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10 30 g cm 3 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 101 102 two proposed forms for dark energy are the cosmological constant a constant energy density filling space homogeneously 103 and scalar fields such as quintessence or moduli 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 to vacuum energy dark matter main article dark matter dark matter is a hypothetical kind of matter that is invisible to the entire electromagnetic 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 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 78 104 ordinary matter main article matter masses and sizes of objects in the universe 105 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 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 106 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 107 108 109 ordinary matter commonly exists in four states or phases solid liquid gas and plasma 110 however advances in experimental techniques have revealed other previously theoretical phases such as bose einstein condensates and fermionic condensates 111 112 ordinary matter is composed of two types of elementary particles quarks and leptons 113 for example the proton is formed of two up quarks and one down quark the neutron is formed of two down quarks and one up quark and the electron is a kind of lepton an atom consists of an atomic nucleus made up of protons and neutrons both of which are baryons and electrons that orbit the nucleus 114 1476 soon after the big bang primordial protons and neutrons formed from the quark gluon plasma of the early universe as it cooled below two trillion degrees a few minutes later in a process known as big bang nucleosynthesis nuclei formed from the primordial protons and neutrons this nucleosynthesis formed lighter elements those with small atomic numbers up to lithium and beryllium but the abundance of heavier elements dropped off sharply with increasing atomic number some boron 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 of heavier elements resulted from stellar nucleosynthesis and supernova nucleosynthesis 115 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 ν e and electron e muon neutrino ν μ and muon μ tau neutrino ν τ and tau τ and the z 0 and w carriers of the weak force mass charge and spin are listed for each particle main article particle physics ordinary matter and the forces that act on matter can be described in terms of elementary particles 116 these particles are sometimes regarded as fundamental because they have no known substructure 117 118 in most contemporary models they are thought of as points in space 119 all elementary particles are described by quantum mechanics and exhibit wave particle duality their behavior has both particle like and wave like aspects manifest under different circumstances 120 of central importance is the standard model a theory that is concerned with electromagnetic interactions and the weak and strong nuclear interactions 121 the standard model is supported by the experimental confirmation of the existence of particles that compose matter quarks and leptons and their corresponding antimatter duals as well as the force particles that mediate interactions the photon the w and z bosons and the gluon 117 the standard model predicted the existence of the recently discovered higgs boson a particle that is a manifestation of a field within the universe that can endow particles with mass 122 123 because of its success in explaining a wide variety of experimental results the standard model is sometimes regarded as a theory of almost everything 121 the standard model does not however accommodate gravity a true force particle theory of everything has not been attained 124 hadrons main article hadron a hadron is a composite particle made of quarks held together by the strong force 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 bound within atomic nuclei are stable other hadrons are unstable under ordinary conditions and are thus insignificant constituents of the modern universe 125 118 123 from approximately 10 6 seconds after the big bang during a period known as the hadron 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 by hadrons 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 particle antiparticle annihilation reactions leaving a small residual of hadrons by the time the universe was about one second old 125 244 266 leptons main article lepton a lepton is an elementary half integer spin 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 126 two main classes of leptons exist charged leptons also known as the electron like leptons and neutral leptons better known as neutrinos electrons are stable and the most common charged lepton in the universe whereas muons and taus are unstable particles that quickly decay after being produced in high energy collisions such as those involving cosmic rays or carried out in particle accelerators 127 128 charged leptons can combine with other particles to form various composite particles such as atoms and positronium the electron governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties neutrinos rarely interact with anything and are consequently rarely observed neutrinos stream throughout the universe but rarely interact with normal matter 129 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 130 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 131 132 photons main article photon epoch a photon is the quantum of light and all other forms of electromagnetic radiation it is the carrier for the electromagnetic force 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 114 1470 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 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 subsequent structure formation took place 125 244 266 v t e timeline of the big bang chronology of the universe big bang planck epoch grand unification epoch electroweak epoch inflation baryogenesis quark epoch nucleosynthesis matter domination recombination dark ages reionization cosmological phase transition fate of the universe heat death of the universe big rip big crunch big bounce big slurp cosmological models model of the universe based on general relativity main article solutions of the einstein field equations see also big bang and ultimate fate of the universe general relativity is the geometric theory of gravitation formulated by albert einstein in 1915 and remains the standard description of gravity in modern physics it extends special relativity and newton s law of universal gravitation by describing gravity as a manifestation of the curvature of space and time spacetime in this framework the curvature of spacetime is determined by the energy and momentum of matter and radiation 133 this relationship is expressed through the einstein field equations which link the distribution of matter and energy to the geometry of spacetime the resulting geometry governs the motion of matter so that solutions of these equations describe how the universe evolves over time 133 under the cosmological principle which assumes that the universe is homogeneous and isotropic on large scales the field equations admit a class of solutions described by the metric tensor known as the friedmann lemaître robertson walker metric in this description the universe is characterized by two quantities a scale factor which describes how its overall size changes with time and a curvature index which specifies its spatial geometry the curvature can be flat positively curved or negatively curved 134 the evolution of the scale factor depends on both the spatial curvature and the cosmological constant which represents the energy density of empty space and may be associated with dark energy 133 79 the relation governing this evolution is known as the friedmann equation introduced by alexander friedmann citation needed the curvature determines the global geometry of space a positively curved universe has a finite volume and can be visualized as a three dimensional sphere a flat or negatively curved universe is spatially infinite 135 although this may seem counterintuitive models with flat or negative curvature allow an infinite universe to emerge from an initial state in which the scale factor vanishes consistent with the cosmological principle analogies include an infinite plane flat or other geometries such as a torus the ultimate fate of the universe depends on both the curvature and the cosmological constant a sufficiently dense universe with positive curvature would eventually recollapse in a big crunch possibly followed by a big bounce in contrast a flat or negatively curved universe would expand indefinitely approaching a big freeze and eventual heat death of the universe observations indicate that the expansion of the universe is accelerating raising the possibility of a big rip current data suggest that the universe is close to flat with a density near the critical value separating recollapse from eternal expansion 136 multiverse hypotheses main articles multiverse and eternal inflation some speculative theories have proposed that our universe is but one of a set of disconnected universes collectively denoted as the multiverse 137 138 an easily visualized metaphor of these concepts 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 139 according to one common terminology each soap bubble of spacetime is denoted as a universe whereas humans particular spacetime is denoted as the universe 137 just as humans call earth s moon the moon the entire collection of these separate spacetimes is denoted as the multiverse 137 max tegmark and brian greene have proposed different classification schemes for multiverse ideas in tegmark s scheme multiverses might result from the immense size of the spacetime from cosmological processes that produce spacetime bubbles 140 from quantum mechanical unitarity or because we live in a mathematical construct 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 that our nearest so called doppelgänger is 10 10 115 metres away from us a double exponential function larger than a googolplex 141 the physical basis of these ideas have been challenged 142 143 historical conceptions see also cosmology timeline of cosmological theories 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 o...