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Our Universe


The Universe is commonly defined as the totality of existence,including planets, stars, galaxies, the contents of intergalactic space, and all matter and energy. Definitions and usage vary and similar terms include the cosmos, the world and nature.
Scientific observation of the Universe, which is believed to be at least 93 billion light years in diameter, has led to inferences of its earlier stages. These observations suggest that the Universe has 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 that describes the early development of the Universe, which in physical cosmology is believed to have occurred about 13.77 billion years ago.

There are various multiverse hypotheses, in which physicists have suggested that the Universe might be one among many universes that likewise exist.The farthest distance that it is theoretically possible for humans to see is described as the observable Universe. Observations have shown that the Universe appears to be expanding at an accelerating rate, and a number of models have arisen to predict its ultimate fate.Throughout recorded history, several 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. Over the centuries, more precise observations and improved theories of gravity led to Copernicus's heliocentric model and the Newtonian model of the Solar System, respectively.


Further improvements in astronomy led to the realization that the Solar System is embedded in a galaxy composed of billions of stars, the Milky Way, and that other galaxies exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology. Discovery of the red shift and cosmic microwave background radiation suggested that the Universe is expanding and had a beginning.

According to a still-more-restrictive definition, the Universe is everything within our connected space-time that could have a chance to interact with us and vice versa. 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 ongoing expansion 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. Strictly speaking, the observable Universe depends on the location of the observer. By traveling, an observer can come into contact with a greater region of space-time than an observer who remains still, so that the observable Universe for the former is larger than for the latter. 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. The Universe is immensely large and possibly infinite in volume. The region visible from Earth the observable Universe is a sphere with a radius of about 47 billion light years, based on where the expansion of space has taken the most distant objects observed.

For comparison, the diameter of a typical galaxy is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years.As an example, our Milky Way Galaxy is roughly 100,000 light years in diameter, and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away. There are probably more than 100 billion galaxies in the observable Universe. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion  stars, all orbiting the galaxy's center of mass. A 2010 study by astronomers estimated that the observable Universe contains 300 sextillion  stars.The observable matter is spread homogeneously uniformly throughout the Universe, when averaged over distances longer than 300 million light-years. 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, the largest-scale structures such as the Great Wall of galaxies.

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. The Universe is also bathed in a highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.725 kelvin. The hypothesis that the large-scale Universe is homogeneous and isotropic is known as the cosmological principle, which is supported by astronomical observations.The present overall density of the Universe is very low, roughly 9.9 × 10 grams per cubic centimetre. This mass-energy appears to consist of 73% dark energy, 23% cold dark matter and 4% ordinary matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume. The properties of dark energy and dark matter are largely unknown. Dark matter gravitates as ordinary matter, and thus works to slow the expansion of the Universe; by contrast, dark energy accelerates its expansion.

The current estimate of the Universe's age is 13.772 ±0.059 billion years old.Independent estimates based on measurements such as radioactive dating agree at 13–15 billion years. The Universe has not been the same at all times in its history; for example, the relative populations of quasars and galaxies have changed and space itself appears to have expanded. This expansion accounts for how Earth-bound scientists 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. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; the photons 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 and corroborated by other data.
The Universe appears to have a smooth space-time continuum consisting of three spatial dimensions and one temporal timedimension. 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. Spacetime also appears to have a simply 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.

The Universe appears to behave in a manner that regularly follows a set of physical laws and physical constants. According to the prevailing Standard Model of physics, 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 by general relativity. The first two interactions can be described by renormalized quantum field theory, and are mediated by gauge bosons that correspond to a particular type of gauge symmetry.


A renormalized quantum field theory of general relativity has not yet been achieved, although various forms of string theory seem promising. 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 the conservation of charge, momentum, angular momentum and energy; in many cases, these conservation laws can be related to symmetries or mathematical identities.

Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the then-available data and conceptions of the Universe. Historically, cosmologies and cosmogonies were based on narratives of gods acting in various ways. Theories of an impersonal Universe governed by physical laws were first proposed by the Greeks and Indians. 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; however, still more careful measurements are required to determine which theory is correct.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 creation idea is caused 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 goddess Coatlicue myth, the ancient Egyptian god Atum story, or the Genesis creation narrative. In another type of story, the world 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 from Tiamat 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,[49] or the yin and yang of the Tao.


A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, and interstellar medium of gas and dust, and, it is hypothesized, an important but poorly understood component called dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Examples of galaxies range from dwarfs with as few as ten million  stars to giants with a hundred trillion  stars, each orbiting their galaxy's own center of mass.Galaxies contain varying numbers of star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object.



Galaxies have been historically categorized according to their apparent shape; usually referred to as their visual morphology. A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped with dusty, curving arms. Those with irregular or unusual shapes are known as irregular galaxies and typically originate from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in a merging, sometimes induce significantly increased incidents of star formation leading to starburst galaxies. Smaller galaxies lacking a coherent structure are referred to as irregular galaxies.


There are probably more than 170 billion  galaxies in the
observable Universe.
Most are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs or megaparsecs. Intergalactic space the space between galaxies is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations known as groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids.
On December 12, 2012, astronomers, working with the Hubble Space Telescope, reported that the most distant known galaxy, UDFj-39546284, is now estimated to be even further away than previously believed. The galaxy, which is estimated to have formed around "380 million years" after the Big Bang about 13.77 billion years ago, and has a z ,redshift of 11.9, is approximately 13.37 billion light years from Earth.

The Greek philosopher Democritus (450–370 BC) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.Aristotle (384–322 BC), however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 AD) was scientifically critical of this view, arguing that if the Milky Way were sublunary it should appear different at different times and places on the Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world

Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. In 1750 the English astronomer Thomas Wright, in his An original theory or new hypothesis of the Universe, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the solar system but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk.In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.


Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region. A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars.

Like the stars, the spiral arms rotate around the center, but they do so with constant
angular velocity. The spiral arms are thought to be areas of high-density matter, or "density waves".
As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. The velocity returns to normal after the stars depart on the other side of the arm. This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.


Despite the prominence of large elliptical and spiral galaxies, most galaxies in the Universe appear to be dwarf galaxies. These galaxies are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. Dwarf galaxies may also be classified as elliptical, spiral, or irregular.

Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called
dwarf spheroidal galaxies instead.
A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses, regardless of whether the galaxy has thousands or millions of stars. This has led to the suggestion that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale


Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, known as a starburst. Should they continue to do so, however, they would consume their reserve of gas in a time frame lower than the lifespan of the galaxy. Hence starburst activity usually lasts for only about ten million years, a relatively brief period in the history of a galaxy. Starburst galaxies were more common during the early history of the Universe, and, at present, still contribute an estimated 15% to the total star production rate.Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create H II regions.

These massive stars produce
supernova explosions, resulting in expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger a chain reaction of star building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the starburst activity come to an end.
Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is M82, which experienced a close encounter with the larger M81. Irregular galaxies often exhibit spaced knots of starburst activity


An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by disorders in gravitational pull. Irregular galaxies also contain abundant amounts of gas and dust.

There are two major Hubble types of irregular galaxies:

  • An Irr-I galaxy (Irr I) is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence. De Vaucouleurs subtypes this into galaxies that have some spiral structure Sm, and those that do not Im.
  • An Irr-II galaxy (Irr II) is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence.

A third classification of irregular galaxies are the dwarf irregulars, labelled as dI or dIrrs. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of metallicity and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe. They may represent a local and therefore more recent version of the faint blue galaxies known to exist in deep field galaxy surveys.Some of the irregular galaxies are small spiral galaxies that are being distorted by the gravity of a larger neighbor.The Magellanic Cloud galaxies were once classified as irregular galaxies, but have since been found to contain barred spiral structures, and have been since re-classified as "SBm", a fourth type of barred spiral galaxy, the barred Magellanic spiral type.

 The faint blue galaxy (F.B.G.) problem in
astrophysics first arose with observations starting in 1978 that there were more galaxies with a bolometric magnitude > 22 than then-current theory predicted.
Galaxies can appear faint because they are small or because they are far away. Neither explanation, nor any combination, initially matched the observations. The distribution of these galaxies has since been found to be consistent with Cosmic Inflation, measurements of the Cosmic Microwave Background, and a nonzero cosmological constant, that is, with the existence of the now-accepted dark energy.

It thus serves as a confirmation of supernova observations requiring dark energy.A second problem arose in 1988, with even deeper observations showing a much greater excess of faint galaxies. These are now interpreted as dwarf galaxies experiencing large bursts of stellar formation, resulting in blue light from young, massive stars.Thus F.B.G.s are extremely bright for their size and distance.Most F.B.G.s appear between red-shift 0.5 and 2. It is believed that they disappear as separate objects by merger with other galaxies


The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years  from Earth in the Andromeda constellation. Also known as Messier , it is often referred to as the Great Andromeda Nebula in older texts. The Andromeda Galaxy is the nearest spiral galaxy to our Milky Way galaxy, but not the closest galaxy overall. It gets its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda. The Andromeda Galaxy is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, the Andromeda Galaxy may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and could be the most massive in the grouping.

The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion stars: at least twice the number of stars in the Milky Way galaxy, which is estimated to be 200–400 billion.The Andromeda Galaxy is estimated to be  solar masses. In comparison a 2009 study estimated that the Milky Way and M31 are about equal in mass, while a 2006 study put the mass of the Milky Way at ~80% of the mass of the Andromeda Galaxy. The two galaxies are expected to collide in 3.75 billion years, eventually merging to form a giant elliptical galaxy.At an apparent magnitude of 3.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects, making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope.

A supermassive black hole is the largest type of black hole in a galaxy, on the order of hundreds of thousands to billions of solar masses. Most and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the density of water in the case of some supermassive black holes. This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have lower average density. Also, the tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut traveling towards the black hole center would not experience significant tidal force until very deep into the black hole.


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