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In Big Bang cosmology, the observable universe (also called the Hubble Volume) is the region of space bounded by a sphere, centred on the observer, that is small enough that we might observe objects in it, i.e. there has been sufficient time for light emitted by an object to arrive at the observer. Every position has its own observable universe which may or may not overlap with the one centered around the Earth.

The word observable used in this sense has nothing to do with whether modern technology actually permits us to detect radiation from an object in this region. It simply means that it is possible for light or other radiation from the object to reach an observer on earth.

In practice, we can only observe objects as far as the surface of last scattering, when the universe became transparent. However, it may be possible to infer information from before this time through the detection of gravitational waves.

The universe versus the observable universe

Both popular and professional research articles in cosmology often use

the term "universe" to mean "observable universe". This can be justified on the grounds that we can never know anything about any part of the universe that is causally disconnected from us. No one believes, however, that the observable universe is precisely the entire universe; that would imply that the Earth is exactly at the center of the universe, violating a fundamental assumption of astronomy (and indeed all of science). It is likely that the galaxies within our visible universe represent only a minuscule fraction of the galaxies in the universe.

It is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies are actually duplicate images of nearby galaxies, formed by light that has circumnavigated the universe. It is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different. A 2004 paper [1] claims to establish a lower bound of 24 gigaparsecs (78 billion[2] light-years) on the diameter of the universe, based on matching-circle analysis of the WMAP data. However, other sources have stated that the outer boundary is 29 gigaparsecs (93 billion light-years) distant

The Hubble Limit[4] is a concept in physical cosmology that is related to the Big Bang Theory. It refers to the limit where objects receding from the observer are receding at the speed of light. It is named after the astronomer Edwin Hubble, who was the first to discover that objects on a galactic scale are moving away from us. In the aftermath of the Big Bang everything in the universe is flying apart, and due to the fact that the speed of light is constant, farther objects appear to be receding at a higher velocity. Eventually an object will appear to have a velocity which is the speed of light, and an object at this point is known to be at the Hubble Limit.

Misconceptions

Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these are listed below.

13.7 billion light-years. The age of the universe is about 13.7 billion years. While it is commonly understood that nothing travels faster than light, it is a common misconception that the radius of the observable universe must therefore amount to only 13.7 billion light-years. This would make sense in the flat spacetime of special relativity. But in the real universe spacetime is highly curved at cosmological scales (general relativity), and light does not move rectilinearly. If a distance is obtained from the product of the speed of light times a cosmological time interval, it has no direct physical significance. [5]

15.8 billion light-years. This is obtained in the same way as the 13.7 billion light-year figure, but starting from an incorrect age of the universe which was reported in the popular press in mid-2006[6] [7] [8]. For an analysis of this claim and the paper that prompted it, see [9].

27 billion light-years. This is a diameter obtained from the (incorrect) radius of 13.7 billion light-years.

78 billion light-years. This figure, as mentioned above, is a lower bound on the diameter of the whole universe. It yields a lower bound on the radius of 39 billion light-years, which is less than the comoving radius of 46.5 billion light-years. For the 39 billion light-year radius to be correct, light must have circumnavigated the universe, and some regions of space would be visible twice, in opposite directions. This has yet to be proven.

156 billion light-years. This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius. Since 78 billion light-years is already a diameter (or rather a circumference), the doubled figure is meaningless even in its original context. This figure was very widely reported.

180 billion light-years. This estimate accompanied the age estimate of 15.8 billion years in some sources; it was obtained by incorrectly adding 15% to the incorrect figure of 156 billion light-years.

Big - Bang

In physical cosmology, the Big Bang is the scientific theory that the universe emerged from an enormously dense and hot state nearly 14 billion years ago. The Big Bang is a consequence of the observed Hubble's law velocities of distant galaxies that when taken together with the cosmological principle implies that space is expanding according to the Friedmann model of general theory of relativity. Extrapolated into the past, these observations show that the universe has expanded from a primeval state, in which all the matter and energy in the universe was at an immense temperature and density. Physicists do not widely agree on what happened before this, although general relativity predicts a gravitational singularity.

The term Big Bang is used both in a narrow sense to refer to a point in time when the observed expansion of the universe (Hubble's law) began—calculated to be 13.7 billion (1.37 × 1010) years ago—and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and expansion of the universe, as well as the composition of primordial matter through nucleosynthesis as predicted by the Alpher-Bethe-Gamow_theory. One consequence of the Big Bang is that the conditions of today's universe are different from the conditions in the past or in the future. From this model, George Gamow in 1948 was able to predict the existence of cosmic microwave background radiation (CMB). The CMB was discovered in the 1960s and served as a confirmation of the Big Bang theory over its chief rival, the steady state theory.

History

The Big Bang theory developed from observations and theoretical considerations. Observationally, in the 1910s, Vesto Slipher and later Carl Wilhelm Wirtz determined that most spiral nebulae were receding from Earth, but they weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way. Also in the 1910s, Albert Einstein's theory of general relativity was found to admit no static cosmological solutions given the basic assumptions of cosmology described below. The universe was described by a metric tensor that was either expanding or shrinking, a result that Einstein himself considered wrong and he tried to fix by adding a cosmological constant. The first person to seriously apply general relativity to cosmology without the stabilizing cosmological constant was Alexander Friedmann, whose equations describe the Friedmann-Lemaître-Robertson-Walker universe.

In 1927, the Belgian Catholic priest Georges Lemaître independently derived the Friedmann-Lemaître-Robertson-Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"—what was later called the Big Bang.

In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. Hubble proved that the spiral nebulae were galaxies and measured their distances by observing Cepheid variable stars which were earlier determined to be standard candles by Henrietta Leavitt. He discovered that, relative to the earth, the galaxies are receding in every direction at speeds directly proportional to their distance from the earth. This fact is now known as Hubble's law (see Edwin Hubble: Mariner of the Nebulae by Edward Christianson). Given the cosmological principle whereby the universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places, Hubble's law suggested that the universe was expanding. This idea allowed for two opposing possibilities. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other possibility was Fred Hoyle's steady state model in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time. It was actually Hoyle who coined the name of Lemaître's theory, referring to it sarcastically as "this 'big bang' idea" during a 1949 BBC radio program, The Nature of Things, the text of which was published in 1950.

For a number of years the support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. Since the discovery of the cosmic microwave background radiation in 1965 it has been regarded as the best theory of the origin and evolution of the cosmos. Before the late 1960s, many cosmologists thought the infinitely dense and physically paradoxical singularity at the starting time of Friedmann's cosmological model could be avoided by allowing for a universe which was contracting before entering the hot dense state and starting to expand again. This was formalized as Richard Tolman's oscillating universe. In the sixties, Stephen Hawking and others demonstrated that this idea was unworkable, and the singularity is an essential feature of the physics described by Einstein's gravity. This led the majority of cosmologists to accept the notion that the universe as currently described by the physics of general relativity has a finite age. However, due to a lack of a theory of quantum gravity, there is no way to say whether the singularity is an actual origin point for the universe or whether the physical processes that govern the regime cause the universe to be effectively eternal in character.

Virtually all theoretical work in cosmology now involves extensions and refinements to the basic Big Bang theory. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding what happened at the Big Bang, and reconciling observations with the basic theory.

Huge advances in Big Bang cosmology were made in the late 1990s and the early 21st century as a result of major advances in telescope technology in combination with large amounts of satellite data such as that from COBE, the Hubble Space Telescope and WMAP. These data have allowed cosmologists to calculate many of the parameters of the Big Bang to a new level of precision and led to the unexpected discovery that the expansion of the universe appears to be accelerating. (See dark energy.)

Overview

Based on measurements of the expansion of the universe using Type Ia supernovae, measurements of the lumpiness of the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a measured age of 13.7 ± 0.2 billion years. The agreement of these three independent measurements is considered strong evidence for the so-called Lambda-CDM model that describes the detailed nature of the contents of the universe.

The early universe was filled homogeneously and isotropically with a incredibly high energy density and concomitantly huge temperatures and pressures. It expanded and cooled, going through phase transitions analogous to the condensation of steam or freezing of water as it cools, but related to elementary particles.

Approximately 10-35 seconds after the Planck epoch, a phase transition caused the universe to experience exponential growth during a period called cosmic inflation. After inflation stopped, the material components of the universe were in the form of a quark-gluon plasma (also including all other particles—and perhaps experimentally produced recently as a quark-gluon liquid [1]) in which the constituent particles were all moving relativistically. As the universe continued growing in size, the temperature dropped. At a certain temperature, by an as-yet-unknown transition called baryogenesis, the quarks and gluons combined into baryons such as protons and neutrons, somehow producing the observed asymmetry between matter and antimatter. Still lower temperatures led to further symmetry breaking phase transitions that put the forces of physics and elementary particles into their present form. Later, some protons and neutrons combined to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. As the universe cooled, matter gradually stopped moving relativistically and its rest mass energy density came to gravitationally dominate that of radiation. After about 300,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is the cosmic microwave background.

Over time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The three possible types are known as cold dark matter, hot dark matter, and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the universe is cold dark matter. The other two types of matter make up less than 20% of the matter in the universe.

The universe today appears to be dominated by a mysterious form of energy known as dark energy. Approximately 70% of the total energy density of today's universe is in this form. This component of the universe's composition is revealed by its property of causing the expansion of the universe to deviate from a linear velocity-distance relationship by causing spacetime to expand faster than expected at very large distances. Dark energy in its simplest formation takes the form of a cosmological constant term in Einstein's field equations of general relativity, but its composition is unknown and, more generally, the details of its equation of state and relationship with the standard model of particle physics continue to be investigated both observationally and theoretically.

All these observations are encapsulated in the Lambda-CDM model of cosmology, which is a mathematical model of the big bang with six free parameters. Mysteries appear as one looks closer to the beginning, when particle energies were higher than can yet be studied by experiment. There is no compelling physical model for the first 10-33 seconds of the universe, before the phase transition called for by grand unification theory. At the "first instant", Einstein's theory of gravity predicts a gravitational singularity where densities become infinite. To resolve this paradox, a theory of quantum gravity is needed. Understanding this period of the history of the universe is one of the greatest unsolved problems in physics.

Theoretical underpinnings

As it stands today, the Big Bang is dependent on three assumptions:

The universality of physical laws

The cosmological principle

The Copernican principle

When first developed, these ideas were simply taken as postulates, but today there are efforts underway to test each of them. Tests of the universality of physical laws have found that the largest possible deviation of the fine structure constant over the age of the universe is of order 10-5. The isotropy of the universe that defines the Cosmological Principle has been tested to a level of 10-5 and the universe has been measured to be homogenous on the largest scales to the 10% level. There are efforts underway to test the Copernican Principle by means of looking at the interaction of galaxy clusters with the CMB through the Sunyaev-Zeldovich effect to a level of 1% accuracy.

The Big Bang theory uses Weyl's postulate to unambiguously measure time at any point as the "time since the Planck epoch". Measurements in this system rely on conformal coordinates in which so-called comoving distances and conformal times remove the expansion of the universe, parameterized by the cosmological scale factor, from consideration of spacetime measurements.

The comoving distances and conformal times are defined so that objects moving with the cosmological flow are always the same comoving distance apart and the particle horizon or observational limit of the local universe is set by the conformal time.

As the universe can be described by such coordinates, the Big Bang is not an explosion of matter moving outward to fill an empty universe; what is expanding is spacetime itself. It is this expansion that causes the physical distance between any two fixed points in our universe to increase.

Objects that are bound together (for example, by gravity) do not expand with spacetime's expansion because the physical laws that govern them are assumed to be uniform and independent of the metric expansion. Moreover, the expansion of the universe on today's local scales is so small that any dependence of physical laws on the expansion is unmeasurable by current techniques.

Observational evidence

It is generally stated that there are three observational pillars that support the Big Bang theory of cosmology. These are the Hubble-type expansion seen in the redshifts of galaxies, the detailed measurements of the cosmic microwave background, and the abundance of light elements. (See Big Bang nucleosynthesis.) Additionally, the observed correlation function of large-scale structure of the cosmos fits well with standard Big Bang theory.

Hubble's law expansion

Observations of distant galaxies and quasars show that these objects are redshifted, meaning that the light emitted from them has been shifted to longer wavelengths. This is seen by taking a frequency spectrum of the objects and then matching the spectroscopic pattern of emission lines or absorption lines corresponding to atoms of the elements interacting with the light. From this analysis, a redshift corresponding to a Doppler shift for the radiation can be measured which is explained by a recessional velocity.

When the recessional velocities are plotted against the distances to the objects, a linear relationship, known as Hubble's law, is observed: where v is the recessional velocity of the galaxy or other distant object D is the distance to the object and H0 is Hubble's constant, measured to be 71 ± 4 km/s/Mpc by the WMAP probe.

The Hubble's Law observation has two possible explanations. One is that we are at the center of an explosion of galaxies, a position which is untenable given the Copernican principle. The second explanation is that the universe is uniformly expanding everywhere as a unique property of spacetime. This type of universal expansion was developed mathematically in the context of general relativity well before Hubble made his analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann-Lemaître-Robertson-Walker.

Features, issues and problems

A number of problems have arisen within the Big Bang theory throughout its history. Some of them are mainly of historical interest today, and have been avoided either through modifications to the theory or as the result of better observations. Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as they can be addressed through refinements of the theory.

There are a small number of proponents of non-standard cosmologies who believe that there was no Big Bang at all. They claim that solutions to standard problems in the Big Bang involve ad hoc modifications and addenda to the theory. Most often attacked are the parts of standard cosmology that include dark matter, dark energy, and cosmic inflation. However, while explanations for these features remain at the frontiers of inquiry in physics, together they are suggested by independent observations of big bang nucleosynthesis, the cosmic microwave background, large scale structure and Type Ia supernovae. The gravitational effects of these features are understood observationally and theoretically but they have not yet been successfully incorporated into the Standard Model of particle physics. Though some aspects of the theory remain inadequately explained by fundamental physics, the vast majority of astronomers and physicists accept that the close agreement between Big Bang theory and observation have firmly established all the basic parts of the theory.

Big Bang "problems" and puzzles:

The horizon problem

The horizon problem results from the premise that information cannot travel faster than light, and hence two regions of space which are separated by a greater distance than the speed of light multiplied by the age of the universe cannot be in causal contact. The observed isotropy of the cosmic microwave background (CMB) is problematic in this regard, because the horizon size at that time corresponds to a size that is about 2 degrees on the sky. If the universe has had the same expansion history since the Planck epoch, there is no mechanism to cause these regions to have the same temperature.

This apparent inconsistency is resolved by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at a time 10-35 seconds after the Planck epoch. During inflation, the universe undergoes exponential expansion, and regions in causal contact expand so as to be beyond each other's horizons. Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the universe. After inflation, the universe expands according to a Hubble Law, and regions that were out of causal contact come back into the horizon. This explains the observed isotropy of the CMB. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian which has been accurately confirmed by measurements of the CMB.

Magnetic monopoles

The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted point defects in space that would manifest as magnetic monopoles with a density much higher than was consistent with observations, given that searches have never found any monopoles. This problem is also resolvable by cosmic inflation, which removes all point defects from the observable universe in the same way that it drives the geometry to flatness.

Baryon asymmetry

It is not yet understood why the universe has more matter than antimatter. It is generally assumed that when the universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and anti-baryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of matter. An unknown process called baryogenesis created the asymmetry. For baryogenesis to occur, the Sakharov conditions, which were laid out by Andrei Sakharov, must be satisfied. They require that baryon number not be conserved, that C-symmetry and CP-symmetry be violated, and that the universe depart from thermodynamic equilibrium. All these conditions occur in the big bang, but the effect is not strong enough to explain the present baryon asymmetry. New developments in high energy particle physics are necessary to explain the baryon asymmetry.

Globular cluster age

In the mid-1990s, observations of globular clusters appeared to be inconsistent with the Big Bang. Computer simulations that matched the observations of the stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13.7-billion-year age of the universe. This issue was generally resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. There still remain some questions as to how accurately the ages of the clusters are measured, but it is clear that these objects are some of the oldest in the universe.

Dark matter

During the 1970s and 1980s various observations (notably of galactic rotation curves) showed that there was not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is not normal or baryonic matter but rather dark matter. In addition, assuming that the universe was mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe is far less lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now a widely accepted part of standard cosmology due to observations of the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and x-ray measurements from galaxy clusters. Dark matter has only been detected through its gravitational signature; no particles that might make it up have yet been observed in laboratories. However, there are many particle physics candidates for dark matter, and several projects to detect them are underway.

Dark energy

In the 1990s, detailed measurements of the mass density of the universe revealed a value that was 30% that of the critical density. Since the universe is flat, as is indicated by measurements of the cosmic microwave background, fully 70% of the energy density of the universe was left unaccounted for. This mystery now appears to be connected to another one: Independent measurements of Type Ia supernovae have revealed that the expansion of the universe is undergoing a non-linear acceleration rather than following a strict Hubble Law. To explain this acceleration, general relativity requires that much of the universe consist of an energy component with large negative pressure. This dark energy is now thought to make up the missing 70%. Its nature remains one of the great mysteries of the Big Bang. Possible candidates include a scalar cosmological constant and quintessence. Observations to help understand this are ongoing.

Chaotic Inflation

brane cosmology models, including the ekpyrotic model in which the Big Bang is the result of a collision between branes an oscillatory universe in which the early universe's hot, dense state resulted from the Big Crunch of a universe similar to ours. The universe could have gone through an infinite number of big bangs and big crunches. The cyclic extension of the ekpyrotic model is a modern version of such a scenario.

models including the Hartle-Hawking boundary condition in which the whole of space-time is finite. Some of these scenarios are qualitatively compatible with one another. Each involves untested hypotheses.

Philosophical and religious interpretations

There are a number of interpretations of the Big Bang theory that are entirely speculative or extra-scientific. Some of these ideas purport to explain the cause of the Big Bang itself (first cause), and have been criticized by some naturalist philosophers as being modern creation myths. Some people believe that the Big Bang theory lends support to traditional views of creation, for example as given in Genesis, while others believe that all Big Bang theories are inconsistent with such views.

The Big Bang as a scientific theory is not associated with any religion. While certain fundamentalist interpretations of religions conflict with the Big Bang history of the universe, there are more liberal interpretations that do not.

The following is a list of various religious interpretations of the Big Bang theory:

A number of Christian apologists, the Roman Catholic Church in particular, have accepted the Big Bang as a description of the origin of the universe, interpreting it to allow for a philosophical first cause. Pope Pius XII was an enthusiastic proponent of the Big Bang even before the theory was scientifically well established.

Some students of Kabbalah, deism and other non-anthropomorphic faiths concord with the Big Bang theory, for example connecting it with the theory of "divine retraction" (tzimtzum) as explained by the Jewish scholar Moses Maimonides.

Some modern Islamic scholars believe that the Qur'an parallels the Big Bang in its account of creation, described as follows: "Do not the unbelievers see that the heavens and the earth were joined together as one unit of creation, before We clove them asunder?" (Ch:21,Ver:30). The Qur'an also appears to describe an expanding universe: "The heaven, We have built it with power. And verily, We are expanding it." (Ch:51,Ver:47). Parallels with the Big Crunch and an oscillating universe have also been suggested: "On the day when We will roll up the heavens like the rolling up of the scroll for writings, as We originated the first creation, (so) We shall reproduce it; a promise (binding on Us); surely We will bring it about." (Ch:21,Ver:104).

Certain theistic branches of Hinduism, such as the Vaishnava-traditions, conceive of a theory of creation with similarities to the theory of the Big Bang. The Hindu mythos, narrated for example in the third book of the Bhagavata Purana (primarily, chapters 10 and 26), describes a primordial state which bursts forth as the Great Vishnu glances over it, transforming into the active state of the sum-total of matter ("prakriti").

Buddhism has a concept of a universe that has no creation event. The Big Bang, however, is not seen to be in conflict with this since there are ways to get an eternal universe within the paradigm. A number of popular Zen philosophers were intrigued, in particular, by the concept of the oscillating universe.

Using the Big Bang model it is possible to calculate the concentration of helium-4, helium-3, deuterium and lithium-7 in the universe as ratios to the amount of ordinary hydrogen, H.

All the abundances depend on a single parameter, the ratio of photons to baryons. The ratios predicted (by mass, not by number) are about 0.25 for 4He/H, about 10-3 for 2H/H, about 10-4 for 3He/H and about

10-9 for 7Li/H.

The measured abundances all agree with those predicted from a single value of the baryon-to-photon ratio. This is considered strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements.

Indeed there is no obvious reason outside of the Big Bang that, for example, the young universe (i.e. before star formation, as determined by studying matter essentially free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than 3He, and in constant ratios, too.

The shape of the Universe is an informal name for a subject of investigation within physical cosmology.

Cosmologists and astronomers describe the geometry of the universe which includes both local geometry and global geometry.

It is loosely divided into curvature and topology, even though strictly speaking, it goes beyond both.

Considerations of the shape of the universe can be split into two parts; the local geometry relates especially to the curvature of the observable universe, while the global geometry relates especially to the topology of the universe as a whole—which may or may not be

within our ability to measure.

The extrapolation of the local geometry of space to the geometry of the whole universe is not without a specific ontological stance regarding how space and time coexist. Current thinking demands that space and time be considered as two aspects of a single entity 'spacetime'.

Nevertheless it still makes sense to speak about three-dimensional concepts referring to the universe, like the Hubble volume.

The local geometry is the curvature describing any arbitrary point in the observable universe (averaged on a sufficiently large scale). Many astronomical observations, such as those from supernovae and the Cosmic Microwave Background (CMB) radiation, show the observable universe to be very close to homogeneous and isotropic and infer it to be accelerating. In General Relativity, this is modelled by the Friedmann-Lemaître-Robertson-Walker (FLRW) model. This model, which can be represented by the Friedmann equations, provides a curvature (often referred to as geometry) of the universe based on the mathematics of fluid dynamics, i.e. it models the matter within the universe as a perfect fluid. Although stars and structures of mass can be introduced into an "almost FLRW" model, a strictly FLRW model is used to approximate the local geometry of the observable universe.

Another way of saying this is that if all forms of dark energy are ignored, then the curvature of the universe can be determined by measuring the average density of matter within it, assuming that all matter is evenly distributed (rather than the distortions caused by 'dense' objects such as galaxies). This assumption is justified by the observations that, while the universe is "weakly" inhomogeneous and anisotropic (see the large-scale structure of the cosmos), it is on average homogeneous and isotropic. The homogeneous and isotropic universe allows for a spatial geometry with a constant curvature. One aspect of local geometry to emerge from General Relativity and the FLRW model is that the density parameter, Omega (O), is related to the curvature of space. Omega is the average density of the universe divided by the critical energy density, i.e. that required for the universe to be flat (zero curvature). The curvature of space is a mathematical description of whether or not the Pythagorean theorem is valid for spatial coordinates. In the latter case, it provides an alternative formula for expressing local relationships between distances.

If the curvature is zero, then O = 1, and the Pythagorean theorem is correct. If O > 1, there is positive curvature, and if O < 1 there is negative curvature; in either of these cases, the Pythagorean theorem is invalid (but discrepancies are only detectable in triangles whose sides' lengths are of cosmological scale). If you measure the circumferences of circles of steadily larger diameters and divide the former by the latter, all three geometries give the value p for small enough diameters but the ratio departs from p for larger diameters unless O = 1. For O > 1 (the sphere, see diagram) the ratio falls below p: indeed, a great circle on a sphere has circumference only twice its diameter. For O < 1 the ratio rises above p.

Astronomical measurements of both matter-energy density of the universe and spacetime intervals using supernova events constrain the spatial curvature to be very close to zero, although they do not constrain its sign. This means that although the local geometries are generated by the theory of relativity based on spacetime intervals, we can approximate it to the familiar Euclidean geometry

There are three categories for the possible spatial geometries of constant curvature, depending on the sign of the curvature. If the curvature is exactly zero, then the local geometry is flat; if it is positive, then the local geometry is spherical, and if it is negative than the local geometry is hyperbolic.

The local geometry of the universe is determined by whether Omega is less than, equal to or greater than 1. From top to bottom: a spherical universe, a hyperbolic universe, and a flat universe.The geometry of the universe is usually represented in the system of comoving coordinates, according to which the expansion of the universe can be ignored. Comoving coordinates form a single frame of reference according to which the universe has a static geometry of three spatial dimensions.

Under the assumption that the universe is homogeneous and isotropic, the curvature of the observable universe, or the local geometry, is described by one of the three "primitive" geometries:

3-dimensional Euclidean geometry, generally annotated as E3

3-dimensional spherical geometry with a small curvature, often annotated as S3

3-dimensional hyperbolic geometry with a small curvature, often annotated as H3

Even if the universe is not exactly spatially flat, the spatial curvature is close enough to zero to place the radius at approximately the horizon of the observable universe or beyond.

Global geometry covers the geometry, in particular the topology, of the whole universe—both the observable universe and beyond. While the local geometry does not determine the global geometry completely, it does limit the possibilities, particularly a geometry of a constant curvature. For a flat spatial geometry, it used to be thought that the scale of any properties of the topology is arbitrary, though recent research suggests that the three spatial dimensions may tend to equalise in length.[1] The length scale of a flat geometry may or may not be directly detectable. For spherical and hyperbolic spatial geometries, the probability of detection of the topology by direct observation depends on the spatial curvature. Using the radius of curvature or its inverse as a scale, a small curvature of the local geometry, with a corresponding radius of curvature greater than the observable horizon, makes the topology difficult or impossible to detect if the curvature is hyperbolic. A spherical geometry with a small curvature (large radius of curvature) does not make detection difficult.

Two strongly overlapping investigations within the study of global geometry are:

whether the universe is infinite in extent or is a compact space whether the universe has a simply or non-simply connected topology

A compact space is a general topological definition that encompasses the more applicable notion of a bounded metric space. In cosmological models, it requires either one or both of: the space has positive curvature (like a sphere), and/or it is "multiply connected", or more strictly non-simply connected.

If the 3-manifold of a spatial section of the universe is compact then, as on a sphere, straight lines pointing in certain directions, when extended far enough in the same direction will reach the starting point and the space will have a definable "volume" or "scale". If the geometry of the universe is not compact, then it is infinite in extent with infinite paths of constant direction that, generally do not return and the space has no definable volume, such as the Euclidean plane.

If the spatial geometry is spherical, the topology is compact. Otherwise, for a flat or a hyperbolic spatial geometry, the topology can be either compact or infinite.

In a flat universe, all of the local curvature and local geometry is flat. In general it can be described by Euclidean space, however there are some spatial geometries which are flat and bounded in one or more directions. These include, in two dimensions, the cylinder, the torus, and the Mobius Strip. Similar spaces in three dimensions also exist.

A positively curved universe is described by spherical geometry, and can be thought of as a three-dimensional hypersphere.

One of the endeavors in the analysis of data from the Wilkinson Microwave Anisotropy Probe (WMAP) is to detect multiple "back-to-back" images of the distant universe in the cosmic microwave background radiation. Assuming the light has enough time since its origin to travel around a bounded universe, multiple images may be observed. While current results and analysis do not rule out a bounded topology, if the universe is bounded then the spatial curvature is small, just as the spatial curvature of the surface of the Earth is small compared to a horizon of a thousand kilometers or so.

Based on analyses of the WMAP data, cosmologists during 2004-2006 focused on the Poincaré dodecahedral space (PDS), but also considered horn topologies to be compatible with the data.

A hyperbolic universe (frequently but confusingly called "open") is described by hyperbolic geometry, and can be thought of as something like a three-dimensional equivalent of an infinitely extended saddle shape. For hyperbolic local geometry, many of the possible three-dimensional spaces are informally called horn topologies.

The ultimate fate of an open universe is that it will continue to expand forever, ending in a Heat Death, a Big Freeze or a Big Rip. This topology is consistent with astrophysical measurements made in the late 1990's.

WMAP image of the cosmic microwave background radiationThe Big Bang theory predicted the existence of the cosmic microwave background radiation or CMB which is composed of photons emitted during baryogenesis.

Because the early universe was in thermal equilibrium, the temperature of the radiation and the plasma were equal until the

plasma recombined. Before atoms formed, radiation was constantly absorbed and reemitted in a process called Compton scattering: the early universe was opaque to light.

However, cooling due to the expansion of the universe allowed the temperature to eventually fall below 3000 K at which point electrons and nuclei combined to form atoms and the primordial plasma turned into a neutral gas. This is known as photon decoupling.

A universe with only neutral atoms allows radiation to travel largely unimpeded.

Because the early universe was in thermal equilibrium, the radiation from this time had a blackbody spectrum and freely streamed through space until today, becoming redshifted because of the Hubble expansion. This reduces the high temperature of the blackbody spectrum. The radiation should be observable at every point in the universe to come from all directions of space.

In 1964, Arno Penzias and Robert Wilson, while conducting a series of diagnostic observations using a new microwave receiver owned by Bell Laboratories, discovered the cosmic background radiation. Their discovery provided substantial confirmation of the general CMB

predictions—the radiation was found to be isotropic and consistent with a blackbody spectrum of about 3 K —and it pitched the balance of opinion in favor of the Big Bang hypothesis. Penzias and Wilson were awarded the Nobel Prize for their discovery.

In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and the initial findings, released in 1990, were consistent with the Big Bang's predictions regarding the CMB. COBE found a residual temperature of 2.726 K and determined that the CMB was isotropic to about one part in 105. During the 1990s, CMB anisotropies were further investigated by a large number of ground-based experiments and the universe was shown to be geometrically flat by measuring the typical angular size (the size on the sky) of the anisotropies. (See shape of the universe.)

In early 2003 the results of the Wilkinson Microwave Anisotropy satellite (WMAP) were released, yielding what were at the time the most accurate values for some of the cosmological parameters. (see cosmic microwave background radiation experiments). This satellite also disproved several specific cosmic inflation models, but the results were consistent with the inflation theory in general.

The flatness problem is an observational problem that results from considerations of the geometry associated with Friedmann-Lemaître-Robertson-Walker metric.

In general, the universe can have three different kinds of geometries: hyperbolic geometry, Euclidean geometry, or elliptic geometry.

The geometry is determined by the total energy density of the universe (as measured by means of the stress-energy tensor): the hyperbolic results from a density less than the critical density, elliptic from a density greater than the critical density, and Euclidean from exactly the critical density.

The universe is measured to be required to be within one part in 1015 of the critical density in its earliest stages. Any greater deviation would have caused either a Heat Death or a Big Crunch, and the universe would not exist as it does today.

The resolution to this problem is again offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that any residual curvature associated with it would have been smoothed out to a high degree of precision. Thus, inflation drove the universe to be flat.

The future according to the Big Bang theory

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe is above the critical density, then the universe would reach a maximum size and then begin to collapse.

It would become denser and hotter again, ending with a state that was similar to that in which it started—a Big Crunch. Alternatively, if the density in the universe is equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as the universe grows less dense.

The average temperature of the universe would asymptotically approach absolute zero. Black holes would evaporate.

The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death. Moreover, if proton decay exists, then hydrogen, the predominant form of baryonic matter in the universe today, would disappear, leaving only radiation.

Modern observations of accelerated expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The Lambda-CDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the universe cools and expands. Other explanations of dark energy—so-called phantom energy theories—suggest that ultimately galaxy clusters and eventually galaxies themselves will be torn apart by the ever-increasing expansion in a so-called Big Rip.

Speculative physics beyond the Big Bang

While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest universe, when inflation is hypothesized to have occurred.

There may also be parts of the universe well beyond what can be observed in principle. In the case of inflation this is required: exponential expansion has pushed large regions of space beyond our observable horizon. It may be possible to deduce what happened when we better understand physics at very high energy scales. Speculations about this often involve theories of quantum gravity.

In general relativity, a naked singularity is a gravitational singularity without an event horizon. The singularities inside black holes are always surrounded by an area which does not allow light to escape,

and therefore cannot be directly observed. A naked singularity, by contrast, is a point in space where the density is infinite and which is observable from the outside.

The theoretical existence of naked singularities is important because their existence would mean that it would be possible to observe the collapse of an object to infinite density.

Computer simulations of the collapse of a disk of dust have indicated that these objects can exist, and thus the cosmic censorship hypothesis (stating that singularities are always hidden) does not hold. Stephen Hawking lost a bet about this question.

This is, of course, an example of a mathematical difficulty (divergence to infinity of the density) which reveals a more profound problem in our understanding of the relevant physics involved in the process. A workable theory of quantum gravity should be able to solve problems such as this.

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a massive galaxy) between the source object and the observer.

The process is known as gravitational lensing, and was one of the predictions made by Einstein's general relativity.

Description

In a gravitational lens, the gravity from the massive object bends light like a lens. As a result, the path of the light from the source is curved,

distorting its image, and the apparent location of the source may be different from its actual position.

In addition, the observer may see multiple images of a single source. If the source, massive object, and the observer lie on a straight line, the source will appear as a ring behind the massive object. This phenomenon was first mentioned by Chwolson in 1924, and quantified by Einstein in 1936.

It is usually referred to in the literature as an Einstein ring, since Chwolson did not concern himself with the flux or radius of the ring image. More commonly, the massive galaxy is off-center, creating a number of images according to the relative positions of the source, lens, and observer, and the shape of the gravitational well of the lensing galaxy.

There are three classes of gravitational lensing:

Strong lensing: where there are easily visible distortions such as the formation of Einstein rings, arcs, and multiple images.

Weak lensing: where the distortions of background objects are much smaller and can only be detected by analysing large numbers of objects to find distortions of only a few percent.

Microlensing: where no distortion in shape can be seen but the amount of light received from a background object changes in time.

Typically, both the background source and the lens are stars in the Milky Way.

The effect is weak, such that (in the case of strong lensing) a galaxy having a mass of over 100 billion solar masses will produce multiple images separated by only a few arcseconds. Galaxy clusters can produce separations of several arcminutes. In both cases the galaxies and sources are quite distant, many hundreds of megaparsecs away from our Galaxy.

Gravitational lenses act on all kinds of electromagnetic radiation, not just visible light. Weak lensing effects are being studied for the cosmic microwave background and strong lenses have been observed in radio and x-ray regimes as well.

History

According to general relativity, gravitational fields "warp" space-time and therefore bend light as a result. This theory was confirmed in 1919 during a solar eclipse, when Arthur Eddington observed the light from stars passing close to the sun was slightly bent, so that stars appeared slightly out of position.

Einstein realized that it was also possible for astronomical objects to bend light, and that under the correct conditions, one would observe multiple images of a single source, called a gravitational lens or sometimes a gravitational mirage.

However, as he only considered gravitational lensing by single stars, he concluded that the phenomenon would most likely remain unobserved for foreseeable future.

In 1937, Fritz Zwicky first considered the case where a galaxy could act as a lens, something that according to his calculations should be well within the reach of observations.

It was not until 1979 that the first gravitational lens would be discovered. It became known as the "Twin Quasar" since it initially looked like two identical quasars; it is officially named Q0957+561. This gravitational lens was discovered accidentally by Dennis Walsh, Bob Carswell, and Ray Weymann using the Kitt Peak National Observatory 2.1 meter telescope.

The study of gravitational lenses is an important part of the future of astronomy and astrophysics.

Cosmological applications

Gravitational lenses may be used to examine objects at distances at which they would not normally be visible, providing information from further back in time than otherwise possible (see below). Also, not just the object being lensed but the lens itself can provide useful information. By inverting the lens equations information can be gathered on the mass and distribution of the lensing body.

In weak lensing large scale maps of dark matter distributions may be produced, and these techniques are particularly important to cosmology as they provide a measure of the mass directly, without relying on any assumptions about the link between distributions of dark matter and visible. Lensing therefore can give a way of constraining the amount of dark matter in the universe and the manner in which it clusters together.

The statistics of strong gravitational lenses can also be used to measure values of cosmological parameters such as the cosmological constant and the mean density of matter in the universe. Presently, the statistics do not place very strong limits on cosmological parameters, partly because the number of strong lenses found is relatively small (less than a hundred).

Another parameter that may come out of the study of gravitational lenses is Hubble's constant which encodes the age and size of the universe. It can be determined, in theory, by measuring two quantities: the angular separation between two images, and the time delay between these images.

There are two contributions to the time delay:

the first is the obvious delay due to the difference in optical path length between the two rays. The second is a general relativistic effect, the Irwin Shapiro time-delay, that causes a change in the rate that clocks tick as they pass through a gravitational field.

Because the two rays travel through different parts of the potential well created by the deflector, the clocks carrying the source's signal will differ by a small amount.

Astronomical applications

Gravitational lenses can be used as gravitational telescopes, because they magnify objects seen behind them. Researchers at Caltech have used the gravitational lensing afforded by the Abell 2218 cluster of galaxies to detect the most distant galaxy known (February 15, 2004) through imaging with the Hubble Space Telescope.

Gravitational microlensing can provide information on comparatively small astronomical objects, such as MACHOs within our own galaxy, or extrasolar planets (planets beyond the solar system). Two extrasolar planets have been found in this way, and this technique has the promise of finding Earth-mass planets around sunlike stars in the not too distant future.

Detail observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang.

A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the big bang, and since then larger structures have been forming, such as galaxy clusters and superclusters.

Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state).

Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang.

These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions, and larger structures agree well with Big Bang simulations of the formation of structure in the universe and are helping to complete details of the theory

The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kuklos; or simply "the Galaxy") is a barred spiral galaxy in the Local Group, and has special significance to humanity as the location of the solar system, which is located near the Orion Arm of the galaxy.

The term "milky" originates from the hazy band of white light appearing across the celestial sphere visible from Earth, which comprises stars and other material lying within the galactic plane.

The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the solar system lies close to the galactic plane.

As a guide to the relative physical scale of the Milky Way, even if the galaxy were only 130 km (80 mi) in diameter, the solar system would be a mere 2 mm (0.08 in) in diameter. Also, a beam of light transmitted around the circumference of the Milky Way would take nearly 250,000 years to complete one revolution.

Age

The Galaxy is currently estimated to be about 13.6 billion (109) years old, which is nearly as old as the Universe itself.

This estimate is based upon research performed in 2004 by a team of astronomers: Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton. They used the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in globular cluster NGC 6397.

This allowed them to deduce the time elapsed between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster, at 200 million to 300 million years. They added in the estimated age of the stars in the globular cluster: 13.4 ± 0.8 billion years. The sum is their estimated age of the Milky Way Galaxy: 13.6 ± 0.8 billion years.

Structure

Observed structure of the Milky Way's spiral armsAs of 2005, the Milky Way is thought to comprise a large barred spiral galaxy of Hubble type SBbc (loosely wound barred spiral) with a total mass of about 1012 solar masses (M?), comprising 200-400 billion stars.

It was only in the 1980s that astronomers began to suspect that the Milky Way is a barred spiral rather than an ordinary spiral, which observations in 2005 with the Spitzer Space Telescope have since confirmed, showing that the galaxy's central bar is larger than previously suspected.

The galactic disk has an estimated diameter of about 100,000 light-years (see 1 E20 m for a list of comparable distances). The distance from the Sun to the galactic center is estimated at about 27,700 light-years. The disk bulges outward at the center.

As with most galaxies, it is suspected that the galactic center harbours a supermassive black hole, with Sagittarius A* being thought to be the most plausible candidate for the location of this extreme concentration of mass.

As is typical for many galaxies, the distribution of mass in the Milky Way is such that the orbital speed of most stars in the galaxy does not depend strongly on its distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is 210 and 240 km/s [3]. Hence the orbital period of the typical star is directly proportional only to the length of the path travelled.

This is unlike in the solar system where different orbits are also expected to have significantly different velocities associated with them.

The galaxy's bar is thought to be about 27,000 light years long, running through the center of the galaxy at a 44±10 degree angle to the line

between our sun and the center of the galaxy. It is composed primarily of red stars, believed to be ancient.

Observed and extrapolated structure of the spiral arms. Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees.

There are believed to be four major spiral arms which all start at the Galaxy's center. These are named as follows, according to the image at right:

2 and 8 - 3kpc and Perseus Arm

3 and 7 - Norma and Cygnus Arm (Along with a newly discovered extension - 6)

4 and 10 - Crux and Scutum Arm

5 and 9 - Carina and Sagittarius Arm

There are at least two smaller arms or spurs, including:

11 - Orion Arm (which contains the solar system and the Sun - 12)

Outside of the major spiral arms is the Outer Ring or Monoceros Ring, a ring of stars around the Milky Way proposed by astronomers Brian Yanny and Heidi Jo Newberg, which consists of gas and stars torn from other galaxies billions of years ago.

The galactic disk is surrounded by a spheroid halo of old stars and globular clusters. While the disk contains gas and dust obscuring the view in some wavelengths, the halo does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk.

The Sun's place in the Milky Way

The Sun (and therefore the Earth and Solar System) may be found close to the inner rim of the Orion Arm, in the Local Fluff, 8.5±0.5 kpc from the galactic center. The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years (see [4]).

The Apex of the Sun's Way, or the solar apex, refers to the direction that the Sun travels through space in the Milky Way. The general direction of the sun's galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 86 degrees to the direction of the galactic center.

The sun's orbit around the galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. We are presently about 8.5 kpc from the center of the galaxy and roughly 1/8 of an orbit before perigalacton (the sun's closest approach to the center, ~8.3 kpc).

It would take the solar system about 200-250 million years to complete one orbit ("galactic year"), and so is thought to have completed about 20-25 orbits during its lifetime. The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days.

The Milky Way is orbited by a number of dwarf galaxies in the Local Group. The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light years. The smallest, Carina Dwarf, Draco Dwarf, and Leo II are only 500 light years in diameter. The other dwarfs orbiting our galaxy are the Small Magellanic Cloud; Canis Major Dwarf, the closest; Sagittarius Dwarf Elliptical Galaxy, previously thought to be the closest; Ursa Minor Dwarf; Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo

Mythology

There are many creation myths around the world regarding the Milky Way. In particular, there are two similar ancient Greek stories that explain the etymology of the name Galaxias and its association with milk.

Some myths associate the constellation with a herd of cattle whose milk gives the sky its blue glow. In Eastern Asia, people believed that the hazy band of stars was the "Silvery River" of Heaven.

A galaxy is a large gravitationally bound system of stars, interstellar gas and dust, unseen dark matter, and possibly dark energy.

Typical galaxies contain 10 million to one trillion (107 to 1012) or more stars, all orbiting a common center of gravity.

In addition to single stars and a tenuous interstellar medium, most galaxies contain a large number of multiple star systems and star clusters as well as various types of nebulae.

Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years.

Although so called dark matter and dark energy appear to account for well over 90% of the mass of most galaxies, the nature of these unseen components is not well understood. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies.

Intergalactic space, the space between galaxies, is a near vacuum with an average density of less than one atom per cubic meter of gas or dust. There are probably more than 1011 galaxies in the visible universe.

Larger scale structures

Only few galaxies exist by themselves; these are known as field galaxies. Most galaxies are gravitationally bound to a number of other galaxies.

Structures containing up to about 50 galaxies are called groups of galaxies, and larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters.

Clusters of galaxies are often dominated by a single giant elliptical galaxy, which over time tidally destroys its satellite galaxies and adds their mass to its own.Superclusters are giant collections containing tens of thousands of galaxies, found in clusters, groups and sometimes individually; at the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids. Above this scale, the universe appears to be isotropic and homogeneous.

Our galaxy is a member of the Local Group, which it dominates together with the Andromeda Galaxy; overall the Local Group contains about 30 galaxies in a space about one megaparsec across. The Local Group is part of the Virgo Supercluster, which is dominated by the Virgo Cluster (of which our Galaxy is not a member).

History

In 1610, Galileo Galilei used a telescope to study the bright band on the night sky known as the Milky Way and discovered that it was composed of a huge number of faint stars. In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, 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 much larger scales.

The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate galaxies.

Towards the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae, later followed by a catalog of 5000 nebulae assembled by William Herschel. In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.

However, the nebulae were not universally accepted as distant separate galaxies until the matter was settled by Edwin Hubble in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.

The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter ~15 kiloparsecs) ellipsoid galaxy with the Sun close to the center.

A different method by Harlow Shapley based on the cataloging of globular clusters lead to a radically different picture: a flat disk with diameter ~70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane; once Robert Julius Trumpler had quantified this effect in 1930 by studying open clusters, the present picture of our galaxy as described above emerged.

In 1944, Hendrik van de Hulst predicted microwave radiation at a wave length of 21 centimetres, resulting from interstellar atomic hydrogen gas; this radiation was observed in 1951. This radiation allowed for much improved study of the Galaxy, since it is not affected by dust absorption and its Doppler shift can be used to map the motion of the gas in the Galaxy.

These observations led to the postulation of a rotating bar structure in the center of the Galaxy. With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s it was realized that the total visible mass of galaxies (from stars and gas) does not properly account for the speed of

the rotating gas, thus leading to the postulation of dark matter.

Etymology

The word galaxy was derived from the Greek term for our own galaxy, kyklos galaktikos meaning "milky circle" for the system’s appearance in the sky. When astronomers speculated that certain objects previously classified as spiral nebulae were actually vast congeries of stars, this was called the "island universe theory"; but this was an obvious misnomer, since universe means everything there is. Consequently, this term fell into disuse, replaced by applying the term galaxy generically to all such bodies.

Types of galaxies

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types is given by the Hubble sequence which serves well for many purposes. While the Hubble sequence does encompass all galaxies, it is entirely based upon visual morphological type. Hence it may miss the importance of certain characteristics of galaxies such as star formation rate.

Our own galaxy, the Milky Way, sometimes simply called the Galaxy (with uppercase), is a large disk-shaped barred spiral galaxy about 30 kiloparsecs or 100,000 light years in diameter and 3,000 light years in thickness. It contains about 3x1011 stars and has a total mass of about 6x1011 times the mass of the Sun.

In spiral galaxies, the spiral arms 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 also rotate around the center, but they do so with constant angular velocity. That means that stars pass in and out of spiral arms. The spiral arms are thought to be areas of high density or density waves. As stars move into an arm, they slow down, thus creating a higher density; this 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 they therefore harbor many bright and young stars.

Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). The system of galaxy classification is called the Hubble "tuning fork" diagram, and is the Hubble sequence.

The "tuning fork" system

The Hubble "tuning fork" diagram starts from the left with elliptical galaxy as its base. Elliptical galaxies can be named from E0 to E7. E stands for elliptical while the number indicates how oval-shaped the ellipse is with 0 being ball shape (in other words, a giant globular cluster) to 7 being discus shape. Technically speaking, the number is ten times the eccentricity. For example, an E7 galaxy has an eccentricty of 0.7.

After the elliptical galaxies the diagram splits into two branches. The upper branch covers spiral galaxy also called lenticular galaxies. It starts off with S0. The "S" means lenticular, the "0" means no arms, and the subscript number indicates how heavily a stripe is absorbed out of the image of the galaxy by dust in the galactic disc. On the same branch are the next 3 types which all have spiral arms. The "S" here also means lenticular, but the lower case letter after it tell how wound up the arms are. They range from "a" to "d" having the following meanings:

Sa - tightly-wound, smooth arms, and a bright central disc

Sb - better defined spiral arms than Sa

Sc - much more loosely wound spiral arms than Sb

Sd - very loose arms, most of the luminosity is in the arms and not the disc

The lower branch of the diagram covers spiral-barred galaxy given the symbol "SB". This branch starts with SBO galaxies which is followed by a subscript number that indicates how heavily defined the bar is. After that the branch continues with the SB galaxies which have lower case letters after them that indicates how heavily defined the bar is. They range from "a" to "c" having the following meanings:

SBa - a bright center and tight spirals

SBb - better defined arms than SBa galaxy and are more loosely wound

SBc - even looser arms, and a much dimmer central portion of the galaxy

The Milky Way Galaxy is now believed to be an SBb galaxy; previously, it was thought to be Sb like its giant companion, the Andromeda Galaxy.

Star clusters are groups of stars which are gravitationally bound. Two distinct types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are often very young.

Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even

though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.

Globular clusters

Globular clusters are roughly spherical groups of anything between 10,000 and several million stars in a region about 10 to 30 light years across. They generally consist of very old Population II stars, just a few million years younger than the universe itself. The constituent stars tend to be yellow and red, and weigh less than about two solar masses. This is because the hotter, more massive stars have either exploded as supernovae or passed through a planetary nebula phase to become white dwarfs. However, some anomalous blue stars are found in globulars, and are believed to have been formed by stellar mergers in the dense inner regions of the cluster. These stars are known as blue stragglers.

In our galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the galactic centre, orbiting the centre in highly elliptical orbits. In 1917, the astronomer Harlow Shapley was able to estimate the Sun's distance from the galactic centre based on the distribution of globular clusters; previously the Sun's location within the Milky Way was by no means well established.

Until recently, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However, greatly improved distance measurements to globular clusters using the Hipparcos satellite and increasingly accurate measurements of the Hubble constant resolved the paradox, giving an age for the universe of about 13 billion years and an age for the oldest stars of a few hundred million years less.

Our galaxy has about 150 globular clusters, some of which may have been captured from small galaxies disrupted by the Milky Way, as seems to be the case for the globular cluster M79. Some galaxies are much richer in globulars: the giant elliptical galaxy M87 contains over a thousand.

A few of the brightest globular clusters are visible to the naked eye, with the brightest, Omega Centauri, having been known since antiquity and catalogued as a star before the telescopic age. The best known globular cluster in the northern hemisphere is M13 (modestly called the Great Globular Cluster in Hercules).

Open clusters

The Pleiades, an open cluster dominated by hot blue stars surrounded by reflection nebulosityOpen clusters are very different to globular clusters.

Unlike the spherically-distributed globulars, they are confined to the galactic plane, and are almost always found within spiral arms. They are generally young objects, up to a few tens of millions of years old.

They form from H II regions such as the Orion Nebula.

Open clusters usually contain up to a few hundred members, within a region up to about 30 light years across. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of giant molecular clouds and other clusters. Close encounters between cluster members can also result in the ejection of stars, a process known as 'evaporation'.

The most prominent open clusters are the Pleiades and Hyades in Taurus. The Double Cluster of h+Chi Persei can also be prominent under dark skies. Open clusters are often dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die.

Stellar associations

Once an open cluster has become gravitationally unbound, the constituent stars will continue to move on similar paths through space. The group is then known as a stellar association, or a moving group. Several of the brightest stars in Ursa Major are members of a former open cluster, and have similar proper motions. Other bright stars across the sky, including Sirius and Alpha Ophiuchi, seem to also be related to this group. Our Sun lies within this stream of stars at the moment, but isn't a true member as shown by its different galactic orbit. Another stellar association is that surrounding Mirfak (a Persei), which is very prominent in binoculars. Distant moving clusters can't readily be detected since the proper motions of the stars need to be known.

Astronomical significance of clusters

The study of star clusters is very important in many areas of astronomy. Because the stars were all born at roughly the same time, the different properties of all the stars in a cluster are a function only of mass, and so stellar evolution theories rely on observations of open and globular clusters.

Clusters are also a crucial step in determining the distance scale of the universe. A few of the nearest clusters are close enough for their distances to be measured using parallax. A Hertzsprung-Russell Diagram can be plotted for these clusters which has absolute values known on the luminosity axis. Then, when similar diagrams are plotted for clusters whose distance is not known, the position of the main sequence can be compared to that of the first cluster and the distance estimated.