THE ORIGIN OF THE UNIVERSE

 Beyond the Milky Way

 Life and Death of Stars

 Big Bang - Big Crunch ?

We are living in the Universe's prime, long after most of the exciting things have happened. Gaze into the sky on a starry night and you will see a few thousand (about seven thousand stars.) Most of the stars that we see around us belong to a great swath called the Milky Way. This is all the ancients knew of the Universe.

Gradually, as telescopes of greater and greater size and resolution have been developed, a universe of unimagined vastness has swum into view, revealing a great cosmic drama.

Beyond the Milky Way

Our Sun is one of at least 100,000 million stars that make up the pinwheel-shaped system known as the galaxy. Our galaxy is given the special name 'Milky Way'.(The word 'galaxy' comes from the Greek 'galactos' meaning milk) It was William Herschel (1738-1822), one of the greatest observers of all time, who first revealed the structure of the galaxies. He concluded that the stars were arranged in a lens-shaped system, about five times as wide as it was thick, with the Sun approximately at the centre. He called this overall system of stars the Galaxy, and the galaxy was assumed to be all that existed in the Universe: beyond was just empty space. Later studies, specially by the American astronomers Harlow Shapley (1885-1972) and Edwin Powell Hubble (1889-1953), showed that our galaxy - The Milky Way - is not the only such star system in the Universe, but that space is filled with countless other galaxies as far as telescopes could penetrate. On current estimates, the Milky Way is about 100,000 light years in diameter, and the Sun lies about 25000 light years from the centre. ( A light year is the distance that a beam of light travels per year at a velocity of 2.99792458 x 10 ^{- 8} m/s) The centre of our Milky Way is called its nucleus. 50,000 light years from the centre lies a region known as the galactic halo, which is a sparsely populated and roughly spherical. The halo contains globular clusters and has very little matter in contrast to the disc and the central bulge.

Nothing is at rest in the Universe. The earth's average speed about the Sun is 30 km/s, whereas the whole Solar system moves through the Milky Way at about 20 km/s. It takes the Sun about 225 million years to complete one revolution about the centre of the Milky Way - the time period known as an interstellar year.

What lies outside our galaxy - The Milky Way? Astronomers had long known of the existence of certain fuzzy patches in space, called Nebulae from the Latin word meaning cloud. Some of these nebulae - for instance, the famous nebula in Orion - obviously are glowing clouds of gas within the Milky Way. But others show a distinct spiral shape, and astronomers were far less sure about the nature of these. The controversy was resolved in 1923 by Hubble when he observed the individual stars in the great spiral galaxy in Andromeda.

The stars were so faint that he realized them to be far off in the universe. It is, in fact, a separate spiral - shaped island, or galaxy, of stars like our own. According to latest measurements the Andromeda galaxy is 2.2 million light years away, so that the light entering our eyes today actually left that galaxy 2.2 million years ago, when our ape-man ancestors were roaming the plains of Africa. The Andromeda galaxy is the farthest object visible to the naked eye.

A galaxy is a family of stars held together by their mutual gravitational attraction, and with a distinct identity separating it from other galaxies. The sizes and shapes of galaxies vary. According to their shapes they can be classified as spirals, barred spirals, ellipticals and irregulars.

The Milky Way is believed to be an average spiral. From afar our milky way would look like the Andromeda spiral. Spiral galaxies have a dense core of old stars, while their outer spiral regions contain younger stars plus much dust and gas, like the orion Nebula, which still has to form into stars. Most of the clearly visible galaxies fall into this category. Elliptical galaxies, o6n the other hand, contain mainly old stars, and almost no dust or gas. For them, star formation has finished.

Astronomers once assumed that galaxies evolved from one type into another as they aged, but they now believe that all the galaxies were formed at the same time and remain the same type throughout their lives. Whether a galaxy becomes a spiral or an elliptical, probably depends on how quickly the gas cloud was spinning that gave birth to the galaxy with spiral galaxies being the fastest spinners and ellipticals the slowest.

Spiral galaxies range from about 20,000 to 100,000 light years or more in diameter, and contain from 1000 million to over 100,000 million stars. So our own milky way is among the largest spirals. Most of the remaining galaxies are ellipticals, which fall into two classes, giants and dwarfs. Giant ellipticals, which are rare, include the biggest and brightest galaxies in the universe. They can be up to several hundred thousand light years across and contain the mass of 10 million, million stars. The most massive galaxy known, the elliptical M-87 (M stands for Messier who first proposed the relevant classification system) contains 3000 billion solar masses.(About 15 times that of our own galaxy) By contrast, dwarf ellipticals are the smallest galaxies known, measuring only about 5000 light years across and containing a few million stars. They are the most abundant. The 3-dimensional shape of elliptical galaxies can be spheroidal or virtually spherical. Irregular galaxies, which have no distinct shape at all, make up a small percentage (about one fourth) of known galaxies.

A very small number of galaxies have unusual structure, often attributed to a gravitational interaction with another galaxy. Others emit exceptionally large amounts of energy and exhibit other evidence such as variability, suggesting that unusual and violent processes are at work. Such active galaxies include Seyfert galaxies and radio galaxies.

Life and Death of Stars

Let us now look at the life cycle of a star. When we look up at the stars in the sky, we get the impression that they are changeless. Although the sky we see today is not very different from the view that our ancestors had 5000 years ago, the stars do change. Like human beings, they are born; they live; and they die.

Astronomers can work out the story of a star's life by picking out stars at different stages of their lives. The theory of stellar evolution is one of the great achievements of science in the twentieth century.

Stars are born from the tenuous gas which fills the whole of space. This gas is composed mainly of hydrogen atoms, with a sprinkling of helium. In some places the gas clumps together in rather more dense interstellar gas clouds. According to gravitational theory the gas cloud's own gravity makes it attract itself, compressing the cloud to ever higher densities. The centre of the cloud should be the most compressed region. When a gas is compressed, it becomes hotter. So, the temperature at the centre rises to 10 million C - hot enough to start nuclear reactions. These reactions turn hydrogen to helium and create vast amounts of energy. As a result the condensed mass begins to shine; a star is born.

The formation of a star (the Sun) from an interstellar gas cloud is intimately connected with the birth of our planets according to some astronomers. According to this theory, a collapsing cloud of gas and dust forms a dense core (a protostar) surrounded by a disc of gas and dust known as the accretion disc. Outflows of hot gas then drive away the remains of the original cloud. As nuclear reactions begin in the protostar, it becomes a star, and the matter in the disc eventually condenses into planets orbiting the new star. The solar wind which blows at a speed of 500 km/s drives away the hot gases from the primordial atmospheres of the planets.

The formation of the solar system from an interstellar gas cloud has wide acceptability over other ideas. One such theory was that the planets were formed by the condensation of gaseous extracts from a star which passed near our sun. This idea did not gain much ground since it did not respect some of the conservation laws of physics such as the conservation of momentum.

Another theory links the formation of the planets to binary star systems. Telescopes show that many stars are not single stars but have companions. Such twin-star systems are known as double stars. In some cases the two stars lie in the same line of sight when viewed from Earth, but may actually be at vastly differing distances from us. These are optical doubles. The great majority of stellar twins are, however, physically related orbiting each other over long periods of time, and are called binary stars. In a binary star, the brighter star is known as the primary, while the fainter star is the secondary. The double nature of very close pairs of stars may only be revealed by analysis of their light in spectroscopes. These are spectroscopic binaries.

There are more complicated stellar families - triples, quadruples, and even larger groups. The maximum that has been observed so far is a system consisting of six stars.

Although many stars belong to binary systems, astronomers have failed to detect a companion for our Sun. Hence, they suggest that the planets were born out of the companion star of the Sun. Some even speculate that the planet Jupiter might be the remnant of that star. Indeed if Jupiter had a mass ten times that of its currently measured mass it would have had the capability to start a nuclear reaction of its own. According to some astronomers however, the suns companion does exist and this may well be the reason for the origin of the comet rains known to come from the hypothetical - Oort-Opik cloud (after J.H.Oort and E.Opik) every 230 million years. A popular belief holds that the incipient comets were formed near the present location of the outer planets and ejected to a much greater distance later. It may be that the Sun's companion passes through or near the Oort cloud which causes its comets to agitate.

Some attribute the comet rain from the Oort-Opik cloud to be the reason for the instant extinction of dinosaurs.

Let us return to our discussion on the life of the stars. Ordinary telescopes cannot actually show us stars being born in the interstellar gas clouds. The reason is dust. In the denser clouds of gas, where the dust is more concentrated, dust particles absorb light passing through the cloud. As a result, we can see the clouds as dark silhouettes against a background of distant stars. To overcome this problem astronomers have built infrared telescopes which pick infrared radiation instead of visible light. The dust particles in space do not absorb infrared radiation and the infrared astronomical satellite (IRAS) found thousands of young stars hidden deep within the interstellar clonds. More elaborate methods exist to detect stars which are hidden from the ones that are closer to us and also in line of sight. The use of gravitational lenses, allows to detect such objects through the bending of their light near the gravitational field of massive objects similar to the bending of light rays by refraction when they pass through a glass lens.Generally, double or multiple images are observed of quasars (these are supposed to be nucleii of younger galaxies) whose light passes through intervening galaxies before reaching us.

Once the star is shining, it produces a powerful 'wind' of hot gas that forces its way outwards in opposite directions, above and below the accretion disc. The wind drives away most of the original gas cloud that hides the star from view. They light up the final tatters of gas from the original cloud, making it glow as a bright nebula. Nebulae, each surrounding a 'nursery' of young stars, form some of the most beautiful sights in the sky. Most famous is the Orion nebula in the constellation of Orion.

The horse head nebula in the same constellation is dark since it does not give birth to stars.

When a star is born it is a ball of hot gases, composed mainly of hydrogen. To this extent, all newborn stars are the same. The main thing that marks out one star from another is its mass. The mass of a star is fixed at its birth and it determines both a star's lifetime and its ultimate fate.

The stars in our skies have a bewildering range of properties: there are giant stars and dwarf stars; bright stars and dim stars; hot stars and cool stars. In 1914 the Danish astronomer, Ejnar Hertzsprung (1873 - 1967) and an American, Henry Norris Russell (1877 - 1957) found the most useful kind of graph in classifying the stars now known as Hertzsprung Russell diagram,or H-R diagram. On an H - R diagram, the vertical axis represents the luminosity or brilliance of a star. The horizontal axis represents the star's temperature. In an H - R diagram most of the stars occupy a narrow strip on the graph running from top left to lower right. This is known as the 'main sequence'. These are the stars that derive their energy by turning hydrogen into helium. Astronomers categorize the stars of the main sequence according to their surface temperatures (spectral type) as O, B, A, F, G, K, M ( remembered as Oh Be A Fine Girl Kiss Me!) Where O stars are the hottest and M stars are the coolest. Red stars are the coolest; orange and yellow stars are hotter; while white and blue-white stars are the hottest of all.

Our Sun is a very typical star, currently in the prime of its life (aged 4.6 billion years). Nuclear reactions run fastest in the heaviest stars, because their centres are the hottest and most compressed. So the heavier stars are the brighter stars, with hotter surfaces. Light weight stars have a surface temperature of about 3000 ºC. The Sun is in the middle of the main sequence ( the sun is a G2 star ) with a surface temperature of 6000 ºC, and an internal temperature of million ºC. The heavyweight stars shining as brightly as 100,000 suns, has a surface temperature of 30.000 ºC or more.

As mentioned above the life-span of a star depends very critically on how heavy the star is. Taking the sun's lifetime as 10 billion years the heaviest stars survive for only one thousandth of this time whereas the very light weight stars can last for 100 times longer than the sun.

When a star starts to run out of hydrogen at its core, having turned it all to helium, the hydrogen burning moves out into the surrounding zone. When this happens, the star gets hotter inside, and the result of this extra energy release is that the star swells up in size. As it swells, its surface temperature drops so that it becomes red in colour. The star has become a red giant.

A red giant can be as much as 100 times the size of the present sun. When our sun reaches this stage in about 5 billion years from now, it will engulf the Earth, thereby ending all life on our planet. Compared with the main sequence stars, red giant stars are not very common. However, because they are red and appear bright, they stand out conspicuously in our skies. The most famous is Betelgeuse in the constellation Orion. A red giant finds it difficult to hold on to its huge outer regions. The star becomes unstable and eventually the outer gas drifts off in to space. Before completely disappearing the gas forms a bubble around the dying star - the effect is like a glowing smoke ring in space. Astronomers call these bubbles planetary nebulae, because they look rather like a planet when observed from a small telescope. After the star's outer regions have disappeared, the tiny, very hot core remain. It is only one hundredth the diameter of the Sun - no larger than the Earth, and is so hot that it shines whitehot. This is called a white dwarf.

A white dwarf no longer produces any energy. As time passes, it gradually cools down, fading through yellow, orange and red, until it fades from sight altogether. This is the ultimate fate of the Sun.

White dwarfs can be detected when they are a companion to another star, through a phenomenon known as a nova. Novae are thought to be close binary systems (one companion is a white dwarf) in which gas flows from one star to the white dwarf and ignites in a nuclear eruption, causing the sudden surge in brightness and throwing off a shell of gas. This sudden increase in brightness was once thought to be new stars, hence the name nova which means 'new'.

White dwarfs are formed from stars which have masses less than 1.4 times that of the Sun. The limit of 1.4 is known as the Chandrasekhar limit after the Indian physicist Subrahmanyan Chandrasekhar (1911 - 1995). It has been found that only about 2.5% of the stars have solar masses greater than that of 1.4 .

Stars with more than about 3.2 solar masses die a spectacular death. They turn into red supergiants, larger and brighter even than red giants, and then a series of run-away nuclear reactions sets in at their core. In the middle of such a massive star, the pressure and temperature keep on rising until helium atoms begin to fuse into a heavier element like carbon. Eventually, the increasing temperature and pressure force the carbon to change to even heavier elements, such as neon, silicon and iron. But the process cannot carry on indefinitely and the star's centre collapses entirely. The shock wave of the collapsing core blows the star apart, in a gigantic nuclear holocaust known as a Supernova.

Some stars blow themselves completely to bits in a supernova. But in many cases the heavy core of the dead star remains as an object even smaller, denser, and more amazing than a white dwarf. The dense, heavy stellar core left behind by a supernova collapses under the inward pull of its own gravity and the force of the explosion above it crushing together the protons and electrons of its atoms to form the neutrons. The object becomes a neutron star. A thimbleful of neutron star material would weigh a staggering 1000 million tonnes. It has been found that a neutron star can be formed by a star having a mass of 1.4 - 3.2 solar masses without a supernova.

The existence of neutron stars were first predicted theoretically in 1939 by Fritz Zwicky and Walter Baade. In 1969, two radio astronomers Tony Hewish and Jocelyn Bell, picked up regular signals coming from the sky. They realized that the signals must have come from a cosmic lighthouse that was emitting beams of radio waves. These objects were termed pulsars although we now know that both the pulsars and the neutron stars are the same since only neutron stars are small enough to spin once a second or less.

In 1968, a pulsor was discovered at the centre of the crab nebula. The crab pulsar flashes at the rate of 30 times per second, one of the fastest pulsars known. The flashes of pulsars may be in the region radio, visible, x-ray or even in gamma ray wavelengths.

Supernovae do not only represent death and destruction. The blast from a supernova sweeps up the gases in space, compressing them into dense clouds. These gas clouds give birth to new stars and planets, as mentioned earlier. Thus the death of a star as a supernova can trigger off the birth of a new generation of stars as well as life forms.

When a star dies - as a planetary nebula or a supernova - it seeds space with the new elements that it has created during its lifetime (-elements such as carbon, iron,gold and even uranium and other radioactive elements). Astronomers believe that in the very early Universe, the gases consisted almost entirely of Hydrogen and Helium. Dying stars have formed all the other elements and several generations of birth and death may predoce the necessary ingredients to trigger complex life on a suitable planet. In this regard our Sun is considered as a third generation star, since we on earth are supposed to be an intelligent life form! In other words we all consist of a little bit of stardust.

What if the remnant core of the supernova has a mass more than 3.2 times that of the Sun? If the collapsing core of a supernova is too massive, it cannot end up as a neutron star. Its own gravity is so powerful that the core continues to shrink, until it becomes a mathematical point, (a singularity) with no size at all and infinite density. Surrounding this point is a region a few kilometers across where gravity is so strong that nothing can escape- not even light. A black hole is formed. It is 'black' because it does not let light escape. It is a 'hole' because anything that you throw into it can never emerge again.

The existence of black holes was first predicted in 1796 by Pierre Simon de Laplace who called them corps obscurs. Armed with Albert Einstein's (1879-1955) general relativity modern physicists concluded that they do exist; the name black hole being coined by the physicist John Wheeler. Consider the light radiated from the surface of a neutron star. As the surface gravity increases as the star collapses¸ the deflection of light emitted normal to its surface increases. Eventually, the star reaches a size at which a horizontal beam of light enters a circular orbit. ( If an object is thrown horizontally at the speed of about 7900 ms -1, just above the surface of the earth, it will circulate the earth indefinitely trapped in the earth's gravitational field. No fuel needed for the process!). A surface of that radius is called the photon sphere. When the radius of the star is about two thirds that of the photon sphere, no light can escape at all. At this point, the velocity of escape from the star (the escape velocity) equals the speed of light. (The escape velocity of earth is about 11.2 km/s) As the star contracts still more, light and everything else is trapped inside, unable to escape through that surface where the escape velocity is the speed of light. That surface is called the event horizon, and its radius is the Schwarzschild radius, named for Karl Schwarzschild, who first described the situation a few years after Einstein introduced general relativity. It is this surface that is the boundary of the black hole. All that is inside is hidden forever from us; as the star shrinks through the black hole, it literally 'disappears' from the Universe.

If the Sun is squeezed to a size of 2.9 km in radius, it will become a black hole. (the radius of the Sun is 696,000 km.) But it will never end up as a black hole since it does not have enough mass. For the earth to become a black hole, it would have to be compressed to a radius of only 1 cm. Stars with a solar mass of about ten times that of the Sun will become black holes if they are compressed to a radius of 29 km and this is a possibility since they have enough mass to shrink to this size in contrast to our sun.

If black holes are invisible how can we detect them? Fortunately, they give their existence away by swallowing gas from the space around them. It has been mentioned that many stars are double. In cases where one star has died and formed a black hole, it may continue to orbit a companion star that is still shining normally. Gas from the companion star streams into the intense gravitational field of the black hole, heating up to many millions of degrees.

At such temperatures, the gas emits x-rays which can be detected by observations satellites. A possible candidate is in the constellation Cygnus. The powerful source of x-rays there is named as Cyg x-1. Astronomers have found a star at this point in the sky. The star itself is quite ordinary, and cannot be producing the x-rays. But it is not on its own. It is swinging around a companion star that is invisible in ordinary telescopes. By observing the visible star carefully, astronomers found that its invisible companion was exerting the gravitational pull of an object as heavy as ten Suns. This is much too heavy to be a neutron star, and so the only possibility is that it is a black hole.

Though it is mentioned earlier that black holes are invisible, ( in the sense that they attract even light) the British physicist Stephen Hawking (1942- ) had suggested that the black holes ain't so "black". The phenomenon now known as Hawking Radiation is rather involved and we will skip the discussion about it. However, the basic idea of Hawking radiation is that if an electron and a positron ( the positron is the anti-particle of the electron and has the exact properties of the electron but differs only in sign of the charge. Every particle has its corresponding anti-particle and these are collectively known as anti- matter.) come into existence momentarily in the vicinity of the black hole (event horizon) there is a chance that one or the other will fall into the hole and hence not be able to annihilate with its anti-particle. Its anti-particle therefore, can escape unscathed. But many such positrons and electrons so created near black holes and escaping from them, do annihilate each other, creating energy. That energy cannot come from nothing; according to Hawking's Theory, it must come from the black hole itself. Robbing the black hole of energy in this way, robs it of mass (according to E = mc²), so the black hole must slowly evaporate through this process of pair production. Hawking radiation is not yet observed although the speculation remains an interesting possibility.

Big Bang - Big Crunch ?

Edwin Hubble announced in 1929, that the galaxies seem to be moving apart from each other at speeds that increased with distance. The relation was that the speed (v) between two galaxies is directly proportional to the distance (d) between them.

ie., v a d

v = H d

where H is known as the Hubble constant. According to him the Universe is like a balloon being inflated. The effect that Hubble observed is known as the Gravitational Red Shift, because as the stars are receding from us, their light is shifted towards longer wavelengths (Doppler Effect).

The Hubble constant is a measure of how fast the Universe is expanding, and its value is expected to lie in the region 40 - 100 km/s (mega parsec per second) (1 parsec = 3.26 light years). The precise value is yet to be determined.

What is the cause of this running away of clusters of galaxies from each other, or simply what is the cause of this expansion? The Belgian cosmologist, Georges LeMaitre (1894-1966) proposed that once upon a time the Universe was compressed into a small, super dense blob which for some unknown reason exploded. The explosion termed the Big Bang, marked the origin of the Universe as we know it, and the components of this Universe have been rushing apart ever since. The galaxies are fragments flung outwards in all directions from the explosion.

If we could determine the exact value of H 0, then 1/H 0 is the time since the Big Bang and the current calculation show the Universe to be about 15 billion years old.

According to the Big Bang Theory physics starts after 10^-43 seconds from explosion. The temperature is estimated to be above 1032 K at this point. Before this time, the laws of physics breaks and it is assumed that the fundamental forces of nature, the strong force, the elctromagnetic force, the weak force and the gravitational force were all unified before this time. At 10^-43 secs. after the explosion the diameter of our Universe was 10^-28 cm.

Finely tuned course was set by 10^-32 seconds after the Big Bang. Much later - when it was a millionth of a second old, the Universe had cooled sufficiently for quarks to clump into protons and neutrons. At about one second, a ghostly particle called a neutrino broke free. It is these neutrinos many scientists believe are responsible for the hidden mass of the Universe.

Three minutes after the Big Bang, the temperature of the expanding Universe had dropped to one billion degrees. Protons and neutrons could then clump to form atomic nucleii. Hydrogen and Helium nucleii appeared. After 100,000 years, the temparature had dropped to 3000 degrees. Electrons bound with the nucleii, creating full-fledged atoms.

Turning back to the hidden mass, astronomers believe that at least 90% and possibly 99% of all the matter in the Universe is completely invisible. Astronomers call this invisible stuff 'dark matter'. What constitutes dark matter? This is one of the greatest puzzles of astronomy today, and two possible candidates are neutrinos and WIMPS (Weakly Interacting Massive Particles). A discussion of these particles and their behaviour is rather involved and we will leave the subject of dark matter and related topics in particle physics for a discussion on future date.

Opposed to the big bang theory Thomas Gold (1920 -), Hermann Bondi (1919 -) and Fred Hoyle (1915 -) proposed that the universe has always existed in much the same state as today, and that there was no single instant of creation. Instead, they said, new matter is being slowly created everywhere to fill space as the universe expands. This is known as the steady state theory.

The steady state theory received a blow in 1965 when the American physicists Arno Penzias (1933 -) and Robert Wilson (1936 -) detected what is known as the 'echo' of the big bang. The 'echo' is known as the cosmic background radiation which is now in the microwave region. The reason for this radiation is that space is not entirely cold, but has a temperature two or three degrees above absolute zero (ie. about -270 C). This slight warmth pervading the universe is interpreted as being energy left over from the intense fireball of the big bang, and its existence is completely inexplicable on the basis of the steady state theory.

Although now it is accepted that the universe is expanding, there's no fixed background space into which the universe can expand. The universe contains all the space there is! The problem is whether the state of expansion we witness in the universe will continue indefinitely. For an ever expanding universe the launch speed of the material must exceed the critical launch value (much like the escape velocity) at the start. The gravitational pull of all the material in such a universe will not be able to halt the expansion, and will keep expanding forever. On the other hand if the speed is less than the critical value, eventually the expansion will halt and reverse, culminating in a contraction back to zero size, known as the big crunch, the very same state in which it apparently began. A big bang may again follow this big crunch leading to an oscillating universe.

In between, there exist the speed which is exactly the critical launch speed. That is, the smallest value that will keep it expanding forever. One of the great mysteries about our universe is that it is currently expanding tantalizingly close to this critical case. So close infact, that we cannot yet say for sure on which side of the critical divide it lies. The amount of dark matter plays a crucial role in determining the fate of the universe and this is why the research on neutrinos and WIMPs have been more important. However for now we do not know what the long-range forecast is.

We discussed earlier how the elements formed in the stars may trigger complex life forms after several generations of Supernovae.Through these processes an untold number of Sun-centered planetary systems - perhaps much like our own - may have formed. Of the 100,000 million stars in the Milky Way perhaps no more than one out of every hundred would have a planet in orbit suitable to give birth for life. It has been estimated that only one star in every 18,000 has the potential for supporting a planet similar to earth.

The search for Extra Terrestrial Intelligence is one of the most thouroughly engaged human projects ever undertaken. Although we have developed incredibly complex and efficient communication systems on Earth, a civilization hundreds - or millions - of years more advanced may have devised systems vastly more efficient. Indeed, their messages could now be reaching earth in a form unfamiliar to us.

Radio wave transmissions now seem the most likely method of interstellar communication. But the radio portion of the electromagnetic spectrum covers a broad range of wave lengths. Which frequency would advanced civilizations most likely choose for broadcasting?

There are several ' natural ' frequencies, including that at 1,420 MHz. This frequency is emitted by a Hydrogen atom when the electron in orbit around the atom's nucleus reverses its spin (spin-flip transition). Most astronomers believe that this frequency may be a fundamental frequency for interstellar communication and that most advanced civilizations must have thought about using this frequency.

The Pioneer 10 and 11 spacecrafts, both used to explore the outer planets of the Solar

System, carry plaques bearing messages for unknown recipients somewhere in space.

The plaques are identical. Each six inch by nine inch gold coated plate is engraved with the time and date of launch, earth's cosmic location, and a graphic representation of humans.

In the upper left hand corner, there is a representation of the spin-flip transition of Hydrogen that can be translated into a unit of measurement for time and distance.

Below the Hydrogen engraving, 14 pulsars are indicated, with their rates of pulsation. The pulsars are arranged on the plaque around a central point - our Sun - with their distances from that point specified. The figures of the man and woman are shown in scale with a representation of the Pioneer spacecraft. A diagram of our Solar System showing the course travelled by Pioneer through the planets is engraved along the bottom of the plaque. Very possibly, the plaques on either Pioneer 10 or 11 will not be deciphered for millions of years. By then, there may be no earthlings left to take credit for their creation.

Voyager 1 and 2 each have a long-playing record that plays the 'sounds of earth' including greetings in many languages (including Sinhala.), with pictures also encoded into its grooves.

Some scientists have said we should not be trying to contact other civilizations. If we succeed, and 'they' are much more advanced than us, then our culture might be overwhelmed just as the native cultures of South America disappeared under the onslaught of European culture.

If we do receive a message from a far-off world, what might it say? Astronomer Carl Sagan believes our first communication may contain information about how to prevent earth's destruction by runaway technology. But the physicist Freeman Dyson wants instead, that any civilization contacting us may well be itself 'a technology run wild' looking for new worlds to conquer. Since all creatures in the Universe must face death, when the life-giving 'Sun' runs out of fuel and dies, unless they can escape to another, more healthy solar system perhaps, the first interstellar message we receive will contain neither advice nor threats. It may instead, contain a cry for help; the last goop of a dying civilization, received too late.

The study of the origins of the Universe is fascinating, and poses a number of deep questions. It is doubtful whether we could ever resolve this grand enigma. And as we try to construct the history of the Universe, searching for the fossil remnants of its youth and adolescence, we find that by the coming together of the largest and the smallest as specks of the physical world, our appreciation of the unity of the Universe becomes more impressive and complete.