Blogs of Professor Pranab Kumar Bhattacharya MD(cal); FICpath(Ind): Fate of a Star-: supernovas and mechanism of explo...

Blogs of Professor Pranab Kumar Bhattacharya MD(cal); FICpath(Ind): Fate of a Star-: supernovas and mechanism of explo...: Authors are _: * Mr. Rupak Bhattacharya -Bsc(cal), Msc(JU), 7/51 Purbapalli, Sodepur, Dist 24 Parganas(north), Kol-110,West Bengal, Indi...

Fate of a Star-: supernovas and mechanism of explosion of supernovas and what is relations of it with our Expanding universe and dark energy?

Authors are_:
*Mr.Rupak Bhattacharya-Bsc(cal), Msc(JU), 7/51 Purbapalli, Sodepur, Dist 24Parganas(north), Kol-110,West Bengal, India**Professor Pranab kumar Bhattacharya- MD(cal) FIC Path(Ind), Now Professor & Head, Department of Pathology [ also Convener  In-charge of DCP &DLT course of WBUHS,  School of Tropical Medicine, 108 C.R Avenue, Kolkata-73, W.B, India  & EX Add. Professor of Pathology, Institute of Post Graduate Medical Education & Research,244  AJC Bose Road, Kolkata-20, West Bengal, India]**Miss Upasana Bhattacharya- Only daughter of Prof.P. K Bhattacharya *Mr.Ritwik Bhattacharya B.Com(cal)* Miss Rupsa Bhattacharya-Student, *Somayak Bhattacharya  BHM; MSC HM  PUSHA  New Delhi, all Resident of  7/51 Purbapalli, Sodepur, Dist 24 parganas(north) ,Kolkata-110,West Bengal, India *** Mrs. Dalia Mukherjee BA(hons) Cal, Swamiji Road, South Habra, 24 Parganas(north), West Bengal, India*** Miss Oaindrila Mukherjee- BA (hons) Student*** Mr Debasis Mukherjee BSC(cal) ,Swamiji Road, South Habra, 24 Parganas(north), West Bengal, India***

 Correspondence author Professor Pranab kumar Bhattacharya  E mail Profpkb@yahoo.co.in 

Stars last too long in this universe. For an astronomers to see any evolution of a star or death of a star, in the course of his/their life time, unless he/they is/are lucky enough to see one star destroying itself in a supernova or in a nova explosion or turning towards a Red giant . My 84 years old and in 2009 diseased father , late Mr. Bholanath Bhattacharjee  of 7/51 purbapali, po-sodepur,24 parganas(north) kol-110, West Bengal, India ,used to teach our brothers and sister in our young ages, with his built up notion like this”….. Stars are long lived objects with ages, they are as old as our galaxy is, as old as our universe is and they are symbol of eternity they may be 2.5 billion years – 3 billion years old from a first generation stars explosions and  are almost  perfect cosmic mile markers even very close to Big Bang.  Today we know that looking at a supernova of a very distant star almost at horizon of the universe, or of a Nebula, we can understand the mystery of creation of the Universe, the Big Bang it self. They are really the symbol of the eternity.  Edington suspected, that the nuclear reactions in the interior of the stars are primary sources of energy for it’s luminosity and fusion of hydrogen to make Helium and that can take place in it, in time bound scale for this ranges, from millions to millions years. Our sun has lost it’s brightness by more then 1% from it’s birth, due to change in it’s internal structure for past 10⁷ years. But  the question remains  how these supernovas explode? What is the mechanism behind it?  No physics, Asronomy, probably answered it!. Here may be some explanations by my brothers Rupak Bhattacharya and Ritwik Bhattacharya  the authors

If we consider the mode of generation of energy in the star, nuclear process provide the only source of energy adequate to keep the stars ongoing luminous. The nuclear fusion in which Hydrogen is built up into Helium, can function sufficient fast at temperature, like those at central core of star (12-25 million degrees). The Helium burning process are important 1) Carbon Nitrogen cycle at which a carbon-12 nucleus (¹²C) capture proton and is converted into ¹³C, Nitrogen-4 and nitrogen –15. At a final temperature, a proton leads to a fusion yielding original ¹²C nucleus to a Helium nucleus .2) The Proton- Proton process, in which protons are built direct Helium nuclei through steps, involving first in production of a deuterium and helium³ nuclei to form Helium⁴ nucleus and two protons. 3) Carbon burning process where ¹²C nucleus undergoes fusion reaction in the interior of a star producing neutron, proton, and Alfa particles with huge temperature. The first reaction probably dominates into the star, applicable to more massive stars then Sun. The second and third reaction is applicable for Sun and in less massive stars then Sun respectively. Thermonuclear reactions like those in a hydrogen bomb are powering the Sun in a contained and continuous explosions converting some four hundred millions tons (4x10¹⁴ grams) of hydrogen into helium. When we look up in the sky in night and see the stars we see them shining because of distant nuclear fusion in them .But hydrogen fusion can not continue for ever.  Our Sun is ~ 4.710⁹ years old star. The energy produced in our ordinary star Sun in each second, is equivalent to the destruction of 41/2 millions tons of hydrogen mass in every second, a mere fleabite compared with the mass of the Sun which is two thousand billion and billion tones. In the Sun or in any other stars, there is limited so much hydrogen in it’s hot interior. Although Helium is predominating as net fusing of Hydrogen, other elements like “carbon”, “Iron”, “L element” “Manganese” “Chromium”, EU, yttrium, Magnesium, SR, Nickel, Osmium are also built up in the interior of the stars. Arnett and Truran [Arnett W.D and Truran. JW –Astrophysics.J-Vol157;P339,1969] showed that nuclear reaction net work in the sun when ¹²C nuclei  began to under go the fusion reaction in the interior of sun many elements are produced such as

¹²C+¹²C----  ²³Na+P+2.238mev----²³Mg+ n+2.623mev------²⁰Ne+ ²⁷Al +4.616mev and the reaction goes on endlessly. A large number of computed reactions are possible as the liberated neutron and gamma particles begin reaction with all the nuclear species generated within the hydrogen fusion. In fact Arnett and Truran produced 99 different reactions only in ¹²C carbon burning net work and  ²³Na,²⁰Ne, ²⁴Mg,²⁷Al,²⁹Si, and some³¹P  elements are also produced. Beside these Li, Be, B ( Known as leptons)are also produced in the stars due to hydrogen burning. Another most elementary particles are produced in huge quantities. They are Neutrinos or ghost particles due to hydrogen burning procedure ( Professor Pranab Bhattacharya & Mr. Rupak Bhattacharyya). Conversion of hydrogen into helium in the center of the stars or of the Sun, not only accounts for Sun’s brightness in photons of visible light. It also produces a radiance of a more ghostly kind. The sun glows faintly in neutrinos  , which like photons, weight nothing and travel at speed of light. Neutrinos emitted from Sun carry an intrinsic angular momentum or spin while photons has no spin. Matter is transparent to neutrinos which can pass effortlessly through the earth and through the Sun. Only a tiny fraction of them is stopped by intervening matter. As you look up our sun, a billions neutrinos pass through your eye ball. They are not stopped by Retina as ordinary photons do ,but continue unmolested through the back of your head. The curious part is that if at night if I look down at ground, towards the place where sun would be, almost exactly same numbers of solar neutrinos pass through my eye ball, pouring through an interposed earth which is as transparent to neutrinos as a plane of clear glass is to visible light. Neutrinos on very rare occasion convert chlorine atoms into argon atoms with the same number of protons and neutrons. Davis first used a beautiful technique of Pontecours and Alvarez based on the reaction  37C₁(V,e^-)³⁷Ar to place an upper limit on the solar neutrinos flux on earth

The previous view regarding the “L atoms elements” was that  each star makes it’s own share of these “L atoms elements”i.e (autogenously origin). But the concept of autogenic view has been now abandoned, because highest abundance values for stellar Li & Be have shown to be not larger than interstellar upper limit. The formation of each “L atoms” requires the acceleration of about 1erg fast proton. To account auto genetically for lithium abundance in T. Tauri stars (L₁/H=10⁹), the time integrated amount production of energy into particle acceleration must be comparable with gravitational release, implying an unlikely high efficiency for acceleration mechanism. So nuclear mechanism is responsible for generation of “L atoms” in the star. It involves high-energy process (Thermonuclear reactions). These L atoms” can be formed in two different ways within the stellar interiors. By the collision of incident light particles on the heavier atoms of interstellar gas (For instance fast protons on stationary C, N, O) or the reverse (for instance fast C, N, O on hydrogen at rest). In the first case the Products “ L atoms are to remain in rest, while in the second case, the products are moving at a velocity comparable with that of cosmic rays. The fate of  “ L atoms” generated by fast protons on stationary C, N, O stationary atoms and are all rapidly thermalised and become part of ISM.

  “L atoms” generated by reverse process have a fate which depends on the initial energy of “L atoms”. L atoms with energy E<0.2 Gev nucleon^-1 will stop in galactic gas (ISM) while L atoms with E > 0.3Gev neucleon^-1 will suffer nuclear transformation of various elements in the stellar interior.

Analysis of Old stars can give us some idea that heavy elements are produced in the interior of the stars and are subsequently ejected into the ISM either through the supernova explosion or through stellar winds or through cosmic rays. The total mass loss, from all stars in a galaxy will be roughly 1MO per year. A fraction of these accumulate in the galactic nuclei, which are center of the gravitational attraction. The halo of our galaxy is nearly spherical region containing very old stars, which have a smaller content of heavy elements than our sun has. It is usually assumed that some how cloud of gas condensed to form our galaxy and that the halo stars were formed during the collapse process and left with a nearly spherical distribution. These stars are ultra high velocity stars. These stars show weak spectral lines corresponding to abundance of carbon and heavier elements [relative to hydrogen] that are lower than our Sun. Because these stars are oldest in our galaxy quite distinct type of nuclear process have been postulated for different groups of elements. The most abundant nuclei are ³²S and ⁵⁸Fe those can be formed by silicon burning process while ¹⁶O, ²⁰Ne,²³Na ²⁴Mg,²⁸S may be produced by explosive carbon burning process. When heavier elements notably Sr, Y, Zr, Ba etc require neutron capture on slow time scale, by iron group nucleotide already present in the star. A peculiar type of star 73 DRA has been investigated for many a time. It is full of chromium with europium and strontium. The star showed the presence of Cr, Eu, Sr and also Mn, Fe, Ni, in gaseous form while osmium (z=76) is present in both neutral and ionized form. The importance of these heavy elements is that, some of them such as Iridium, gold, uranium are also produced in the stars in the gamma process of nucleus synthesis [Neutron capture slow process]

 So Helium, L atoms, Carbon, Iron, gold, chromium, nickel, silicon and many other elements are built up in the stellar interior. Although the net fusing of hydrogen into helium dominates however at this stage. Helium builds up in the core. The supply of hydrogen fuel diminishes and eventually becomes in sufficient to provide energy to hold up the strain position. As the energy production decreases, the core of the star contracts and heats up through release of gravitational energy. With a hotter center there is a greater outward pressure and the outer layer of the star expands, so that the star now becomes a RED GIANT. The red giant has a radius hundred times that of a sun. Mean while in the hotter core a new series of fusion reactions begins and with the helium as the fuel many elements like carbon oxygen, neon, magnesium. When helium will exhaust as a fuel, the carbon burning process will start as 12C as a fuel in the star. In any star the internal temperature and density and therefore the rate at which the energy is generated depend sensitively on the opacity of the stellar material or in other words, on the ease with which the photons can escape from the stellar core. In simple terms you can say greater the opacity harder it is for heat to get out making core hotter. Opacities in normal star can be calculated reliably from knowledge for the abundance of the constituent elements and their ionization site

Suernova-: Another important thing in our universe are the supernovas or novas. The supernovas are the explosion of the central core or outer core of a giant massive star. These supernovas are found in the binary star system. A star may end its life cycle either in the form of a RED GIANT or in the form of a white dwarf or in the form of a “ black Dwarf” or in the form of “ neutron Star” or in Black Hole” or in the form of Supernova Explosion”. When the explosion of a star occurs in small scale, we call it Nova. In Big bang concept, apart from hydrogen, a little helium was produced. Every atom of every element had been built up by the nuclear fusion reaction in the stellar pressure cooker. The elements only could arrive in the interstellar space to mingle in the clouds of forming protostars is through this supernovas  Novas are however quite different from supernovas. Novas occur in binary star system and are powered by silicon or carbon fusion. Supernovas occur in single associated with old population II stellar system such as elliptic galaxies and in globular clusters. The classical supernovas are therefore a subset of the cataclysmic variable class of objects, which undergoes out bursts with peak luminiocity ~ 5x10³⁷ to 5x 10³⁸ ergs S^-1 in every 10⁴ to 10⁵ years. Around 10^-5 to 10^-4 MO material are ejected at velocity typically 1000 Kms^-1 at each outburst of supernova. The central system is a semi detached binary stars, containing a white dwarf . Classical supernova out burst was observed in 1901, where as dwarf nova out burst was first observed in 1986.

Supernovas are two types Type-1(SN-I) and Type 2(SN_II) supernovas.  Most astronomers  must agree with us that a type 1a supernova starts with formation of a white dwarf — an aging star that crams as much mass as the sun into a volume no bigger than Earth. Most white dwarfs are cold and inert. But if the star has a companion, it will siphon mass off the neighbor star until tipping the scales at about 1.4 solar masses. At that mass, the white dwarf becomes dense and hot enough to initiate an explosion. mass accreting white dwarfs, A far more exciting end awaited a white dwarf that is part of a binary star system, which is fairly common in our universe. In this case, the white dwarf’s strong gravity robs the companion star of its gas. However, when the white dwarf  grew  to 1.4  our solar masses, it no longer manages to hold together. When this happens, the interior of the dwarf becomes sufficiently hot for runaway fusion reactions to start, and the star gets ripped apart in seconds. The nuclear fusion products emit strong radiation that increases rapidly during the first weeks after the explosion, only to decrease over the following months. So there is a rush to find supernovae – their violent explosions are brief. Across the visible Universe, about ten type Ia supernovae occur every minute in our universe. But the Universe is huge. In a typical galaxy only one or two supernova explosions occur in a thousand years. In September 2011, we were lucky to observe one such supernova in a galaxy close to the Big Dipper, visible just through a pair of regular binoculars. But most supernovae are much farther away and thus dimmer. So where and when would you look in the canopy of the sky.  We knew they had to comb the heavens for distant supernovae. The trick was to com­pare two images of the same small piece of the sky, corresponding to a thumbnail at arm’s length. The first image has to be taken just after the new moon and the second three weeks later, before the moonlight swamps out starlight. Then the two images was compared in the hope of discovering a small dot of light – a pixel among others in the CCD image – that could be a sign of a supernova in a galaxy far  far away. Only supernovae farther than a third of the way across the visible Universe were used, in order to eliminate local distortions. The researchers had many other problems to deal with. Type Ia supernovae are not quite as reliable as they ini­tially appeared – the brightest explosions fade more slowly. Furthermore, the light of the supernovae needed to be extracted from the background light of their host galaxies. Another important task was to obtain the correct brightness. The intergalactic dust between us and the stars changes starlight. This affects the results when calculating the maximum brightness of supernovae. Chasing supernovae challenged not only the limits of science and technology but also those of logistics. First, the right kind of supernova had to be found. Second, its red shift and brightness had to be measured. The light curve had to be analyzed over time in order to be able to compare it to other supernovae of the same type at known distances. This required a network of scientists that could decide quickly whether a particular star was a worthy candidate for observation. They needed to be able to switch between telescopes and have observation time at a telescope granted without delay, a procedure that usually takes months. They needed to act fast because a supernova fades  so quickly. At times, the two competing research teams discreetly crossed each other’s paths. The potential pitfalls had been numerous, and the scientists actually were reassured by the fact that they had reached the same amazing results: all in all, they found some 50 distant supernovae whose light seemed weaker than expected. This was contrary to what they had envisioned. If cosmic expansion had been losing speed, the supernovae should appear brighter. However, the supernovae were fading as they were carried faster and faster away, embedded in their galaxies. The surprising conclusion was that the expansion of the Universe is not slowing down – quite to the contrary, it is  rather accelerating.in close binary system of stars are Type-1 supernovas, while  low mass (M70t <5MO) binary X ray sources are known as Type II supernovas. Supernovas are the brightest source of IRAS and radio noises. Supernovas are the sources of Cosmic rays also .The bulk of the cosmic rays with high intensity are local cosmic rays and they are derived from many such supernovas in past distributed through our galactic disks. Historical supernovas are all too recent and too distant , to be significant contributor of cosmic rays. In 6^th  April of 1947 ( almost 9 years before I was born in this planet) a supernova, in a satellite of famous Whirlpool galaxy called MSI was observed . A star in that galaxy  had a sudden maximum Brightness and following that within a few weeks  it faded out  and had been then overlooked. A supernova appears in the spiral galaxy  on an average once in  400 years approximately. The most distant supernova so far detected is 10 billion light years away from our earth,  the first generation star it was. How much my father was correct, I often think

 The remnants of the Exploding stars or supernovas are called Supernova remnants. They are easily identified by radio astronomers up to millions years after their explosion. The optical ultra violets and X-rays continue are produced by the supernova explosion and interaction of the resting debris (Supernova remnants), with dense Circumstellar gas shell, previously formed by the stellar wind of the progenitor supernova. L. Stavely Smith , in 1992 showed the birth of the radio noise supernova remnants SNR1987A, following radio outbursts of Supernova 1987A[ Nature Vol-355 1992]. In the mid 1990, about 1200 earthen days after the supernova radio emission was detected at frequencies 843 MHZ and 8.6 GHG and this radio emission was within 0.5 arsc of the optical supernovas

Although both young and old star can give rise to supernovas, the massive stars, none of which is thought to live more than several million years, are also thought to end their life in this way. Supernovas can occur in conjunction with their satellite planet or binary stars. One of You among readers of our thesis may obviously ask me that in what conditions could a star or a planet can survive such a nearby explosion? Several simplifying assumption can be made to answer this question.

1)   The time of mass ejection will be small compared with the orbiting period

2) The mass of the second star or planet will be smaller than that of the pre- supernova star. The first simplification was based on that the ejection velocity of major fraction of the matter from a supernova will be comparable with or larger than its initial escape velocity from the pre- supernova star. Because the binary number is necessarily at larger radius, its orbital velocity will be less than the average ejection velocity. If the combined mass is reduced to less than a half by the supernova, in the limit, where the mass ejection is sudden and where the mass of the secondary is small, the system will un-banned regardless of the effects of the collision of the ejected matter within the satellite of the planet.

   The fractional mass ejection by supernova is known for the thermonuclear supernova model. No remnant of star remain on the other hand models of neutron star supernova, predicts various fractional mass ejection depending on mass and structure of the initial star. But all the models, be it thermonuclear or neutron star or cataclysmic variables predicts a small fractional mass ejection, for the models slightly more massive mass than C.S. Limit, and are self consistent, in that the mass of the remnant neutron star does not exceed current stability limit. A star of initial mass 1.5MO leaves a remnant neutron star of mass 1MO. The link between supernova explosion and formation of a neutron star has to be rather established even if Type II supernovas are expected to leave to a stellar component. Only five example of Pulsar Supernova remnant association are known on our galaxy and in large Megallenic cloud.

 Previously as we the authors told, that mass accreting white dwarf in close binary system can be considered to be Type-1 supernovas progenitors. Low mass (Mtot≈5MO) binary X ray sources( Known as type II supernovas) appear to be descendents of cataclysmic variables and thus they have been produced by collapse of a mass  accreting white dwarf. The fashionable model of explaining the out burst involves central deflagration of a white dwarf in close binary system living no remnant. But this model implies a unique configuration and do allow for variation. Slow supernovas show higher peak of luminosity, higher velocity in their ejecta and slower decline in their light curve. Fast Supernovas are dimmer velocities of expanding material are low and light curve decay is faster.

Super humps_: They are periodic increase in Brightness (up to 40%) that are observed during occasional super out bursts lasting about 12 days, that are additional to normal outbursts of a subclass of white dwarf Nova. Dwarf Nova’s are characterized by two distinct class of out bursts. Normal  outbursts of duration ~2 days and less frequent super outbursts which lasts for ~12 days. During super outbursts, super hemps are observed which modulate the visible light by≤40% with a period 3-7% longer than biniary period.

Mechanism of Explosion in Supernova-a mechanism by Professor Pranab Kumar Bhattacharya  Mr, Rupak Bhattacharya_: What is the mechanism of a supernova explosion in a star? It is not Known and yet explained very well. Possibly one of the standard mechanisms of a supernova is the collapse& out going shock due to collapse, leaving behind a neutron star, is the collapse of the iron core of a massive star. During the initial phase of the collapse, a sizable portion of the star transfers into neutrinos with emissions of ve energy≤10Mev.[Rupak Bhattacharya and professor Pranab Kumar Bhattacharya’s theory] The collapse phase lasts until the infiltrating matter becomes opaque to neutrinos. A few~10⁵⁶ ve neutrinos are emitted during this phase. As the collapsed core reaches nuclear matter densities, an out going shock develop. When the shock reaches the dense layer that are still transparent to neutrinos, another ~10⁵⁶V mostly Ve are expected to be emitted. The second one of the bursts , produce neutrinos with an average energy~10Mev^5~10. The whole process lasts much less than a second. The remainder of the gravitational energy (~2x10⁵³ergs) is emitted in the form of vv pairs of all flavors. Although the neutron star contain vv pairs of very high energy (100Mev), the only low energy one is eliminated, because the neutrinos mean the free path, is strongly energy dependent. The energy is larger for µ, e R neutrinos (Rupak neutrinos). There are dozen of neutrinos particles of 7-35 mev mass in that energy. There charges is smaller than about 10^-17 times the charge of electron. [Such a neutrinos is R particles or R neutrinos-a near zero mass, conceptualized by Rupak Bhattacharya and hence nomenclatured according to his name]

 What is the Key thermonuclear feature of an expanding star that will end it in supernovas? The ignition of helium in the hydrogen as soon as exhausted, in core of a low mass star, in the presence of a degenerate electron gas which is providing the bulk of the pressure support of the star, the expansion of the star core starts. Because pressure of such a gas does not increase ,substantially when temperature rises, where as the rates of thermonuclear reaction increase dramatically with increasing temperature, a brief run away in thermonuclear activity ensues. After this the star in there core quickly expands. After only a small degree of nuclear burning to an adjusted configuration, where burning can proceed in hydrostatic equilibrium with subsequent discovery of very effective cooling of stellar interior due to neutrinos emission, it has become apparent that intrinsically more explosive nuclear fuel namely ¹²C and¹⁶O may also ignite in a very degenerate electron gas and that in that case, the run away in nuclear reaction may be great enough completely to disrupt the star via a thermonuclear explosion. The high temperature of the explosion which lasts only a fraction of second produces such a high degree nuclear processing that expelled thermonuclear product are vastly different than the composition of the mass zone of the star, before the explosion. The key thermonuclear feature of explosive burning is that several fuel combust at temperature considerably higher than those at which same fuel burn in an object in hydrostatic equilibrium with considerable effect in abundances of ejected matter. The over heating may result either from the fact that the fuels first ignite in a degenerate electron gas for the non central mass zones from the compression heating, produced as a strong pressure and have propagation outward fro an expanding core. In either case large amount of thermal energy are liberated in a time short compared with star’s ability to compensate hydro dynamically, with the result that the entire star may be given with positive energy sufficient to disrupt it in explosion.

Before the explosion, the gas is virtually half and consisted of ¹²C and ¹⁶O. The first indication of importance of dynamics of the explosion of the final nuclear product came in a study of carbon burning phenomenon of Arnet, who established a numerical scheme for solving the nuclear reaction net work that result when¹²C nuclear reaction began to undergo the fusion reaction in the interior of the star before supernova[ Arnet W.D & Truran J.W Astrophysics Jpurnal V157;P339;1969]

12C+12C        23Na+P+2.238mev             23Mg+tn    -2.623mev         20 Net+α+4.616Mev

A large numbers of computed reactions are thus possible, as the  fusion reaction liberated proton, neutron, neutrinos and alpha particles and began to react with all of the nuclear species generated within the gas. Before the explosion, the gas is virtually half and half of the ¹²C and ¹⁶O as produced in previous epoch on helium burning plus 2% of ¹⁸O which is the result of earlier conversion, within the same star, of all of the original CNO nuclei in to ¹⁸O by hydrogen burning and helium burning in turn. Carbon burn furiously for about 1/10^th  of second at which time reactions are frozen by falling temperature, associated with vigerous expansion of gas. Most of the carbon and virtually all of the initial oxygen remain unburned, so that the final ratio of ¹²C/²⁴Mg matches the solar ratio. More subsequently the nuclei  ^2One,²³Na, ²⁴Mg, ²⁶Mg,²⁷Al, ²⁹Si and ³O and some time ³¹P are produced. So today whatever elementary nucluie we know in our earth or in earth’s atmosphere is the fusion-burning product of a supernova explosion in a dying star.

1987 A Supernova-:  and Recently detected Supernovas[  Picture by Geoff Brumfiel on July 09, 2009 at The Great Beyond, Nature.com]

 

In our galaxy there were evidences of eight supernovas. They are in the years 185,393,1006 AD and in 1054,1181,1572,1604,and very recently one is 1987.Only supernovas 1006,1572,1604 were observed by European Astronomers. The supernova of 1054 was as a cloudy patch, and remains still as Crab nebulae as the legs of a crab. It is the remnant of that supernova. It is at a distance of 4500 light years away and is left over gases that has a diameter of about 6 miles.[Mitra A.K- Space Light first year 2^nd quarter1997 P10]. Supernova 1987A occurred in the large Megaloionic Cloud (MLC).It was a supernova of a giant star SK69202 that exploded. The star was the star of multiple star systems instead of a binary star system. SN 1987a was 18 solar mass blue giant Sanduleak -69° 202a, a mere 0.000168 billion light-years distant. This star had lost a considerable mass of M₂₀O due to the explosion. The other members of this giant multiple system is now visible as supernova remnant. The SK 69202 was probably a red super giant 10⁴—10⁵ years ago. The outer envelop, blue star giant progenitor star is preserved during the rapid supersonic un turbulence outflow of the supernova. The huge amount of R- neutrinos (Rupak Particles) are now emitted by this 1987A supernova proposes the formation of a neutron star, inside this supernova some have reported that central region of this supernova was a central pulsar. However the history of this supernova 1987A is today 23 years old. In the supernova 1987A there is evidence of presence of H₃+ in the envelope or in the shell of it. The infrared L window spectrum of supernova 1987A is between 2.95-4.15um were obtained by hydrogen re combination line ( Mcikle.W. PS Not.R. Astr Society V283;P193-223;1989). But from 110 days onward there were an evidence of hydrogen recombination line between spectrum 3.41-3.53um. These were possibilities in wave length at which H3+ announces most strongly presence of a planet like Jupiter (Okata.T etal- Astrophysics J-Vol351;P253-56;1990).When the first explosion of supernova 1987A happened there were a brief initial outburst of radio emissions that lasted for more than a few days. The expanding Nebulae were set into motion by the ejection and cooling of ejected material and its interaction with circumstellar material that surrounded the progenitor& was then non visible at the available radio frequencies. Evolution of radio-supernova remnant over the last years provided us the information about the progress of the expanding supernova remnant. The nature of its unusual progenitor star which was first a   red giant and then a blue giant, before it exploded. In the red giant phase, the star threw off a dense slow moving wind which was succeeded by a more tenuous but faster wind from the blue giant. The circumstellar material of the progenitor at the moment of the explosion there fore consisted of a hot thin gas- cocooned inside a cooler thicker shell with a supersonic shock wave created at the boundary, as the blue  giant wind ran into red giant wind. The first brief flash of radio emission was a very minor part of the initial supernova outbursts and was probably attributable to the propagation of shock wave from explosion through the thin material immediately surrounding what had been progenitor star.

 Supernova are now routinely observed in other galaxies . During the life time of a galaxy about 10 billion years, a hundred millions of stars exploded. Amongst them, David Helfend and Knoxlong reported an extremely intense burst of hard X-ray and gamma rays which was also recorded by nine interplanetary space crafts and which was also probably Supernova N49 remnant in large Megellanic Cloud [ MLC is a small satellite galaxy of Milky Way 18,0000 light year distant)[ Nature march5,1979 & decemb6 1979]. The recent nova which had been detected in Cygx-1 galaxy. It was Nova V404 cygni- Low mass x ray binary emits x ray and x ray behavior is similar to black hole system. In April 6,1947 discovered a supernova in a satellite of famous whirlpool galaxy M51- A star suddenly had maximum brightness and had then overlooked. On 9th January 2008, while viewing of galaxy NGC 2770 an unexpected transient burst of Xray was detected in one of the galaxies spiral arm. Further observation showed that the burst was a first sighting of new type of Ibc supernova duly was named as SN200D.  Most distant Supernovae are super bright, and that makes them easy to see from far away. Very far away. 11 Billion Light years to be exact from earth. almost at time of birth of first generation stars and galaxy.

 Pulsar formation is generally attributed to supernova events and two pulsars are till associated with known supernova events. They are Crab Nebulae pulsar NP0532 and Vela. Other pulsars are close enough to supernova remnants to suggest an association but only if they are moving away from the remnants at velocities of order 10³ Km/second. Fowler KA and Hogel F in 1963 suggested that supernova core may well be too massive to form a gravitationally  stable object(neutron Star) and gravitationally Collapsing Object(black Hole too). They suggested that systemic ejection of Radio luminous material from galaxy could be caused by symmetrical process occurring in the collapse of very massive objects, thus the more massive core could fission into several less massive objects. The same process can be applied to supernova events where core fragments into some distribution of neutron stars” Black holes’ and general debris[ Fowler WA, Hoyel F Nature 197; 533;1963]. There is a pulsar PSR 1509-58 located near the center of radio supernova remnant MSH 15-52, a supernova remnant of supernova AD1054. This pulsar is young only about 1700 years. The near coincidence of this age with that of supernova of AD 185 strongly suggest that PSR1509-58 was born in AD1054 supernova explosion.

  Why Supernova is So important before physicists?

 Supernova means accelerating expansion of the universe! The discovery of the accelerating expansion of the universe was a real milestone for cosmology,  ultra high Physics as significant as the discovery of the minute temperature variations in the Cosmic Microwave Background (CMB) radiation with the COBE satellite (Nobel Prize in Physics 2006, won & Received by John Mather{ See the link at  Nobel prize org face book ask a question to  Jhon Mather at 
http://www.facebook.com/note.php?note_id=163524746719}  and prof. George Smoot NL. By studying the CMB, we can learn about the early history of our Universe and the origins of all structures we see around us, whereas the expansion history of the universe gives us insights into its evolution and possibly its ultimate fate{ It  was predicted in Mayan calendar & by some shallow knowledgeable  people & Physicists that our world will see its end in 2012 December either through a collision with a giant planet approaching towards us or  through  strong solar storms}. The expansion of the universe was originally discovered by Vesto Slipher, Carl Wirtz, Knut Lundmark, Georges Lemaître himself and Edwin Hubble in the 1920’s[ I wonder often why George Lemaitre was not awarded Nobel prize for his BIG Bang theory!]. The expansion rate depends on the energy content – a universe containing only matter should eventually slow down due to the attractive force of gravity. However, observations of type Ia supernovae ( often it is designated as “SNe”) at distances of about 6 billion light years by two independent research groups, led by Saul Perlmutter and by Brian Schmidt and Adam Riess respectively, revealed that presently the expansion rate instead is accelerating rapidly . Within the framework of the standard  Big Bang cosmological model, the acceleration is generally believed to be caused by the vacuum energy (sometimes we call it as ”Dark energy”) which is again – based on concordant data from the SNe, the observations of the anisotropies in the CMB and surveys of the clustering of galaxies – accounts for about 73% of the total energy density of our universe. Of the remainder, about 23% is due to an unknown form of matter ( we call it  ”dark matter”). Only about 4% of the energy density corresponds to ordinary matter like atoms. Is that not so strange!

In our everyday life, the effects of the vacuum energy are so tiny, but measurable – observed

for instance in the form of shifts of the energy levels of the hydrogen atom, the Lamb

shift ( Lamb won Nobel Prize in Physics 1955).  The evolution of the universe was  also described by Einstein’s theory of general relativity- the essence of that theory was that. In relativistic field theories, the vacuum energy contribution is given by an expression mathematically similar to the famous “cosmological constant “in Einstein’s theory. The question of whether the vacuum energy term is truly time independent like the cosmological constant, or varies with time, is currently a very hot research topic.

General Relativity and the our Universe!

The stars in the night sky  always fascinated human beings( males and females), Poets Literature, Dramatists, scientists, lovers and in worlds top Romantics. Why Stars in dark space time of night? We can only guess what the people of ancient times speculated about when they saw the stars return every night to the same spots in the sky! We know of Greek philosophers who proposed a heliocentric astronomical model with the Sun in the middle and the planets circulating around it as early as the 3rd century B.C., but it was in fact Nicolaus Copernicus, who in the 16th century developed the first modern version of a model. It took Galileo Galilei’s genius in the beginning of the next century to really observe and understand the underlying facts, building one of the first telescopes for astronomy and hence laying the ground for modern astronomy. For the next three hundred years, astronomers collected ever more impressive tables of observations of the visible stars. In the Copernican system, the stars were assumed to be fixed to a distant sphere and nothing in the observations indicated anything to the contrary. In 1718, Edmund Halley discovered that stars actually could move in the sky, but it was believed that this happened in a static, fixed universe. Throughout the 18th and 19th century, the study of celestial bodies was placed on an ever-firmer footing with the famous laws of Kepler and Newton. In November 1915, Albert Einstein ( He received Nobel Prize in Physics 1921) presented his theory of gravity, which he nicknamed as General Relativity (GR) , an extension of his theory of special relativity. This was one of the greatest achievements no doubt in the history of science, a modern milestone and put Albert Einstein immortality for centuries together as ever most genius and intelligent. His brain preserved in museum till days  & evidenced that his brain had increased Zyrus size in  his frontal lobe. It was based on the Equivalence Principle, which states that the gravitational mass of a body is the same as its inertial mass. You cannot distinguish gravity from acceleration! Einstein had already checked that this could explain the precession of the peri helion of Mercury, a problem of Newtonian mechanics. The new insight was that gravity is really geometric in nature and that the curving of space and time, space-time, makes bodies move as if they were affected by a force. But Einstein did not say of what “Gravity was made of”- a most basic question and  exactly when and why it appeared or what caused it to appear in space time vacuum? The crucial physical parameters are the metric of space-time, a matrix that allows us to compute infinitesimal distances (actually infinitesimal line elements - or proper times in the language of special relativity.) It became immediately clear that Einstein’s theory could be applied to cosmological situations, and Karl Schwarzschild very soon found the general solution for the metric around a massive body such as the Sun or a star [2].

In 1917, Einstein applied the GR equations to the entire Universe , making the

implicit assumption that the universe is homogenous isotropic ; if we consider cosmological

scales large enough such that local clusters of matter are evened out. He argued that this

assumption fited well with his theory and he was not bothered by the fact that the

observations at the time did not really substantiate his conjecture. Remarkably, the

solutions of the equations indicated that the universe could not be stable. This was

contrary to all the thinking of the time and bothered much Einstein. He soon found a solution, however. His theory of 1915 was not the most general one consistent with the

Equivalence  Principle. He could also introduce a cosmological constant, a constant

energy density component of the Universe. With this Einstein could balance the

Universe to make it static. In the beginning of the 1920s, the Russian mathematician and physicist Alexander Friedmann NL  studied the problem of the dynamics of the universe using essentially the same assumptions as Einstein, and found in 1922 that Einstein’s steady state solution was really unstable [4]. Any small perturbation would make the universe non-static. At first Einstein did not believe Friedmann’s results and submitted his criticism to

Zeitschrift für Physik, where Friedmann’s paper had been published first. However, a year

later Einstein found that he had made a mistake and submitted a new letter to the journal

acknowledging this fact. Even so, Einstein did not like the concept of an expanding

 Universe and is said to have found the idea “abominable”. In 1924, Friedmann

presented his full equations , but after he died in 1925 his work remained essentially

neglected or mostly unknown, even though it had been published in a prestigious journal. We

have to remember that a true revolution was going on in physics during these years with

the advent of the new quantum mechanics, and most physicists were busy with this

process. In 1927, the Belgian priest and physicist Georges Lemaître working

independently from Friedmann performed similar calculations based on GR and arrived

at the same results [6].  Very Unfortunately, Lemaître’s paper was published in a local Belgian

journal and again the results did not spread far, even though Einstein knew of them and

discussed them with Lemaître. In the beginning of the 20th century it was generally believed that the entire universe only consisted of our galaxy, the Milky Way. The many nebulae which had been found in the sky were thought to be merely gas clouds in distant parts of the Milky Way. In1912, Vesto Slipher [7], while working at the Lowell Observatory, pioneered

measurements of the shifts towards red of the light from the brightest of these spiral

nebulae. The red shift of an object depends on its velocity radially away from us, and

Slipher found that the nebulae seemed to move faster than the Milky Way escape

velocity. In the following years, the nature of the spiral nebulae was intensely debated. Could there be more than one galaxy? This question was finally settled in the 1920s with

Edwin Hubble as a key figure. Using the new 100-inch telescope at Mt Wilson, Hubble

was able to resolve individual stars in the Andromeda nebula and some other spiral

nebulae, discovering that some of these stars were Cepheids, dimming and brightening

with a regular period [8].

The Cepheids are pulsating giants with a characteristic relation between luminosity and

the time interval between peaks in brightness, discovered by the American astronomer

Henrietta Leavitt in 1912. This luminosity-period relation, calibrated with nearby

Cepheids whose distances are known from parallax measurements, allows the

determination of a Cepheid’s true luminosity from its time variation – and hence its

distance (within ~10%) from the inverse square law. Hubble used Leavitt’s relation to estimate the distance to the spiral nebulae, concluding that they were much too distant to be part of the Milky Way and hence must be galaxies of their own. Combining his own measurements and those of other astronomers he was able to plot the distances to 46 galaxies and found a rough proportionality of an object’s distance with its red shift. In 1929, he published what is today known as ‘Hubble’s law’: a galaxy’s distance is proportional to its radial recession velocity [9]. Even though Hubble’s data were quite rough and not as precise as the modern ones, the law became generally accepted, and Einstein had to admit that the universe is indeed expanding. It is said, that he called the introduction of the cosmological constant his “greatest mistake” (Eselei in German). From this time on, the importance of the

cosmological constant faded, although it reappeared from time to time.4 (17)

It should be noted for the historic records that Lemaître in his 1927 paper correctly

derived the equations for an expanding Universe obtaining a relation similar to Hubble’s and found essentially the same proportionality constant (the “Hubble constant”) as Hubble did two years later. After Hubble’s result had spread, Arthur Eddington had Lemaître’s paper translated into English in 1931, without the sections about Hubble’s law. In a reply to Eddington, Lemaître [10] also pointed out a logical consequence of an expanding Universe: The Universe must have existed for a finite time only, and must have emerged from an initial single quantum (in his words). In this sense, he paved the way for the concept of the Big Bang (a name coined much later by Fred Hoyle). It should also be noted that Carl Wirtz in 1924 [11] and Knut Lundmark in 1925 [12] had found that nebulae farther away recede faster than closer ones.

Hubble’s and others’ results from 1926 to 1934, even though not very precise, were

encouraging indications of a homogeneous Universe and most scientists were quick to

accept the notion. The concept of a homogeneous and isotropic universe is called the

Cosmological Principle. This goes back to Copernicus, who stated that the Earth is in

no special, favored place in the Universe. In modern language it is assumed that the

Universe looks the same on cosmological scales to all observers, independent of their

location and independent of in which direction they look in. The assumption of the

Cosmological Principle was inherent in the work of Friedmann and Lemaître but

virtually unknown in large parts of the scientific society. Thanks to the work of Howard

Robertson in 1935-1936 [13] and Arthur Walker in 1936 [14] it became well known.

Robertson and Walker constructed the general metric of space time consistent with the

Cosmological Principle and showed that it was not tied specifically to Einstein’s

equations, as had been assumed by Friedmann and Lemaître. Since the 1930s, the

evidence for the validity of the Cosmological Principle has grown stronger and stronger,

and with the 1964 discovery of the CMB radiation by Arno Penzias and Robert Wilson

(Nobel Prize in Physics 1978), the question was finally settled [15]. The recent

observations of the CMB show that the largest temperature anisotropies (on the order of

10^-3) arise due to the motion of the Milky Way through space. Subtracting this dipole

component, the residual anisotropies are a hundred times smaller.

Einstein’s Equations for a Homogeneous and Isotropic Universe

In Einstein’s theory [1], gravity was described by the space time metric g_uv , where the

indices run over the time and the three space coordinates, and where the metric varies in

space time. The infinitesimal, invariant, line element dτ is given  bydτ² = g_μν (x)dx^μ dx^ν . (1)

There are then ten(10) gravity fields over the four space time coordinates. However, the

symmetries of the theory stemming from the “Equivalence Principle” reduce that to two

 independent degrees of freedom.   Professor Albert Einstein  himself used the mathematical theory of differential geometry to find  out the relevant tensors quadratic in space time derivatives of the metric field, the Ricci tensor particle R_μν and the curvature scalar R, to derive the dynamical equations for the metric tensor. In the modified form with a cosmological constant Λ, the equations were R μν − Rg μν + Λ gμν = π GT μν…… (2) where G was Newton’s constant, which determined  the strength of the gravity as  a force, and T μν is the energy-momentum tensor particle. Here, as in the following, we have  then set the velocity of light to unity (c = 1). Einstein’s equations (2) represented then(10) ten coupled differential equations. With the Friedmann-Lemaître-Robertson-Walker assumption about the Cosmological Principlethe metric simplifies to  dτ² = dt² − a²(t) dr²/1− kr² + r²dθ ² + r² sin²θ dϕ ^{2 …}……….(3)

where a(t) is a scale factor and k is a constant that depends on the curvature of space time. The constant k has been normalized to the values -1, 0 or 1 describing either an open, flat or closed type Universe. The variables r, θ and ϕ are so called co-moving coordinates, in which a typical galaxy had fixed values. The physical cosmological distance for galaxies separated by r at a given time t (in the case of k = 0) is a(t)r, which grows with time as the scale factor a(t) in an  rapidly expanding Universe. In order to solve Einstein’s equations for this metric one also must assume a form for the matter density. The Cosmological Principle implies that the energy-momentum tensor particle has a form similar to that of the energy-momentum tensor in relativistic hydrodynamic, for a totally homogeneous and isotropic fluid with density ρ and pressure p (which both may depend on time). It is, in the rest frame of the fluid, a diagonal tensor with the diagonal elements (ρ, p, p, p). If we will insert the metric above and the energy-momentum tensor into the equations (2) , we get then two independent.  Friedmann equations H ²Ξ(a/a ) ²=8 πGρ/3- k/ a²+ a/3 And ä/a=− 4π G/3( ρ+3 p) + Λ /3……………(4) where a dot means a time derivative and H is the expansion rate of our Universe, called the Hubble parameter, or the Hubble constant, with its present value H₀. It is seen to depend on both the energy density of the universe as well as its curvature and a possible cosmological constant. When k and Λ set to zero, one defines the critical density as ρc=3 H2/8 πG………(5) In 1934, Lemaître [16] himself had already pointed out that the cosmological constant could be considered as a vacuum energy and hence a contribution to the energy density of the form ρΛ=Λ/8 π G. We will then assume that the Universe is composed of a set of components i, each having a fraction, Ωi , of the critical density, Ωi= ρi/ ρc. The two Friedmann equations were not enough to fully solve for the energy density, the pressure and the scale factor. We also then needed an equation of state, ρ = f(p), which could usually be written as wi = pi / ρi. For example, wi takes the value 0 for normal, non-relativistic, matter and 1/3 for photons. Since we now consider the cosmological constant as a part of the energy-momentum tensor we can compare the expression for the energy-momentum tensor for a perfect fluid in the rest frame, with diagonal elements (ρ, p, p, p), to the cosmological term ρΛ gμν, with diagonal elements ρΛ (1, -1, -1, -1). We concluded  that pΛ = -ρΛ, i.e., = −1 Λ w . The cosmological constant can hence be seen as a fluid with negative pressure. From Eq. (5), it  was clear that a static universe cannot be stable. Eq. (5) determines the deceleration or acceleration of the Universe. Since the expansion of the Universe was (wrongly) assumed to be slowing down (i.e., a negative sign of the acceleration), a parameter q0, called the deceleration parameter, was defined by

q0=− ä a/ ä2=− ä/ aH0 ²……………………..(6)

From Eq:s (4) and (5) it then follows that  q0=1/2 ΣiΩi(1+ 3 w i )……………….(6)

When we measured the light coming from a  long distant object, we can obtain two pieces of information apart from the direction to the object. We can measure the red shift and the apparent luminosity of the object: It is straightforward to measure the wavelength of light (e.g. from a given atomic spectral line) that a distant object emits. From Eq. (3) one can easily compute the relation between the wavelength an object emits, λ1 at time t1 and the wavelength observed here λ0 at time  t0  λ0/ λ1= a(t0)/ a(t1)-

 This is conventionally expressed in terms of a red shift parameter z as  z= λ0−λ1/ λ1= a(t0 )/ a(t1 )- 1.

For small z, we can then interpret the red shift z was the radial velocity of the object (it would correspond to a Doppler effect), and we find again Hubble’s law. For cosmological distances, the interpretation is less simple. Once we find standard candles luminous enough, however, measurements of red shift are relatively straightforward.

Measuring a cosmological distance in the universe is not straightforward. We must use a light signal that is emitted at a certain time and detected at another. During this time the universe has expanded. There are different distance measures introduced, but the one used for standard candles, i.e., objects with known intrinsic luminosity is the luminosity distance d_L, defined by d_L = (L/4πl)1/2 , where L is the absolute luminosity of the standard candle and l is the apparent luminosity. Luminosity distance can be computed in terms of the parameters in which we are interested, and for small z we can expand it as

dL=1/ H0     [   z +1/2(1−q0)z2)+ ... .] Again, to be completely clear, d_L is not an unambiguous measure of the distance to the standard candle, but it is a measure sensitive to the parameters we want to determine. In order to use it we need to know of celestial objects with known absolute luminosity.

From Eq. , we can see that in the nearby universe, the luminosity distances scale linearly with red shift, with 1/H0 as the constant of proportionality. In the more distant Universe, d_L depends to first order on the rate of deceleration, or equivalently on the amount and types of matter that make up the Universe. The general expression has to be written in terms of an integral over the redshift” z’ of the propagating photon as it travels from redshift z to us, at z = 0. In the case that relates to 2011 Nobel Prize in Physics, we may assume a flat Universe, k = 0 (as indicated to good accuracy by CMB measurements), and since radiation gives only a tiny contribution today, we may as approximation keep only the matter contribution ΩM and that of dark energy ΩΛ. The expression for the luminosity distance then becomes

If we could measure d_L accurately for low z as well as for higher red  shifts, we could

both measure the Hubble constant and determine the energy components of our Universe, in particular the value of ΩΛ [17]. It may be noted from the expression under the square root in Eq. (8) that when one measures very high redshift objects, the influence of the cosmological constant is reduced, and the optimal range is roughly for 0.3 < z < 2.

 The Standard Candles in Astronomy

A well-known class of standard candles, as mentioned above, is the Cepheid variable

stars, which nowadays can be identified out to distances of about 10 Mpc. To obtain a

record of the expansion history of the Universe, one needs, however, standard candles

that can be identified over distances at least 100 times larger. Already in 1938, Walter

Baade [18], working closely with Fritz Zwicky at the Mt Wilson Observatory, suggested that supernovae are promising as distance indicators: they are extremely bright and can, over a few weeks, outshine an entire galaxy. Therefore, they would be visible over a considerable redshift interval. The SNe that have been discussed over the past decades as standard candles [19] are designated type Ia (SNe Ia). According to William Fowler (Nobel Prize in Physics 1983) and Fred Hoyle [20], type Ia supernovae occur occasionally in binary systems, when a low-mass white dwarf accreting matter from a nearby companion approaches the limit of 1.4 solar masses (Nobel Prize in Physics 1983, Indian born  Subramanyan Chandrasekhar), and becomes unstable. A thermonuclear explosion ensues and an immense amount of energy is suddenly released. The evolution of the supernova brightness with time – the so-called light curve– can be observed over a few weeks. In a typical galaxy, supernovae occur a few times in thousand years. In our galaxy, supernovae have been observed with the naked eye,

e.g., by Chinese astronomers in 1054 and by Tycho Brahe in 1572. The supernova

1987A (not of type Ia) in the nearby galaxy the Large Magellanic Cloud, at a distance of160 000 light years, was observed both in light and in neutrinos (Nobel Prize in Physics 2002). For a review of supernova Ia properties, and their use as standard candles, see,e.g., the review by David Branch and Gustav Tammann [21].SNe Ia are identified through their spectral signatures: The absence of hydrogen features and the presence of a silicon absorption line. Their spectra and light curves are amazingly uniform, indicating a common origin and a common intrinsic  luminosity. The small deviations from uniformity can be investigated and corrected. Observations of how the brightness of these SNe varies with red shift, therefore, allow studies of the expansion history of the Universe. And because – according to theory –the expansion rate is determined by the energy-momentum density of the Universe andthe curvature of spacetime, discovery of the ultimate fate of the Universe appearspossible.

Detection of Type Ia Supernovae

The homogeneity of SNe Ia spectra makes this class of objects eminent standard candle

candidates. Because the peak luminosity occurs after only a short time, a supernova

must be observed early on after the explosion in order to determine the peak magnitude

with high precision. There is also another catch: SNe Ia are rare, occurring only a

couple of times per millennium in any given galaxy. However, to get a statistically

significant determination of cosmological parameters, a large observational sample is needed, including SNe at fairly high red shifts (z > 0.3).The first systematic search for SNe Ia at high red shifts was made during the late 1980s by a Danish-British collaboration [22] working at the 1.5 m Danish telescope at La Silla, Chile. Two years of observations resulted in the discovery of two distant SNe –one of them of Type Ia, the SN1988U at z = 0.31. However, this supernova was observed after its maximum which hampered the precision of the peak brightness determination. So, it seemed that discovery of distant SNe was possible but difficult. Obviously larger and faster instruments were needed to ensure the required statistics. The Supernova Cosmology Project (SCP) was initiated in 1988 by Saul Perlmutter of the Lawrence Berkeley National Laboratory (LBNL), USA, with the aim of measuring the presumed deceleration of the Universe - using SNe Ia as standard candles. In an expanding Universe dominated by matter, gravity should eventually cause the expansion to slow down. To address the problem of sufficient statistics, Perlmutter and collaborators developed a strategy that they dubbed Supernova on Demand. Using aCCD-based wide-field imager at a 4 m telescope, the group would observe thousands of galaxies over two to three nights just after new Moon. Imaging the same patches of the sky about three weeks later and using improved image-processing techniques, allowed selection of entire batches of about a dozen or so new SNe at a time. The timing ensured that many SNe would be close to peak brightness, making essential calibration possible. And, because the SNe were guaranteed, timely follow-up observations on the world’s largest telescopes in Chile, Hawaii and La Palma could be scheduled in advance for apre-defined date. The first high-z SN was discovered in 1992, and by 1994, the total number found by SCP reached seven. The first results were published in 1995 [23].In the mean time, light curves of several nearby type Ia SNe were measured by astronomers led by Mario Hamuy at the Cerro Calán Tololo Inter american Observatory in Chile [24]. Using this and other data, it was shown by Mark Phillips [25] that arelation between peak brightness and fading time could be used to recalibrate the SNe to a standard profile. The brighter ones grew and faded slower – the fainter ones faster,and the relation allowed to deduce the peak brightness from the time scale of the light curve. The few ”abnormal” occurrences were filtered out.Prompted by the success of the Supernova on Demand strategy and motivated by theimportance of the quest for q0, Brian Schmidt of the Mount Stromlo Observatory inAustralia organized, in 1994, a competing collaboration, consisting of supernova

experts, backed by the renowned scientist Robert Kirshner – the High-z Supernova

Search Team (HZT). Over the following years the two collaborations independently

searched for supernovae, often but not always at the same telescopes. Like SCP, HZT

could successfully demonstrate the validity of the chosen strategy, finding batches of

SNe at or close to maximum light that then could be followed up by spectroscopic

observations.In the beginning of 1998, both groups published scientific papers and gave talks at conferences, cautiously pointing out that their observations seemed consistent with a

low matter density Universe.The two breakthrough papers [27, 28] implying that the expansion of the Universe doesnot slow down but actually accelerates, were submitted for publication later that year.

The HZT article is based on observations of 16 SNe Ia mainly analyzed by Adam Riess,

then a postdoctoral researcher at University of California at Berkeley, whereas the SCP

paper, with Perlmutter as the driving force, includes 42 Type Ia SNe.

The fact that both groups independently presented similar - albeit extraordinary - results

was a crucial aspect for their acceptance within the physics and astronomy community.

The Observations

The larger the magnitude, the fainter is the object. On the red shift scale, z = 1

corresponds to a light travel time of almost 8 billion light years. The data was compared to

a number of cosmological scenarios with and without vacuum energy (or cosmological

constant). The data at z < 0.1 is from [26]. At redshifts z > 0.1 (i.e., distances greater

than about a billion light years), the cosmological predictions started to diverge.

Compared to an unrealistic empty Universe (ΩM = ΩΛ = 0) with a constant expansion

rate, the SNe for a given high red shift are observed to be about 10 - 15% fainter. If the

Universe were matter dominated (ΩM = 1), the high-z supernovae should have been

about 25% brighter than what is actually observed. The conclusion was that the

deceleration parameter q0 is negative, and that the expansion at the present epoch

unexpectedly accelerates. The result of the analyses of the twocollaborations, showing that ΩΛ = 0 is excluded with high significance, and that the expansion of the Universe accelerates,.

Could the dimness of the distant supernovae be the effect of intervening dust? Or might

the SNe Ia in the early Universe have had different properties from the nearby, recent

ones?Such questions have been extensively addressed by both collaborations, indicating that dust is not a major problem and that the spectral properties of near and distant SNe arevery similar. Although not as evident at the time of the discovery, later studies of SNe

beyond z = 1 [29], from the time when the Universe was much denser and ΩM

dominated, indicate that at that early epoch, gravity did slow down the expansion as

predicted by cosmological models. Repulsion only set in when the Universe was about

half its present age.

What is Dark Energy?

The driving force behind the acceleration is however yet unknown, but the current belief is that the cause of the expansion is vacuum energy (in this context called dark energy) – as it was suggested by Lemaître already in 1934 [16]. The SN results emerged at a time when some cosmologists, for many different reasons, argued that the universe might be

vacuum dominated. Others were, however, reluctant to accept such a claim implying a

non-zero cosmological constant. The SN observations were the crucial link in support of

vacuum dominance, directly testing models with Λ > 0. The currently accepted

cosmological standard model – the Concordance Model or the Λ CDM model – includes

both a cosmological constant Λ and Cold (i.e. non-relativistic) Dark Matter. The SNe

results combined with the CMB data and interpreted in terms of the Concordance Model

allow a precise determination of ΩM and ΩΛ . The predictions of the Concordance Model agree, within the experimental uncertainties, with all the presently available data. None of the alternative models proposed to explain the SN observations, based on inhomogeneities of the Universe at large scales, extra dimensions or modifications of general relativity, seem to convincingly account for all observations.

The very successful Standard Model for Particle Physics, which describes nature at the

smallest scales where we can measure, has two inherent sources for vacuum energy,

quantum fluctuations and spontaneous symmetry breaking. In relativistic quantum

physics the vacuum is not empty but filled with quantum fluctuations, allowed by

Heisenberg’s uncertainty principle (Nobel Prize in Physics 1932). A naïve estimate of

the size of the vacuum energy density, using the gravity constant G, Planck’s constant and the velocity of light, c, would imply a contribution to the energy density ρΛ of the

order of

3

2

~

P

P

l

M c

Λ ρ ,

where MP is the Planck mass (~ 1019 GeV/c2) and lP is the Planck length (~ 10-33 cm),

i.e., about 10118 GeV/cm3. This is to be compared to the present-day critical density of ~

0.5·10-5 GeV/cm3. Since the energy density of the Universe according to measurements

seems very close to critical, the naïve estimate is wrong by 122 orders of magnitude.

Prior to the discovery of the accelerated expansion of the Universe, particle physicists

believed, that there must be a symmetry principle forbidding a cosmological constant.

There is, however, another mechanism in the Standard Model that generates vacuum

energy. In order to explain how the Universe can be so homogeneous with different

parts that seemingly cannot have been in causal contact with each other, the idea of an

inflationary phase in the early Universe was put forward [30]. It states that at a very

early stage, the Universe went through a phase transition, breaking certain symmetries,

spontaneously generating a time-dependent, huge vacuum energy density that during a

very short time made the Universe expand enormously. A similar effect may still be at

work, leading to the vacuum energy that we see today. This so-called quintessence may

perhaps be detectable, as such a vacuum energy would have a weak time dependence

(see [31], and references therein).

Other important but yet unanswered questions are why ΩΛ has its measured value – and

why ΩΛ and ΩM at the present epoch in the history of the Universe are of the same order

of magnitude. At present we have no theoretical understanding of the value of ΩΛ.

Conclusion

The study of distant supernovae specially Type Ia constituted a crucial contribution to  the cosmology. Together with galaxy clustering and the CMB anisotropy measurements, it allowed precise determination of cosmological parameters. The observations presented us with a challenge, however: What is the source of the dark energy that drives the accelerating expansion of the Universe? Or whether our understanding of gravity as described by general relativity was insufficient? Or was Einstein’s “mistake” of introducing the cosmological constant one more stroke of his genius? Many new experimental efforts are underway to help shed light on these question in future.

References

1]” Did our universe started in a Big Bang gospel or Just Be?” Authors: Professor Pranab Kumar Bhattacharya, Mr. Rupak Bhattacharya,  Mr. Ritwik Bhattacharya  Mrs. Dalia Mukherjee & Miss Upasana Bhattacharya in the chapter” Fate of a Star” published at www.unipathos.com as E book in July 2004

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32] Scientific Background on the Nobel Prize in Physics 2011THE aCCELERATING UNIVERSEcompiled by the Class for Physics of the Royal Swedish Academy of Sciences- Advance report
http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/advanced-physicsprize2011.pdf

Links to see the  Publication

http://phys.org/news162651549.html  at Physics org

http://www.sciencenews.org/view/generic/id/46298 At Science News

http://www.facebook.com/note.php?note_id=398324121719 

Ask a Nobel Laureate on Facebook, David Gross, Nobel Prize in Physics 2004by Nobelprize.org on Monday, May 31, 2010 at 12:21am

  Acknowledgement- To  diseased late Mr. Bholanath Bhattacharya and  late Mrs Bani Bhattacharya of residence 7/51 purbapalli, Po-sodepur Dist 24 parganas(north) , Kolkata-110,WestBengal, India,  for their initial teaching  for us about the universe, Big Bang]  and  the English version of “Scientific Background on the Nobel Prize in Physics 2011”THE accelerating UNIVERSE” compiled by the Class for Physics of the Royal Swedish Academy of Sciences Published in www.nobel prize.org

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