137-729: Martin John Rees, Baron Rees of Ludlow , (born 23 June 1942) is a British cosmologist and astrophysicist . He is the fifteenth Astronomer Royal , appointed in 1995, and was Master of Trinity College, Cambridge , from 2004 to 2012 and President of the Royal Society between 2005 and 2010. He has received various physics awards including the Wolf Prize in Physics in 2024 for fundamental contributions to high-energy astrophysics, galaxies and structure formation, and cosmology. Rees
274-659: A Fellow of the Royal Society in 1979. From 1992 to 2003, he was Royal Society Research Professor, and from 2003 Professor of Cosmology and Astrophysics . He was Master of Trinity College, Cambridge, during 2004–2012. He is an Honorary Fellow of Darwin College , King's College , Clare Hall , Robinson College and Jesus College, Cambridge . Rees is a member of the Board of the Institute for Advanced Study , in Princeton and
411-656: A "small cloud". In 964, he probably mentioned the Large Magellanic Cloud in his Book of Fixed Stars , referring to "Al Bakr of the southern Arabs", since at a declination of about 70° south it was not visible where he lived. It was not well known to Europeans until Magellan 's voyage in the 16th century. The Andromeda Galaxy was later independently noted by Simon Marius in 1612. In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there might be other galaxies outside that were formed into galactic clusters that were minuscule parts of
548-452: A chain reaction of star-building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the activity 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. A radio galaxy
685-459: A common feature at the centres of galaxies. Galaxies are categorised according to their visual morphology as elliptical , spiral , or irregular . The Milky Way is an example of a spiral galaxy. It is estimated that there are between 200 billion ( 2 × 10 ) to 2 trillion galaxies in the observable universe . Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years ) and are separated by distances in
822-684: A debate took place between Harlow Shapley and Heber Curtis , the Great Debate , concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift. In 1922, the Estonian astronomer Ernst Öpik gave
959-512: A description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin , structure, evolution , and ultimate fate . Cosmology as a science originated with the Copernican principle , which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics , which first allowed those physical laws to be understood. Physical cosmology, as it
1096-536: A distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. Using the new 100-inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables , thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of
1233-453: A factor of ten, due to not knowing about the types of Cepheid variables. Given the cosmological principle, Hubble's law suggested that the universe was expanding. Two primary explanations were proposed for the expansion. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other explanation was Fred Hoyle 's steady state model in which new matter is created as the galaxies move away from each other. In this model,
1370-921: A few have well-established names, such as the Andromeda Galaxy , the Magellanic Clouds , the Whirlpool Galaxy , and the Sombrero Galaxy . Astronomers work with numbers from certain catalogues, such as the Messier catalogue , the NGC ( New General Catalogue ), the IC ( Index Catalogue ), the CGCG ( Catalogue of Galaxies and of Clusters of Galaxies ), the MCG ( Morphological Catalogue of Galaxies ),
1507-528: A few nearby bright galaxies, like the Andromeda Galaxy , began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called island universes , but this term quickly fell into disuse, as the word universe implied the entirety of existence. Instead, they became known simply as galaxies. Millions of galaxies have been catalogued, but only
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#17327656486931644-418: A function of their radius (or distance from their cores) falls off more slowly than their smaller counterparts. The formation of these cD galaxies remains an active area of research, but the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters, or even those outside of clusters with random overdensities. These processes are the mechanisms that drive
1781-680: A host of other revelations. After holding postdoctoral research positions in the United Kingdom and the United States, he was a professor at Sussex University , during 1972–1973. He later moved to Cambridge , where he was the Plumian Professor at the University of Cambridge until 1991, and the director of the Institute of Astronomy . He was professor of astronomy at Gresham College , London, in 1975 and became
1918-429: A low portion of open clusters and a reduced rate of new star formation. Instead, they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller globular clusters . The largest galaxies are
2055-404: A modification of gravity on the largest scales. The effect on cosmology of the dark energy that these models describe is given by the dark energy's equation of state , which varies depending upon the theory. The nature of dark energy is one of the most challenging problems in cosmology. A better understanding of dark energy is likely to solve the problem of the ultimate fate of the universe . In
2192-417: 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
2329-400: A plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter which extends beyond the visible component, as demonstrated by the universal rotation curve concept. 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
2466-438: A reserve of cold gas that forms giant molecular clouds . Some galaxies have been observed to form stars at an exceptional rate, which is known as a starburst . If they continue to do so, they would consume their reserve of gas in a time span less than the galaxy's lifespan. Hence starburst activity usually lasts only about ten million years, a relatively brief period in a galaxy's history. Starburst galaxies were more common during
2603-435: A rotating bar structure in the center of this galaxy. With improved radio telescopes , hydrogen gas could also be traced in other galaxies. In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter . Beginning in
2740-580: A sequence of stellar nucleosynthesis reactions, smaller atomic nuclei are then combined into larger atomic nuclei, ultimately forming stable iron group elements such as iron and nickel , which have the highest nuclear binding energies . The net process results in a later energy release , meaning subsequent to the Big Bang. Such reactions of nuclear particles can lead to sudden energy releases from cataclysmic variable stars such as novae . Gravitational collapse of matter into black holes also powers
2877-488: A single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. Most of the information we have about dwarf galaxies come from observations of the local group , containing two spiral galaxies, the Milky Way and Andromeda, and many dwarf galaxies. These dwarf galaxies are classified as either irregular or dwarf elliptical / dwarf spheroidal galaxies . A study of 27 Milky Way neighbors found that in all dwarf galaxies,
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#17327656486933014-502: A small, positive cosmological constant. The solution is an expanding universe; due to this expansion, the radiation and matter in the universe cool and become diluted. At first, the expansion is slowed down by gravitation attracting the radiation and matter in the universe. However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the universe starts to accelerate rather than decelerate. In our universe this happened billions of years ago. During
3151-419: A smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy NGC 3923 has over 20 shells. Spiral galaxies resemble spiraling pinwheels . Though the stars and other visible material contained in such a galaxy lie mostly on
3288-437: A unified description of gravity as a geometric property of space and time. At the time, Einstein believed in a static universe , but found that his original formulation of the theory did not permit it. This is because masses distributed throughout the universe gravitationally attract, and move toward each other over time. However, he realized that his equations permitted the introduction of a constant term which could counteract
3425-522: A universe with a larger cosmological constant. Many cosmologists find this an unsatisfying explanation: perhaps because while the weak anthropic principle is self-evident (given that living observers exist, there must be at least one universe with a cosmological constant (CC) which allows for life to exist) it does not attempt to explain the context of that universe. For example, the weak anthropic principle alone does not distinguish between: Other possible explanations for dark energy include quintessence or
3562-528: Is 3C 236 , with lobes 15 million light-years across. It should however be noted that radio emissions are not always considered part of the main galaxy itself. A giant radio galaxy is a special class of objects characterized by the presence of radio lobes generated by relativistic jets powered by the central galaxy's supermassive black hole . Giant radio galaxies are different from ordinary radio galaxies in that they can extend to much larger scales, reaching upwards to several megaparsecs across, far larger than
3699-442: Is a fourth "sterile" species of neutrino. The ΛCDM ( Lambda cold dark matter ) or Lambda-CDM model is a parametrization of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted by Lambda ( Greek Λ ), associated with dark energy, and cold dark matter (abbreviated CDM ). It is frequently referred to as the standard model of Big Bang cosmology. The cosmic microwave background
3836-516: Is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from its active galactic nucleus . Radio galaxies are classified according to their Fanaroff–Riley classification . The FR I class have lower radio luminosity and exhibit structures which are more elongated; the FR II class are higher radio luminosity. The correlation of radio luminosity and structure suggests that
3973-420: Is a version of MOND that can explain gravitational lensing. If the universe is flat , there must be an additional component making up 73% (in addition to the 23% dark matter and 4% baryons) of the energy density of the universe. This is called dark energy. In order not to interfere with Big Bang nucleosynthesis and the cosmic microwave background, it must not cluster in haloes like baryons and dark matter. There
4110-562: Is also used to observe distant, red-shifted galaxies that were formed much earlier. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy . The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies . The Earth's atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. The ionosphere blocks signals below this range. Large radio interferometers have been used to map
4247-651: Is an atheist but has criticized militant atheists for being too hostile to religion. Rees is a lifelong supporter of the Labour Party , but has no party affiliation when sitting in the House of Lords. [REDACTED] This article incorporates text available under the CC BY 4.0 license. Physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model , or simply cosmology , provides
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4384-412: Is called baryogenesis . Three required conditions for baryogenesis were derived by Andrei Sakharov in 1967, and requires a violation of the particle physics symmetry , called CP-symmetry , between matter and antimatter. However, particle accelerators measure too small a violation of CP-symmetry to account for the baryon asymmetry. Cosmologists and particle physicists look for additional violations of
4521-531: Is called the Galactic Center . The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter . Consequently, these galaxies also have
4658-402: Is changed only by the increase in volume, but the energy density of radiation is changed both by the increase in volume and by the increase in the wavelength of the photons that make it up. Thus the energy of radiation becomes a smaller part of the universe's total energy than that of matter as it expands. The very early universe is said to have been 'radiation dominated' and radiation controlled
4795-461: Is given by the Hubble sequence . Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies . Many galaxies are thought to contain a supermassive black hole at their center. This includes the Milky Way, whose core region
4932-438: Is known about dark energy. Quantum field theory predicts a cosmological constant (CC) much like dark energy, but 120 orders of magnitude larger than that observed. Steven Weinberg and a number of string theorists (see string landscape ) have invoked the 'weak anthropic principle ': i.e. the reason that physicists observe a universe with such a small cosmological constant is that no physicists (or any life) could exist in
5069-476: Is now understood, began in 1915 with the development of Albert Einstein 's general theory of relativity , followed by major observational discoveries in the 1920s: first, Edwin Hubble discovered that the universe contains a huge number of external galaxies beyond the Milky Way ; then, work by Vesto Slipher and others showed that the universe is expanding . These advances made it possible to speculate about
5206-444: Is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way. In the astronomical literature, the capitalised word "Galaxy" is often used to refer to the Milky Way galaxy, to distinguish it from the other galaxies in the observable universe . The English term Milky Way can be traced back to a story by Geoffrey Chaucer c. 1380 : See yonder, lo,
5343-699: Is part of the Local Group , which it dominates along with the Andromeda Galaxy . The group is part of the Virgo Supercluster . At the largest scale , these associations are generally arranged into sheets and filaments surrounded by immense voids . Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea . The word galaxy was borrowed via French and Medieval Latin from
5480-469: Is radiation left over from decoupling after the epoch of recombination when neutral atoms first formed. At this point, radiation produced in the Big Bang stopped Thomson scattering from charged ions. The radiation, first observed in 1965 by Arno Penzias and Robert Woodrow Wilson , has a perfect thermal black-body spectrum. It has a temperature of 2.7 kelvins today and is isotropic to one part in 10 . Cosmological perturbation theory , which describes
5617-456: Is roughly equal to the age of the universe at each point in time. Observations suggest that the universe began around 13.8 billion years ago. Since then, the evolution of the universe has passed through three phases. The very early universe, which is still poorly understood, was the split second in which the universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth. Therefore, while
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5754-447: Is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into filaments , superclusters and voids . Most simulations contain only non-baryonic cold dark matter , which should suffice to understand the universe on the largest scales, as there is much more dark matter in the universe than visible, baryonic matter. More advanced simulations are starting to include baryons and study
5891-392: Is smaller than, or comparable to, the time scale of the expansion of the universe. The time scale that describes the expansion of the universe is 1 / H {\displaystyle 1/H} with H {\displaystyle H} being the Hubble parameter , which varies with time. The expansion timescale 1 / H {\displaystyle 1/H}
6028-417: 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. The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense. In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals ,
6165-457: Is strong observational evidence for dark energy, as the total energy density of the universe is known through constraints on the flatness of the universe, but the amount of clustering matter is tightly measured, and is much less than this. The case for dark energy was strengthened in 1999, when measurements demonstrated that the expansion of the universe has begun to gradually accelerate. Apart from its density and its clustering properties, nothing
6302-497: Is that dark energy is just the vacuum energy , a component of empty space that is associated with the virtual particles that exist due to the uncertainty principle . There is no clear way to define the total energy in the universe using the most widely accepted theory of gravity, general relativity. Therefore, it remains controversial whether the total energy is conserved in an expanding universe. For instance, each photon that travels through intergalactic space loses energy due to
6439-570: Is the author of more than 500 research papers. He is an author of books on astronomy and science intended for the lay public and gives many public lectures and broadcasts. In 2010 he was invited to deliver the Reith Lectures for the BBC , now published as From Here to Infinity: Scientific Horizons . Rees has made contributions to the origin of cosmic microwave background radiation , as well as to galaxy clustering and formation. His studies of
6576-515: Is to measure the brightness of an object and assume an intrinsic luminosity , from which the distance may be determined using the inverse-square law . Due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own Milky Way , nor did they speculate about the cosmological implications. In 1927, the Belgian Roman Catholic priest Georges Lemaître independently derived
6713-626: Is to measure the basic parameters of the Lambda-CDM model with increasing accuracy, as well as to test the predictions of the Big Bang model and look for new physics. The results of measurements made by WMAP, for example, have placed limits on the neutrino masses. Newer experiments, such as QUIET and the Atacama Cosmology Telescope , are trying to measure the polarization of the cosmic microwave background. These measurements are expected to provide further confirmation of
6850-605: Is unstable to small perturbations—it will eventually start to expand or contract. It was later realized that Einstein's model was just one of a larger set of possibilities, all of which were consistent with general relativity and the cosmological principle . The cosmological solutions of general relativity were found by Alexander Friedmann in the early 1920s. His equations describe the Friedmann–Lemaître–Robertson–Walker universe, which may expand or contract, and whose geometry may be open, flat, or closed. In
6987-452: Is what caused the universe to contain far more matter than antimatter . Cosmologists can observationally deduce that the universe is not split into regions of matter and antimatter. If it were, there would be X-rays and gamma rays produced as a result of annihilation , but this is not observed. Therefore, some process in the early universe must have created a small excess of matter over antimatter, and this (currently not understood) process
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#17327656486937124-437: Is zero or negligible compared to their kinetic energy , and so move at the speed of light or very close to it; non-relativistic particles have much higher rest mass than their energy and so move much slower than the speed of light. As the universe expands, both matter and radiation become diluted. However, the energy densities of radiation and matter dilute at different rates. As a particular volume expands, mass-energy density
7261-519: The Cosmic Background Explorer in the early 1990s, few cosmologists have seriously proposed other theories of the origin and evolution of the cosmos. One consequence of this is that in standard general relativity, the universe began with a singularity , as demonstrated by Roger Penrose and Stephen Hawking in the 1960s. An alternative view to extend the Big Bang model, suggesting the universe had no beginning or singularity and
7398-477: The Greek term for the Milky Way, galaxías (kúklos) γαλαξίας ( κύκλος ) 'milky (circle)', named after its appearance as a milky band of light in the sky. In Greek mythology , Zeus places his son, born by a mortal woman, the infant Heracles , on Hera 's breast while she is asleep so the baby will drink her divine milk and thus become immortal. Hera wakes up while breastfeeding and then realises she
7535-973: The Oxford Martin School . He co-founded the Centre for the Study of Existential Risk and serves on the Scientific Advisory Board for the Future of Life Institute . He has formerly been a Trustee of the British Museum , the Science Museum , the Gates Cambridge Trust and the Institute for Public Policy Research (IPPR) . His doctoral students have included Roger Blandford , Craig Hogan , Nick Kaiser Priyamvada Natarajan , and James E. Pringle . Rees
7672-787: The Royal Institution of Great Britain until 2010. Rees has received honorary degrees from a number of universities including Hull, Sussex, Uppsala, Toronto, Durham, Oxford, Cambridge, Harvard, Yale, Melbourne and Sydney. He belongs to several foreign academies, including the US National Academy of Sciences , the Russian Academy of Sciences , the Pontifical Academy of Sciences , the Royal Netherlands Academy of Arts and Sciences ,
7809-839: The Science Academy of Turkey and the Japan Academy . He became president of the Royal Society on 1 December 2005 and continued until the end of the Society's 350th Anniversary Celebrations in 2010. In 2011, he was awarded the Templeton Prize . In 2005, Rees was elevated to a life peerage , sitting as a crossbencher in the House of Lords as Baron Rees of Ludlow, of Ludlow in the County of Shropshire. In 2005, he
7946-487: The Sloan Digital Sky Survey . Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars. Aristotle (384–322 BCE), however, believed the Milky Way was 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
8083-502: The Solar System . Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a thousand stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass . Most of the mass in a typical galaxy is in the form of dark matter , with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are
8220-525: The University of Nottingham , used 20 years of Hubble images to estimate that the observable universe contained at least two trillion ( 2 × 10 ) galaxies. However, later observations with the New Horizons space probe from outside the zodiacal light reduced this to roughly 200 billion ( 2 × 10 ). Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance
8357-544: The University of St Andrews . He made two TED talks on existential risks. Rees thinks the search for extraterrestrial intelligence is worthwhile and has chaired the advisory board for the " Breakthrough Listen " project, a programme of SETI investigations funded by the Russian/US investor Yuri Milner . In August 2014, Rees was one of 200 public figures who were signatories to a letter to The Guardian expressing their hope that Scotland would vote to remain part of
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#17327656486938494-579: The atmosphere , in the region of the World that is continuous with the heavenly motions ." Neoplatonist philosopher Olympiodorus the Younger ( c. 495 –570 CE) was critical of this view, arguing that if the Milky Way was sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax , which it did not. In his view,
8631-417: The cosmic microwave background , structure formation, and galaxy rotation curves suggests that about 23% of the mass of the universe consists of non-baryonic dark matter, whereas only 4% consists of visible, baryonic matter . The gravitational effects of dark matter are well understood, as it behaves like a cold, non-radiative fluid that forms haloes around galaxies. Dark matter has never been detected in
8768-491: The mathematical tripos at Trinity College, Cambridge , graduating with first class honours . He then undertook post-graduate research at Cambridge and completed a PhD supervised by Dennis Sciama in 1967. Rees' post-graduate work in astrophysics in the mid-1960s coincided with an explosion of new discoveries, with breakthroughs ranging from confirmation of the Big Bang , the discovery of neutron stars and black holes , and
8905-464: The redshift effect. This energy is not transferred to any other system, so seems to be permanently lost. On the other hand, some cosmologists insist that energy is conserved in some sense; this follows the law of conservation of energy . Different forms of energy may dominate the cosmos— relativistic particles which are referred to as radiation , or non-relativistic particles referred to as matter. Relativistic particles are particles whose rest mass
9042-603: The spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes ) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in the Zone of Avoidance (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies. A 2016 study published in The Astrophysical Journal , led by Christopher Conselice of
9179-519: The type-cD galaxies . First described in 1964 by a paper by Thomas A. Matthews and others, they are a subtype of the more general class of D galaxies, which are giant elliptical galaxies, except that they are much larger. They are popularly known as the supergiant elliptical galaxies and constitute the largest and most luminous galaxies known. These galaxies feature a central elliptical nucleus with an extensive, faint halo of stars extending to megaparsec scales. The profile of their surface brightnesses as
9316-579: The "Great Andromeda Nebula", as the Andromeda Galaxy, Messier object M31 , was then known. Searching the photographic record, he found 11 more novae . Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within this galaxy. As a result, he was able to come up with a distance estimate of 150,000 parsecs . He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies. In 1920
9453-452: The "cosmic dark ages" ended when the first stars formed. Since the 1970s he has been interested in anthropic reasoning, and the possibility that our visible universe is part of a vaster " multiverse ". In addition to expansion of his scientific interests, Rees has written and spoken extensively about the problems and challenges of the 21st century, and interfaces between science, ethics, and politics. In his books Our Final Hour and On
9590-400: The 1910s, Vesto Slipher (and later Carl Wilhelm Wirtz ) interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth. However, it is difficult to determine the distance to astronomical objects. One way is to compare the physical size of an object to its angular size , but a physical size must be assumed in order to do this. Another method
9727-525: The 1990s, the Hubble Space Telescope yielded improved observations. Among other things, its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars. The Hubble Deep Field , an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion ( 1.25 × 10 ) galaxies in the observable universe. Improved technology in detecting
9864-549: The Big Bang cosmology, which is presented in Timeline of the Big Bang . The early, hot universe appears to be well explained by the Big Bang from roughly 10 seconds onwards, but there are several problems . One is that there is no compelling reason, using current particle physics, for the universe to be flat , homogeneous, and isotropic (see the cosmological principle ) . Moreover, grand unified theories of particle physics suggest that there should be magnetic monopoles in
10001-487: The CP-symmetry in the early universe that might account for the baryon asymmetry . Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and experiment , rather than through observations of the universe. Big Bang nucleosynthesis is the theory of the formation of the elements in the early universe. It finished when
10138-480: 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 " —which was later called the Big Bang. In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of
10275-562: The Future , Rees warns that humanity faces significant existential risks in the 21st century due to technological advancements, particularly in bioengineering and artificial intelligence . Although he remains optimistic that if it is managed successfully, technology could drastically improve standards of living. In 2007, he delivered the Gifford Lectures on 21st Century Science: Cosmic Perspective and Terrestrial Challenges at
10412-550: The Galaxyë Which men clepeth the Milky Wey , For hit is whyt. Galaxies were initially discovered telescopically and were known as spiral nebulae . Most 18th- to 19th-century astronomers considered them as either unresolved star clusters or anagalactic nebulae , and were just thought of as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of
10549-432: The Hubble classification scheme, spiral galaxies are listed as type S , followed by a letter ( a , b , or c ) which 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
10686-462: The Milky Way galaxy was "a myriad of tiny stars packed together in the sphere of the fixed stars." Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study it and discovered it was composed of a huge number of faint stars. In 1750, English astronomer Thomas Wright , in his An Original Theory or New Hypothesis of
10823-431: The Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways. Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae. In 1845, Lord Rosse examined the nebulae catalogued by Herschel and observed
10960-493: The Milky Way was celestial. According to Mohani Mohamed, Arabian astronomer Ibn al-Haytham (965–1037) made the first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." Persian astronomer al-Biruni (973–1048) proposed the Milky Way galaxy was "a collection of countless fragments of
11097-481: The Milky Way. Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies. They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars. Blue compact dwarf galaxies contains large clusters of young, hot, massive stars . Ultra-compact dwarf galaxies have been discovered that are only 100 parsecs across. Many dwarf galaxies may orbit
11234-402: The Milky Way. In 1926 Hubble produced a classification of galactic morphology that is used to this day. Advances in astronomy have always been driven by technology. After centuries of success in optical astronomy , infrared astronomy was attempted in the 1830s, but only blossomed in the early 1900s. Radio astronomy was born in the 1930s, and matured by the 1950s and 1960s. The problem
11371-648: The UGC ( Uppsala General Catalogue of Galaxies), and the PGC ( Catalogue of Principal Galaxies , also known as LEDA). All the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 (or "M109") is a spiral galaxy having the number 109 in the catalogue of Messier. It also has the designations NGC 3992, UGC 6937, CGCG 269–023, MCG +09-20-044, and PGC 37617 (or LEDA 37617), among others. Millions of fainter galaxies are known by their identifiers in sky surveys such as
11508-561: The United Kingdom in September's referendum on that issue . To mark the 300th anniversary of the Board of Longitude in 2014, he instigated a programme of new challenge prizes of £5-10m under the name " Longitude Prize 2014" for which he chairs the advisory board. The themes of the first two prizes are the reduction of inappropriate antibiotic use, and enhancing the safety and independence of dementia sufferers. The Longitude Prize on Dementia
11645-483: The Universe , correctly speculated that it 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, and that the resulting disk of stars could be seen as a band on the sky from a perspective inside it. In his 1755 treatise, Immanuel Kant elaborated on Wright's idea about the Milky Way's structure. The first project to describe
11782-407: The active jets emitted from active nuclei. Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole. The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies
11919-473: The age of the universe is infinite, has been presented. In September 2023, astrophysicists questioned the overall current view of the universe , in the form of the Standard Model of Cosmology , based on the latest James Webb Space Telescope studies. The lightest chemical elements , primarily hydrogen and helium , were created during the Big Bang through the process of nucleosynthesis . In
12056-441: The attractive force of gravity on the cosmic scale. Einstein published his first paper on relativistic cosmology in 1917, in which he added this cosmological constant to his field equations in order to force them to model a static universe. The Einstein model describes a static universe; space is finite and unbounded (analogous to the surface of a sphere, which has a finite area but no edges). However, this so-called Einstein model
12193-418: The basic features of this epoch have been worked out in the Big Bang theory, the details are largely based on educated guesses. Following this, in the early universe, the evolution of the universe proceeded according to known high energy physics . This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background
12330-407: The brightness of Cepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance from Earth. This fact is now known as Hubble's law , though the numerical factor Hubble found relating recessional velocity and distance was off by
12467-498: The center. A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the centre. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane ; but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters ,
12604-749: The central mass is approximately 10 million solar masses , regardless of whether it has thousands or millions of stars. This suggests 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. Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution . Near misses between galaxies result in warping distortions due to tidal interactions , and may cause some exchange of gas and dust. Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies usually do not collide, but
12741-442: The core, or else due to a tidal interaction with another galaxy. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms. Our own galaxy, the Milky Way , is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×10 ) stars and has a total mass of about six hundred billion (6×10 ) times
12878-399: The core, then merges into the spiral arm structure. In the Hubble classification scheme, these are designated by an SB , followed by a lower-case letter ( a , b or c ) which indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from
13015-449: The current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger than superclusters from forming. It is not known whether the acceleration will continue indefinitely, perhaps even increasing until a big rip , or whether it will eventually reverse, lead to a Big Freeze , or follow some other scenario. Gravitational waves are ripples in the curvature of spacetime that propagate as waves at
13152-405: The deceleration of expansion. Later, as the average energy per photon becomes roughly 10 eV and lower, matter dictates the rate of deceleration and the universe is said to be 'matter dominated'. The intermediate case is not treated well analytically . As the expansion of the universe continues, matter dilutes even further and the cosmological constant becomes dominant, leading to an acceleration in
13289-763: The development of a standard model of cosmology . This model requires the universe to contain large amounts of dark matter and dark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations. Cosmology draws heavily on the work of many disparate areas of research in theoretical and applied physics . Areas relevant to cosmology include particle physics experiments and theory , theoretical and observational astrophysics , general relativity, quantum mechanics , and plasma physics . Modern cosmology developed along tandem tracks of theory and observation. In 1916, Albert Einstein published his theory of general relativity , which provided
13426-438: The distribution of quasars challenged the now-rejected steady state theory . He was one of the first to propose that enormous black holes power quasars, and that superluminal astronomical observations can be explained as an optical illusion caused by an object moving partly in the direction of the observer. Since the 1990s, Rees has worked on gamma-ray bursts , especially in collaboration with Péter Mészáros , and on how
13563-436: The earliest moments of the universe, the average energy density was very high, making knowledge of particle physics critical to understanding this environment. Hence, scattering processes and decay of unstable elementary particles are important for cosmological models of this period. As a rule of thumb, a scattering or a decay process is cosmologically important in a certain epoch if the time scale describing that process
13700-414: The evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angular power spectrum of the radiation, and it has been measured by the recent satellite experiments ( COBE and WMAP ) and many ground and balloon-based experiments (such as Degree Angular Scale Interferometer , Cosmic Background Imager , and Boomerang ). One of the goals of these efforts
13837-400: The formation of fossil groups or fossil clusters, where a large, relatively isolated, supergiant elliptical resides in the middle of the cluster and are surrounded by an extensive cloud of X-rays as the residue of these galactic collisions. Another older model posits the phenomenon of cooling flow , where the heated gases in clusters collapses towards their centers as they cool, forming stars in
13974-436: The formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy. Other, complementary observations to measure the distribution of matter in the distant universe and to probe reionization include: These will help cosmologists settle the question of when and how structure formed in the universe. Evidence from Big Bang nucleosynthesis ,
14111-546: The formation of the early universe shortly after the Big Bang. In 2016, the LIGO Scientific Collaboration and Virgo Collaboration teams announced that they had made the first observation of gravitational waves , originating from a pair of merging black holes using the Advanced LIGO detectors. On 15 June 2016, a second detection of gravitational waves from coalescing black holes
14248-590: The galaxies' original morphology. If one of the galaxies is much more massive than the other, the result is known as cannibalism , where the more massive larger galaxy remains relatively undisturbed, and the smaller one is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy . Stars are created within galaxies from
14385-448: The gas and dust within the two forms interacts, sometimes triggering star formation. A collision can severely distort the galaxies' shapes, forming bars, rings or tail-like structures. At the extreme of interactions are galactic mergers, where the galaxies' relative momentums are insufficient to allow them to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to
14522-470: 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. A majority of spiral galaxies, including the Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of
14659-492: The laboratory, and the particle physics nature of dark matter remains completely unknown. Without observational constraints, there are a number of candidates, such as a stable supersymmetric particle, a weakly interacting massive particle , a gravitationally-interacting massive particle, an axion , and a massive compact halo object . Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations ( MOND ) or an effect from brane cosmology. TeVeS
14796-492: The largest objects, such as superclusters, are still assembling. One way to study structure in the universe is to survey the visible galaxies, in order to construct a three-dimensional picture of the galaxies in the universe and measure the matter power spectrum . This is the approach of the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey . Another tool for understanding structure formation
14933-421: The mass of the Sun. Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 87,400 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than
15070-408: The most energetic processes, generally seen in the nuclear regions of galaxies, forming quasars and active galaxies . Cosmologists cannot explain all cosmic phenomena exactly, such as those related to the accelerating expansion of the universe , using conventional forms of energy . Instead, cosmologists propose a new form of energy called dark energy that permeates all space. One hypothesis
15207-446: The nature of nebulous stars." Andalusian astronomer Avempace ( d. 1138) proposed that it was composed of many stars that almost touched one another, and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects were near. In the 14th century, Syrian-born Ibn Qayyim al-Jawziyya proposed
15344-517: The order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly) and is separated from the Andromeda Galaxy , its nearest large neighbour, by just over 750,000 parsecs (2.5 million ly). The space between galaxies is filled with a tenuous gas (the intergalactic medium ) with an average density of less than one atom per cubic metre. Most galaxies are gravitationally organised into groups , clusters and superclusters . The Milky Way
15481-478: The origin of the universe, and allowed the establishment of the Big Bang theory, by Georges Lemaître , as the leading cosmological model. A few researchers still advocate a handful of alternative cosmologies ; however, most cosmologists agree that the Big Bang theory best explains the observations. Dramatic advances in observational cosmology since the 1990s, including the cosmic microwave background , distant supernovae and galaxy redshift surveys , have led to
15618-472: The present picture of the Milky Way galaxy emerged. A few galaxies outside the Milky Way are visible on a dark night to the unaided eye , including the Andromeda Galaxy , Large Magellanic Cloud , Small Magellanic Cloud , and the Triangulum Galaxy . In the 10th century, Persian astronomer Abd al-Rahman al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as
15755-474: The process, a phenomenon observed in clusters such as Perseus , and more recently in the Phoenix Cluster . A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs
15892-475: The shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center . Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to
16029-414: The signal can be entirely attributed to interstellar dust in the Milky Way. Understanding the formation and evolution of the largest and earliest structures (i.e., quasars, galaxies, clusters and superclusters ) is one of the largest efforts in cosmology. Cosmologists study a model of hierarchical structure formation in which structures form from the bottom up, with smaller objects forming first, while
16166-439: The sources in these two types of galaxies may differ. Radio galaxies can also be classified as giant radio galaxies (GRGs), whose radio emissions can extend to scales of megaparsecs (3.26 million light-years). Alcyoneus is an FR II class low-excitation radio galaxy which has the largest observed radio emission, with lobed structures spanning 5 megaparsecs (16×10 ly ). For comparison, another similarly sized giant radio galaxy
16303-438: The speed of light, generated in certain gravitational interactions that propagate outward from their source. Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves to collect observational data about sources of detectable gravitational waves such as binary star systems composed of white dwarfs , neutron stars , and black holes ; and events such as supernovae , and
16440-576: The spiral structure of Messier object M51 , now known as the Whirlpool Galaxy. In 1912, Vesto M. Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high Doppler shifts , indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us. In 1917, Heber Doust Curtis observed nova S Andromedae within
16577-669: The theory as well as information about cosmic inflation, and the so-called secondary anisotropies, such as the Sunyaev-Zel'dovich effect and Sachs-Wolfe effect , which are caused by interaction between galaxies and clusters with the cosmic microwave background. On 17 March 2014, astronomers of the BICEP2 Collaboration announced the apparent detection of B -mode polarization of the CMB, considered to be evidence of primordial gravitational waves that are predicted by
16714-588: The theory of inflation to occur during the earliest phase of the Big Bang. However, later that year the Planck collaboration provided a more accurate measurement of cosmic dust , concluding that the B-mode signal from dust is the same strength as that reported from BICEP2. On 30 January 2015, a joint analysis of BICEP2 and Planck data was published and the European Space Agency announced that
16851-414: The universe is roughly the same at any point in time. For a number of years, 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. The discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model, and since the precise measurements of the cosmic microwave background by
16988-498: The universe that extended far beyond what could be seen. These views "are remarkably close to the present-day views of the cosmos." In 1745, Pierre Louis Maupertuis conjectured that some nebula -like objects were collections of stars with unique properties, including a glow exceeding the light its stars produced on their own, and repeated Johannes Hevelius 's view that the bright spots were massive and flattened due to their rotation. In 1750, Thomas Wright correctly speculated that
17125-443: The universe was about three minutes old and its temperature dropped below that at which nuclear fusion could occur. Big Bang nucleosynthesis had a brief period during which it could operate, so only the very lightest elements were produced. Starting from hydrogen ions ( protons ), it principally produced deuterium , helium-4 , and lithium . Other elements were produced in only trace abundances. The basic theory of nucleosynthesis
17262-442: The universe's early history, but still contribute an estimated 15% to total star production. 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 stars produce supernova explosions, creating expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger
17399-499: The universe's expansion. The history of the universe is a central issue in cosmology. The history of the universe is divided into different periods called epochs, according to the dominant forces and processes in each period. The standard cosmological model is known as the Lambda-CDM model . Within the standard cosmological model , the equations of motion governing the universe as a whole are derived from general relativity with
17536-612: The universe, which have not been found. These problems are resolved by a brief period of cosmic inflation , which drives the universe to flatness , smooths out anisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles. The physical model behind cosmic inflation is extremely simple, but it has not yet been confirmed by particle physics, and there are difficult problems reconciling inflation and quantum field theory . Some cosmologists think that string theory and brane cosmology will provide an alternative to inflation. Another major problem in cosmology
17673-625: The uses and abuses of advanced technology and on issues such as assisted dying , preservation of dark skies, and reforms to broaden the post-16 and undergraduate curricula in the UK. He is also a current member of the House of Lords Science and Technology Committee. He has been president of the Royal Astronomical Society (1992–94) and the British Science Association (1995–96), and was a Member of Council of
17810-483: Was announced in 2022. In 2015, he was co-author of the report that launched the Global Apollo Programme , which calls for developed nations to commit to spending 0.02% of their GDP for 10 years, to fund coordinated research to make carbon-free baseload electricity less costly than electricity from coal by the year 2025. In his general writings and in the House of Lords , his focus has been on
17947-428: Was announced. Besides LIGO, many other gravitational-wave observatories (detectors) are under construction. Cosmologists also study: Galaxy A galaxy is a system of stars , stellar remnants , interstellar gas , dust , and dark matter bound together by gravity . The word is derived from the Greek galaxias ( γαλαξίας ), literally 'milky', a reference to the Milky Way galaxy that contains
18084-633: Was awarded the Crafoord Prize. Other awards and honours include: The Asteroid 4587 Rees and the Sir Martin Rees Academic Scholarship at Shrewsbury International School are named in his honour. In June 2022, to celebrate his 80th birthday, Rees was the subject of the BBC programme The Sky at Night , in conversation with Professor Chris Lintott . Rees married the anthropologist Caroline Humphrey in 1986. He
18221-464: Was born on 23 June 1942 in York , England. After a peripatetic life during the war his parents, both teachers, settled with Rees, an only child, in a rural part of Shropshire near the border with Wales. There, his parents founded Bedstone College , a boarding school based on progressive educational concepts. He was educated at Bedstone College, then from the age of 13 at Shrewsbury School . He studied for
18358-405: Was confirmed through X-ray astronomy. In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas; and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in this galaxy. These observations led to the hypothesis of
18495-495: Was developed in 1948 by George Gamow, Ralph Asher Alpher , and Robert Herman . It was used for many years as a probe of physics at the time of the Big Bang, as the theory of Big Bang nucleosynthesis connects the abundances of primordial light elements with the features of the early universe. Specifically, it can be used to test the equivalence principle , to probe dark matter , and test neutrino physics. Some cosmologists have proposed that Big Bang nucleosynthesis suggests there
18632-469: Was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the first stars and quasars , and ultimately galaxies, clusters of galaxies and superclusters formed. The future of the universe is not yet firmly known, but according to the ΛCDM model it will continue expanding forever. Below, some of the most active areas of inquiry in cosmology are described, in roughly chronological order. This does not include all of
18769-493: Was that air only allows visible light and radio waves to pass, with a little bit of near infrared. The first ultraviolet telescope was launched in 1968, and since then there's been major progress in all regions of the electromagnetic spectrum . The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared , which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail. Infrared
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