Hubble Space Telescope
The Hubble Space Telescope is a space telescope, launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile and is well known as both a vital research tool and a public relations boon for astronomy; the HST is named after the astronomer Edwin Hubble and is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory and the Spitzer Space Telescope. With a 2.4-meter mirror, Hubble's four main instruments observe in the ultraviolet and near infrared regions of the electromagnetic spectrum. Hubble's orbit outside the distortion of Earth's atmosphere allows it to take high-resolution images, with lower background light than ground-based telescopes. Hubble has recorded some of the most detailed visible light images allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe.
The HST was built by the United States space agency NASA, with contributions from the European Space Agency. The Space Telescope Science Institute selects Hubble's targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft. Space telescopes were proposed as early as 1923. Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, the Challenger disaster; when launched in 1990, Hubble's main mirror was found to have been ground incorrectly, creating a spherical aberration, compromising the telescope's capabilities. The optics were corrected to their intended quality by a servicing mission in 1993. Hubble is the only telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired and replaced systems on the telescope, including all five of the main instruments; the fifth mission was canceled on safety grounds following the Columbia disaster.
However, after spirited public discussion, NASA administrator Mike Griffin approved the fifth servicing mission, completed in 2009. The telescope is operating as of 2019, could last until 2030–2040. After numerous delays, its successor, the James Webb Space Telescope, is scheduled to be launched in March 2021. In 1923, Hermann Oberth—considered a father of modern rocketry, along with Robert H. Goddard and Konstantin Tsiolkovsky—published Die Rakete zu den Planetenräumen, which mentioned how a telescope could be propelled into Earth orbit by a rocket; the history of the Hubble Space Telescope can be traced back as far as 1946, to the astronomer Lyman Spitzer's paper "Astronomical advantages of an extraterrestrial observatory". In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes. First, the angular resolution would be limited only by diffraction, rather than by the turbulence in the atmosphere, which causes stars to twinkle, known to astronomers as seeing.
At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.05 arcsec for a telescope with a mirror 2.5 m in diameter. Second, a space-based telescope could observe infrared and ultraviolet light, which are absorbed by the atmosphere. Spitzer devoted much of his career to pushing for the development of a space telescope. In 1962, a report by the US National Academy of Sciences recommended the development of a space telescope as part of the space program, in 1965 Spitzer was appointed as head of a committee given the task of defining scientific objectives for a large space telescope. Space-based astronomy had begun on a small scale following World War II, as scientists made use of developments that had taken place in rocket technology; the first ultraviolet spectrum of the Sun was obtained in 1946, the National Aeronautics and Space Administration launched the Orbiting Solar Observatory to obtain UV, X-ray, gamma-ray spectra in 1962.
An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, in 1966 NASA launched the first Orbiting Astronomical Observatory mission. OAO-1's battery failed after three days, it was followed by OAO-2, which carried out ultraviolet observations of stars and galaxies from its launch in 1968 until 1972, well beyond its original planned lifetime of one year. The OSO and OAO missions demonstrated the important role space-based observations could play in astronomy, in 1968, NASA developed firm plans for a space-based reflecting telescope with a mirror 3 m in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope, with a launch slated for 1979; these plans emphasized the need for manned maintenance missions to the telescope to ensure such a costly program had a lengthy working life, the concurrent development of plans for the reusable Space Shuttle indicated that the technology to allow this was soon to become available.
The continuing success of the OAO program encouraged strong consensus within the astronomical community that the LST should be a major goal. In 1970, NASA established two committees, one to plan the engineering side of the space telescope project, the other to determine the scientific goals of the mission. Once these had been established, the next hurdle for NASA was to obtain funding for the instrument, which would be far more costly than any Earth-bas
Richard C. Tolman
Richard Chace Tolman was an American mathematical physicist and physical chemist, an authority on statistical mechanics. He made important contributions to theoretical cosmology in the years soon after Einstein's discovery of general relativity, he was a professor of physical chemistry and mathematical physics at the California Institute of Technology. Tolman was born in West Newton and studied chemical engineering at the Massachusetts Institute of Technology, receiving his bachelor's degree in 1903 and Ph. D. in 1910 under A. A. Noyes, he married Ruth Sherman Tolman in 1924. In 1912, he conceived of the concept of relativistic mass by writing that "the expression m0−1/2 is best suited for the mass of a moving body."In a 1916 experiment with Thomas Dale Stewart, Tolman demonstrated that electricity consists of electrons flowing through a metallic conductor. A by-product of this experiment was a measured value of the mass of the electron. Overall, however, he was known as a theorist. Tolman was a member of the Technical Alliance in 1919, a forerunner of the Technocracy movement where he helped conduct an energy survey analyzing the possibility of applying science to social and industrial affairs.
Tolman was elected a Fellow of the American Academy of Arts and Sciences in 1922. The same year, he joined the faculty of the California Institute of Technology, where he became professor of physical chemistry and mathematical physics and dean of the graduate school. One of Tolman's early students at Caltech was the theoretical chemist Linus Pauling, to whom Tolman taught pre-Schrödinger quantum theory. In 1927, Tolman published a text on statistical mechanics whose background was the old quantum theory of Max Planck, Niels Bohr and Arnold Sommerfeld. In 1938, he published a new detailed work that covered the application of statistical mechanics to classical and quantum systems, it was the standard work on the subject for many remains of interest today. In the years of his career, Tolman became interested in the application of thermodynamics to relativistic systems and cosmology. An important monograph he published in 1934 titled Relativity and Cosmology demonstrated how black body radiation in an expanding universe cools but remains thermal – a key pointer toward the properties of the cosmic microwave background.
In this monograph, Tolman was the first person to document and explain how a closed universe could equal zero energy. He explained how all mass energy is positive and all gravitational energy is negative and they cancel each other out, leading to a universe of zero energy, his investigation of the oscillatory universe hypothesis, which Alexander Friedmann had proposed in 1922, drew attention to difficulties as regards entropy and resulted in its demise until the late 1960s. During World War II, Tolman served as scientific advisor to General Leslie Groves on the Manhattan Project. At the time of his death in Pasadena, he was chief advisor to Bernard Baruch, the U. S. representative to the United Nations Atomic Energy Commission. Each year, the southern California section of the American Chemical Society honors Tolman by awarding its Tolman Medal "in recognition of outstanding contributions to chemistry." Tolman's brother was the behavioral psychologist Edward Chace Tolman. List of notable textbooks in statistical mechanics Tolman–Oppenheimer–Volkoff equation Tolman–Oppenheimer–Volkoff limit Oscillatory universe Tolman length Tolman surface brightness test Lemaître–Tolman metric Cyclic model Tachyonic antitelephone Statistical mechanics with applications to physics and chemistry.
New York: The Chemical Catalog Company. 1927. Relativity and Cosmology. Oxford: Clarendon Press. 1934. LCCN 340-32023. Reissued New York: Dover ISBN 0-486-65383-8; the Principles of Statistical Mechanics. Oxford: Clarendon Press. 1938. Reissued New York: Dover ISBN 0-486-63896-0. 1987 Dover reprint. Short biography from the Online Archive of California Short biography from the "Tolman Award" page of the Southern California Section of the American Chemical Society. Works by Richard C. Tolman at Project Gutenberg Works by or about Richard C. Tolman at Internet Archive Biographical memoir, National Academy of Sciences. Includes a complete bibliography of Tolman's writings. Retrieved July 14, 2017
According to the theory of relativity, time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently situated relative to a gravitational field. As a result of the nature of spacetime, a clock, moving relative to an observer will be measured to tick slower than a clock, at rest in the observer's own frame of reference. A clock, under the influence of a stronger gravitational field than an observer's will be measured to tick slower than the observer's own clock; such time dilation has been demonstrated, for instance by small disparities in a pair of atomic clocks after one of them is sent on a space trip, or by clocks on the Space Shuttle running slower than reference clocks on Earth, or clocks on GPS and Galileo satellites running faster. Time dilation has been the subject of science fiction works, as it technically provides the means for forward time travel. Time dilation by the Lorentz factor was predicted by several authors at the turn of the 20th century.
Joseph Larmor, at least for electrons orbiting a nucleus, wrote "... individual electrons describe corresponding parts of their orbits in times shorter for the system in the ratio: 1 − v 2 c 2 ". Emil Cohn related this formula to the rate of clocks. In the context of special relativity it was shown by Albert Einstein that this effect concerns the nature of time itself, he was the first to point out its reciprocity or symmetry. Subsequently, Hermann Minkowski introduced the concept of proper time which further clarified the meaning of time dilation. Special relativity indicates that, for an observer in an inertial frame of reference, a clock, moving relative to him will be measured to tick slower than a clock, at rest in his frame of reference; this case is sometimes called special relativistic time dilation. The faster the relative velocity, the greater the time dilation between one another, with the rate of time reaching zero as one approaches the speed of light; this causes massless particles that travel at the speed of light to be unaffected by the passage of time.
Theoretically, time dilation would make it possible for passengers in a fast-moving vehicle to advance further into the future in a short period of their own time. For sufficiently high speeds, the effect is dramatic. For example, one year of travel might correspond to ten years on Earth. Indeed, a constant 1 g acceleration would permit humans to travel through the entire known Universe in one human lifetime.. With current technology limiting the velocity of space travel, the differences experienced in practice are minuscule: after 6 months on the International Space Station an astronaut would have aged about 0.005 seconds less than those on Earth. The cosmonauts Sergei Krikalev and Sergei Avdeyev both experienced time dilation of about 20 milliseconds compared to time that passed on Earth. Time dilation can be inferred from the observed constancy of the speed of light in all reference frames dictated by the second postulate of special relativity; this constancy of the speed of light means that, counter to intuition, speeds of material objects and light are not additive.
It is not possible to make the speed of light appear greater by moving towards or away from the light source. Consider a simple clock consisting of two mirrors A and B, between which a light pulse is bouncing; the separation of the mirrors is L and the clock ticks once each time the light pulse hits either of the mirrors. In the frame in which the clock is at rest, the light pulse traces out a path of length 2L and the period of the clock is 2L divided by the speed of light: Δ t = 2 L c. From the frame of reference of a moving observer traveling at the speed v relative to the resting frame of the clock, the light pulse is seen as tracing out a longer, angled path. Keeping the speed of light constant for all inertial observers, requires a lengthening of the period of this clock from the moving observer's perspective; that is to say, in a frame moving relative to the local clock, this clock will appear to be running more slowly. Straightforward application of the Pythagorean theorem leads to the well-known prediction of special relativity: The total time for the light pulse to trace its path is given by Δ t ′ = 2 D c.
The length of the half path can be calculated as a function of known quantities as D = 2 + L 2. Elimination of the variables D and L from these three equations results in Δ t ′ = Δ t 1 − v 2 c 2, which expresses the fact that the moving observer's period of the clock Δ t ′
Expansion of the universe
The expansion of the universe is the increase of the distance between two distant parts of the universe with time. It is an intrinsic expansion; the universe does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric governing the geometry of spacetime itself that changes in scale. Although light and objects within spacetime cannot travel faster than the speed of light, this limitation does not restrict the metric itself. To an observer it appears that space is expanding and all but the nearest galaxies are receding into the distance. During the inflationary epoch about 10−32 of a second after the Big Bang, the universe expanded, its volume increased by a factor of at least 1078, equivalent to expanding an object 1 nanometer in length to one 10.6 light years long. A much slower and gradual expansion of space continued after this, until at around 9.8 billion years after the Big Bang it began to expand more and is still doing so. The metric expansion of space is of a kind different from the expansions and explosions seen in daily life.
It seems to be a property of the universe as a whole rather than a phenomenon that applies just to one part of the universe or can be observed from "outside" it. Metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the Friedmann-Lemaître-Robertson-Walker metric and is a generic property of the universe we inhabit. However, the model is valid only on large scales, because gravitational attraction binds matter together enough that metric expansion cannot be observed at this time, on a smaller scale; as such, the only galaxies receding from one another as a result of metric expansion are those separated by cosmologically relevant scales larger than the length scales associated with the gravitational collapse that are possible in the age of the universe given the matter density and average expansion rate. Physicists have postulated the existence of dark energy, appearing as a cosmological constant in the simplest gravitational models as a way to explain the acceleration.
According to the simplest extrapolation of the currently-favored cosmological model, the Lambda-CDM model, this acceleration becomes more dominant into the future. In June 2016, NASA and ESA scientists reported that the universe was found to be expanding 5% to 9% faster than thought earlier, based on studies using the Hubble Space Telescope. While special relativity prohibits objects from moving faster than light with respect to a local reference frame where spacetime can be treated as flat and unchanging, it does not apply to situations where spacetime curvature or evolution in time become important; these situations are described by general relativity, which allows the separation between two distant objects to increase faster than the speed of light, although the definition of "separation" is different from that used in an inertial frame. This can be seen. Light, emitted today from galaxies beyond the cosmological event horizon, about 5 gigaparsecs or 16 billion light-years, will never reach us, although we can still see the light that these galaxies emitted in the past.
Because of the high rate of expansion, it is possible for a distance between two objects to be greater than the value calculated by multiplying the speed of light by the age of the universe. These details are a frequent source of confusion among amateurs and professional physicists. Due to the non-intuitive nature of the subject and what has been described by some as "careless" choices of wording, certain descriptions of the metric expansion of space and the misconceptions to which such descriptions can lead are an ongoing subject of discussion within education and communication of scientific concepts. In 1912, Vesto Slipher discovered that light from remote galaxies was redshifted, interpreted as galaxies receding from the Earth. In 1922, Alexander Friedmann used Einstein field equations to provide theoretical evidence that the universe is expanding. In 1927, Georges Lemaître independently reached a similar conclusion to Friedmann on a theoretical basis, presented the first observational evidence for a linear relationship between distance to galaxies and their recessional velocity.
Edwin Hubble observationally confirmed Lemaître's findings two years later. Assuming the cosmological principle, these findings would imply that all galaxies are moving away from each other. Based on large quantities of experimental observation and theoretical work, the scientific consensus is that space itself is expanding, that it expanded rapidly within the first fraction of a second after the Big Bang; this kind of expansion is known as "metric expansion". In mathematics and physics, a "metric" means a measure of distance, the term implies that the sense of distance within the universe is itself changing; the modern explanation for the metric expansion of space was proposed by physicist Alan Guth in 1979 while investigating the problem of why no magnetic monopoles are seen today. Guth found in his investigation that if the universe contained a field that has a positive-energy false vacuum state according to general relativity it would generate an exponential expansion of space. I
W. M. Keck Observatory
The W. M. Keck Observatory is a two-telescope astronomical observatory at an elevation of 4,145 meters near the summit of Mauna Kea in the U. S. state of Hawaii. Both telescopes feature 10 m primary mirrors among the largest astronomical telescopes in use. With a concept first proposed in 1977, telescope designers at the University of California and Lawrence Berkeley Labs had been developing the technology necessary to build a large, ground-based telescope. With a design in hand, a search for the funding began. In 1985, Howard B. Keck of the W. M. Keck Foundation gave $70 million to fund the construction of the Keck I telescope. Construction of Keck I began in September 1985, with first light occurring on 24 November 1990 using only nine of the eventual 36 segments. With construction of the first telescope well advanced, further donations allowed the construction of a second telescope starting in 1991; the Keck I telescope began science observations in May 1993, while first light for Keck II occurred on October 23, 1996.
The key advance that allowed the construction of the Keck Observatory's large telescopes was the ability to operate smaller mirror segments as a single, contiguous mirror. In the case of the Keck Observatory telescopes each of the primary mirrors is composed of 36 hexagonal segments that work together as a single unit; each segment is 1.8 meters wide, 7.5 centimeters thick, weighs half a ton. The mirrors were made from Zerodur glass-ceramic by the German company Schott AG. On the telescope, each segment is kept stable by a system of active optics, which uses rigid support structures in combination with three actuators under each segment. During observation, the computer-controlled system of sensors and actuators adjusts the position of each segment, relative to its neighbors, to an accuracy of four nanometers; this twice-per-second adjustment counters the effect of gravity as the telescope moves, in addition to other environmental and structural effects that can affect the mirror shape. Each Keck Observatory telescope sits on an altazimuth mount.
Most current 8–10 m class telescopes use altazimuth designs due to the reduced structural requirements compared to older equatorial designs. This mounting style provides the greatest strength and stiffness for the least amount of steel, for Keck Observatory, totals about 270 tons per telescope; the total weight of each telescope is more than 300 tons. Two of the proposed designs for the next generation 30 and 40 m telescopes use the same basic technology pioneered at Keck Observatory, a hexagonal mirror array coupled with an altazimuth mounting; the primary mirrors of each of the two telescopes are 10 meters in diameter smaller than the Gran Telescopio Canarias. However, all of the light collected by the Keck Observatory primary mirrors is sent to the secondary mirror and the instruments, compared to GTC's primary mirror, which has an effective light-collection area of 73.4 m2, or 2.36 m2 less than each of the Keck Observatory primary mirrors. Because of this fundamental difference in design, Keck Observatory's telescopes arguably remain the largest steerable, optical/infrared telescopes on Earth.
The telescopes are equipped with a suite of instruments, both cameras and spectrometers that allow observations across much of the visible and near infrared spectrum. The Keck Observatory is managed by the California Association for Research in Astronomy, a non-profit 501 organization whose board of directors includes representatives from Caltech and the University of California. Construction of the telescopes was made possible through private grants totaling more than $140 million provided by the W. M. Keck Foundation; the National Aeronautics and Space Administration joined the partnership in October 1996, at the time Keck II commenced observations. Telescope time is allocated by the partner institutions. Caltech, the University of Hawaii System, the University of California accept proposals from their own researchers. NASA accepts proposals from researchers based in the United States. Jerry Nelson was the project scientist for the Keck Telescope, he contributed to multi-mirror projects until he died in June 2017.
Nelson was behind one of the key innovations of the Keck telescope, the use of multiple thin segments acting as one mirror to provide the reflecting surface. MOSFIRE MOSFIRE is a third generation instrument for the W. M. Keck Observatory. MOSFIRE was delivered to Keck Observatory on February 8, 2012 and first light on the Keck I telescope was obtained on April 4, 2012. A Multi-Object Spectrograph For Infra-Red Exploration and wide-field camera for the near-infrared, MOSFIRE's special feature is the cryogenic Configurable Slit Unit, reconfigurable under remote control in less than 6 minutes without any thermal cycling. Bars move in from each side to form up to 46 short slits; when the bars are removed MOSFIRE becomes a wide-field imager. The instrument was developed by teams from the University of California, Los Angeles, the California Institute of Technology and the University of California, Santa Cruz; the Co- Principal Investigators are Ian S. McLean and Charles C. Steidel, the project was managed by WMKO Instrument Program Manager, Sean Adkins.
MOSFIRE was funded in part by the Telescope System Instrumentation Program, operated by AURA and funded by the National Science Foundation, by a private donation to WMKO by Gordon and Betty Moore. DEIMOS The Deep Extragalactic Imaging Multi-Object Spectrograph is capable of gathering spectra from 130 galaxies or more in a single exposure. In "Mega Mask" mode, DEIMOS
Tired light is a class of hypothetical redshift mechanisms, proposed as an alternative explanation for the redshift-distance relationship. These models have been proposed as alternatives to the models that require metric expansion of space of which the Big Bang and the Steady State cosmologies are the most famous examples; the concept was first proposed in 1929 by Fritz Zwicky, who suggested that if photons lost energy over time through collisions with other particles in a regular way, the more distant objects would appear redder than more nearby ones. Zwicky himself acknowledged that any sort of scattering of light would blur the images of distant objects more than what is seen. Additionally, the surface brightness of galaxies evolving with time, time dilation of cosmological sources, a thermal spectrum of the cosmic microwave background have been observed — these effects should not be present if the cosmological redshift was due to any tired light scattering mechanism. Despite periodic re-examination of the concept, tired light has not been supported by observational tests and has been consigned to consideration only in the fringes of astrophysics.
Tired light was an idea that came about due to the observation made by Edwin Hubble that distant galaxies have redshifts proportional to their distance. Redshift is a shift in the spectrum of the emitted electromagnetic radiation from an object toward lower energies and frequencies, associated with the phenomenon of the Doppler effect. Observers of spiral nebulae such as Vesto Slipher observed that these objects exhibited redshift rather than blueshifts independent of where they were located. Since the relation holds in all directions it cannot be attributed to normal movement with respect to a background which would show an assortment of redshifts and blueshifts. Everything is moving away from the Milky Way galaxy. Hubble's contribution was to show that the magnitude of the redshift correlated with the distance to the galaxies. Basing on Slipher's and Hubble's data, in 1927 Georges Lemaître realized that this correlation fit non-static solutions to the equations of Einstein's theory of gravity, the Friedmann–Lemaître solutions.
However Lemaître's article was appreciated only after Hubble's publication of 1929. The universal redshift-distance relation in this solution is attributable to the effect an expanding universe has on a photon traveling on a null spacetime interval. In this formulation, there was still an analogous effect to the Doppler effect, though relative velocities need to be handled with more care since distances can be defined in different ways in expanding metrics. At the same time, other explanations were proposed. Edward Milne proposed an explanation compatible with special relativity but not general relativity that there was a giant explosion that could explain redshifts. Others proposed. Along this line, Fritz Zwicky proposed a "tired light" mechanism in 1929. Zwicky suggested that photons might lose energy as they travel vast distances through a static universe by interaction with matter or other photons, or by some novel physical mechanism. Since a decrease in energy corresponds to an increase in light's wavelength, this effect would produce a redshift in spectral lines that increase proportionally with the distance of the source.
The term "tired light" was coined by Richard Tolman in the early 1930s as a way to refer to this idea. Tired light mechanisms were among the proposed alternatives to the Big Bang and the Steady State cosmologies, both of which relied on the general relativistic expansion of the universe of the FRW metric. Through the middle of the twentieth century, most cosmologists supported one of these two paradigms, but there were a few scientists those who were working on alternatives to general relativity, who worked with the tired light alternative; as the discipline of observational cosmology developed in the late twentieth century and the associated data became more numerous and accurate, the Big Bang emerged as the cosmological theory most supported by the observational evidence, it remains the accepted consensus model with a current parametrization that specifies the state and evolution of the universe. Although the proposals of "tired light cosmologies" are now more-or-less relegated to the dustbin of history, as a alternative proposal tired-light cosmologies were considered a remote possibility worthy of some consideration in cosmology texts well into the 1980s, though it was dismissed as an unlikely and ad hoc proposal by mainstream astrophysicists.
By the 1990s and on into the twenty-first century, a number of falsifying observations have shown that "tired light" hypotheses are not viable explanations for cosmological redshifts. For example, in a static universe with tired light mechanisms, the surface brightness of stars and galaxies should be constant, that is, the farther an object is, the less light we receive, but its apparent area diminishes as well, so the light received divided by the apparent area should be constant. In an expanding universe, the surface brightness diminishes with distance; as the observed object recedes, photons are emitted at a reduced rate because each photon has to travel a distance, a little longer than the previous one, while its energy is reduced a little because of increasing redshift at a larger distance. On the other hand, in an expanding universe, the object appears to be larger than it is, because it was closer to us when the photons started their travel; this causes a difference in surface brilliance of objects between a static and an expanding Unive
In physics, redshift is a phenomenon where electromagnetic radiation from an object undergoes an increase in wavelength. Whether or not the radiation is visible, "redshift" means an increase in wavelength, equivalent to a decrease in wave frequency and photon energy, in accordance with the wave and quantum theories of light. Neither the emitted nor perceived light is red. Examples of redshifting are a gamma ray perceived as an X-ray, or visible light perceived as radio waves; the opposite of a redshift is energy increases. However, redshift is a more common term and sometimes blueshift is referred to as negative redshift. There are three main causes of red in astronomy and cosmology: Objects move apart in space; this is an example of the Doppler effect. Space itself expands; this is known as cosmological redshift. All sufficiently distant light sources show redshift corresponding to the rate of increase in their distance from Earth, known as Hubble's Law. Gravitational redshift is a relativistic effect observed due to strong gravitational fields, which distort spacetime and exert a force on light and other particles.
Knowledge of redshifts and blueshifts has been used to develop several terrestrial technologies such as Doppler radar and radar guns. Redshifts are seen in the spectroscopic observations of astronomical objects, its value is represented by the letter z. A special relativistic redshift formula can be used to calculate the redshift of a nearby object when spacetime is flat. However, in many contexts, such as black holes and Big Bang cosmology, redshifts must be calculated using general relativity. Special relativistic and cosmological redshifts can be understood under the umbrella of frame transformation laws. There exist other physical processes that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; the history of the subject began with the development in the 19th century of wave mechanics and the exploration of phenomena associated with the Doppler effect. The effect is named after Christian Doppler, who offered the first known physical explanation for the phenomenon in 1842.
The hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot in 1845. Doppler predicted that the phenomenon should apply to all waves, in particular suggested that the varying colors of stars could be attributed to their motion with respect to the Earth. Before this was verified, however, it was found that stellar colors were due to a star's temperature, not motion. Only was Doppler vindicated by verified redshift observations; the first Doppler redshift was described by French physicist Hippolyte Fizeau in 1848, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The effect is sometimes called the "Doppler–Fizeau effect". In 1868, British astronomer William Huggins was the first to determine the velocity of a star moving away from the Earth by this method. In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In 1887, Vogel and Scheiner discovered the annual Doppler effect, the yearly change in the Doppler shift of stars located near the ecliptic due to the orbital velocity of the Earth.
In 1901, Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors. The earliest occurrence of the term red-shift in print appears to be by American astronomer Walter S. Adams in 1908, in which he mentions "Two methods of investigating that nature of the nebular red-shift"; the word does not appear unhyphenated until about 1934 by Willem de Sitter indicating that up to that point its German equivalent, was more used. Beginning with observations in 1912, Vesto Slipher discovered that most spiral galaxies mostly thought to be spiral nebulae, had considerable redshifts. Slipher first reports on his measurement in the inaugural volume of the Lowell Observatory Bulletin. Three years he wrote a review in the journal Popular Astronomy. In it he states that "the early discovery that the great Andromeda spiral had the quite exceptional velocity of –300 km showed the means available, capable of investigating not only the spectra of the spirals but their velocities as well."
Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable "positive" velocities. Subsequently, Edwin Hubble discovered an approximate relationship between the redshifts of such "nebulae" and the distances to them with the formulation of his eponymous Hubble's law; these observations corroborated Alexander Friedmann's 1922 work, in which he derived the Friedmann-Lemaître equations. They are today considered strong evidence for the Big Bang theory; the spectrum of light that comes from a single source can be measured. To determine the redshift, one searches for features in the spectrum such as absorption lines, emission lines, or other variations in light intensity. If found, these featur