7 Amazing Facts About James Webb Space Telescope

1) James Webb Space Telescope will help astronomers and cosmologists get higher resolution images of distant galaxies and star systems. JWST is the advanced successor of Hubble Space Telescope. It is named after NASA's second administrator James E. Webb.

2) JWST is primarily an infrared observatory designed for near to mid infrared range. For this purpose a large solar shield made of five sheets of silicon and aluminum coated Kapton will keep JWST mirror and 4 science instruments below 50K(-220 degree C) .

3) Primary mirror of this space telescope is made of 18 gold coated beryllium mirror segments. The mirror segments are hexagonal in shape. At a diameter of 6.5 meter, this telescope is much larger than Hubble space telescope.

4) The Integrated Science Instrument Module or ISIM for short holds four science instruments, the near infrared camera, the near infrared spectrograph, the mid infrared instrument, the fine guidance sensor and near infrared imager and slitless spectrograph and a guide camera.

5) The ISIM provides electrical power, computing resources, cooling capability and structural stability to the Webb telescope. It is made of bonded graphite epoxy composite connected to underside of telescope.

6) Webb telescope planned launch date is March 30, 2021 on Ariane 5 rocket. It is a collaboration among NASA, European space agency and Canadian space agency. Primary mission time is 5 years, extendable to ten years at second lagrange point.

7) Northrop Grumman Aerospace Systems is the primary contractor responsible for building the spacecraft. NASA's Goddard Space Flight Center is leading the project. Ball Aerospace is subcontracted to build the optical telescope.

Reference:

1) https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Infrared_astronomy
2) https://www.jwst.nasa.gov
3) https://www.space.com/21925-james-webb-space-telescope-jwst.html

Amazing Things to Know About ESA's Euclid Dark Universe Mission

     I.        Euclid Dark Universe Mission, named after Greek Mathematician ‘Euclid’, known as Father of Geometry, is first space based mission dedicated to collect data for understanding Dark Matter and Dark Energy, led by European Space Agency, ESA. Euclid spacecraft will look as far back in time as 10 billion years. In Astronomy, it is known as look back time. A Look back time of about 10 billion years is equivalent to observing objects with redshift close to 2. Weak Gravitational Lensing and parameters related to Galaxy Clustering such as redshift, will be measured to infer insight into nature of Dark Matter and Dark Energy.
  

II.        Euclid will operate for 6.25 years after its launch in last quarter of 2020. During that period, it will cover more than a third of extragalactic space which is equivalent to covering more than 15000 deg² of sky, excluding Solar System and Milky Way. The spacecraft will also peer about 10 times deeper, for 3 times during its operation, covering 40 deg² of space for calibration and performance monitoring purposes, during which it will be observing objects with redshift higher than 2, which includes distant Quasars and Galaxies. Euclid will cover about 10 billion objects, measuring weak gravitational lensing of more than 1 billion and redshift of about 50 million of them.

III.    Thales Alenia Space, which is Europe’s largest Satellite manufacturer, headquartered in Cannes, France, is chosen to make the Satellite and its service module. Payload Module and telescope, which includes 1.2 m Silicon Carbide primary mirror, Korsch Telescope, covering an area of 0.5 deg² with a focal length of 24.5 m, will be built by Airbus Defenseand Space. Euclid Consortium which is an International Consortium of Scientists, will make very broad band R+I+Z filter, visible CCD imager- VIS, with pixel size of .1 arcsecond, near infrared, broad band Y,J,H filter Photometer- NISP P and a slitless Spectrograph- NISP S, with common field of view of .53 deg². Data will be collected by the Spacecraft using these instruments and will be sent to Earth at 855 Gbit/s in 4 hr daily slots in K band (25.5-27 Ghz). Onboard storage capacity will be more than 300 GB. The Spacecraft will have an exposure time of up to 4500 sec/field.

IV.   Visible CCD detectors will be a mosaic of 36 (6×6), 4000×4000 pixel each, e2v charge coupled detectors, operating in visible wavelength (550-900 nm). They will be used for measuring shape of Galaxies. Near Infrared detector will be a mosaic of 4×4 Teledyne H2RG detectors, 2000×2000 pixels each, operating in (900-2000) nm wavelengths. It will provide low accuracy redshifts of over a billion galaxies using multicolor photometry and high accuracy redshifts of millions of Galaxies using Spectrometry.

V.      Solar Panels will supply power and provide stability to orientation of telescope. Thermal Insulation will be done to protect against radiation heat. The Spacecraft will weigh 2100 kg and it will be 4.5 m long and 3.1 m in diameter. It will be launched to L2 Sun-Earth Lagrangian point- halo orbit using Soyuz ST-2.1B rocket from Kourou launch site, Guiana Space Center. Travel time to orbit is 30 days.

VI.     Nasa is collaborating with ESA on Euclid Mission. From JPL Lab in Pasadena, California, NASA will put up infrared flight detectors for Euclid science instrument. Goddard Space Flight Center will be used for testing these detectors. Three US science teams totaling 40 scientists are nominated to add to planning and analysis of data.

VII.     Dark Matter is the main contributor to Weak Gravitational Lensing effect of Galaxies as it constitutes most of Galactic matter content. A measurement of bending of light by Galaxies, therefore, gives information about the Dark Matter that it contains. By measuring this effect at this large scale and accuracy, scientists will try to gain additional insight into Physics of Dark Matter. Also, clustering of Galaxies is influenced by Dark Energy. A measurement of clustering is hoped to help in understanding the nature of this mysterious type of Energy.

References:

1) http://sci.esa.int/euclid/

2) https://www.euclid-ec.org/

3) https://arxiv.org/ftp/arxiv/papers/1110/1110.3193.pdf

Image credits goes to respective sources.

Dark Energy and Accelerating Expansion of Universe: Part 2

Click here to go to first part of this article

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.

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.  

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.                

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

Image credits goes to respective sources.          

Dark Energy and Accelerating Expansion of Universe

Cepheid variables are stars whose luminosity/brightness increases and reduces with time. 18^th century Astronomers were well aware of such stars. In 1893, Harvard Astronomer Edward Charles Pickering (Jul 19, 1846-Feb 3, 1919) recruited a recent grad of ‘Radcliffe College’ then called- ‘the society for the collegiate instruction for women’ to work as a ‘human computer’ to analyze photographic plate collection of Harvard College Observatory to study these stars with variable brightness/luminosity. She was Henrietta Swan Leavitt (Jul 4, 1868 – Dec 12, 1921), daughter of a congregational church minister. While observing those plates, she noticed a pattern in some of variables. Brighter variables appeared to have longer periods than less bright ones. Leavitt used the method of trigonometric parallax to calculate distance to these Cepheids located in small and large Magellanic Clouds. She assumed all Cepheids in a Magellanic Cloud to be at same distance from Earth. With these distances she was able to calculate the maximum luminosity of stars from their observed apparent brightness or apparent magnitude in photographic plates using inverse square law.

B = L/(4πd²)

Where, B= Brightness of star measured using magnitude scale

           L= Luminosity or energy output of star per unit time also known as

                intrinsic brightness of the star measured in Suns

           d= Distance from the star

Also, apparent magnitude m and absolute magnitude M can be linked as

M = m + 5 - 5logd

Where, d= Distance from star

           Apparent magnitude m= Brightness as observed through telescope      without atmospheric interference.

           Absolute magnitude M= Apparent brightness of star at 10 parsec from

                                              observer.

A star’s luminosity in Suns can be linked to its absolute magnitude M as

logL = -0.4M + 1.884

In Astronomy, Brightness is measured using a calibrated magnitude scale first created by Greek Astronomer Hipparchus in around 130-120 BC. SI Unit for apparent brightness is W/m² and is defined as measure of amount of energy coming from a star per unit area per unit time to Earth. Magnitude of two stars and their brightness b₁ and b₂ can be linked as

b₁/b₂ = 10^(2/5)(m2-m1)

When she plotted luminosity against period of each variable, the pattern was noticeable. Period of these variable stars was directly proportional to their maximum luminosity. She analyzed 1777 of these variables to come to her conclusion and published her result in ‘Annals of the Astronomical Observatory of Harvard College’ in 1908 titled ‘1777 Variables in the Magellanic Clouds’. After further study she gave confirmation to her initial conclusion in 1912. As Leavitt put it- “A straight line can readily be drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods”. Leavitt’s plot is known as ‘period luminosity relationship’ or ‘Leavitt’s law’ and is used by Astronomers to determine absolute brightness/luminosity/magnitude of Cepheid, having obtained its period through telescopic observation.

Trigonometric Parallax method can be used to determine distance to stars which are up to 200 parsec or 650 light-years away. In this method, position of a nearby star is obtained either visually or photographically with respect to a background star further away. 6 months later, when Earth is at diagonally opposite point in its orbit around sun, position is obtained again. If the angle between the two lines of sight is 2p, distance of the star from sun is ‘d’ and distance between Earth and Sun                                                        is ‘r’, then

Tanp = r/d

From this,

d = r/(Tanp)

Distance of star from Earth will be

D = √(r² + d²)

Here, p = parallax

         r = 149 million km.

Parallax is a very small angle and is usually measured in seconds of arc. A star with parallax of 1 second of arc as observed from Earth is said to be at 1 parsec from Sun. 1 Second of arc or 1 arcsecond is 1/3600 of a degree. A parsec is equal to 3.0857×10¹³ km or about 19 trillion miles. Astronomical unit is distance between Earth and Sun and equals to about 149,597,870,700 meters. Yet another unit is a lightyear which is the distance light travels in a year and equals to 9.4607×10¹² km or about 6 million million miles. A parsec is about 3.26 lightyears in length. Appropriate unit is used in case of small and large distances.

Hipparchus classified stars he could see in night sky according to their visual brightness. He assigned a magnitude of 1 to the brightest stars and magnitude 6 to least bright ones. Ptolemy refined Hipparchus system in 140AD. This magnitude scale is still in use having modernized and improved from time to time. Galileo using his telescope introduced seventh magnitude star. In 1856, Oxford Astronomer Norman Robert Pogson (Mar 23, 1829-Jun 23, 1891) set first magnitude star to be 100 times as bright as sixth magnitude and established the logarithmic magnitude scale. In Pogson’s scheme, difference of one magnitude is same as brightness difference of (100)^1/5 which is 2.512- known as Pogson’s ratio. Pogson’s scale assigned Polaris a magnitude of 2. It was used as zero point of the scale. Later, on discovering that Polaris is slightly variable, Vega was used as standard reference with an assigned magnitude of 0, a standard still in use. Stars brighter than Vega are assigned negative magnitudes. Hubble space telescope could see stars up to a magnitude of 31. In late 19^th century, after the introduction of photography in stellar photometry, Astronomers found that some stars appearing similar in brightness to eye were differing on photographic plates. It was happening because photographic emulsions used on photographic plates were more sensitive to blue light than red. This led to the creation of photographic magnitudes denoted as m_p whereas visual magnitudes are denoted as m_v. Henrietta Swan Leavitt analyzed 299 plates from 13 telescopes to construct her logarithmic scale, spanning 17 magnitudes. Difference between star’s photographic and visual magnitudes is called ‘color index’ which is a measure of star’s color. Color index of blue stars came to be of negative value while that of yellow, orange and red stars is increasingly positive. Now days, magnitude is obtained using standard photoelectric photometer through standard color filters, most common of which is UBV. U stands for near ultraviolet filter, B stands for Blue filter and V stands for visual band filter. Color index is obtained by subtracting V magnitude from B magnitude. This way the color index of yellow sun came to be 0.63. UBV system was extended to red and near infrared filters, becoming UBVRI system. System was extended beyond infrared to J, K, L, N, Q bands. An object’s real brightness is known as its bolometric magnitude m_bol which is a measure of total radiation the object is emitting. Modern Photometry makes use of CCD sensors. CCD stands for charge coupled devices. CCD Sensors can receive more than 95% of incoming light. 

Spectrum is obtained when stellar light is shown through a prism. Prism or a diffraction grating splits light into its constituents. Continuous spectrum is obtained when there is no intervening matter between the light source and prism or diffraction grating. When dust or gas cloud is heated by a proto star or active galactic nucleus, Electrons of their atoms absorb specific amount of energy and emit it in specific quantas. Such re emitted radiation when passes through prism gives of a particular type of spectrum known as emission spectrum, having bright lines corresponding to specific wavelengths depending upon type of atom that emitted the radiation. By comparing such stellar spectrum with spectrum of known elements, composition of stellar dust or gas is known. When radiation of a star is absorbed by electrons in the atoms of surrounding colder dust or gas, it gets re radiated with somewhat reduced intensity and in random direction. When such re radiated light passes through prism it gives of absorption spectrum which is a continuous spectrum except for dark lines at wavelengths where elements in dust or gas would have their bright emission lines. Therefore such spectra can also be used to obtain elemental composition of intervening dust or gas. Helium line was discovered in 1868 in solar spectra independently by Norman Lockyer and Pierre Janssen and was found on Earth in 1895. Modern Spectroscopy makes use of Holographic gratings and gratings made using Lithographic techniques. Reactive Ion Etching is another advanced method in use now for making gratings. 
     

Click here to read The Dark Matter Story to know more about measuring wavelengths and redshifts

By the year 1913, Edwin Powell Hubble (Nov 20, 1889 – Sep 28, 1953) was sure that he wanted to get into Astronomy. He gave up law practice and went back to his alma mater- the University of Chicago, to get doctorate in Astronomy. While finishing his doctoral work in early 1917, he was invited by George Ellery Hale to work with recently finished 100 inch telescope at Mount Wilson observatory, Pasadena, California. Hubble accepted the commission as an army captain instead. After war, he joined Mount Wilson Observatory in summer of 1919. Hubble was well aware of spectroscopic work done by Vesto Melvin Slipher (Nov 11, 1875 – Nov 8, 1969) at Lowell Observatory in Arizona and his 1912 discovery of shifts in spectral lines toward red band indicating that most of the nebulae are moving away from us. Also, Slipher was first to observe the rotation of spiral Galaxies in 1914. General consensus among Physicists of those days was that the Universe is static. Even Albert Einstein (Mar 14, 1879-Apr 18, 1955) believed that Universe is unmoving which led him to introduce cosmological constant in his field equation of General Relativity to exactly balance out the crunching effect of gravity which he thought would cause the Universe to collapse on itself. General relativity showed how stress-energy causes spacetime to curve. His equations in their original form indicated an expanding or shrinking Universe which Einstein couldn’t believe.

Nebulae, which later became known as Galaxies were the great mystery of those days. They appeared as fuzzy patches in those old telescopes and were subject of great debate between Harlow Shapley and Heber Curtis in 1920. Shapley believed that the Milky Way is 300,000 lightyears wide and is our entire Universe, Sun is not at center and Nebulae are part of Milky Way system. Curtis argued that Milky Way is only 30,000 lightyears across and Nebulae are Island Universes, separate Galaxies beyond Milky Way. Hubble was interested in studying these Nebulae and so he started taking photographic plates of these objects. On the night of Oct 5, 1923, he observed 3 Novae close to Andromeda Nebula- M31. On comparing this plate with earlier plates, he noticed that one of the Novae is actually a variable star. Further observations confirmed that the variable star matches characteristic of a Cepheid variable. Using the period luminosity graph for Cepheids, Hubble was able to calculate distance to the variable star and got a value of about 900,000 lightyears for distance of Andromeda Nebula. The actual value is close to 2.2 million lightyears. Earlier, Harlow Shapley had found a value close to 300,000 lightyears for the diameter of Milky Way, current value being 100,000-120,000 lightyears. This conveniently put M31 well beyond the boundaries of Milky Way and therefore Heber Curtis’s point of view was partially confirmed that M31 and other such Nebulae are separate star systems comparable to Milky Way. Hubble published his discovery first in Nov 23, 1924 issue of New York Times and then in front of American Astronomical Society on Jan1, 1925.

Hubble continued his work on Nebulae, calculating distance to 22 Nebulae. He also calculated their velocities using shifts in their spectral lines, 4 of which were determined by his assistant Milton Lasalle Humason. He further calculated distance to 22 more Nebulae from their radial velocities, assisted by Humason. Observing the photographic plates, Hubble could see that small appearing Nebulae have greater radial velocities than bigger ones. Assuming that all Nebulae are more or less the same size, he concluded that more distant Nebulae are moving at much greater velocities than the nearer ones. Together with 2 earlier estimates by Harlow Shapley, he had data for total 46 Nebulae. He plotted the radial velocities corrected for solar motion against distance calculated using luminosities for 24 Nebulae and also for the 22 Nebulae whose distance could not be calculated individually. He found an almost linear variation in radial velocity with distance. This indicated that with increasing distance the radial velocity of Nebulae also increases by a common factor, a term which later became known as Hubble Constant. Hubble concluded that Nebulae are going away from each other and the Universe is expanding. He communicated these results in his Jan 17, 1929 paper titled ‘A Relation Between Distance and Radial Velocity Among Extra Galactic Nebulae’. The relation can be mathematically expressed as,

v = H₀×d

This is known as Hubble’s law, where,

v = recessional velocity of Galaxy

d = Distance to the Galaxy

H₀ = Hubble’s constant    

Hubble got a value of about 500 km/s/Mpc for his proportionality constant, current value being 67.8±0.77 km/s/Mpc which means that expansion of Universe increases by about 67.8 km/s for every 3.26 million lightyears in any direction.

In 1917, Albert Einstein introduced cosmological constant to curvature side of his field equation of Gravitation to make the equations predict a static Universe as a dynamic Universe was considered absurd at that time. Russian Physicist Alexander Friedmann (Jun 16, 1888-Sep 16, 1925), worked on Einstein’s field equations without presumptions and showed that Universe might be expanding at a rate which can be calculated using the equations. Friedmann’s equation can be obtained by putting metric for homogeneous and isotropic Universe in Einstein field equations. He presented his equations in 1922. Georges Lemaître (Jul 17, 1894-Jun 20, 1966), a Belgian priest and astronomer, found something similar from Einstein’s field equations in 1927. Lemaître made the first empirical determination of Hubble constant H₀ using these equations. He also suggested that if Universe is expanding then it must have been unimaginably small back in time, a state he called ‘cosmic egg’. He met Einstein and showed his results to him. Einstein held on to the popular belief of a static Universe and disapproved Lemaître’s result. After Hubble published his findings in 1929, Einstein visited him. He saw the data and was convinced that the Universe is expanding. He then dropped cosmological constant, restoring the field equations to their original form.    

After Hubble, almost everyone accepted an expanding Universe. Astronomers and Physicists believed that the expansion must be slowing down due to gravitational pull of matter-energy density. Rate of slowdown was named deceleration parameter with symbol q₀. Allan Rex Sandage (Jun 18, 1926 – Nov 13, 2010), a student of Walter Baade and Hubble, published a paper in 1961 titled ‘The Ability of the 200 inch Telescope to Discriminate Between Selected World Models’. In this paper he advocated that cosmology is search for two parameters- Hubble constant H₀ and deceleration parameter q₀. Earlier in 1958 he had measured Hubble constant to be 75 km/s/Mpc, which is its first good estimate.

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References:

1)https://www.famousscientists.org/henrietta-swan-leavitt/

2)https://apod.nasa.gov/debate/debate20.html

3)http://www.amnh.org/explore/resource-collections/cosmic-horizons/profile-georges-lemaitre-father-of-the-big-bang/

4)http://skyserver.sdss.org/dr1/en/astro/universe/universe.asp

5)http://w.astro.berkeley.edu/~mwhite/darkmatter/hubble.html

Image credits goes to respective sources.

The Dark Matter Story

Mount Wilson observatory, built on top of Mount Wilson in Southern California by George Ellery Hale, a prolific astronomer who discovered magnetic field in sunspots, houses the 100 inch telescope, largest from November 2, 1917 to January26, 1949. During early 1930’s, Fritz Zwicky, then associate professor of physics at Caltech, was using this 100 inch telescope to look at galaxies in Coma Cluster- a rich cluster of approximately 1000 galaxies about 330 million light years away. Zwicky gathered light from galaxies through the telescope and obtained spectrum by placing spectrograph at focus of telescope. Spectrograph is a Spectrometer used to obtain spectrum which is a graph showing intensity as function of wavelength. Spectrograph of those days were light tight boxes comprising a prism or diffraction grating plate to which light was let through a narrow opening and a detector placed at appropriate distance and angle to record the spectrum. 

Diffraction Gratings, pioneered by David Rittenhouse in 1785 and furthered by Joseph von Fraunhofer in 1821, are plates with many uniformly spaced, parallel, opaque, grooves per millimeter, engraved upon them. Transparent plates are used for making transmission gratings and metal coated opaque plates are used for reflective gratings. Space between grooves could be of the order of microns. Groovings can be sinusoidal or triangular. Triangular groove gratings are also known as blazed gratings because of higher brightness of the spectra they produce. Grooves do the diffraction. Light falling on gratings diffracts and splits into constituent wavelengths. In most directions, light diffracted from one groove cancels out light diffracted from another, known as destructive interference. In certain number of directions though, constructive interference takes place. These directions correspond to diffraction order ‘m’. Many such orders exist when wavelength of light diffracted is much smaller compared to spacing between adjacent grooves often denoted by ‘d’. Fewer such orders exist in case ‘d’ is comparable to wavelength. Wavelength ‘λ’ of diffracted light depends on angle of incidence w.r.t normal to grating substrate, angle of diffraction w.r.t normal to grating substrate, spacing between consecutive grooves and diffraction order. Gratings can be reflective or transmissive. Reflective gratings are better suited for commercial spectrography. The detector used to be a photographic plate which was glass plate coated with special emulsion. 

mλ = d(sinθ_i + sinθ_r)

Where, θ_i = Angle of incidence measured w.r.t grating normal, anticlockwise

           θ_r = Angle of diffraction measured w.r.t grating normal, clockwise

Zwicky obtained graph of intensity of light as function of wavelength. This was usually done using a Microdensitometer which shines a compact ray of light through the photographic plate to a light sensitive photo multiplier tube. The tube evaluates and registers amount of light at each wavelength as light crosses the photographic plate, usually in form of an intensity amplitude and wavelength graph. Once wavelengths were obtained, Zwicky calculated redshift for each galaxy and then radial velocities and using that velocity dispersion for the cluster. Link between redshift and velocity of galaxies can be expressed as:

z = (λ – λ₀)/λ₀ = v/c = (R_Present/R_Emit)-1

For z << 1.

In much broader sense:

z = {(λ/ λ₀)² – 1}/{(λ/ λ₀)² + 1} = v/c

Here, z = Redshift

         λ = Measured wavelength

         λ₀ = Emitted wavelength

         v = Radial velocity of object

         c = Speed of light

         R_Present = Radius of curvature of Universe at present = 1 (Very-Very close)

         R_Emit = Radius of curvature of Universe when radiation was emitted

Velocity v calculated this way is the radial velocity or line of sight velocity of the object.

V_Radial = V_Recessional ± V_Peculiar

Here, V_Recessional is velocity due to accelerating expansion of spacetime and V_Peculiar or sometimes infall velocity is velocity of object due to net gravitational effect of surrounding objects. For higher radial velocities or large distances, V_Radial can be approximated to V_Recessional. Peculiar velocities and Galactocentric velocity of sun become significant at radial velocities below about 1500km/s or in other words for closer objects. Moreover, several Earth related velocities should also be considered for higher precision. Distance d of object from viewer can be linked to its recessional velocity using the equation:

V_Recessional = H₀ × d

Known as Hubble’s law. Here, H₀ is Hubble constant with current value of 67.8 km/s/Mpc, Mpc stands for megaparsec and equals to 1 million parsecs or 3.262 million light years or 3.086×10¹⁹ km. Hubble constant is speed with which a galaxy at 1 Mpc distance, is moving away from us in any direction, assuming our Universe is homogeneous and isotropic at large scales. Inverse of Hubble constant is called Hubble time T_h.

In his 1933 paper titled ‘The redshift of extragalactic nebulae’ Zwicky considered Coma cluster to be virialized which means the cluster is neither expanding nor collapsing, it has reached a state of dynamic equilibrium. Further, he counted the number of Galaxies in cluster to be approximately 800 each having a mass of the order of 10⁹ Solar masses. He thus calculated the approximate total mass M of cluster.

M ∼ 800 × 10⁹ × 2 × 10³⁰ kg = 1.6 × 10⁴² kg

He then calculated mean potential energy of system using:

P.E_Mean = (P.E_Total)/M ∼ -64 ×10⁸ m²/s²

Where, P.E_Total = -(3/5) × (GM²)/R

R is radius of cluster, about a million light years or 10²² m.

Since cluster is considered virialized, virial theorem can be applied, according to which:

K.E_Mean = -(1/2) × P.E_Mean = 32×10⁸ m²/s²

Also, K.E_Mean = (Mean v²)/2

From this, (Mean v²)^1/2 = 80 km/s

Also, mean v² = 3 × σ², where σ is radial velocity dispersion. From this,

σ = ((Mean v²)^1/2)/√3

Zwicky found a radial velocity dispersion of 1019 ± 360 km/s for Coma cluster which he calculated from observed radial velocity of 8 Galaxies using their spectral redshift, Comparing the two results, Zwicky concluded that average density of Coma cluster must be at least 400 times greater than density due to luminous matter alone, for a velocity dispersion of over 1000 km/s. This directly indicated presence of non luminous matter in the cluster. Zwicky called it Dunkle Materie which is Swiss for dark matter. Since Zwicky considered Hubble parameter H₀ to be 558 km/s/Mpc, his estimates are different than more recent ones, but meaningful nonetheless.

In 1973, Physicist James Peebles and Astronomer Jeremiaha Ostriker were trying to simulate the evolution of Galaxies using N Body Simulation. They programmed 300 mass points to represent stars in a Galaxy rotating about a central point with more mass points towards center and fewer toward boundary. Simulation was based on movement of mass points due to Newtonian gravitational force between them. In less than a rotation period most of the mass points were collapsing into a bar shaped blob near central region. However, they were able to obtain recognizable spiral or elliptical shapes on adding a uniform mass distribution 10 times the size of the 300 mass points. This indicated that Galaxies might be harboring non luminous matter about 10 times the mass of visible matter. They presented their results in 1974 paper titled ‘The size and mass of Galaxies, and the mass of the Universe’. They also gave a criterion known as Ostriker-Peebles criterion, according to which if T is first kinetic energy component and W is total kinetic energy, then, a Galaxy will become barred if T/W > 0.15.

At Carnegie institution in Washington, Astronomer Vera Cooper Rubin was collaborating with instrument maker Kent Ford. Ford had created one of the most sensitive spectrometer of those days. Together they used this spectrometer to obtain reliable spectrum of Hydrogen gas clouds orbiting in different parts of Andromeda, including the boundary regions. They obtained the velocities and plotted a chart. What they obtained was a curve which looked almost flat indicating that the gas near boundary region is orbiting as fast as the gas near central region of Galaxy. They noted an almost constant gas velocity outside visible boundary of Galaxy from their plot. This couldn’t be explained by Newtonian gravity. Gas orbiting that fast near boundary region couldn’t be held by the gravity of luminous mass of Galaxy alone. This indicated presence of non luminous matter in large quantities, about 10 times more than luminous matter according to Rubin’s calculations. Rubin noted that if Andromeda obeyed Newton’s law then it must contain Dark Matter with quantities increasing with increasing distance from its center. Rubin and Ford announced their result first in 1975 at a meeting of the American Astronomical Society. In 1980, Rubin published these results in a widely reviewed paper.

In 2004 NASA’s space based orbiting X-ray observatory recorded an image which came to be known as galaxy cluster 1E 065756 or Bullet Cluster in common usage. A deeper look at object 1E 065756 revealed that it’s in fact two Galaxy clusters that underwent a collision about 100 million years ago. Gas in two clusters underwent friction as the two clusters passed through each other and got superheated emitting X-ray captured by the observatory. Hubble space telescope recorded optical image of the object. Scientists also used gravitational lensing effect of colliding clusters to obtain an image of gravitational mass of object. On combining the three images it was clear that X-ray emitting portion of object is lagging behind mass concentration, indicating that weakly interactive dark matter and heavy compact objects passed right through without colliding but the gas was slowed down.

At end of cosmic inflation, at about 10^-32s after big bang, inflation field decayed into Quark-Gluon plasma. This phenomenon is named Reheating. Between 10^-12s to 10^-6s after big bang, W and Z Bosons and Photons separated and Higgs field manifested and particles interacting with this field acquired mass via Higgs mechanism. Between 10^-6s and 1s after big bang, Universe was cool enough for Quarks to combine using Gluons forming Protons and Neutrons, collectively known as Hadrons. Between 1s and 10s after big bang most of Hadrons and Antihadrons annihilated each other leaving a Universe primarily filled with Leptons and Antileptons. Approximately 10s after, creation of new Lepton-Antilepton pairs stopped as the Universe further expanded and cooled. A small residue of Leptons remained at the end of mutual annihilation. Between 10s and about 380000 years after big bang, Photons kept colliding with charged electrons, protons and nuclei because of low mean free path. Nucleosynthesis took place during this period forming heavier nuclei. 70000 Years after Big Bang, Cold Dark Matter was dominating. Small variations were present in the density of matter and dark matter, owing to quantum mechanical fluctuations. Both normal matter and dark matter were pulled toward higher density regions by gravity making dense regions denser and rare regions rarer. Dark matter kept getting concentrated around center of these quantum mechanical fluctuations without any obstruction as it didn’t interact with Photons, but normal matter while falling in under the effect of gravity was getting hit by Photons causing it to move away. 

When photon pressure was more, normal matter moved away and when gravity was stronger, it fell in creating an oscillating effect known as baryonic acoustic oscillation. When the normal matter fell in it grew denser and therefore hotter and when it was pushed out, it cooled off. Also areas where matter concentrated grew hotter compared to areas from where it moved out giving rise to hotter and colder regions in Universe which we see as hot and cold spots of different sizes in CMB map. About 380000 years after Big Bang, the Universe was so big it became cool enough for electrons and protons to combine to form neutral atoms in a process known as recombination. The process was fast and faster for Helium than for Hydrogen. Due to recombination the mean free path of Photons became infinite and they for the first time were able to travel throughout the Universe. This phenomenon is known as decoupling. The pattern of temperature variation and therefore the baryonic acoustic oscillations and information about fluctuations that rose during inflation was encoded into this light which we today call cosmic microwave background radiation as the wavelength of this primordial light has shifted to microwave band after billions of years of traveling through an expanding Universe. This is why an analysis of cosmic microwave background is sometimes called a baby picture of Universe. It shows the seeds of large scale structures that we find in Universe today. Planck CMB data gives an effective temperature of CMB as 2.7 degree Kelvin with variations of 1 part per 100,000. The angular size of cold and hot spots observed in CMB and extent of temperature variation indicates a dark matter density of 26.8%, normal matter density of 4.9% and a dark energy density of 68.3%.

Efforts are ongoing around world to detect dark matter directly. Scientists have hypothesized a fundamental particle having all known properties of dark matter known as WIMP and are trying to detect it through underground experiments in deep mines such as UK’s Boulby mines. Now,  USA’s large underground xenon or LUX experiment and Europe’s ZonEd proportional scintillation in liquid noble gases or Zeplin experiment are collaborating to combine both experiments to increase sensitivity to WIMPs by more than 100 times. LZ experiment is second generation direct dark matter detection experiment. 7 ton purified liquid xenon at ultra low temperature with an active system to suppress non WIMP signals is used in this experiment to detect faint effect of a WIMP on a Xenon nucleus. The experiment uses high voltage feed through, 120 veto photo multiplier tubes, 488 photo multiplier tubes, additional 180 Xenon skin photomultiplier tubes and Gadolinium loaded liquid scintillator veto.  System is housed inside a water tank shield. The LZ collaboration has 190 scientists in 32 institutions.

Meanwhile scientists working on dark energy survey at Cerro Tololo Inter-American Observatory, in Chilean Andes, are using the 570 megapixel Dark Energy camera or DE cam mounted on Blanco 4 meter telescope there, to create detailed maps of dark matter by utilizing effect of Dark Energy and strong and weak gravitational lensing effect of said dark matter, in order to understand the nature of dark energy through analysis of clumpiness of dark matter in those maps. DE cam has about 3 ft wide mirrors and weighs between 4 to 5 tons. It is the largest digital camera ever built. The survey started on 31 Aug 2013 and will utilize 525 nights of observation till 2018 to record information from 300 million galaxies. It is supposed to create the most detailed dark matter map of Universe. Dark Matter or cold Dark Matter in this case, bends light through its gravitational effect and the bending is directly proportional to strength of gravitational field which is directly proportional to the concentration of dark matter. Therefore a measure of bending in light could be used to create a density map of Dark Matter through careful calculations.

References:
1) http://blair.pha.jhu.edu/spectroscopy/measure.html
2) https://arxiv.org/pdf/astro-ph/9904251.pdf
3) http://adsabs.harvard.edu/full/1999ApJ...525C1223T
4) https://ned.ipac.caltech.edu/level5/Sept03/Einasto/paper.pdf
5) https://www.sciencealert.com/this-timeline-shows-the-entire-history-of-the-universe-and-where-it-s-headed

Image credits goes to respective sources.