedΛCDM cosmological model, dark matter accounts for about25.8%±1.1% of the mass and energy in the universe while about69.2%±1.2% isdark energy, a mysterious form of energy responsible for theacceleration of theexpansion of the universe.[17] Ordinary ('baryonic') matter therefore composes only4.84%±0.1% of the universe.[17] Stars, planets, and visible gas clouds only form about 6% of this ordinary matter.[18]
There are many competing hypotheses about theultimate 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 ever be accessible. Some physicists have suggested variousmultiverse hypotheses, in which the universe might be one among many.[3][19][20]
The universe is often defined as "the totality of existence", oreverything that exists, everything that has existed, and everything that will exist.[24] 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.[26][27][28] The worduniverse may also refer to concepts such asthecosmos,theworld, andnature.[29][30]
Etymology
The worduniverse derives from theOld French wordunivers, which in turn derives from theLatin worduniversus, meaning 'combined into one'.[31] The Latin word 'universum' was used byCicero and later Latin authors in many of the same senses as the modernEnglish word is used.[32]
Synonyms
A term foruniverse among the ancient Greek philosophers fromPythagoras onwards wasτὸ πᾶν (tò pân) 'the all', defined as all matter and all space, andτὸ ὅλον (tò hólon) 'all things', which did not necessarily include the void.[33][34] Another synonym wasὁ κόσμος (ho kósmos) meaning 'theworld, thecosmos'.[35] Synonyms are also found in Latin authors (totum,mundus,natura)[36] and survive in modern languages, e.g., theGerman wordsDas All,Weltall, andNatur foruniverse. The same synonyms are found in English, such as everything (as in thetheory of everything), the cosmos (as incosmology), the world (as in themany-worlds interpretation), andnature (as innatural laws ornatural philosophy).[37]
The prevailing model for the evolution of the universe is theBig Bang theory.[38][39] The Big Bang model states that the earliest state of the universe was an extremely hot and dense one, and that the universe subsequently expanded and cooled. The model is based ongeneral relativity and on simplifying assumptions such as thehomogeneity andisotropy of space. A version of the model with acosmological constant (Lambda) andcold dark matter, known as theLambda-CDM model, is the simplest model that provides a reasonably good account of various observations about the universe.
In this schematic diagram, time passes from left to right, with the universe represented by a disk-shaped "slice" at any given time. Time and size are not to scale. To make the early stages visible, the time to the afterglow stage (really the first 0.003%) is stretched and the subsequent expansion (really by 1,100 times to the present) is largely suppressed.
The initial hot, dense state is called thePlanck epoch, a brief period extending from time zero to onePlanck 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, andgravity—currently the weakest by far of thefour known forces—is believed to have been as strong as the other fundamental forces, and all the forces may have beenunified. The physics controlling this very early period (includingquantum gravity in the Planck epoch) is not understood, so we cannot say what, if anything, happenedbefore time zero. Since the Planck epoch,the universe has been expanding to its present scale, with a very short but intense period ofcosmic inflation speculated to have occurred within the first10−32 seconds.[40] This initial period of inflation would explain why space appears to bevery flat.
Within the first fraction of a second of the universe's existence, the four fundamental forces had separated. As the universe continued to cool from its inconceivably hot state, various types ofsubatomic particles were able to form in short periods of time known as thequark epoch, thehadron epoch, and thelepton epoch. Together, these epochs encompassed less than 10 seconds of time following the Big Bang. Theseelementary particles associated stably into ever larger combinations, including stableprotons andneutrons, which then formed more complexatomic nuclei throughnuclear fusion.[41][42]
This process, known asBig Bang nucleosynthesis, lasted for about 17 minutes and ended about 20 minutes after the Big Bang, so only the fastest and simplest reactions occurred. About 25% of theprotons and all theneutrons in the universe, by mass, were converted tohelium, with small amounts ofdeuterium (aform ofhydrogen) and traces oflithium. Any otherelement was only formed in very tiny quantities. The other 75% of the protons remained unaffected, ashydrogen nuclei.[41][42]: 27–42
After nucleosynthesis ended, the universe entered a period known as thephoton epoch. During this period, the universe was still far too hot for matter to form neutralatoms, so it contained a hot, dense, foggyplasma of negatively chargedelectrons, neutralneutrinos and positive nuclei. After about 377,000 years, the universe had cooled enough that electrons and nuclei could form the first stableatoms. This is known asrecombination for historical reasons; electrons and nuclei were combining for the first time. Unlike plasma, neutral atoms aretransparent to manywavelengths of light, so for the first time the universe also became transparent. The photons released ("decoupled") when these atoms formed can still be seen today; they form thecosmic microwave background (CMB).[42]: 15–27
In the earliest stages of the universe, tiny fluctuations within the universe's density led toconcentrations ofdark matter gradually forming. Ordinary matter, attracted to these bygravity, formed large gas clouds and eventually, stars and galaxies, where the dark matter was most dense, andvoids where it was least dense. After around 100–300 million years,[43]: 333 the firststars formed, known asPopulation III stars. These were probably very massive, luminous,non metallic and short-lived. They were responsible for the gradualreionization of the universe between about 200–500 million years and 1 billion years, and also for seeding the universe with elements heavier than helium, throughstellar nucleosynthesis.[44]
The universe also contains a mysterious energy—possibly ascalar field—calleddark energy, the density of which 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 presentdark-energy-dominated era.[45] In this era, the expansion of the universe isaccelerating due to dark energy.
Of the fourfundamental interactions,gravitation is the dominant at astronomical length scales. Gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on astronomical length scales. The remaining two interactions, theweak andstrong nuclear forces, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.[46]: 1470
The universe appears to have much morematter thanantimatter, an asymmetry possibly related to theCP violation.[47] This imbalance between matter and antimatter is partially responsible for the existence of all matter existing today, since matter and antimatter, if equally produced at theBig Bang, would have completely annihilated each other and left onlyphotons as a result of their interaction.[48]
Illustration of the observable universe, centered on the Sun. The distance scale islogarithmic. Due to the finite speed of light, we see more distant parts of the universe at earlier times.
Due to the finitespeed of light, there is a limit (known as theparticle horizon) to how far light can travel over theage of the universe.The spatial region from which we can receive light is called theobservable universe. Theproper distance (measured at a fixed time) between Earth and the edge of the observable universe is 46 billion light-years[49][50] (14 billionparsecs), making thediameter of the observable universe about 93 billion light-years (28 billion parsecs).[49] Although the distance traveled by light from the edge of the observable universe is close to theage of the universe times the speed of light, 13.8 billion light-years (4.2×10^9 pc), the proper distance is larger because the edge of the observable universe and the Earth have since moved further apart.[51]
For comparison, the diameter of a typicalgalaxy is 30,000 light-years (9,198parsecs), and the typical distance between two neighboring galaxies is 3 millionlight-years (919.8 kiloparsecs).[52] As an example, theMilky Way is roughly 100,000–180,000 light-years in diameter,[53][54] and the nearest sister galaxy to the Milky Way, theAndromeda Galaxy, is located roughly 2.5 million light-years away.[55]
Because humans cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite.[3][56][57] An estimate from 2011 suggests that if thecosmological principle holds, the whole universe must be more than 250 times larger than aHubble sphere.[58] Some disputed[59] estimates for the total size of the universe, if finite, reach as high as megaparsecs, as implied by a suggested resolution of theNo-Boundary Proposal.[60][b]
Assuming that theLambda-CDM model is correct, the measurements of the parameters using a variety of techniques by numerous experiments yield a best value of the age of the universe at 13.799± 0.021 billion years, as of 2015.[2]
Over time, the universe and its contents have evolved. For example, the relative population ofquasars and galaxies has changed[61] and theuniverse has expanded. This expansion is inferred from the observation that the light from distant galaxies has beenredshifted, which implies that the galaxies are receding from us. Analyses ofType Ia supernovae indicate that theexpansion is accelerating.[62][63]
The more matter there is in the universe, the stronger the mutualgravitational pull of the matter. If the universe weretoo dense then it would re-collapse into agravitational singularity. However, if the universe contained toolittle matter then the self-gravity would be too weak for astronomical structures, like galaxies or planets, to form. Since the Big Bang, the universe has expandedmonotonically.Perhaps unsurprisingly, our universe hasjust the right mass–energy density, equivalent to about 5 protons per cubic meter, which has allowed it to expand for the last 13.8 billion years, giving time to form the universe as observed today.[64][65]
There are dynamical forces acting on the particles in the universe which affect the expansion rate. Before 1998, it was expected that the expansion rate 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 thedeceleration parameter, which most cosmologists expected to be positive and related to the matter density of the universe. In 1998, the deceleration parameter was measured by two different groups to be negative, approximately −0.55, which technically implies that the second derivative of the cosmicscale factor has been positive in the last 5–6 billion years.[66][67]
Modern physics regardsevents as being organized intospacetime.[68] This idea originated with thespecial theory of relativity, which predicts that if one observer sees two events happening in different places at the same time, a second observer who is moving relative to the first will see those events happening at different times.[69]: 45–52 The two observers will disagree on the time between the events, and they will disagree about the distance separating the events, but they will agree on thespeed of light, and they will measure the same value for the combination.[69]: 80 The square root of theabsolute value of this quantity is called theinterval between the two events. The interval expresses how widely separated events are, not just in space or in time, but in the combined setting of spacetime.[69]: 84, 136 [70]
The special theory of relativity describes a flat spacetime. Its successor, thegeneral theory of relativity, explainsgravity as curvature ofspacetime arising due to its energy content. A curved path like an orbit is not the result of a force deflecting a body from an ideal straight-line path, but rather the body's attempt to fall freely through a background that is itself curved by the presence of other masses. A remark byJohn Archibald Wheeler that has become proverbial among physicists summarizes the theory: "Spacetime tells matter how to move; matter tells spacetime how to curve",[71][72] and therefore there is no point in considering one without the other.[15] TheNewtonian theory of gravity is a good approximation to the predictions of general relativity when gravitational effects are weak and objects are moving slowly compared to the speed of light.[73]: 327 [74]
The relation between matter distribution and spacetime curvature is given by theEinstein field equations, which requiretensor calculus to express.[75]: 43 [76] The universe appears to be a smooth spacetime continuum consisting of threespatialdimensions and one temporal (time) dimension. Therefore, an event in the spacetime of the physical universe can be identified by a set of four coordinates:(x,y,z,t).
The three possible options for the shape of the universe
Cosmologists often work withspace-like slices of spacetime that are surfaces of constant time incomoving coordinates. The geometry of these spatial slices is set by thedensity parameter, Omega (Ω), defined as the average matter density of the universe divided by a critical value. This selects one of three possiblegeometries depending on whether Ω is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes.[77]
The fine-tuned universe hypothesis is the proposition that the conditions that allow the existence of observablelife in the universe can only occur when certain universalfundamental physical constants lie within a very narrow range of values. According to this hypothesis, if any of several fundamental constants were only slightly different, the universe would have been unlikely to be conducive to the establishment and development ofmatter, astronomical structures, elemental diversity, or life as it is understood. Whether this is true, and whether that question is even logically meaningful to ask, are subjects of much debate.[84] The proposition is discussed amongphilosophers,scientists,theologians, and proponents ofcreationism.[85]
The proportions of all types of matter and energy have changed over the history of the universe.[89] The total amount of electromagnetic radiation generated within the universe has decreased by 1/2 in the past 2 billion years.[90][91] Today, ordinary matter, which includes atoms, stars, galaxies, andlife, accounts for only 4.9% of the contents of the universe.[8] The present overalldensity of this type of matter is very low, roughly 4.5 × 10−31 grams per cubic centimeter, corresponding to a density of the order of only one proton for every four cubic meters of volume.[6] The nature of both dark energy and dark matter is unknown. Dark matter, a mysterious form of matter that has not yet been identified, accounts for 26.8% of the cosmic contents. Dark energy, which is the energy of empty space and is causing the expansion of the universe to accelerate, accounts for the remaining 68.3% of the contents.[8][92][93]
The formation of clusters and large-scalefilaments in thecold dark matter model withdark 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 andvoids nearest to Earth
Matter, dark matter, and dark energy are distributed homogeneously throughout the universe over length scales longer than 300 million light-years (ly) or so.[94] However, over shorter length-scales, matter tends to clump hierarchically; manyatoms are condensed intostars, most stars into galaxies, most galaxies intoclusters, superclusters and, finally, large-scalegalactic filaments. The observable universe contains as many as an estimated 2 trillion galaxies[95][96][97] and, overall, as many as an estimated 1024 stars[98][99] – more stars (and earth-like planets) than all thegrains of beach sand on planetEarth;[100][101][102] but less than the total number of atoms estimated in the universe as 1082;[103] and the estimated total number of stars in aninflationary universe (observed and unobserved), as 10100.[104] Typical galaxies range fromdwarfs with as few as ten million[105] (107) stars up to giants with onetrillion[106] (1012) stars. Between the larger structures arevoids, which are typically 10–150 Mpc (33 million–490 million ly) in diameter. TheMilky Way is in theLocal Group of galaxies, which in turn is in theLaniakea Supercluster.[107] This supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years.[108] The universe also has vast regions of relative emptiness; the largest known void measures 1.8 billion ly (550 Mpc) across.[109]
Comparison of the contents of the universe today to 380,000 years after the Big Bang, as measured with 5 year WMAP data (from 2008).[110] Due to rounding, the sum of these numbers is not 100%.
The observable universe isisotropic on scales significantly larger than superclusters, meaning that the statistical properties of the universe are the same in all directions as observed from Earth. The universe is bathed in highly isotropicmicrowaveradiation that corresponds to athermal equilibriumblackbody spectrum of roughly 2.72548kelvins.[7] The hypothesis that the large-scale universe is homogeneous and isotropic is known as thecosmological principle.[111] A universe that is both homogeneous and isotropic looks the same from all vantage points and has no center.[112][113]
An explanation for why the expansion of the universe is accelerating remains elusive. It is often attributed to the gravitational influence of "dark energy", an unknown form of energy that is hypothesized to permeate space.[114] On amass–energy equivalence basis, the density of dark energy (~ 7 × 10−30 g/cm3) 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.[115][116]
Two proposed forms for dark energy are thecosmological constant, aconstant energy density filling space homogeneously,[117] andscalar fields such asquintessence ormoduli,dynamic quantities whose energy density can vary in time and space while still permeating them enough to cause the observed rate of expansion. 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 tovacuum energy.
Dark matter is a hypothetical kind ofmatter that is invisible to the entireelectromagnetic spectrum, 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 thelarge-scale structure of the universe. Other thanneutrinos, a form ofhot dark matter, dark matter has not been detected directly, making it one of the greatest mysteries in modernastrophysics. Dark matter neitheremits nor absorbs light or any otherelectromagnetic 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.[92][118]
The remaining 4.9% of the mass–energy of the universe is ordinary matter, that is,atoms,ions,electrons and the objects they form. This matter includesstars, which produce nearly all of the light we see from galaxies, as well as interstellar gas in theinterstellar andintergalactic media,planets, and all the objects from everyday life that we can bump into, touch or squeeze.[119] The great majority of ordinary matter in the universe is unseen, since visible stars and gas inside galaxies and clusters account for less than 10 percent of the ordinary matter contribution to the mass–energy density of the universe.[120][121][122]
Soon after theBig Bang, primordial protons and neutrons formed from thequark–gluon plasma of the early universe as it cooled below two trillion degrees. A few minutes later, in a process known asBig Bang nucleosynthesis, nuclei formed from the primordial protons and neutrons. This nucleosynthesis formed lighter elements, those with small atomic numbers up tolithium andberyllium, but the abundance of heavier elements dropped off sharply with increasing atomic number. Someboron may have been formed at this time, but the next heavier element,carbon, was not formed in significant amounts. Big Bang nucleosynthesis shut down after about 20 minutes due to the rapid drop in temperature and density of the expanding universe. Subsequent formation ofheavier elements resulted fromstellar nucleosynthesis andsupernova nucleosynthesis.[127]
Particles
Standard model of elementary particles: the 12 fundamental fermions and 4 fundamental bosons. Brown loops indicate which bosons (red) couple to which fermions (purple and green). Columns are three generations of matter (fermions) and one of forces (bosons). In the first three columns, two rows contain quarks and two leptons. The top two rows' columns contain up (u) and down (d) quarks, charm (c) and strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and gluon (g), respectively. The bottom two rows' columns contain electron neutrino (νe) and electron (e), muon neutrino (νμ) and muon (μ), tau neutrino (ντ) and tau (τ), and the Z0 and W± carriers of the weak force. Mass, charge, and spin are listed for each particle.
Ordinary matter and the forces that act on matter can be described in terms ofelementary particles.[128] These particles are sometimes described as being fundamental, since they have an unknown substructure, and it is unknown whether or not they are composed of smaller and even more fundamental particles.[129][130] In most contemporary models they are thought of as points in space.[131] All elementary particles are currently best explained byquantum mechanics and exhibitwave–particle duality: their behavior has both particle-like andwave-like aspects, with different features dominating under different circumstances.[132]
Of central importance is theStandard Model, a theory that is concerned withelectromagnetic interactions and theweak andstrong nuclear interactions.[133] The Standard Model is supported by the experimental confirmation of the existence of particles that compose matter:quarks andleptons, and their corresponding "antimatter" duals, as well as the force particles that mediateinteractions: thephoton, theW and Z bosons, and thegluon.[129] The Standard Model predicted the existence of the recently discoveredHiggs boson, a particle that is a manifestation of a field within the universe that can endow particles with mass.[134][135] Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a "theory of almost everything".[133] The Standard Model does not, however, accommodate gravity. A true force–particle "theory of everything" has not been attained.[136]
A hadron is acomposite particle made ofquarksheld together by thestrong force. Hadrons are categorized into two families:baryons (such asprotons andneutrons) made of three quarks, andmesons (such aspions) made of one quark and oneantiquark. Of the hadrons, protons are stable, and neutrons bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions and are thus insignificant constituents of the modern universe.[137]: 118–123
From approximately 10−6 seconds after theBig Bang, during a period known as thehadron epoch, the temperature of the universe had fallen sufficiently to allow quarks to bind together into hadrons, and the mass of the universe was dominated byhadrons. Initially, the temperature was high enough to allow the formation of hadron–anti-hadron pairs, which kept matter and antimatter inthermal 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 particle–antiparticleannihilation reactions, leaving a small residual of hadrons by the time the universe was about one second old.[137]: 244–266
A lepton is anelementary,half-integer spin particle that does not undergo strong interactions but is subject to thePauli exclusion principle; no two leptons of the same species can be in exactly the same state at the same time.[138] Two main classes of leptons exist:charged leptons (also known as theelectron-like leptons), and neutral leptons (better known asneutrinos). Electrons are stable and the most common charged lepton in the universe, whereasmuons andtaus are unstable particles that quickly decay after being produced inhigh energy collisions, such as those involvingcosmic rays or carried out inparticle accelerators.[139][140] Charged leptons can combine with other particles to form variouscomposite particles such asatoms andpositronium. Theelectron governs nearly all ofchemistry, as it is found inatoms and is directly tied to allchemical properties. Neutrinos rarely interact with anything, and are consequently rarely observed. Neutrinos stream throughout the universe but rarely interact with normal matter.[141]
Thelepton epoch was the period in the evolution of the early universe in which theleptons dominated the mass of the universe. It started roughly 1 second after theBig Bang, after the majority of hadrons and anti-hadrons annihilated each other at the end of thehadron 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.[142] Most leptons and anti-leptons were then eliminated inannihilation reactions, leaving a small residue of leptons. The mass of the universe was then dominated byphotons as it entered the followingphoton epoch.[143][144]
The photon epoch started after most leptons and anti-leptons wereannihilated 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 denseplasma 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 and the universe became transparent. The highly redshifted photons from this period form the cosmic microwave background. Tiny variations in the temperature of the CMB correspond to variations in the density of the universe that were the early "seeds" from which all subsequentstructure formation took place.[137]: 244–266
The relation is specified by theEinstein field equations, a system ofpartial differential equations. In general relativity, the distribution of matter and energy determines the geometry of spacetime, which in turn describes theacceleration of matter. Therefore, solutions of the Einstein field equations describe the evolution of the universe. Combined with measurements of the amount, type, and distribution of matter in the universe, the equations of general relativity describe the evolution of the universe over time.[146]
where (r, θ, φ) correspond to aspherical coordinate system. This metric has only two undetermined parameters. An overalldimensionless lengthscale factorR describes the size scale of the universe as a function of time (an increase inR is theexpansion of the universe),[147] and a curvature indexk describes the geometry. The indexk is defined so that it can take only one of three values: 0, corresponding to flatEuclidean geometry; 1, corresponding to a space of positivecurvature; or −1, corresponding to a space of positive or negative curvature.[148] The value ofR as a function of timet depends uponk and thecosmological constantΛ.[146] The cosmological constant represents the energy density of the vacuum of space and could be related to dark energy.[93] The equation describing howR varies with time is known as theFriedmann equation after its inventor,Alexander Friedmann.[149]
The solutions forR(t) depend onk andΛ, but some qualitative features of such solutions are general. First and most importantly, the length scaleR of the universe can remain constantonly if the universe is perfectly isotropic with positive curvature (k = 1) and has one precise value of density everywhere, as first noted byAlbert Einstein.[146]
Second, all solutions suggest that there was agravitational singularity in the past, whenR went to zero and matter and energy were 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, thePenrose–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 (whenR had a small, finite value); this is the essence of theBig Bang model of the universe. Understanding the singularity of the Big Bang likely requires aquantum theory of gravity, which has not yet been formulated.[150]
Third, the curvature indexk determines the sign of the curvature of constant-time spatial surfaces[148] averaged over sufficiently large length scales (greater than about a billionlight-years). Ifk = 1, the curvature is positive and the universe has a finite volume.[151] A universe with positive curvature is often visualized as athree-dimensional sphere embedded in a four-dimensional space. Conversely, ifk is zero or negative, the universe has an infinite volume.[151] It may seem counter-intuitive that an infinite and yet infinitely dense universe could be created in a single instant whenR = 0, but exactly that is predicted mathematically whenk is nonpositive and thecosmological principle is satisfied. By analogy, an infinite plane has zero curvature but infinite area, whereas an infinite cylinder is finite in one direction and atorus is finite in both.
Theultimate fate of the universe is still unknown because it depends critically on the curvature indexk and the cosmological constantΛ. If the universe were sufficiently dense,k would equal +1, meaning that its average curvature throughout is positive and the universe will eventually recollapse in aBig Crunch,[152] possibly starting a new universe in aBig Bounce. Conversely, if the universe were insufficiently dense,k would equal 0 or −1 and the universe would expand forever, cooling off and eventually reaching theBig Freeze and theheat death of the universe.[146] Modern data suggests that theexpansion of the universe is accelerating; if this acceleration is sufficiently rapid, the universe may eventually reach aBig Rip. Observationally, the universe appears to be flat (k = 0), with an overall density that is very close to the critical value between recollapse and eternal expansion.[153]
Some speculative theories have proposed that our universe is but one of aset of disconnected universes, collectively denoted as themultiverse, challenging or enhancing more limited definitions of the universe.[19][154]Max Tegmark developed a four-partclassification scheme for the different types of multiverses that scientists have suggested in response to various problems inphysics. An example of such multiverses is the one resulting from thechaotic inflation model of the early universe.[155]
Another is the multiverse resulting from themany-worlds interpretation of quantum mechanics. In this interpretation, parallel worlds are generated in a manner similar toquantum superposition anddecoherence, with all states of thewave functions being realized in separate worlds. Effectively, in the many-worlds interpretation the multiverse evolves as auniversal 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.[156] Whether scientifically meaningful probabilities can be extracted from this picture has been and continues to be a topic of much debate, and multiple versions of the many-worlds interpretation exist.[157][158][159] The subject of theinterpretation of quantum mechanics is in general marked by disagreement.[160][161][162]
The least controversial, but still highly disputed, category of multiverse in Tegmark's scheme isLevel I. The multiverses of this level are composed by distant spacetime events "in our own universe". Tegmark and others[163] have argued that, if space is infinite, or sufficiently large and uniform, identical instances of the history of Earth's entireHubble volume occur every so often, simply by chance. Tegmark calculated that our nearest so-calleddoppelgänger is 1010115 metres away from us (adouble exponential function larger than agoogolplex).[164][165] However, the arguments used are of speculative nature.[166]
It is possible to conceive of disconnected spacetimes, each existing but unable to interact with one another.[164][167] An easily visualized metaphor of this concept is a group of separatesoap bubbles, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle.[168] According to one common terminology, each "soap bubble" of spacetime is denoted as auniverse, whereas humans' particular spacetime is denoted asthe universe,[19] just as humans call Earth's moontheMoon. The entire collection of these separate spacetimes is denoted as the multiverse.[19]
With this terminology, differentuniverses are notcausally connected to each other.[19] In principle, the other unconnecteduniverses may have differentdimensionalities andtopologies of spacetime, different forms ofmatter andenergy, and differentphysical laws andphysical constants, although such possibilities are purely speculative.[19] Others consider each of several bubbles created as part ofchaotic inflation to be separateuniverses, though in this model these universes all share a causal origin.[19]
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.[13] Ancient Chinese philosophy encompassed the notion of the universe including both all of space and all of time.[169] 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 withAlbert Einstein's 1915general 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 predictedBig Bang.[170]
Many cultures havestories describing the origin of the world and universe. Cultures generally regard these stories as having sometruth. 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).[171]
Thepre-Socratic Greek philosophers andIndian philosophers developed some of the earliest philosophical concepts of the universe.[13][175] 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, orarche. The first to do so wasThales, who proposed this material to bewater. Thales' student,Anaximander, proposed that everything came from the limitlessapeiron.Anaximenes proposed the primordial material to beair on account of its perceived attractive and repulsive qualities that cause thearche to condense or dissociate into different forms.Anaxagoras proposed the principle ofNous (Mind), whileHeraclitus proposedfire (and spoke oflogos).Empedocles proposed the elements to be earth, water, air and fire. His four-element model became very popular. LikePythagoras,Plato believed that all things were composed ofnumber, with Empedocles' elements taking the form of thePlatonic solids.Democritus, and later philosophers—most notablyLeucippus—proposed that the universe is composed of indivisibleatoms moving through avoid (vacuum), althoughAristotle did not believe that to be feasible because air, like water, offersresistance to motion. Air will immediately rush in to fill a void, and moreover, without resistance, it would do so indefinitely fast.[13]
Although Heraclitus argued for eternal change,[176] his contemporaryParmenides emphasized changelessness. Parmenides' poemOn Nature has been read as saying that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature, or at least that the essential feature of each thing that exists must exist eternally, without origin, change, or end.[177] His studentZeno of Elea challenged everyday ideas about motion with several famousparadoxes. Aristotle responded to these paradoxes by developing the notion of a potential countable infinity, as well as the infinitely divisible continuum.[178][179]
TheIndian philosopherKanada, founder of theVaisheshika school, developed a notion ofatomism and proposed thatlight andheat were varieties of the same substance.[180] In the 5th century AD, theBuddhist atomist philosopherDignāga proposedatoms 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.[181]
Pantheism is thephilosophicalreligious belief that the universe itself is identical todivinity and asupreme being or entity.[183] The physical universe is thus understood as an all-encompassing,immanent deity.[184] The term 'pantheist' designates one who holds both that everything constitutes a unity and that this unity is divine, consisting of an all-encompassing, manifestedgod orgoddess.[185][186]
LaterGreek philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the universe based more profoundly onempirical evidence. The first coherent model was proposed byEudoxus of Cnidos, a student of Plato who followed Plato's idea that heavenly motions had to be circular. In order to account for the known complications of the planets' motions, particularlyretrograde movement, Eudoxus' model included 27 differentcelestial spheres: four for each of the planets visible to the naked eye, three each for the Sun and the Moon, and one for the stars. All of these spheres were centered on the Earth, which remained motionless while they rotated eternally. Aristotle elaborated upon this model, increasing the number of spheres to 55 in order to account for further details of planetary motion. For Aristotle, normalmatter was entirely contained within the terrestrial sphere, and it obeyed fundamentally different rules fromheavenly material.[191][192]
The post-Aristotle treatiseDe Mundo (of uncertain authorship and date) 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".[193] This model was also refined byCallippus and after concentric spheres were abandoned, it was brought into nearly perfect agreement with astronomical observations byPtolemy.[194] 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 (theFourier modes). Other Greek scientists, such as thePythagorean philosopherPhilolaus, postulated (according toStobaeus' account) that at the center of the universe was a "central fire" around which theEarth,Sun,Moon andplanets revolved in uniform circular motion.[195]
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.[196]
Aristarchus thus believed the stars to be very far away, and saw this as the reason whystellar parallax had not been observed, that is, the stars had not been observed to move relative each other as the Earth moved around the Sun. The stars are in fact much farther away than the distance that was generally assumed in ancient times, which is why stellar parallax is only detectable with precision instruments. The geocentric model, consistent with planetary parallax, was assumed to be the explanation for the unobservability of stellar parallax.[197]
The only other astronomer from antiquity known by name who supported Aristarchus's heliocentric model wasSeleucus of Seleucia, aHellenistic astronomer who lived a century after Aristarchus.[198][199][200] According to Plutarch, Seleucus was the first to prove the heliocentric system throughreasoning, but it is not known what arguments he used. Seleucus' arguments for a heliocentric cosmology were probably related to the phenomenon oftides.[201] According toStrabo (1.1.9), Seleucus was the first to state that the tides are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.[202] Alternatively, he may have proved heliocentricity by determining the constants of ageometric model for it, and by developing methods to compute planetary positions using this model, similar toNicolaus Copernicus in the 16th century.[203] During theMiddle Ages,heliocentric models were also proposed by thePersian astronomersAlbumasar[204] andAl-Sijzi.[205]
The Aristotelian model was accepted in theWestern world for roughly two millennia, until Copernicus revived Aristarchus's perspective that the astronomical data could be explained more plausibly if theEarth rotated on its axis and if theSun were placed at the center of the universe.[206]
In the center rests the Sun. For who would place this lamp of a very beautiful temple in another or better place than this wherefrom it can illuminate everything at the same time?
— Nicolaus Copernicus, in Chapter 10, Book 1 ofDe Revolutionibus Orbium Coelestrum (1543)
This cosmology was accepted byIsaac Newton,Christiaan Huygens and later scientists.[210] Newton demonstrated that the samelaws of motion and gravity apply to earthly and to celestial matter, making Aristotle's division between the two obsolete.Edmund Halley (1720)[211] andJean-Philippe de Chéseaux (1744)[212] noted independently that the assumption of an infinite space filled uniformly with stars would lead to the prediction that the nighttime sky would be as bright as the Sun itself; this became known asOlbers' paradox in the 19th century.[213] Newton believed that an infinite space uniformly filled with matter would cause infinite forces and instabilities causing the matter to be crushed inwards under its own gravity.[210] This instability was clarified in 1902 by theJeans instability criterion.[214] One solution to these paradoxes is theCharlier universe, in which the matter is arranged hierarchically (systems of orbiting bodies that are themselves orbiting in a larger system,ad infinitum) in afractal way such that the universe has a negligibly small overall density; such a cosmological model had also been proposed earlier in 1761 byJohann Heinrich Lambert.[52][215]
Deep space astronomy
During the 18th century,Immanuel Kant speculated thatnebulae could be entire galaxies separate from the Milky Way,[211] and in 1850,Alexander von Humboldt called these separate galaxiesWeltinseln, or "world islands", a term that later developed into "island universes".[216][217] In 1919, when theHooker Telescope was completed, the prevailing view was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope,Edwin Hubble identifiedCepheid variables in several spiral nebulae and in 1922–1923 proved conclusively thatAndromeda Nebula andTriangulum among others, were entire galaxies outside our own, thus proving that the universe consists of a multitude of galaxies.[218] With this Hubble formulated theHubble constant, which allowed for the first time a calculation of the age of the Universe and size of the Observable Universe, which became increasingly precise with better meassurements, starting at 2 billion years and 280 million light-years, until 2006 when data of theHubble Space Telescope allowed a very accurate calculation of the age of the Universe and size of the Observable Universe.[219]
The modern era ofphysical cosmology began in 1917, whenAlbert Einstein first applied hisgeneral theory of relativity to model the structure and dynamics of the universe.[220] The discoveries of this era, and the questions that remain unanswered, are outlined in the sections above.
Map of the observable universe with some of the notable astronomical objects known as of 2018. The scale of length increases exponentially toward the right. Celestial bodies are shown enlarged in size to be able to understand their shapes.
^Although listed inmegaparsecs by the cited source, this number is so vast that its digits would remain virtually unchanged for all intents and purposes regardless of which conventional units it is listed in, whether it to benanometers orgigaparsecs, as the differences would disappear into the error.
^abZeilik, Michael; Gregory, Stephen A. (1998).Introductory Astronomy & Astrophysics (4th ed.). Saunders College.ISBN978-0-03-006228-5.The totality of all space and time; all that is, has been, and will be.
^Dold-Samplonius, Yvonne (2002).From China to Paris: 2000 Years Transmission of Mathematical Ideas. Franz Steiner Verlag.
^abcdGlick, Thomas F.; Livesey, Steven; Wallis, Faith (2005).Medieval Science Technology and Medicine: An Encyclopedia. Routledge.ISBN978-0-415-96930-7.
^abSteane, Andrew M. (2021).Relativity Made Relatively Easy, Volume 2: General Relativity and Cosmology. Oxford University Press.ISBN978-0-192-89564-6.
^Larson, Richard B. & Bromm, Volker (March 2002)."The First Stars in the Universe".Scientific American.Archived from the original on June 11, 2015. RetrievedJune 9, 2015.
^Ryden, Barbara, "Introduction to Cosmology", 2006, eqn. 6.33
^"Antimatter". Particle Physics and Astronomy Research Council. October 28, 2003. Archived fromthe original on March 7, 2004. RetrievedAugust 10, 2006.
^Crane, Leah (June 29, 2024). de Lange, Catherine (ed.). "How big is the universe, really?".New Scientist. p. 31.
^Crockett, Christopher (February 20, 2013)."What is a light-year?".EarthSky.Archived from the original on February 20, 2015. RetrievedFebruary 20, 2015.
^Friederich, Simon (November 12, 2021)."Fine-Tuning".The Stanford Encyclopedia of Philosophy. Center for the Study of Language and Information (CSLI), Stanford University.Archived from the original on October 10, 2023. RetrievedFebruary 15, 2022.
^"Physics – for the 21st Century".learner.org. Harvard-Smithsonian Center for Astrophysics Annenberg Learner. Archived fromthe original on September 7, 2015. RetrievedJuly 27, 2015.
^abSean Carroll, Ph.D., Caltech, 2007, The Teaching Company,Dark Matter, Dark Energy: The Dark Side of the Universe, Guidebook Part 2. p. 46, Accessed October 7, 2013, "...dark matter: An invisible, essentially collisionless component of matter that makes up about 25 percent of the energy density of the universe... it's a different kind of particle... something not yet observed in the laboratory..."
^Strassler, M. (October 12, 2012)."The Higgs FAQ 2.0".ProfMattStrassler.com.Archived from the original on October 12, 2013. RetrievedJanuary 8, 2013.[Q] Why do particle physicists care so much about the Higgs particle? [A] Well, actually, they don't. What they really care about is the Higgsfield, because it isso important. [emphasis in original]
^Weinberg, Steven (2011).Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature. Knopf Doubleday Publishing Group.ISBN978-0-307-78786-6.
^abcAllday, Jonathan (2002).Quarks, Leptons and the Big Bang (2nd ed.). IOP Publishing.ISBN978-0-7503-0806-9.
^Harari, H. (1977). "Beyond charm". In Balian, R.; Llewellyn-Smith, C.H. (eds.).Weak and Electromagnetic Interactions at High Energy, Les Houches, France, Jul 5 – Aug 14, 1976. Les Houches Summer School Proceedings. Vol. 29.North-Holland. p. 613.
^Harari H. (1977)."Three generations of quarks and leptons"(PDF). In E. van Goeler; Weinstein R. (eds.).Proceedings of the XII Rencontre de Moriond. p. 170. SLAC-PUB-1974.Archived(PDF) from the original on May 13, 2020. RetrievedMay 29, 2020.
^"First few minutes".Eric Chaisson. Harvard Smithsonian Center for Astrophysics.Archived from the original on December 4, 2013. RetrievedJanuary 6, 2016.
^Dick, Steven J. (2020). "The Biophysical Cosmology: The Place of Bioastronomy in the History of Science".Space, Time, and Aliens. Cham: Springer International Publishing. pp. 53–58.doi:10.1007/978-3-030-41614-0_4.ISBN978-3-030-41613-3.
^abcdeZeilik, Michael; Gregory, Stephen A. (1998). "25-2".Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing.ISBN978-0-03-006228-5.
^Gil, Francisco José Soler; Alfonseca, Manuel (2013). "About the Infinite Repetition of Histories in Space".Theoria: An International Journal for Theory, History and Foundations of Science.29 (3): 361.arXiv:1301.5295.doi:10.1387/theoria.9951.hdl:10486/664735.S2CID52996408.
^Leeming, David A. (2010).Creation Myths of the World. ABC-CLIO. p. xvii.ISBN978-1-59884-174-9.In common usage the word 'myth' refers to narratives or beliefs that are untrue or merely fanciful; the stories that make up national or ethnic mythologies describe characters and events that common sense and experience tell us are impossible. Nevertheless, all cultures celebrate such myths and attribute to them various degrees of literal or symbolictruth.
^Eliade, Mircea (1964).Myth and Reality (Religious Traditions of the World). Allen & Unwin.ISBN978-0-04-291001-7.
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"Two systems of Hindu thought propound physical theories suggestively similar to those ofGreece. Kanada, founder of the Vaisheshika philosophy, held that the world is composed of atoms as many in kind as the various elements. TheJains more nearly approximated toDemocritus by teaching that all atoms were of the same kind, producing different effects by diverse modes of combinations. Kanada believed light and heat to be varieties of the same substance;Udayana taught that all heat comes from the Sun; andVachaspati, likeNewton, interpreted light as composed of minute particles emitted by substances and striking the eye."
^Stcherbatsky, F. Th. (1930, 1962),Buddhist Logic, Volume 1, p. 19, Dover, New York:
"The Buddhists denied the existence of substantial matter altogether. Movement consists for them of moments, it is a staccato movement, momentary flashes of a stream of energy... "Everything is evanescent",... says the Buddhist, because there is no stuff... Both systems [Sānkhya, and later Indian Buddhism] share in common a tendency to push the analysis of existence up to its minutest, last elements which are imagined as absolute qualities, or things possessing only one unique quality. They are called "qualities" (guna-dharma) in both systems in the sense of absolute qualities, a kind of atomic, or intra-atomic, energies of which the empirical things are composed. Both systems, therefore, agree in denying the objective reality of the categories of Substance and Quality,... and of the relation of Inference uniting them. There is in Sānkhya philosophy no separate existence of qualities. What we call quality is but a particular manifestation of a subtle entity. To every new unit of quality corresponds a subtle quantum of matter which is calledguna, "quality", but represents a subtle substantive entity. The same applies to early Buddhism where all qualities are substantive... or, more precisely, dynamic entities, although they are also calleddharmas ('qualities')."
^Viney, Donald Wayne (1985). "The Cosmological Argument".Charles Hartshorne and the Existence of God. SUNY Press. pp. 65–68.ISBN978-0-87395-907-0.
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^William P. D. Wightman (1951, 1953),The Growth of Scientific Ideas, Yale University Press. p. 38, where Wightman calls himSeleukos theChaldean.
^de Cheseaux JPL (1744).Traité de la Comète. Lausanne. pp. 223ff.. Reprinted as Appendix II inDickson, F. P. (1969).The Bowl of Night: The Physical Universe and Scientific Thought. Cambridge, Massachusetts: M.I.T. Press.ISBN978-0-262-54003-2.
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