Garden and wife friendly telescope cover
When I moved to my present house eleven years ago I built a summer house which became the ‘observatory.’ In it I kept the telescope, a 200mm LX200, and the computer on which to download the images acquired. To use the telescope it was necessary to carry it out and place it on a pier and clamp it in place, then connect the power and computer cables etc. Over the last couple of years I have found this increasingly difficult and tiresome and was looking for a way of making the whole operation more agreeable without the expense of a second building.
The appearance of the garden is as important to myself, and my wife, as is the need for a nice telescope. So whatever I was to have constructed needed to blend in and after much thought I came up with a design for a dummy dovecote to cover the telescope. After consulting the yellow pages in the glass fibre section I contacted a company called “Artistic Solutions”. The name alone gave some cause for confidence. A telephone call was made, and a drawing posted off to them. The result was that the company agreed to construct the dovecote and produced an estimate that was half what I was prepared to pay.
When installed the telescope cover is very user friendly. The power and computer cable are permanently connected and all that is necessary is to remove the three sections, and for some operations only the two top sections need to be removed. The only thing I did have to build was a step device to enable me to remove the top two sections.
I chose a dummy dovecote, but now wonder what other shapes would enhance the garden if only I had the imagination!
By Ivor Clarke
One of the interesting things about our hobby of astronomy is that you can still get down old books off your bookshelf on the subject and still enjoy reading them. Of cause the older they are the more “wrong” they are. And if you have any books on the stars, cosmology or the solar system that are older than say 50 years: well most of the ideas in them will be well out of date. Such is the speed of change today. Even though they are out of date doesn’t mean that they can’t still be enjoyed today.
I have 4 old books from the end of the Victorian era, all hard bound, all about the size of a paperback, the oldest are two small green books, one called the Sun Moon and Planets, and the companion volume The Stars. Both of these books have a date of 1881 in them, but I can’t find an authors name in either. The former has 46 engravings as illustrations showing aspects of the sun such as various spots and “protuberances seen during a solar eclipse” and drawings of the planets. The frontispiece is a wonderful drawing of the lunar surface with impossible tall mountains and crater walls, its title is Lunar Landscape — Ideal View, see below.
A 1881 drawing of the lunar surface
The Stars book has 40 small engravings showing stars in simple constellations with a few marked stars and a couple of drawings of comets. One of the strange things to our eyes is the shape of the constellations. Back then just the main brightest stars in a constellations were classified as belonging to it with a drawing showing how they were “joined” up.
The fainter stars in every constellation which did not belong in the main designated shape were drawn just as small dots which had letters or numbers depending on who’s map the reference was. Most of the early maps had the boundaries of each constellation wandering around the edge of that group and roughly divided the area between groups equally (not in this book, there are no constellation boundaries marked at all). But many other books of this period show boundaries, though there was no fixed reference point to were each boundary was in the sky. So different maps pinched each others stars, for instance, a faint star on the border between neighbours could be in either constellation depending who had drawn the map you looked at.
The Northern Polar stars
It was not until 1922 that the modern list of 88 constellations was adopted by the newly formed International Astronomical Union, even early editions of works like the Norton’s Star Atlas did not have set boundaries. Just dotted lines wondering about between the stars. It was not until a Belgian astronomer working for the IAU drew up the constellation boundaries in 1930 that they became fixed in space with respect to the stars. But because of precession the lines are now starting to depart from the lines of declination and right ascension as originally drawn.
Because the seeing with most early telescopes was not at all up to todays standards, with optical problems and poor glasses, it did not matter I suppose. If a faint star was in another constellation did it matter to much? Most early astronomy was about mapping the sky. In 1603 the German astronomer Johann Bayer had hit on the idea of giving stars letters of the Greek alphabet starting with the brightest star in a constellation as alpha α, then beta β, and so on down the magnitude range to the faintest stars. John Flamsteed, the Astronomer Royal in his 1725 catalogue gave each constellation star a number listed by its position in order of right ascension. As time went on more catalogues were produced with more ways of listing the stars in the sky. But by the middle of the 19th century things were changing.
Even then, the speed of light was known to a value of 185,000 mps, close enough to todays value for most purposes, but other numbers on the same page in The Stars reveal major differences with today. For instance the distance to the brightest star Sirius is stated to be 1,370,000 times the radius of the Earth’s orbit, making it 22 light years away! In my copy of Norton’s 2000, Sirius is 8.6 ly, nearly three times closer. I don’t know how they got it so wrong.
Back then in 1881, stars were not classified as today, most stars were in just 4 classes, the HR diagram was still 25 years in the future! Objects like galaxies and globular clusters, were all classed as nebula. The largest globular clusters such has NGC 5139, Omega Centauri, which had been first observed by Sir Edmund Halley in 1679 while working on his Catalogue of the Stars of the Southern Hemisphere, could just about be resolved into the myriads of stars in them with the best telescopes available. They were still classed as a type of nebula, even though it was known they were great balls of stars at a great distance.
The universe was a lot smaller then, only the Great Nebula in Andromeda, M 31 was conceded by a few astronomers to be a vast conglomeration of stars which was at a vast distant. For the three and a half centuries before then, no telescope was powerful or sharp enough to see into the Andromeda galaxy and see the individual stars. Even in the 1880’s some astronomers still called them clouds of diffuse and luminous medium along with all the other nebula. Distant star clouds looked fuzzy, gas clouds looked fuzzy, galaxies looked fuzzy, so they all were nebula too.
M 51, The Whirlpool Galaxy in The Stars book
Only a 100 or so years before, Charles Messier had drawn up his famous catalogue of 100 or so objects in the night sky which did not move and so were definitely not comets. In his small telescope most of the objects in the heavens must have looked a bit soft and fuzzy anyway. Even in a slightly later book I have, printed in 1890 called Easy Star Lessons by Richard Proctor, none of Messier’s catalogue numbers appear in the books, for instance, M 51 The Whirlpool Galaxy is just called a Spiral Nebula in Canus Venatici!
M 51 was first seen as a spiral in what was then the largest reflector in the world, the 72” Birr telescope in Ireland (not the world’s best observing site). The Leviathan was built by William Parsons the third Earl of Rosse in 1845. This telescope could only track an object in the sky for about 50 minutes if low, to 2 hours (if an object was near overhead), at a time as it was positioned between two stone towers which supported it.
Other twisted shapes in the sky were much more mysterious. Lots of small faint patches of light bent into every shape possible. Some could be seen to look like cream stirred into a cup of coffee, defiantly a spiral shape. Were these new solar systems and stars forming? A popular idea at this time. No-one had any idea how far away these faint objects were. They could be small, dim and near or large, bright and far away. But which?
It was not until 1897 that the famous 40” (1 meter) refractor at Yerkes came into use and not until 1908 when the 60” reflector on Mount Wilson saw first light did the astronomers have instruments powerful enough to see faint distant objects.
A very small illustration in The Stars book of Orion
Telescopes sharp enough to resolve faint stars soon started to revolutionise astronomy. With larger mirrors came a new understanding of what could be seen. Other help came with increased sensitivity in the photographic plates which helped increase the magnitude which the telescope could see. Also almost all of the early photographic plates could only detect blue and green light and the sensitive dropped off with long exposures due to reciprocity failure, where this caused a doubling of the density in a 5 minute exposure to exposed for a far greater time which could be for over an hour. The engineering required to keep a giant telescope pointing to exactly the same spot of sky for hours was pushing the state-of-the-art boundaries of the day.
It was not until October 5th 1923 when Edwin Hubble, using the new 100” reflector at Mount Wilson, found Cepheid variable stars in the Andromeda Galaxy (M 31) that the universe starts to really get bigger. Cepheid stars had been found in the Small Magellanic Cloud between 1908 / 12 and were found to share a similar light curve with the brightest having the longer light cycle. As all the stars in the Cloud having the same distance, it was a simple matter to draw up a distance chart for them depending on their magnitude. This was and still is, one of the best yard-sticks we have for determining distance. Now with the Hubble Space Telescope we can go out to all the local galaxies.
So the universe in Victorian times was our galaxy, with stars and luminous vapours making up all that was known. The planets and moons of the solar system were also mysterious places, likewise comets which suddenly appeared out of the night skies. Don’t forget how dark Victorian night skies were. On very clear night the milky way would be visible, so even faint comets would be seen. It was a simple matter to watch a new comet for a few days and plot its orbit around the sun.
All other objects were considered to be a part of our galaxy and it was around this time that doubt was being voiced by a few because of the reports coming in from all parts of the world of new discoveries.
For example, the Earth had just got a lot older with Charles Darwin’s publication of The Origin of Species in 1859. Before then most people believed that the Earth, and everything else for that matter, was created about 4004 BC. in just 6 days. Darwin accepted this as the truth when he was young along with everyone else. It took the voyage around the world in the HMS Beagle from 1832 to 1836 for the penny to drop that it could not have started as it was stated in the Bible and the world was millions of years old. Also every where he went he found evidence of long dead animals which had no living relatives today.
With Darwin’s book and other geological evidence coming to light concerning how rocks form and water erodes them, means it was not possible to have some of the land forms visible today. For them to be built up in many layers in such a short time scale of a few thousand years was unbelievable. Cliffs and rivers showed evidence of long time scales of erosion and silting. There was also the problem of large rocks in the northern areas being misplaced by large distances. How could these vast rocks which weighted hundreds of tons be hundreds of miles from similar rock formations?
It was with the findings of the new bands of fossil hunters roaming around quarries and the beaches, picking up old bones of long deceased animals, imprints of plants which are not seen today which was casting a dark shadow over long held beliefs. It was plain the planet was much older, millions of years older than had ever been thought. So if the planet was millions of years old, so the rest of space must be too, along with all the stars and planets and nebula.
This caused a huge problem with the sun. Why was it so bright and hot? It must have been alight for all this time, but how? If it was made of the best coal it would burn out in less than a half a million years! The best theory was that it was contracting with force of gravity squashing the gasses together in the centre and the friction gave off the heat and light. And there were millions of other stars doing just the same thing.
Top : An engraving of a large sunspot from the 1881 book Sun, Moon and Planets showing the central dark umbra and lighter penumbra on the sun’s surface.
Bottom : The diagram shows how the spot changes shape as it nears the limb and displays a depth to its structure.
Other mysteries of the sun were sunspots and the pink bright flares seen at its edge during a solar eclipse. What were they and how were they formed? From the earliest days of Aristotle and the Greek philosophers is was assumed that the sun, like all the stars, were “incorruptible and was ineffably and incomparable pure”. Many refused to look through the early telescopes at sun spots as they could not believe the sun could have a disfigurement on its surface. This must be an error. Galileo got himself into hot water by confirming earlier observation of spots. Later it was realised that the spots rotated around the sun in about 25 days. So the sun must rotate. Other ideas were that the dark central area of a sun spot was a gap in the hot outer layer of the sun’s atmosphere and below it was a land filled with strange creatures. Another was that the spots were lumps of unmelted metal boiling away on the surface.
The spectroscope had been developed from Newton’s work in 1666 when he split the solar spectrum into its component colours. By 1802 the scientist William Wollaston had found that by using a fine slit, the spectrum contained black lines amongst the colours. This research continued in the laboratory to find what caused these mysterious dark lines and during the mid 19th century several of the dark lines had been found to correspond to various substances that when heated give off light. While most of the lines were of unknown origin, a lot of progress had been made. Helium for example was found in the spectrum of the sun in 1868 during a solar eclipse, before it was discovered on Earth.
Let’s look at what these other old books tell us about our solar system. In the Sun Moon and Planets and The Story of the Solar System by G F Chambers, the planets get a chapter each. But reading them today, they do sound very old fashioned with talk of what the inhabitants may be like and how they would view the sky and sun. There is nowhere any reference to what the surface temperate may be like, hot enough to melt lead on Mercury and the icy coldness of the outer planets and moons.
A “birds eye view” of Saturn showing the shadow of the planet on the ring system from Sun, Moon and Planets.
Venus was known to have a lot of clouds covering the planet, but lots of observations note high mountains sticking out above them. Mars was not mapped very well with no mention of canals. The Italian Giovanni Schiaparelli had first recorded straight lines on the surface in 1877, canali is Italian for channel and we all know what happened with that story. Little is said about the Jupiter cloud belts as the constituency of them was unknown, while much is talked about the 4 main moons and the eclipses of them on the planet. The Great Red Spot was strongly coloured for a few years around 1880 and gets a mention along with a description of the main cloud belts. The chapters on Saturn and the rings are interesting as is the story of Galileo not knowing what they could be (it is said he died not knowing what he had seen) and Christiaan Huygens in 1656 realising it was a ring system about the planet tilted at 27° which accounted for its disappearing act every 15 years. Little was known of Uranus and Neptune in the late 19th century, apart from their orbital details. Both books tell the story of William Herschel’s discovery on 13th March 1781 of a new object which he thought was a comet for a few days until it became clear the orbit was circular so it must be a large planet. He spotted two of its moons 6 years later. By 1851, 4 were known to be in a strange retrograde orbit nearly 90° from the ecliptic. Neptune’s discovery by the French is helped by a clever John Adams working out its position and the Astronomer Royal, Airy not looking for it in 1846. Nothing was known about these distant worlds of-cause but they did have the same number of “planets” as we have today! (8)
The facts which are presented in these books are ones which cover the orbital period, distance, the size of the planet, its known moons and rotational periods. In most cases these are accurate, but in the case of all the inner planets, all were thought to rotate once in around 24 hours, with the exception of Mercury were some coincided its rotational period to be the same as its orbit of 88 days. With Mars, the rotation period given was exactly right, 24h 37m 23s. All the planet sizes and orbital positions were near to todays values as were the satellites.
Jupiter only had 5 moons until 1904 when Himalia was discovered, the last of the 100+ km diameter moons. Todays total is 63! Saturn did better with 8, Phoebe was found in 1899, now the number is over 60 and with all the small ringlet moons, this could go into hundreds. Uranus had 6 known moons (27 now) and Neptune one (13 now). These outer planets are so far away and faint it was not until the Voyager 2 spacecraft reached Uranus in 1986 and Neptune in 1989 that we really saw these planets in close-up. And don’t forget that no other probe is going in the near future.
I suppose if we keep todays text books a hundred years, our kids can have a good laugh at our ignorance in the future.
By Paritosh Maulik
In general we agree that that our universe started from a very small but finite size and grew to the vast universe we have today. The diameter of the visible universe is about 92 – 94 *109 light years or 3*1080 cubic metres now; and we do not know how much lies beyond. As far as we can measure, on large scales, the properties of the observable universe are very uniform. How is it possible that something so big, can be so uniform? The best answer we have so far is, in its very early life, the embryonic universe expanded to a very large size in a very short period. The rate of expansion has slowed down since then. The riddle of the size of the present universe was worked out by using quantum mechanics, theory that govern the behaviour of sub-atomic particles – the theory of the small, explained the big universe.
Fixed or Grows with Time
Einstein tried to determine the structure of the universe by using the general theory of relativity. He assumed the universe to be of fixed size. Hence, in order to counterbalance the attraction of gravity to cause the universe to collapses on itself, he introduced the term gravitational constant. It is a repulsive force working against the gravity.
According to the classical theory gravity depends on
i) mass and
But the general theory of relativity introduced a few other terms into the equation of gravity
iii) energy and
When a substance is heated, its energy increases and since energy is equivalent to mass, its mass increase; however for most of the practical purposes this increases in mass from energy is perhaps negligible.
Pressure in this context has a slight different concept than normally we are used to. Let us consider an object with a pressure difference between two surfaces. If the difference is such that the distance between the surfaces contracts, it is called positive pressure and if the distance between the surfaces dilates or expands, it is called negative pressure. When two objects are closed by, the force of gravity acts and the system has a given energy. But when we separate these two objects, we have to work against the gravity, the energy of the system increases. This is why gravity is said to have a negative energy.
For most of the ordinary materials including electrons and protons, the pressure is positive and under everyday condition, pressure from these ordinary matters contributes little to the gravity. Ordinary matter therefore exerts a positive pressure, it causes positive attraction i.e. gravitation or inward motion; but if there is something that causes negative pressure, it leads to repulsive gravity or expansion. Therefore if the repulsive gravity dominates, matter will be pushes away from each other.
In the equation of the universe, the gravitational constant behaves like an energy term filling the space uniformly. Its effect is to create a negative pressure or outward movement and its magnitude is higher than that of the positive pressure causing gravitational inward movement. It also came out that the magnitude of this repulsive force becomes stronger as the distance increases. Therefore for nearby systems like the solar system, the magnitude of this energy is small and obeys the classical gravitational process, but over the large cosmological distance, the repulsive force takes over keeping the universe static. Observations by Hubble showed that the universe expands and Einstein agreed.
However as early as 1920, it was suggested that a static universe is possible, provided we assume that there is no mass in the universe and the cosmological constant provides the energy density (this is the energy to move apart; matter on the other hand provides gravitational energy to move closer). One of the consequences of this theory is, the universe has to expand exponentially to keep the matter density to near zero. Another prediction of this model is redshift i.e. further is the distance, further the object is moving away from us. If a very small matter is added to such an universe, the expansion would continue. This proposal also explained the Olbers paradox. If the universe is static, it should be filled with light from the stars, but if the universe expands, objects are moving away from each other, the light is receding. Here we can mention that Hubble’s observation did show an expanding universe, not on exponential scale, but on a linear scale.
Soon other models of the Universe, based on the general theory of relativity, allowed the expansion of the universe starting from what we now call singularity. All these models accepted that the universe expands, but could not figure out the process which kick started the expansion. Before we go into this, a little background information.
One simple way to think about the field is, as an area of influence. Arrangement of iron filings near a magnet shows the magnetic field. A glass rod charged with static electricity can pick a small piece of paper from a distance. Every field has its own messenger particle, which carry the information from the field.
The most common field around us is electromagnetic field. Electrically charged particles, when interact with each other, it creates an electromagnetic field. The ripples in this field are carried by its messenger particle photons. Photons are without any mass. So these can interact over a large distance.
Gravity is all around us and its message is carried by particles called gravitons. Gravity is a very weak force. We can pick up a pin with a magnet against the combined gravitational pull of the Earth. The presence of graviton is yet to be confirmed experimentally. When a body falls to the ground, it distorts spacetime and it is the gravitons, which carry the message. Gravity is powerful with with large masses and over a long distance.
Strong and Weak nuclear force have their influence over very small distances and are generally not obvious, but we all know of atomic fission and radioactivity. These two forces also have their own fields and their messenger particles. Strong nuclear force binds fundamental particles which make up the proton. Weak nuclear force is the cause of radioactive decay. The messenger particles of the week force are very heavy and therefore are difficult to produce. The weak force is very weak. Initially electrons, neutrons and protons were considered to be the fundamental particles, but the quantum theory suggested that neutrons and protons were made up of other fundamental particles.
Following the work of James Clarke Maxwell, which showed that the electric and magnetic fields are connected, physicists began to think that all the fundamental forces in nature, namely the electromagnetic, the gravity, the strong and the weak forces must be interrelated. To explain this, physicists introduced a concept called scalar field. We only feel the effect of electromagnetic force only if there is a difference in charge or the field is not homogeneous. This field has no potential energy. But the scalar field has potential energy. It changes with time and when it reaches to the minimum value, it fills the universe. It makes it presence known by interacting with other messenger particles. This is called Higgs field.
At time somewhere in between the big bang and 10-35 second after the big bang, the temperature was 1028°. During this period, photons, the messenger particles of electromagnetism and the messenger particles of weak and strong field did not have a separate existence. One messenger particle could change to another. It was called symmetry between the forces. When the temperature dropped below 1028°, the strong force got separated from the electroweek force. At a time of around 10-12 second and at a temperature of 1015°, the week force separated from the electromagnetic force. Now all three forces, the strong force, the weak force and the electromagnetic force got their own identities and began to exist as separate forces. The symmetry was completely broken. This is the concept of Grand Unified Force. At a much earlier time 10-43 second (Planck time), the gravity and Grand Unified force separated. We do not know what happened before the Planck time. Gravity has not yet been successfully introduced in the Grand Unified scheme. Separation of the strong force is also not very clear.
Higgs field is an interesting concept in physics. All matter is ultimately composed of quarks and electrons. These particles, while moving through the Higgs field, feel the drag of the field. Particles find it difficult to change their velocities and this drag appear to us as mass. Since this field is present everywhere, we can not get rid of this particle – field interaction; we are stuck with the mass. There is no friction between the particles and the Higgs field, so particles moving at a constant speed can continue unimpeded and only when we need to increase or decrease the speed of the particles, we have to increase or decrease the force needed to move the particles.
In short, the Higgs field gives mass to the most fundamental particles like quarks and electrons. Quarks combine to form proton and neutron. This process needs energy and since energy and mass are equivalent, this energy also adds to the mass. All matter is ultimately made up of, electrons, neutrons and protons (hydrogen has no neutron, but its heavier isotopes have). So the total mass of a system is due to the interaction of quarks and electrons with the Higgs field and also from the energy needed to combine the quarks to form protons and neutrons.
Spot the Defects and Work Backwards
As the universe went through the different stages of separation of forces, we can assume the universe went from one state to another; as an example, ice—water—steam. We call it phase transition. When a normal material undergoes phase transition, generally it leaves behind some defects or flaws in the material. Theoretically it was suggested that in the case of the universe, one such defect was magnetic monopoles. These are high energy particles with a single magnetic pole. It is symmetric, no north or south pole, one pole can change into another pole. As the energy dropped, magnets got two poles and it became asymmetric. Magnetic monopoles are yet to be experimentally determined.
Alan Guth and Henry Tye, while working on the magnetic monopoles around the late 1970s and early 1980s, came out with the idea that, during cooling from the big bang, the universe did not drop to the lowest possible value (possible at the then temperature), but stuck to an intermediate level for a short period. The situation is akin to water dropping its temperature, below 0°C, and yet to freeze to form ice. This is called supercooling of water. In this supercooled universe, the symmetry between the forces was not broken. In the supercooled state, the universe had a higher energy than the state, if the symmetry was broken.
According to Guth this additional energy acted like a negative pressure and provided the repulsive force to expand the universe, a situation somewhat similar to the cosmological constant providing the outward repulsive force. Guth coined this expansion of the universe as inflation and the term caught on.
There are some subtle differences between the cosmological constant and supercooled energy field worked out by Guth. The cosmological constant as originally suggested, is a constant, but the supercooled Higg’s field is not a constant. It can stay at this high energy state only for a very brief period (10-35 second) and the universe expanded in such a short period by a factor of 1030 to 10100 or more starting from an initial size of about 10-33 cm.
The energy and negative pressure contribution of this supercooled Higgs field is about 10100 times higher than the gravitational constant, which counterbalances the inward attractive gravity with the repulsive gravity
In the inflationary model, the energy of the supercooled universe is constant, but the universe expands. This stored energy is very high. It creates matter and hence the positive energy, but this positive energy is just counterbalanced by the negative energy of the gravity. The total energy remains zero. On the other hand according to the standard big bang expansion, as the universe expands, the energy density decreases.
This model of rapid expansion of the universe came to be known as inflation; it solved a couple of niggling problems raised by the big bang theory.
Let us imagine two galaxies, each 10*109 light year away from us, and we are halfway in between. The distance between these two galaxies is 20*109 light year, but the universe is 13.7*109 years old. So there are regions of universe which are visible to us, but invisible to each other; beyond their cosmic horizon. These two regions are not in “casual contact” now. When we measure the average cosmic background temperature of the universe, it is reasonably uniform around 2.725° K.
The fastest speed of communication is the speed of light. Now the question can be asked, if these two regions are separated such that the light has not yet reached from one to the other, how was it possible for these two regions to communicate with each other, which has made it possible for the temperature to be so uniform today?
According to the inflation model, prior to the time when the inflation or the rapid expansions started, all regions of the universe was very small and all regions were in casual contact with each other; their properties were uniform. But once the inflation occurred, the universe expanded rapidly. The initial properties got frozen. These regions (with their uniform properties) got separated. In reality the space in between these regions expanded and, all regions began to cool down and eventually all regions ended up with the present background temperature.
This is one of the great successes of the inflation model. The model is based on particle physics and quantum mechanics, which deals with sub-microscopic scale, yet it can explain the super size universe. The quantum theory predicted that the pre-inflationary universe should have some small scale regional variations and these should retain their signature in the post-inflationary universe.
These predicted regional variations of the cosmic microwave background have now been detected. These are the speckle pattern in the COBE and the WMAP images. These regional difference are very minute and we had to wait for the sensitive instruments to detect these differences. Additionally, the predicted polarisation pattern of the microwave background has also been confirmed.
Another great success of the inflation model is the possible explanation of the flatness problem. If there is more matter in the universe, the stronger will be the force due to gravity and after a while the expanding universe will close on itself. This universe has a beginning and an end. On the other hand, if there is less matter in the universe, the universe will keep on expanding; the open universe.
However it is possible that the universe has just enough matter so that force of the collapse is balanced by the force of expansion. This density is called critical density. This density depends on the Hubble constant. But in reality, the Hubble constant changes with time and therefore the critical density also changes with time.
Critical density = 3 H2/(8πG); H is Hubble constant and G is gravitational constant.
The present critical density of the universe is 1.06*10-29 g/cc, about six hydrogen atoms per cubic metre. Sometimes it is easier to express it as Ω = Current density ÷ Critical density. A universe with the critical density is a flat universe; Ω = 1.
We know that the Hubble constant, opposing the attraction of gravity, causes the expansion of the universe. If the gravity has slowed down the expansion, then the Hubble constant is lower now. A lower Hubble constant will slow down the rate of expansion of the universe. In other words, the Hubble constant was higher in the earlier times and since the critical density depends on the Hubble constant, the critical density was higher at earlier times.
If the universe started with the then current density greater than the critical density, the present density will be higher than the critical density, (Ω>1). If then current density was lower than the critical density, the present density will be lower than the critical density. (Ω<1); matter would not condense into galaxies.
All the observations to date suggest that the universe is expanding, but, but not as fast as an open universe; the galaxies did form and the universe is not closing in on itself. Ω is very close to 1 and as far as we know, the universe is flat. If we work out the Ω at the Planck time, it turns out that the ratio, Ω = 1±10-60; any small deviation from this value would have caused the universe either to expand for ever or collapse. This finely tuned value of = 1±10-60 seems to be a too much of a coincidence. According to the Inflation model at the Planck time, when the universe expanded exponentially from an initial size of about 10-33 cm to 100100 cm (depending on the model), the curvature of the universe became very large. Hence, even if the universe started as a closed universe, following the inflation, the universe became a very big curved surface and over a small distance, such a surface appears to be near flat. The universe we can see is about 1028 cm and it appears to us flat. It is like a collapsed balloon inflated with gas; any small surface of this full balloon appears to be near flat.
Guth was not the first to introduce such a concept. Starobinsky from Russia suggested a similar theory based on quantum theory of gravity. But it was a difficult model and remained largely unknown outside Russia, mainly because of the isolation of Russia during the late seventies.
One of the outcomes of the Guth’s model was that, the supercooled universe would breakdown into small regions or bubbles and each bubble would expand colliding with each other. No such bubbles have been detected. Since the original work, the model of inflation has been modified
One present models of inflation suggests that there is no need to have the supercooling, or phase transition. The Planck size spacetime region, which grew in size to our universe, was not unique. The whole universe did not start at one given moment and different regions had different energies. Only the regions with suitable condition inflated, while the other regions did not and remained small. Since this model suggests that our universe could have its origin come from other spacetime, it has a ring to the steady state universe. One can argue that the present universe as we know it, just happen to have the right initial conditions, otherwise we could have something completely different.
It is now accepted that at a very early age our universe grew from very small size over a very short period. All the problems have not been solved, and this is the best we have now. One of the challenges is to understand the nature of the dark matter and dark energy, and their role in the critical density and the expansion of universe. It also remains to include gravity properly in the scenario.
The Fabric of the Cosmos, Brian Green, Penguin Books, 2005
The Case of the Missing Neutrinos, John Gribbin, Penguin Books, 1998
A Brief History of Time, Stephen W Hawking, Bantam Press, 1998
Underground Observatories: Why?
By Paritosh Maulik
To get a fuller picture of astrophysical objects; photons, from gamma rays to radio waves, are not enough. The intervening environment such as dust or electro-magnetic fields attenuates the signal; messages can get lost. High energy nuclear processes occur in stars, supernova and active galactic nucleus. Neutrinos are one of the by-products of these processes. Neutrinos do not interact much with most materials and therefore can act as a messenger to bring information from these sources.
To look for these neutrinos one has to go deep underground, to avoid “pollution” from naturally occurring ionising sources. Instead of mirrors or lenses or antenna, these observatories use thousands of tonnes of liquid. We shall look how a neutrino is produced in the Sun and then the detection of the Solar neutrino. Initially the observations did not match the theory, and the reason needed to be explained. We shall also briefly mention neutrinos from other sources.
A brief background
According to the simplified model, at the nucleus of an atom there are:—
Protons, p+, with one positive charge and one unit of mass
Neutrons, n, with no charge, but one unit of mass.
Electrons, e -, with negligible mass and one unit of negative charge, orbit the nucleus.
In order to keep the atom electrically neutral, the number of proton and the number of electrons are same. In hydrogen nucleus there are no neutrons only proton. This is written as 1H1.
The next element to hydrogen is helium. It contains two of protons and two neutrons at the nucleus and two electrons in orbit around the nucleus. So it has a mass of 4 and it is written as 2He4.
Theoretical calculations in 1930 by Paul Dirac suggested that there should be a particle counterpart to electron, but opposite in charge i.e. one unit of positive charge and no mass. This was termed as positron, e+, and the concept of anti-matter began. In 1932, study of cosmic rays confirmed the presence of positrons.
One of the radioactive decay processes, called β decay, generates a stream of electrons. The nucleus undergoing radioactive b decay louses more energy than that can be accounted for by the energy carried away by the beam of electrons. Wolfgang Pauli suggested that there must be another particle which carries away the additional energy. Enrico Fermi worked out that a free neutron outside a nucleus would break down in about 10 minutes according to the reaction
n p+ + e- + (1)
Into a particle called a neutrino, which carries away the energy. Calculations suggested that the neutrino should have a negligible mass and is capable of travelling at near the speed of the light. The concept of neutrino remained somewhat enigmatic.
Like other subatomic particles it could not be found in and around the nucleus, but occurred only as a by-product of nuclear process. Theory suggested that the probability of neutrino interacting with any material is very small and it would pass through most of the materials without leaving behind any trace. There lies the problem, how to detect the neutrons. The solution suggested was, build a very big detector; the bigger the detector, more the chances of interaction with the detector and thus one may detect these illusive particles.
The proof finally came in 1956. They put a 10 ton tank of water next to a nuclear reactor in the Los Alamos laboratory. This reactor was producing weapon grade plutonium. Over a period of one year, the experiment ran for about 100 days and they detected a few gamma (g) ray flashes. This was explained as the reverse of the above reaction (1)
p+ + n + e+ (2)
Proton reacts with neutrino, it produces neutron and positron. The positron and electron reacts to produce g ray and this was detected. (The first detection of solar neutrinos was at Brookhaven National Laboratory in 1964 using a tank of 400,000 litres of dry cleaning fluid. This was at the bottom of a mine 1500 meters below the surface to screen out all other types of radiation. The measured flux was just over 2 per day, just a third of the expected number. Ed) First we shall look at an astronomical source of neutrino close to home, the Sun.
The Powerhouse of the Sun
Rutherford proved that radioactivity and changes in the nuclear structure are linked together i.e. one element can change to other and Francis Aston, at Cambridge, showed that the mass of helium atom is somewhat smaller than the 4 hydrogen atoms needed to make helium. By the 1920s it was accepted that of transformation of one element into another is not alchemy but can be explained by science.
Astrophysicists were trying hard to explain the seemingly inexhaustible energy source of the Sun. The Sun was found to be a vast reservoir of hydrogen. Eddignton suggested that “subatomic energy” (nuclear process) can only be the possible energy source of the Sun; also around the same time Jean Perrin from France proposed that the mass loss occurred in the reaction, when four hydrogen atoms (4 protons) combine to form one helium atom, can be converted into energy according to the equation
E = mc2
One could ask, how is it possible to bring four positively charged protons together to fuse into helium? The answer came from the quantum mechanics. If a large number of particles, hydrogen in the case of the Sun, are brought close together, they collide with each other and get hot. When the temperature is order of 106°, the electrons gets stripped off. This is the state called a plasma.
The hydrogen atoms become proton. The average energy of the protons depends on the temperature. Higher the energy, higher is the chance of collision. Most of the protons will repeal each other, but according to the quantum mechanics a few protons could collide head on with each other and convert into helium atoms. This process releases energy. Note that only a small fraction of protons can take part in the process and therefore the nuclear fusion in the Sun occurs in a “controlled manner”; all the energy is not released in a very short time as in the case of thermo-nuclear explosion.
If the Sun gets too hot, the gas ball of the Sun expands, the temperature drops; the gas ball contracts and the temperature rises again. Thus the Sun maintains it temperature.
Structure of the Sun
At the centre of the Sun there is a ball of hydrogen. This amounts to about half of the mass of the Sun, but its volume is about 1.6% of the total volume of the Sun and the temperature is about 15*106°.
A zone called Radiative Zone surrounds the core. It reaches out about 71% from the centre of the Sun. This layer radiates heat from the core outwards. Outside the radiative zone there is Convective zone. Like a boiling liquid, turbulent flow of gas, carries the heat outwards. The hot material rises and then sinks. It creates granules on the surface of the Sun, visible with optical telescopes. The size of these granules are about 1000 to 1500 km and last for about ten days and eventually dissolves into new ones. There are some larger granules aptly named supergranulars, of about 30,000 km, move horizontally on the surface.
Surrounding the Convective layer there is a thin layer called the Photosphere of about a few hundred km thick; this is what we see as the Sun. Strictly speaking this is not a surface layer, but a thin layer of gas, much thinner compared to the interior.
Behaviour of this “surface” depends on the behaviour of the gases inside the Sun. The temperature at this zone is low enough to combine proton and electrons to form hydrogen atoms. Some of the hydrogen atoms thus formed, pick up free electrons and becomes negatively charged ions. These negatively charged ions absorb radiation from the interior and emit as visible light to us. The amount of hydrogen in this outer surface is so low that it should be transparent, but only because of the formation of these negatively charged ions we can see the visible light.
Reactions at the Core of the Sun
At a temperature of about 10*106° at the core of the Sun, hydrogen atoms loose their electrons and exist as protons. Now and then two protons overcome their repulsive force and collide to form
p+ + p+ D2 (deuterium) +e (low energy neutrino) + e+ (positron) (3)
The probability of this reaction is very low; but the total weight of hydrogen in the Sun makes up for it. Electron and its anti-matter positron cancel each other out releasing energy as a gamma (g) ray
e+ (positron) + e- (electron) g (4)
In the next stage a proton reacts with deuterium to form a lighter version of helium and a g ray. This reaction occurs very quickly, it’s why we do not see any deuterium in the stars.
D2 + p+ He3 (light helium isotope) + g (5)
This helium subsequently converts into other heavier elements.
Two He3 thus formed, fuses into normal helium, He4 and releasing two protons to the solar atmosphere. This reaction takes about one million years to complete.
He3 + He3 He4 + 2 p+ (6)
We can see that we need three protons to form one helium atom He3 (reaction (3) and (5), therefore if we add all of the four reactions (3)–(6) and rearrange slightly, the net reaction occurring in the core of the Sun is
4p He4 + 2e + g (7)
The above scheme was essentially worked out by Hans Bethe during the late thirties and early forties.
Although, the Sun is enormous and keeps us warm, the rate of heat released by per unit weight of the matter in the Sun is far less than that by normal chemical metabolism in a human being!
Reaction in the core of the Sun produces high energy photons as gamma rays, travelling at the speed of the light. But these gamma ray photons can not escape easily. They collide with plasma and bounce forwards and backwards. At each collision the gamma ray photons loose energy and by the time these reach the photosphere they have converted into low energy photons via X-rays, Ultraviolet rays to Visible light. It may take about 170,000 years for the photon starting from the core to reach the bottom of the convective layer; therefore the visible light from the Sun that reaches us has started its life somewhat earlier.
According to the reaction (5), every time a helium atom forms in the Sun, two neutrinos are released. Since the helium atom contains two neutrons and two protons, we can re-write the reaction (5) as
4p+ (2p+ + 2n) + 2e + g (7a)
in other words, conversion of a proton to a neutron releases one neutrino. The formation of helium in the Sun is estimated to be around 1038 nuclei per second. If the sums are correct we should expect twice as many neutrinos to form in the Sun and since neutrinos do not interact with anything, these can travel unimpeded from the core of the Sun at nearly the speed of light; some of these neutrinos also travel to the Earth.
The rate of solar neutrinos passing through the Sunlit side of the Earth is expected to be around 70*1012 per square centimetre per second, and since interaction of neutrinos with other matter are negligible, the majority of these are expected to leave the Earth through the other side. And additionally these solar neutrinos take about 500 seconds (about 8 minutes) to travel to the Earth, almost instantaneous compared to the photons. So we probe the behaviour of the solar neutrino, to understand the Sun.
The source of the majority of the solar neutrons is reaction (7). It is called p – p (proton – proton) reaction. These neutrinos have a range of energies, the maximum energy of about 0.42 MeV. The other neutrino producing reactions in the Sun contributes very little to the process of energy formation in the Sun. These reactions are
He3 + H4 Be7 + radiation;
Be7 (beryllium) captures electron and releases neutrinos at two distinct energy levels
p+ + p+ + e-
produces neutrinos at a very well defined energy level.
Be7 + p+ B8 (boron), radioactive decay 2H4 + e ; this reaction again produces a neutrino with a range of energies up to about 15 MeV maximum.
All we have discussed until now forms the Standard Solar Model as the working of the Sun and it goes something like this
1) Fusion of protons, commonly called Hydrogen burning, occurs in the core of the Sun. This reaction is controlled by temperature, density and composition of at the core. There is very little mixing of material between the core and the outer layer
2) Thermal pressure at the interior of the Sun balances the gravitational collapse.
3) Radiation and convection carries energy from the deep interior to the surface and the bulk of the energy from the Sun is distributed by radiation. The composition of the outer layer controls the radiation process; analogy being transmission of light is affected by the opacity of the medium.
Neutrons produced in the Hydrogen based fusion reactors (still in the early development stage), cause severe radiation damage to the reactor materials. Helium (He3) based fusion reaction does not produce any neutrons. This makes He3 a more desirable candidate for fuel in fusion reactors, but there is hardly any naturally occurring He3 on the Earth.
He3 produced in the Sun (reaction 5), is prevented in reaching the Earth by its atmosphere. On the other hand, since the Moon has no atmosphere, Moon rocks have locked up this helium isotope in abundance. One US and one Russian enterprise are planning to start strip mining on Moon within the next couple of decades to extract He3 as a fuel for fusion reactor.
Sun, Earth and Sky, Lang, K. L, Springer, 1997
The case the Missing Neutrinos, Gribbin, J, Penguin Books, 1998