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Supernovae were first explained by Caltech Astrophysicist Fritz Zwicky and his collaborator for few years- Astronomer Walter Baade, in their 1934 paper using recently discovered Neutrons. Term ‘Supernova’ was introduced by them. Zwicky and Baade argued that since Galaxies are extremely distant to one another, Supernovae must be releasing extremely high amount of energy to be observable. It is now known that Supernovae can sometimes be as bright as entire Galaxies for a couple of weeks. Zwicky envisioned that Supernovae will be used to survey Universe at extremely large distances. He found many Supernovae using wide view 18 inch Schmidt Telescope at Caltech’s Palomar Observatory, San Diego County, California, by looking for them during new moon. In total, he found more than 120 of them for which he also used 48 inch Schmidt at Palomar. Around that time, Cepheid variables were used as standard candle at large distances. At even larger distances, Hubble used brightest stars in Galaxies as standard candle, assuming them to be of same size and brightness which was disapproved in following years. In 1952, at Conference of International Astronomical Union in Rome, Walter Baade revealed that he had found two different types of Cepheid Variables in Andromeda galaxy. This called for a revision of Hubble’s earlier estimates in which he had considered, what turned out to be population2 Cepheid variables, as standard candle. 
Supernovae are very rare events, occurring a very small number of times in a Galaxy over a century. Type1a SNe are even more rare. This is why the need for automated search was felt. First robotic Supernova search was attempted by Stirling Auchincloss Colgate in 1970s, without much success. He was a Physicist at Los Alamos National Laboratory and Prof. of Physics at New Mexico tech. His project was a search for early Supernova in Galaxies with a remote controlled telescope in real time using an IBM 360-44 mainframe computer through a digital microwave link from the New Mexico Tech campus to the school’s Langmuir Laboratory. Later in mid 1980’s, Richard A. Muller, Prof. of Physics at UC Berkeley and Carlton R. Pennypacker, Astrophysicist at same institution, started ‘The Berkeley Real Time Supernova Search’ renamed as ‘The Berkeley Automated Supernova Search’. Muller’s group with their robotic telescope, fitted with new CCD detectors and latest computers, identified 20 Supernovae. They were the first to demonstrate the efficiency of automated supernova search. Muller’s graduate student Saul Perlmutter was a leading member of the team. In 1988, the group made a proposal to use their search technique to find distant supernovae. Goal was to measure deceleration parameter q0 and reveal ultimate fate of Universe using distant Type1a SNe as standard candle. Mass density, expansion history and curvature of Universe was hoped to be found, as well. They faced constant funding problems. In 1991, Muller and Pennypacker handed the leadership to Perlmutter.
In 1986, Danish Astronomer Hans Ulrik Norgaard-Nielsen led a team at La Silla Observatory, Chile, to search for Type1a SNe at large distances. After two years, they only had one Type1a which was already past its peak brightness. This was a dampener for many Astronomers who were hoping to use distant Type1a for measuring cosmological parameters. Berkeley team continued its effort unabated. In 1988 Pennypacker and Perlmutter built a wide field imager for 3.9m Anglo-Australian Telescope at Siding Spring, Australia, to observe thousands of Galaxies in one go. For this they were allocated 12 nights of telescope time to look for distant SNe. Without a convincing search strategy during early years, team had difficulty securing telescope time. They had what is known as catch-22 telescope scheduling problem. They couldn’t get follow up time for obtaining spectra and light curve for a Supernova that may or may not be found and without prescheduled follow up time they couldn’t get time to look for Supernovae. Any search strategy required telescope time to demonstrate its effectiveness. With improved techniques, Berkeley team was successful in identifying its first candidate Supernova in 1992 using Isaac Newton Telescope in La Palma, Canary Islands. This was named SN 1992bi. In 1994, Perlmutter demonstrated that by taking images of adjacent patches of sky containing 10s of thousands of Galaxies just after a new moon and subtracting it from images of same patches of sky taken before next new moon about 29 days later, nearly a dozen new Supernovae could be found as new bright spots. Timing between two new moons ensures that most of the Supernovae found will be still brightening as it takes 21 days for a Type1a to reach peak brightness. After this, scheduling observation time on large telescopes at Keck, Cerro Tololo and Isaac Newton was easy as the team was able to make specific proposals and schedule follow up time with Hubble Space Telescope and other ground based telescopes to confirm findings. At this point the team had grown considerably as prominent Physicists and Astronomers from Institutes around the world joined in. The project was renamed ‘Supernova Cosmology Project’.
More distant SNe have higher redshift than near ones. Therefore same filter cannot be used to measure and compare their brightness to identify the type. Doing so will give incorrect result. This is known as K-correction problem in Astronomy. Light of Supernova is also dimmed by dust and gas in host Galaxy, making it even more difficult to identify their type correctly. SCP Team went about this problem by using correspondingly redshifted filters. By end of 1997, the team had analyzed data for 40 distant Type1a SNe. They found that the SNe were fainter for their redshift than one would expect from a decelerating Universe dominated by matter density. They were fainter, even for an empty Universe, leading them to conclude that expansion of Universe is not decelerating but accelerating. For the acceleration to happen a previously unknown form of energy density should be present in Universe and everyone’s first thought was Einstein’s abandoned cosmological constant, in a somewhat different sense. Cosmologist Michael Turner named it Dark Energy, in analogy with Dark Matter. Perlmutter presented the result first at a press conference sponsored by American Astronomical Society on Jan 8, 1998 in Washington D.C. and then next month in February at UCLA symposium on Dark Matter, in California.
By February 1998, the team had analyzed 16 high redshift Type1a SNe. Their calculations showed a negative matter density which couldn’t be possible. After working out the possibility of overlooked error, only logical conclusion was a negative deceleration parameter, which means an accelerating Universe. Their data was showing the same result as SCP team’s data. Supernovae were too faint for what would be expected from a decelerating Universe. Prof. Alex Filippenko of UC Berkeley, who had done most of the spectroscopic work for measuring redshifts, made the announcement at UCLA Symposium on Dark Matter in California in February 1998. Formal submission of result to ‘The Astronomical Journal’ came on March 13, 1998, in a paper titled ‘Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant’. High Z team was several months early than SCP team in formally publishing their result. Perlmutter, Schmidt and Riess were awarded The 2006 Shaw Prize in Astronomy. In 2011, they were awarded The Nobel Prize in Physics, which they split among their team members, making it clear that everyone's contribution was important in the discovery.
By 1941, Supernovae abbreviated as SNe, were classified into two types. SNe that didn’t have Hydrogen emission lines were called Type1 and those with Hydrogen emission lines were called Type2. By 1985, Type1 SNe were found to be of two subtypes. Type1 with Silicon absorption line at 6150Å were classified as Type1a and those without Silicon absorption line, were classified as Type1b. There is yet another subtype named Type1c. Type1a Supernovae occur when a white dwarf reaches 1.44 Solar mass, accreting matter from its companion star. 1.44 Solar mass limit is known as Chandrasekhar limit in honor of Indian-American Astrophysicist Subrahmanyan Chandrasekhar, who discovered it. Astronomers studied type1a SNe and found that their spectra and light curves were strikingly similar. Swiss Cosmologist Gustav Andreas Tammann and his student Bruno Leibundgut were among the first to notice this similarity. This raised hopes that Type1a can be used as standard candle at large distances. Further detailed study revealed some significant differences in their luminosity and light curve. Mark Phillips of Cerro Tololo Inter-American Observatory, after studying light curve of a number of low redshift Type1a, found that, brighter the Type1a, longer it takes to fade and fainter it is, faster it fades. This observation allowed physicists and Astronomers to know the peak brightness of a type1a with higher precision by observing its light curve- pattern in which it brightens and fades over time and gave them confidence to use type1a as standard candle.
In 1986, Danish Astronomer Hans Ulrik Norgaard-Nielsen led a team at La Silla Observatory, Chile, to search for Type1a SNe at large distances. After two years, they only had one Type1a which was already past its peak brightness. This was a dampener for many Astronomers who were hoping to use distant Type1a for measuring cosmological parameters. Berkeley team continued its effort unabated. In 1988 Pennypacker and Perlmutter built a wide field imager for 3.9m Anglo-Australian Telescope at Siding Spring, Australia, to observe thousands of Galaxies in one go. For this they were allocated 12 nights of telescope time to look for distant SNe. Without a convincing search strategy during early years, team had difficulty securing telescope time. They had what is known as catch-22 telescope scheduling problem. They couldn’t get follow up time for obtaining spectra and light curve for a Supernova that may or may not be found and without prescheduled follow up time they couldn’t get time to look for Supernovae. Any search strategy required telescope time to demonstrate its effectiveness. With improved techniques, Berkeley team was successful in identifying its first candidate Supernova in 1992 using Isaac Newton Telescope in La Palma, Canary Islands. This was named SN 1992bi. In 1994, Perlmutter demonstrated that by taking images of adjacent patches of sky containing 10s of thousands of Galaxies just after a new moon and subtracting it from images of same patches of sky taken before next new moon about 29 days later, nearly a dozen new Supernovae could be found as new bright spots. Timing between two new moons ensures that most of the Supernovae found will be still brightening as it takes 21 days for a Type1a to reach peak brightness. After this, scheduling observation time on large telescopes at Keck, Cerro Tololo and Isaac Newton was easy as the team was able to make specific proposals and schedule follow up time with Hubble Space Telescope and other ground based telescopes to confirm findings. At this point the team had grown considerably as prominent Physicists and Astronomers from Institutes around the world joined in. The project was renamed ‘Supernova Cosmology Project’.More distant SNe have higher redshift than near ones. Therefore same filter cannot be used to measure and compare their brightness to identify the type. Doing so will give incorrect result. This is known as K-correction problem in Astronomy. Light of Supernova is also dimmed by dust and gas in host Galaxy, making it even more difficult to identify their type correctly. SCP Team went about this problem by using correspondingly redshifted filters. By end of 1997, the team had analyzed data for 40 distant Type1a SNe. They found that the SNe were fainter for their redshift than one would expect from a decelerating Universe dominated by matter density. They were fainter, even for an empty Universe, leading them to conclude that expansion of Universe is not decelerating but accelerating. For the acceleration to happen a previously unknown form of energy density should be present in Universe and everyone’s first thought was Einstein’s abandoned cosmological constant, in a somewhat different sense. Cosmologist Michael Turner named it Dark Energy, in analogy with Dark Matter. Perlmutter presented the result first at a press conference sponsored by American Astronomical Society on Jan 8, 1998 in Washington D.C. and then next month in February at UCLA symposium on Dark Matter, in California.
Having heard about the success of Perlmutter’s team in finding Type1a Supernova at about 5 billion lightyears away in early March of 1994, Nicholas Suntzeff of Texas A&M and Brian Schmidt started talking about forming their own team to compete with Perlmutter’s. Newly formed ‘High Z Supernova search team’ had Suntzeff as its principal investigator. Later in 1996, Schmidt took over the leadership. Another leading member of the team was Adam Riess who was working on his doctoral thesis at the time and whose contributions proved crucial in team’s success. In 1993, Adam had worked with Harvard Prof. William Press on a method to reduce error in measuring Luminosity and distance of Type1a SNe, using data from Calan/Tololo survey. This method was called ‘Light Curve Shape’ method. Later, Adam improved this method by making use of filters of different color to reduce error caused by intervening dust. This new method was called ‘Multicolor Light Curve Shape’ method or MLCS for short. With the help of further improved MLCS, number of Type1a SNe required to make reliable calculation of expansion rate of Universe was greatly reduced. This helped in catching up with SCP team. Brian Schmidt wrote software required for automated calculations and was the one to find, team’s first high z Type1a.
By February 1998, the team had analyzed 16 high redshift Type1a SNe. Their calculations showed a negative matter density which couldn’t be possible. After working out the possibility of overlooked error, only logical conclusion was a negative deceleration parameter, which means an accelerating Universe. Their data was showing the same result as SCP team’s data. Supernovae were too faint for what would be expected from a decelerating Universe. Prof. Alex Filippenko of UC Berkeley, who had done most of the spectroscopic work for measuring redshifts, made the announcement at UCLA Symposium on Dark Matter in California in February 1998. Formal submission of result to ‘The Astronomical Journal’ came on March 13, 1998, in a paper titled ‘Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant’. High Z team was several months early than SCP team in formally publishing their result. Perlmutter, Schmidt and Riess were awarded The 2006 Shaw Prize in Astronomy. In 2011, they were awarded The Nobel Prize in Physics, which they split among their team members, making it clear that everyone's contribution was important in the discovery.
Members of both team continued to find SNe at even higher redshift. A new ‘Higher Z’ team was formed by including some new members with some members of old High Z team. Goal was to plot the expansion history of Universe using Type1a SNe that exploded when Universe was young. A comparison of change in redshift of these SNe with change in redshift of more nearby SNe, would effectively give clues about expansion history of Universe. By using the improved Hubble Space Telescope, they found 6 Type1a at redshift greater than 1.25, between 2002 and 2003. Data analysis showed that Universe was decelerating early on but after reaching a particular size during its decelerating expansion phase, started accelerating. A simple explanation is that- matter density dominated in early Universe, but with increasing size it grew weaker against Dark Energy density. Acceleration phase started when Dark Energy density started dominating the weakening matter density. In 2007, with new data from 23 Type1a SNe, this conclusion was confirmed by the ‘Higher Z’ team. This data also indicated that property of Dark Energy didn’t change over time. Another conclusion which came from data is- Density of Dark Energy doesn’t dilute with expanding space, which means, we are living in a Universe which will accelerate forever, taking Galaxies away from each other, ever faster.
Meanwhile Perlmutter and others have been promoting space based Supernova/Acceleration Probe Mission or ‘SNAP’ for getting better data to work with. This mission is now superseded by Widefield Infrared Survey Telescope Mission or ‘WFIRST’. Budget overruns on JWST mission has pushed dates for any Satellite mission for Dark Energy studies to mid 20’s. Ongoing Dark Energy Survey at Cerro Tololo Inter-American Observatory is supposed to provide valuable insight into nature of Dark Energy. European Space Agency is going ahead with ‘Euclid Dark Universe Mission’, which is expected to launch in December 2020. This spacecraft will map 2 billion Galaxies across more than a third of sky providing Astronomers with wealth of data to analyze. A new study by Adam Riess and his team using HST with its wide field camera 3 is indicating that Dark Energy may be growing in strength. Newly measured, expansion rate of Universe is giving a value which is about (5-9)% faster than what is measured from CMB data. They have submitted their paper about this study to arXiv on 5 Apr, 2016. The study will also appear in ‘The Astrophysical Journal’.
Theorists are hard at work trying to figure out Dark Energy. It has been put forth that Dark Energy is a property of space itself and doesn’t dilutes with its expansion and time for Universe to double in size remains same. But that line of thought now seems in jeopardy with the release of recent measurements by Riess and his team. Universe should double in size in about 9.8 billion years, according to these new measurements.
References:
1) http://adsbit.harvard.edu//full/2005ASPC..342...53K/0000053.000.html
2) http://www.astro.princeton.edu/~burrows/pub-html/papers/pnas201422666_7rt2gl.pdf
3) https://arxiv.org/pdf/astro-ph/9812133
4) https://arxiv.org/pdf/astro-ph/9805201.pdf
5) https://www.nobelprize.org/nobel_prizes/physics/laureates/2011/popular-physicsprize2011.pdf
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