The universe comprises all of existence: all forms of matter and energy in space and time, and the structures they form, from subatomic particles to entire galactic filaments. Since the early 20th century, the field of cosmology has established that the universe has been expanding for , starting from a dense fireball in an event called the Big Bang. The observable portion of the universe is approximately 93 billion light-years in diameter at present. The total size of the universe is not known.
Some of the earliest cosmological models of the universe were geocentric, placing Earth at the center. During the Scientific Revolution, astronomical observations led to a heliocentric model. Further observational improvements led to the realization that the Sun is one of a few hundred billion stars in the Milky Way, which is one of a few hundred billion galaxies in the observable universe. At the largest scale, galaxies are distributed uniformly and the same in all directions. At smaller scales, galaxies are distributed in clusters and superclusters, which form immense filaments and voids in space, creating a vast foam-like structure. Discoveries in the early 20th century, including general relativity, led to the modern view of an expanding, isotropic, homogeneous universe. Evidence accumulated supporting the Big Bang theory: an initial hot fireball cooled and becoming less dense as the universe expanded, allowing the first subatomic particles and simple atoms to form. Giant clouds of hydrogen and helium were gradually drawn to the places where matter was most dense, forming the first galaxies, stars, and eventually, everything else.
From studying the effects of gravity on both matter and light, it has been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulae and interstellar gas. This unseen matter is known as dark matter. In the widely accepted ΛCDM cosmological model, dark matter accounts for about of the mass and energy in the universe while about is dark energy, a mysterious form of energy responsible for the acceleration of the expansion of the universe. Ordinary ('baryonic') matter therefore composes only of the universe. Stars, planets, and visible gas clouds only form about 6% of this ordinary matter.
There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang.
Definition and related terms
The universe contains all energy and matter, including therefore planets, moons, stars, galaxies, and the contents of intergalactic space. In the broadest sense, the physical universe has been defined as "The totality of all space and time; all that is, has been, and will be." More restrictive definitions include the condition that the universe is connected, to allow for the possibility of other mutually existing universes.
In philosophy, one of the meanings of the word nature – the opposite of supernatural or unreal - coincides with the meaning the universe. Universe is also one of several meanings of world.
Etymology The word universe derives from the Old French word , which in turn derives from the Latin word , meaning 'combined into one'.The Compact Edition of the Oxford English Dictionary, volume II, Oxford: Oxford University Press, 1971, p. 3518. . The Latin word 'universum' was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.Lewis, C.T. and Short, S (1879) A Latin Dictionary, Oxford University Press, , pp. 1933, 1977–1978.
Synonyms A term for universe among the ancient Greek philosophers from Pythagoras onwards was () 'the all', defined as all matter and all space, and () 'all things', which did not necessarily include the void. Another synonym was () meaning 'the world, the cosmos'. Synonyms are also found in Latin authors (, , ) and survive in modern languages, e.g., the German words , , and for universe. The synonyms found in English, include the cosmos and the world.
Chronology and the Big Bang
The prevailing model for the evolution of the universe is the Big Bang theory. In the Big Bang model, the earliest state of the universe was extremely hot and dense but the universe cooled during subsequent expansion. The model is based on general relativity and on symmetry assumptions such as the homogeneity and isotropy of space. A version of the model with a cosmological constant (Lambda) and cold dark matter, known as the Lambda-CDM model, provides an excellent account of most observations of the universe.
Much of very earliest time is not understood. An intense period of expansion called cosmic inflation is postulated to explain many astronomical observations and set the initial conditions for the Lambda-CDM model.
Within the first fraction of a second of the universe's existence, it was extremely dense, and the high energy meant all the particles of the Standard model were in equilibrium. As the universe cooled due to expansion, the state of the universe went through phase transitions analogous to water freezing. Various types of elementary particles associated stably producing a plasma of electrons, protons, and neutrons, with very energetic photons preventing them from binding until about one minute after the Big Bang.
During the next few minutes, some protons and neutrons combined to form atomic nuclei through nuclear fusion. This process, known as Big Bang nucleosynthesis, lasted for about 15 minutes, produced helium, with small amounts of deuterium (a form of hydrogen) and traces of lithium. No other nuclei formed in significant amounts during this time. All of the neutrons that did not fuse decayed in to protons and electrons.
After nucleosynthesis ended, the universe was still far too hot for matter to form neutral atoms, so it contained a hot, dense, optically opaque plasma of negatively charged electrons, neutral neutrinos and positive nuclei. After about 377,000 years, the universe had cooled enough that electrons and nuclei could form the first stable atoms. This is known as recombination for historical reasons; electrons and nuclei were combining for the first time. Unlike plasma, neutral atoms are transparent to many wavelengths 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 the cosmic microwave background (CMB).
As the universe expanded, the energy density of electromagnetic radiation decreased more quickly than that of matter because the energy of each photon decreased as it is cosmologically redshifted. At around 47,000 years, the energy density of matter became larger than that of photons and neutrinos, and began to dominate the large scale behavior of the universe. This marked the end of the radiation-dominated era and the start of the matter-dominated era.
In the earliest stages of the universe, tiny fluctuations within the universe's density led to concentrations of dark matter gradually forming. Ordinary matter, attracted to these by gravity, formed large gas clouds and eventually, stars and galaxies, where the dark matter was most dense, and voids where it was least dense. After around 100–300 million years, the first stars formed, known as Population III stars. These were probably very massive, luminous, non metallic and short-lived. They were responsible for the gradual reionization of the universe between about 200–500 million years and 1 billion years, and also for seeding the universe with elements heavier than helium, through stellar nucleosynthesis.
The universe contains a mysterious energy—possibly a scalar field—called dark 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 present dark-energy-dominated era.Ryden, Barbara, "Introduction to Cosmology", 2006, eqn. 6.33 In this era, the expansion of the universe is accelerating due to dark energy.
Physical properties The properties of the universe are measured by observational cosmology, a combination of direct observations and observations analyzed in conjunction with models based on general relativity and Einstein's field equations.
Size
Due to the finite speed of light, there is a limit (known as the particle horizon) to how far light can travel over the age of the universe. The spatial region from which we can receive light is called the observable universe. The proper distance (measured at a fixed time) between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs), making the diameter of the observable universe about 93 billion light-years (28 billion parsecs). Although the distance traveled by light from the edge of the observable universe is close to the age of the universe times the speed of light, , the proper distance is larger because the edge of the observable universe and the Earth have since moved further apart.
For comparison, the Milky Way is roughly 87,400 light-years in diameter, and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light-years away.
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.
Age and expansion
Assuming that the Lambda-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.
Over time, the universe and its contents have evolved. For example, the relative population of quasars and galaxies has changed and the universe has expanded. This expansion is inferred from the observation that the light from distant galaxies has been redshifted, which implies that the galaxies are receding from us. Analyses of Type Ia supernovae indicate that the expansion is accelerating.
The more matter there is in the universe, the stronger the mutual gravitational pull of the matter. If the universe were too dense then it would re-collapse into a black hole. However, if the universe contained too little matter then it would expand too quickly for astronomical structures, like galaxies or planets, to form. Since the Big Bang, the universe has expanded monotonically. The mass–energy density of the universe, equivalent to about 5 protons per cubic meter, allowed it to expand for the last 13.8 billion years, giving time to form the universe as observed today.
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 the deceleration 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 cosmic scale factor has been positive in the last 5–6 billion years.
Spacetime