THE UNIVERSE
The Universe can be defined as everything that exists, everything that has existed, and everything
that will exist.[21][22][23] According to our current understanding, the Universe consists of spacetime,
forms of energy(including electromagnetic radiation and matter), and the physical lawsthat relate
them. The Universe encompasses all of life, all of history, and some philosophers and scientists
suggest that it even encompasses ideas such as mathematics and logic.[24][25][26]
Etymology
The word universe derives from the Old French word univers, which in turn derives from
the Latin word universum.[27] The Latin word was used by Cicero and later Latin authors in many of
the same senses as the modern English word is used.[28]
Synonyms
A term for "universe" among the ancient Greek philosophers from Pythagoras onwards was τὸ
πᾶν tò pân ("the all"), defined as all matter and all space, and τὸ ὅλον tò hólon ("all things"), which
did not necessarily include the void.[29][30] Another synonym was ὁ κόσμος ho kósmos(meaning
the world, the cosmos).[31] Synonyms are also found in Latin authors (totum, mundus, natura)[32
333]
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).[33]
Chronology and the Big Bang
Main articles: Big Bang and Chronology of the universe
The prevailing model for the evolution of the Universe is the Big Bang theory.[34][35] The Big Bang
model states that the earliest state of the Universe was extremely hot and dense and that it
subsequently expanded. The model is based on general relativity and on simplifying assumptions
such as 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, is the simplest model
that provides a reasonably good account of various observations about the Universe. The Big Bang
model accounts for observations such as the correlation of distance and redshift of galaxies, the
ratio of the number of hydrogen to helium atoms, and the microwave radiation background.
, In this diagram, time passes from left to right, so at any given time, the Universe is represented by a disk-
shaped "slice" of the diagram.
The initial hot, dense state is called the Planck epoch, a brief period extending from time zero to
one Planck time unit of approximately 10−43seconds. 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
of cosmic inflation which caused the Universe to reach a much larger size in less
than 10−32 seconds.[36]
After the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these
epochs encompassed less than 10 seconds of time following the Big Bang. The observed
abundance of the elements 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 formed atomic
nuclei through nuclear reactions. This process, known as Big Bang nucleosynthesis, led to the
present abundances of lighter nuclei, particularly hydrogen, deuterium, and helium. Big Bang
nucleosynthesis ended about 20 minutes after the Big Bang, when the Universe had cooled enough
so that nuclear fusion could no longer occur. 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.
Eventually, at a 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 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.[37] The Universe also contains a mysterious energy
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.[38] In this era, the
expansion of the Universe is accelerating due to dark energy.
Properties
Main articles: Observable universe, Age of the Universe, and Metric expansion of space
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".[39] 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.
Spacetime events are not absolutely defined spatially and temporally but rather are known relative to
the motion of an observer. Minkowski space approximates the Universe without gravity; the pseudo-
Riemannian manifolds of general relativitydescribe spacetime with matter and gravity. String
theory postulates the existence of additional dimensions.
Of the four fundamental interactions, gravitation is dominant at cosmological length scales, including
galaxies and larger-scale structures. Gravity's effects are cumulative; by contrast, the effects of
The Universe can be defined as everything that exists, everything that has existed, and everything
that will exist.[21][22][23] According to our current understanding, the Universe consists of spacetime,
forms of energy(including electromagnetic radiation and matter), and the physical lawsthat relate
them. The Universe encompasses all of life, all of history, and some philosophers and scientists
suggest that it even encompasses ideas such as mathematics and logic.[24][25][26]
Etymology
The word universe derives from the Old French word univers, which in turn derives from
the Latin word universum.[27] The Latin word was used by Cicero and later Latin authors in many of
the same senses as the modern English word is used.[28]
Synonyms
A term for "universe" among the ancient Greek philosophers from Pythagoras onwards was τὸ
πᾶν tò pân ("the all"), defined as all matter and all space, and τὸ ὅλον tò hólon ("all things"), which
did not necessarily include the void.[29][30] Another synonym was ὁ κόσμος ho kósmos(meaning
the world, the cosmos).[31] Synonyms are also found in Latin authors (totum, mundus, natura)[32
333]
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).[33]
Chronology and the Big Bang
Main articles: Big Bang and Chronology of the universe
The prevailing model for the evolution of the Universe is the Big Bang theory.[34][35] The Big Bang
model states that the earliest state of the Universe was extremely hot and dense and that it
subsequently expanded. The model is based on general relativity and on simplifying assumptions
such as 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, is the simplest model
that provides a reasonably good account of various observations about the Universe. The Big Bang
model accounts for observations such as the correlation of distance and redshift of galaxies, the
ratio of the number of hydrogen to helium atoms, and the microwave radiation background.
, In this diagram, time passes from left to right, so at any given time, the Universe is represented by a disk-
shaped "slice" of the diagram.
The initial hot, dense state is called the Planck epoch, a brief period extending from time zero to
one Planck time unit of approximately 10−43seconds. 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
of cosmic inflation which caused the Universe to reach a much larger size in less
than 10−32 seconds.[36]
After the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these
epochs encompassed less than 10 seconds of time following the Big Bang. The observed
abundance of the elements 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 formed atomic
nuclei through nuclear reactions. This process, known as Big Bang nucleosynthesis, led to the
present abundances of lighter nuclei, particularly hydrogen, deuterium, and helium. Big Bang
nucleosynthesis ended about 20 minutes after the Big Bang, when the Universe had cooled enough
so that nuclear fusion could no longer occur. 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.
Eventually, at a 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 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.[37] The Universe also contains a mysterious energy
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.[38] In this era, the
expansion of the Universe is accelerating due to dark energy.
Properties
Main articles: Observable universe, Age of the Universe, and Metric expansion of space
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".[39] 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.
Spacetime events are not absolutely defined spatially and temporally but rather are known relative to
the motion of an observer. Minkowski space approximates the Universe without gravity; the pseudo-
Riemannian manifolds of general relativitydescribe spacetime with matter and gravity. String
theory postulates the existence of additional dimensions.
Of the four fundamental interactions, gravitation is dominant at cosmological length scales, including
galaxies and larger-scale structures. Gravity's effects are cumulative; by contrast, the effects of
