Universe
From Wikipedia, the free encyclopedia
Not to be confused with Observable universe.
For other uses, see Universe (disambiguation).
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The Universe is the totality of everything that exists, has existed, and ever will exist.[2][3] Similar meaning is sometimes conveyed with the words cosmos, world, reality, and nature. The Universe includes all of spacetime; the entire contents ofouter space; all matter, energy, dark matter, and dark energy; all galaxies, stars, and planets; all humans and every living thing; all molecules, atoms, subatomic particles, photons; all physical constants, physical laws and fundamental interactions. The Universe can even be understood to encompass all of mathematics, all concepts and ideas, and all thoughts and emotions.[4][5]
The part of the Universe that we can see, referred to as the observable universe, has a diameter of about 93 billion light years (28 billion parsecs)[6] and a volume of 4×1083 liters.[7] The size of the whole Universe is not known - it may be infinite.[8] Considering only ordinary matter, the density of the Universe is 9.9 x 10−30 g/cm3, equivalent to a mass density of 5.9 protons per cubic meter.[9] Space is expanding, and the rate of its expansion is increasing. Astronomical observations have led to inferences of the earlier stages of the Universe, which appear to have been governed by the same physical laws and constants throughout most of its extent and history. The Big Bang theory is the prevailing cosmological model describing the early development of the Universe, which is estimated to have begun 13.798 ± 0.037 billion years ago.[10][11] Some physicists have speculated that there is more than one universe.[12][13][14]
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Etymology and historical observation
The word universe derives from the Old French word univers, which in turn derives from the Latin word universum.[15] The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.[16] The Latin word derives from the poetic contraction unvorsum — first used by Lucretius in Book IV (line 262) of his De rerum natura (On the Nature of Things) — which connects un, uni (the combining form of unus, or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning "something rotated, rolled, changed").[16]
Throughout recorded history, cosmologies and cosmogonies have been proposed to account for observations of the Universe. The earliest quantitative geocentric models were developed by the ancient Greek philosophers and Indian philosophers.[17][18] Over the centuries, more precise observations led to Copernicus's heliostatic model of our solar system, and Kepler's heliocentric and elliptical model of our solar system. The theory of gravity led to the Newtonian model of our Solar System. Further improvements in astronomical observations led to the realization that our Solar System is located in a galaxy composed of billions of stars, the Milky Way. And, then, it was subsequently discovered that our galaxy is just one of many. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology. Discovery of the redshift suggested that the Universe is expanding, and the discovery of the cosmic microwave background radiation suggested that the Universe had a beginning.[19]
History
Main article: Chronology of the Universe
According to the prevailing scientific model of the Universe, known as the Big Bang,[20][21] the Universe expanded from an extremely hot, dense phase called the Planck epoch, a brief period extending from time zero to approximately 10−43 seconds (Planck time). During the Planck epoch, all types of matter, all types of energy, and all spacetime were concentrated into a dense state, wheregravitation is believed to have been as strong as the other fundamental forces, and all the forces may have been unified. Since the Planck epoch, the Universe has been expanding to its present form, possibly with a very brief period (less than 10−32 seconds) ofcosmic inflation, which caused the Universe to reach a much larger size almost instantaneously. Several independent experimental measurements support this theoretical expansion.
In the early universe, after the Planck epoch and inflation, came the Quark epoch, Hadron epoch and the Lepton epoch. All of these phases together lasted only up to 10 seconds after the Big Bang. The Photon epoch that followed lasted 380 thousand years. After that, hydrogen and helium atoms began to form as the density of the Universe falls, allowing light to travel freely. That is the earliest light possible to see in the Universe and is known as the cosmic microwave background (CMB), also known as the afterglow of the Big Bang. The Universe continues to expand to this day, studies have shown that this expansion is accelerating due to a mysterious force called Dark Energy.
Under general relativity, space can expand faster than the speed of light, although we can view only a small portion of the Universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the Universe is finite or infinite.
Properties and laws
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Size
The extent of the observable universe is defined by an Earth-centered sphere 93 billion light years in diameter[22] and having a volume of 1.2×1013 Mpc3 (equivalent to 4×1083 liters).[7] For comparison, the diameter of a typical galaxy is 30,000 light-years, and the typical distance between two neighboring galaxies is 3 million light-years.[23] As an example, the Milky Way Galaxy is roughly 100,000 light years in diameter,[24] and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light years away.[25]
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There are probably more than 100 billion (1011) galaxies in the observable Universe.[26] Typical galaxies range from dwarfs with as few as ten million[27] (107) stars up to giants with one trillion[28] (1012) stars, all orbiting the galaxy's center of mass. A 2010 study by astronomers estimated that the observable Universe contains 300 sextillion (3×1023) stars.[29]
The Universe is composed of ordinary baryonic matter (only 4.9% of the contents); which includes atoms, stars and galaxies. The present overall density of the this type of matter is very low, roughly 9.9 × 10−30grams per cubic centimetre, corresponding to a density of the order of only six protons for every four cubic meters of volume.[9] The Universe also contains dark matter (26.8%), a mysterious form of matter that has not yet been identified, and dark energy (68.3%), which is the energy of empty space and that is causing the expansion of the Universe to accelerate.[30] The common use of the "dark matter" and "dark energy"placeholder names for the unknown entities (purported to account for about 95% of the mass-energy densityof the Universe) demonstrates the present observational and conceptual shortcomings and uncertainties concerning the nature and ultimate fate of the Universe.[31]
Ordinary observable matter is spread homogeneously, that is, uniformly, throughout the Universe, when averaged over distances longer than 300 million light-years.[32] However, on smaller length-scales, matter is observed to form "clumps", i.e., to cluster hierarchically; many atoms are condensed into stars, most stars into galaxies, most galaxies into clusters, superclusters and, finally, thelargest-scale structures such as the Sloan great wall. The observable matter of the Universe is also spread isotropically, meaning that no direction of observation seems different from any other; each region of the sky has roughly the same content.[33] The Universe is also bathed in a highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.725 kelvin.[34] The hypothesis that the large-scale Universe is homogeneous and isotropic is known as the cosmological principle,[35]which is supported by astronomical observations.
Age and expansion
The age of the Universe is estimated to be 13.798 ± 0.037 billion years.[11] Over its history, the Universe and its contents have evolved; for example, the relative population of quasars and galaxies has changed and spaceitself has expanded. This expansion accounts for how it is that scientists on Earth can observe the light from a galaxy 30 billion light years away, even if that light has traveled for only 13 billion years; the very space between them has expanded, and that is one of the tools used to calculate the age of the Universe. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; thephotons emitted have been stretched to longer wavelengths and lower frequency during their journey. The rate of this spatial expansion is accelerating, based on studies of Type Ia supernovae.
The more matter there will be in the Universe, the more will be the gravitational pull among them. If the Universe is too dense then it would re-collapse into singularity. However, if the Universe contain too little matter then the expansion is accelerated greatly, thereby leaving no time for planets & solar systems to form. After the Big Bang, the universe is continuously expanding. The rate of expansion is affected by the gravity among the matter present. Surprisingly, our universe has just the right mass density of about 5 protons per cubic meter which has allowed it to expand gently for last 13.8 billion years, giving time to form the universe as we see it today.[36]
Laws
The relative fractions of different chemical elements — particularly the lightest atoms such as hydrogen, deuterium and helium — seem to be identical throughout the Universe and during its observable history.[37] The Universe seems to have much more matter than antimatter, an asymmetry possibly related to the observations of CP violation.[38] The Universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological length scales. The Universe also appears to have neither net momentum nor angular momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and the non-divergence of thestress-energy-momentum pseudotensor, respectively), if the Universe were finite.[39]
The Universe appears to have a smooth spacetime continuum consisting of three spatial dimensions and one temporal (time) dimension. On the average, space is observed to be very nearly flat (close to zero curvature), meaning that Euclidean geometry is experimentally true with high accuracy throughout most of the Universe.[40] Spacetime also appears to have asimply connected topology, at least on the length-scale of the observable Universe. However, present observations cannot exclude the possibilities that the Universe has more dimensions and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or toroidal topologies of two-dimensional spaces.[41]
Our Standard Model of physics seems to follow a universal set of physical laws and physical constants.,[42] where all matter is composed of three generations of leptons and quarks, both of which are fermions. These elementary particles interact via at most three fundamental interactions: the electroweak interaction which includes electromagnetism and the weak nuclear force; the strong nuclear force described by quantum chromodynamics; and gravity, which is best described at present bygeneral relativity. The first two interactions can be described by renormalized quantum field theory, and are mediated bygauge bosons that correspond to a particular type of gauge symmetry. A renormalized quantum field theory of general relativity has not yet been achieved. The theory of special relativity is believed to hold throughout the Universe, provided that the spatial and temporal length scales are sufficiently short; otherwise, the more general theory of general relativity must be applied. There is no explanation for the particular values that physical constants appear to have throughout our Universe, such as Planck's constant h or the gravitational constant G. Several conservation laws have been identified, such as theconservation of charge, momentum, angular momentum and energy; in many cases, these conservation laws can be related to symmetries or mathematical identities.
Shape of the Universe
Main article: Shape of the Universe
The curvature, topology shape or geometry of the Universe includes both local geometry in the observable universe and global geometry, which is possibly measurable. More formally, this practical subject investigates which 3-manifold corresponds to the spatial section incomoving coordinates of the four-dimensional spacetime of the Universe. Cosmologists normally work with a given space-like slice of spacetime called the comoving coordinates. In terms of observation, the section of spacetime that can be observed is the backward light cone, being the time it takes to reach a given observer within the cosmic light horizon. On assumption that the observable universe is smaller than the entire universe, which some models consider is many orders of magnitude smaller, we cannot determine the true global structure by observation alone, but are restricted only to localised regions.
Observational data suggests the cosmological topological model of the Universe is infinite with finite age, supported by the so-calledFriedmann–Lemaître–Robertson–Walker (FLRW) models,[43] including other FLRW models like the Poincaré dodecahedral space[44][45]and the Picard horn.[46] The data fit by these FLRW models of space especially include the Wilkinson Microwave Anisotropy Probe(WMAP) and Planck maps of cosmic background radiation. NASA released the first WMAP cosmic background radiation data in February 2003, while a higher resolution map regarding Planck data was released by ESA in March 2013. Both probes have found almost perfect agreement with inflationary models and the standard model of cosmology, describing a flat, homogeneous universe dominated by dark matter and dark energy.[11][47]
Synonyms and definitions
An alternative interpretation of unvorsum is "everything rotated as one" or "everything rotated by one". In this sense, it may be considered a translation of an earlier Greek word for the Universe, περιφορά, (periforá, "circumambulation"), originally used to describe a course of a meal, the food being carried around the circle of dinner guests.[48] This Greek word refers to celestial spheres, an early Greek model of the Universe. Regarding Plato's Metaphor of the Sun, Aristotle suggests that the rotation of the sphere of fixed starsinspired by the prime mover, motivates, in turn, terrestrial change via the Sun. Careful astronomical and physical measurements (such as the Foucault pendulum) are required to prove the Earth rotates on its axis.
A term for 'universe' in ancient Greece was τὸ πᾶν (tò pán, The All, Pan (mythology)). Related terms were matter, (τὸ ὅλον, tò hólon, see also Hyle, lit. wood) and place (τὸ κενόν,tò kenón).[49][50] Other synonyms for the Universe among the ancient Greek philosophers included κόσμος (cosmos) and φύσις (meaning Nature, from which we derive the wordphysics).[51] The same synonyms are found in Latin authors (totum, mundus, natura)[52] 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).[53]
Broadest definition: reality and probability
The broadest definition of the Universe is found in De divisione naturae by the medieval philosopher and theologian Johannes Scotus Eriugena, who defined it as simply everything: everything that is created and everything that is not created.
Definition as reality
More customarily, the Universe is defined as everything that exists, from its beginning to end.[54] According to our current understanding, the Universe consists of three principles:spacetime, forms of energy, including momentum and matter, and the physical laws that relate them.
Definition as connected spacetime
See also: Eternal inflation
It is possible to conceive of disconnected spacetimes, each existing but unable to interact with one another. An easily visualized metaphor is a group of separate soap bubbles, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle. According to one common terminology, each "soap bubble" of spacetime is denoted as a universe, whereas our particular spacetime is denoted as the Universe, just as we call our moon the Moon. The entire collection of these separate spacetimes is denoted as the multiverse.[55] In principle, the other unconnected universes may have different dimensionalities and topologies of spacetime, different forms ofmatter and energy, and different physical laws and physical constants, although such possibilities are purely speculative.
Definition as observable reality
See also: Observable universe and Observational cosmology
According to a still more restrictive definition, the Universe is everything within our connected spacetime that could have a chance to interact with us and vice versa.[56] According to the general theory of relativity, some regions of space may never interact with ours even in the lifetime of the Universe due to the finite speed of light and the ongoingexpansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the Universe would live forever: space may expand faster than light can traverse it.
Distant regions of space are taken to exist and be part of reality as much as we are, yet we can never interact with them. The spatial region within which we can affect and be affected is the observable universe. The observable Universe depends on the location of the observer. By traveling, an observer can come into contact with a greater region of spacetime than an observer who remains still. Nevertheless, even the most rapid traveler will not be able to interact with all of space. Typically, the observable Universe is taken to mean the Universe observable from our vantage point in the Milky Way Galaxy.
Historical models
See also: Cosmology and Timeline of cosmology
Historically, there have been many ideas of the cosmos (cosmologies) and its origin (cosmogonies). Some cosmogonies were based on narratives of gods. Theories of an impersonal universe governed by physical laws were first proposed by the Greeks and Indians.[18] Over the centuries, improvements in astronomical observations and theories of motion and gravitation led to ever more accurate descriptions of the Universe. The modern era of cosmology began with Albert Einstein's 1915 general theory of relativity, which made it possible to quantitatively predict the origin, evolution, and conclusion of the Universe as a whole. Most modern, accepted theories of cosmology are based on general relativity and, more specifically, the predicted Big Bang.
Creation
Main articles: Creation myth and Creator deity
Many cultures have stories describing the origin of the world, which may be roughly grouped into common types. In one type of story, the world is born from a world egg; such stories include the Finnish epic poem Kalevala, the Chinese story of Pangu or the Indian Brahmanda Purana. In related stories, the Universe is created by a single entity emanating or producing something by him- or herself, as in the Tibetan Buddhism concept of Adi-Buddha, the ancient Greek story of Gaia (Mother Earth), the Aztec goddessCoatlicue myth, the ancient Egyptian god Atum story, or the Genesis creation narrative. In another type of story, the Universe is created from the union of male and female deities, as in the Maori story of Rangi and Papa. In other stories, the Universe is created by crafting it from pre-existing materials, such as the corpse of a dead god — as fromTiamat in the Babylonian epic Enuma Elish or from the giant Ymir in Norse mythology – or from chaotic materials, as in Izanagi and Izanami in Japanese mythology. In other stories, the Universe emanates from fundamental principles, such as Brahman and Prakrti, the creation myth of the Serers,[57] or the yin and yang of the Tao.
Philosophical models
Further information: Cosmology
See also: Pre-Socratic philosophy, Physics (Aristotle), Hindu cosmology, Islamic cosmology, Time and Philosophy of space and time
The pre-Socratic Greek philosophers and Indian philosophers developed some of the earliest philosophical concepts of the Universe.[18][58] The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice to water to steam) and several philosophers proposed that all the physical materials in the world are different forms of a single primordial material, or arche. The first to do so was Thales, who proposed this material is water. Thales' student, Anaximander, proposed that everything came from the limitless apeiron. Anaximenes proposed air on account of its perceived attractive and repulsive qualities that cause the arche to condense or dissociate into different forms. Anaxagoras proposed the principle of Nous (Mind). Heraclitusproposed fire (and spoke of logos). Empedocles proposed the elements: earth, water, air and fire. His four element theory became very popular. Like Pythagoras, Plato believed that all things were composed of number, with Empedocles' elements taking the form of the Platonic solids. Democritus, and later philosophers—most notably Leucippus—proposed that the Universe was composed of indivisible atoms moving through void (vacuum). Aristotle did not believe that was feasible because air, like water, offers resistance to motion. Air will immediately rush in to fill a void, and moreover, without resistance, it would do so indefinitely fast.
Although Heraclitus argued for eternal change, his contemporary Parmenides made the radical suggestion that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature. Parmenides denoted this reality as τὸ ἐν (The One). Parmenides' theory seemed implausible to many Greeks, but his student Zeno of Eleachallenged them with several famous paradoxes. Aristotle responded to these paradoxes by developing the notion of a potential countable infinity, as well as the infinitely divisible continuum. Unlike the eternal and unchanging cycles of time, he believed the world was bounded by the celestial spheres, and thus magnitude was only finitely multiplicative.
The Indian philosopher Kanada, founder of the Vaisheshika school, developed a theory of atomism and proposed that light and heat were varieties of the same substance.[59] In the 5th century AD, the Buddhist atomist philosopher Dignāga proposed atoms to be point-sized, durationless, and made of energy. They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.[60]
The theory of temporal finitism was inspired by the doctrine of creation shared by the three Abrahamic religions: Judaism, Christianity and Islam. The Christian philosopher, John Philoponus, presented the philosophical arguments against the ancient Greek notion of an infinite past and future. Philoponus' arguments against an infinite past were used by the early Muslim philosopher, Al-Kindi (Alkindus); the Jewish philosopher, Saadia Gaon (Saadia ben Joseph); and the Muslim theologian, Al-Ghazali (Algazel). Borrowing from Aristotle's Physics and Metaphysics, they employed two logical arguments against an infinite past, the first being the "argument from the impossibility of the existence of an actual infinite", which states:[61]
- "An actual infinite cannot exist."
- "An infinite temporal regress of events is an actual infinite."
- "
An infinite temporal regress of events cannot exist."
The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:[61]
- "An actual infinite cannot be completed by successive addition."
- "The temporal series of past events has been completed by successive addition."
- "
Both arguments were adopted by Christian philosophers and theologians, and the second argument in particular became more famous after it was adopted by Immanuel Kant in his thesis of the first antinomy concerning time.[61]
Fine tuning
Main article: Fine-tuned Universe
Many of the properties of the Universe have the appearance of having been tuned or selected so as to permit the emergence of intelligent life.[19][62][63] Not all scientists agree that this fine-tuning exists.[64][65] [66] In particular, it is not known under what conditions intelligent life could form and what form or shape that would take. A relevant observation in this discussion is that for an observer to exist to observe fine-tuning, the Universe must be able to support intelligent life. As such the conditional probability of observing a Universe that is fine-tuned to support intelligent life is 1. This observation is known as the anthropic principle and is particularly relevant if the creation of the Universe was probabilistic or if multiple universes with a variety of properties exist (see below). However, the observation that the chemistry of life may have begun shortly after the Big Bang,13.8 billion years ago, during a habitable epoch when the Universe was only 10–17 million years old, may differ, in part, with the anthropic principle.[67][68][69]
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