Universe

Origin of Universe :-

According to the Boshongo people of central Africa, in the beginning, there was only darkness, water, and the great god Bumba. One day Bumba, in pain from a stomach ache, vomited up the sun. The sun dried up some of the water, leaving land. Still in pain, Bumba vomited up the moon, the stars, and then some animals. The leopard, the crocodile, the turtle, and finally, man.This creation myth, like many others, tries to answer the questions we all ask. Why are we here? Where did we come from? The answer generally given was that humans were of comparatively recent origin, because it must have been obvious, even at early times, that the human race was improving in knowledge and technology. So it can't have been around that long, or it would have progressed even more. For example, according to Bishop Usher, the Book of Genesis placed the creation of the world at 9 in the morning on October the 27th, 4,004 BC. On the other hand, the physical surroundings, like mountains and rivers, change very little in a human lifetime. They were therefore thought to be a constant background, and either to have existed forever as an empty landscape, or to have been created at the same time as the humans. Not everyone, however, was happy with the idea that the universe had a beginning.

For example, Aristotle, the most famous of the Greek philosophers, believed the universe had existed forever. Something eternal is more perfect than something created. He suggested the reason we see progress was that floods, or other natural disasters, had repeatedly set civilization back to the beginning. The motivation for believing in an eternal universe was the desire to avoid invoking divine intervention to create the universe and set it going. Conversely, those who believed the universe had a beginning, used it as an argument for the existence of God as the first cause, or prime mover, of the universe.If one believed that the universe had a beginning, the obvious question was what happened before the beginning? What was God doing before He made the world? Was He preparing Hell for people who asked such questions? The problem of whether or not the universe had a beginning was a great concern to the German philosopher, Immanuel Kant. He felt there were logical contradictions, or antimonies, either way. If the universe had a beginning, why did it wait an infinite time before it began? He called that the thesis. On the other hand, if the universe had existed for ever, why did it take an infinite time to reach the present stage? He called that the antithesis. Both the thesis and the antithesis depended on Kant's assumption, along with almost everyone else, that time was Absolute. That is to say, it went from the infinite past to the infinite future, independently of any universe that might or might not exist in this background. This is still the picture in the mind of many scientists today.

However in 1915, Einstein introduced his revolutionary General Theory of Relativity. In this, space and time were no longer Absolute, no longer a fixed background to events. Instead, they were dynamical quantities that were shaped by the matter and energy in the universe. They were defined only within the universe, so it made no sense to talk of a time before the universe began. It would be like asking for a point south of the South Pole. It is not defined. If the universe was essentially unchanging in time, as was generally assumed before the 1920s, there would be no reason that time should not be defined arbitrarily far back. Any so-called beginning of the universe would be artificial, in the sense that one could extend the history back to earlier times. Thus it might be that the universe was created last year, but with all the memories and physical evidence, to look like it was much older. This raises deep philosophical questions about the meaning of existence. I shall deal with these by adopting what is called, the positivist approach. In this, the idea is that we interpret the input from our senses in terms of a model we make of the world. One can not ask whether the model represents reality, only whether it works. A model is a good model if first it interprets a wide range of observations, in terms of a simple and elegant model. And second, if the model makes definite predictions that can be tested and possibly falsified by observation.

In terms of the positivist approach, one can compare two models of the universe. One in which the universe was created last year and one in which the universe existed much longer. The Model in which the universe existed for longer than a year can explain things like identical twins that have a common cause more than a year ago. On the other hand, the model in which the universe was created last year cannot explain such events. So the first model is better. One can not ask whether the universe really existed before a year ago or just appeared to. In the positivist approach, they are the same. In an unchanging universe, there would be no natural starting point. The situation changed radically however, when Edwin Hubble began to make observations with the hundred inch telescope on Mount Wilson, in the 1920s.

Hubble found that stars are not uniformly distributed throughout space, but are gathered together in vast collections called galaxies. By measuring the light from galaxies, Hubble could determine their velocities. He was expecting that as many galaxies would be moving towards us as were moving away. This is what one would have in a universe that was unchanging with time. But to his surprise, Hubble found that nearly all the galaxies were moving away from us. Moreover, the further galaxies were from us, the faster they were moving away. The universe was not unchanging with time as everyone had thought previously. It was expanding. The distance between distant galaxies was increasing with time.The expansion of the universe was one of the most important intellectual discoveries of the 20th century, or of any century. It transformed the debate about whether the universe had a beginning. If galaxies are moving apart now, they must have been closer together in the past. If their speed had been constant, they would all have been on top of one another about 15 billion years ago. Was this the beginning of the universe? Many scientists were still unhappy with the universe having a beginning because it seemed to imply that physics broke down. One would have to invoke an outside agency, which for convenience, one can call God, to determine how the universe began. They therefore advanced theories in which the universe was expanding at the present time, but didn't have a beginning. One was the Steady State theory, proposed by Bondi, Gold, and Hoyle in 1948.

In the Steady State theory, as galaxies moved apart, the idea was that new galaxies would form from matter that was supposed to be continually being created throughout space. The universe would have existed for ever and would have looked the same at all times. This last property had the great virtue, from a positivist point of view, of being a definite prediction that could be tested by observation. The Cambridge radio astronomy group, under Martin Ryle, did a survey of weak radio sources in the early 1960s. These were distributed fairly uniformly across the sky, indicating that most of the sources lay outside our galaxy. The weaker sources would be further away, on average. The Steady State theory predicted the shape of the graph of the number of sources against source strength. But the observations showed more faint sources than predicted, indicating that the density sources were higher in the past. This was contrary to the basic assumption of the Steady State theory, that everything was constant in time. For this, and other reasons, the Steady State theory was abandoned.

Another attempt to avoid the universe having a beginning was the suggestion that there was a previous contracting phase, but because of rotation and local irregularities, the matter would not all fall to the same point. Instead, different parts of the matter would miss each other, and the universe would expand again with the density remaining finite. Two Russians, Lifshitz and Khalatnikov, actually claimed to have proved, that a general contraction without exact symmetry would always lead to a bounce with the density remaining finite. This result was very convenient for Marxist Leninist dialectical materialism, because it avoided awkward questions about the creation of the universe. It therefore became an article of faith for Soviet scientists.

When Lifshitz and Khalatnikov published their claim, I was a 21 year old research student looking for something to complete my PhD thesis. I didn't believe their so-called proof, and set out with Roger Penrose to develop new mathematical techniques to study the question. We showed that the universe couldn't bounce. If Einstein's General Theory of Relativity is correct, there will be a singularity, a point of infinite density and spacetime curvature, where time has a beginning. Observational evidence to confirm the idea that the universe had a very dense beginning came in October 1965, a few months after my first singularity result, with the discovery of a faint background of microwaves throughout space. These microwaves are the same as those in your microwave oven, but very much less powerful. They would heat your pizza only to minus 271 point 3 degrees centigrade, not much good for defrosting the pizza, let alone cooking it. You can actually observe these microwaves yourself. Set your television to an empty channel. A few percent of the snow you see on the screen will be caused by this background of microwaves. The only reasonable interpretation of the background is that it is radiation left over from an early very hot and dense state. As the universe expanded, the radiation would have cooled until it is just the faint remnant we observe today.

Although the singularity theorems of Penrose and myself, predicted that the universe had a beginning, they didn't say how it had begun. The equations of General Relativity would break down at the singularity. Thus Einstein's theory cannot predict how the universe will begin, but only how it will evolve once it has begun. There are two attitudes one can take to the results of Penrose and myself. One is to that God chose how the universe began for reasons we could not understand. This was the view of Pope John Paul. At a conference on cosmology in the Vatican, the Pope told the delegates that it was OK to study the universe after it began, but they should not inquire into the beginning itself, because that was the moment of creation, and the work of God. I was glad he didn't realize I had presented a paper at the conference suggesting how the universe began. I didn't fancy the thought of being handed over to the Inquisition, like Galileo.

The other interpretation of our results, which is favored by most scientists, is that it indicates that the General Theory of Relativity breaks down in the very strong gravitational fields in the early universe. It has to be replaced by a more complete theory. One would expect this anyway, because General Relativity does not take account of the small scale structure of matter, which is governed by quantum theory. This does not matter normally, because the scale of the universe is enormous compared to the microscopic scales of quantum theory. But when the universe is the Planck size, a billion trillion trillionth of a centimeter, the two scales are the same, and quantum theory has to be taken into account.In order to understand the Origin of the universe, we need to combine the General Theory of Relativity with quantum theory. The best way of doing so seems to be to use Feynman's idea of a sum over histories. Richard Feynman was a colorful character, who played the bongo drums in a strip joint in Pasadena, and was a brilliant physicist at the

California Institute

of Technology. He proposed that a system got from a state A, to a state B, by every possible path or history. Each path or history has a certain amplitude or intensity, and the probability of the system going from A- to B, is given by adding up the amplitudes for each path. There will be a history in which the moon is made of blue cheese, but the amplitude is low, which is bad news for mice.

The probability for a state of the universe at the present time is given by adding up the amplitudes for all the histories that end with that state. But how did the histories start? This is the Origin question in another guise. Does it require a Creator to decree how the universe began? Or is the initial state of the universe, determined by a law of science? In fact, this question would arise even if the histories of the universe went back to the infinite past. But it is more immediate if the universe began only 15 billion years ago. The problem of what happens at the beginning of time is a bit like the question of what happened at the edge of the world, when people thought the world was flat. Is the world a flat plate with the sea pouring over the edge? I have tested this experimentally. I have been round the world, and I have not fallen off. As we all know, the problem of what happens at the edge of the world was solved when people realized that the world was not a flat plate, but a curved surface. Time however, seemed to be different. It appeared to be separate from space, and to be like a model railway track. If it had a beginning, there would have to be someone to set the trains going. Einstein's General Theory of Relativity unified time and space as spacetime, but time was still different from space and was like a corridor, which either had a beginning and end, or went on forever. However, when one combines General Relativity with Quantum Theory, Jim Hartle and I realized that time can behave like another direction in space under extreme conditions. This means one can get rid of the problem of time having a beginning, in a similar way in which we got rid of the edge of the world. Suppose the beginning of the universe was like the South Pole of the earth, with degrees of latitude playing the role of time. The universe would start as a point at the South Pole. As one moves north, the circles of constant latitude, representing the size of the universe, would expand. To ask what happened before the beginning of the universe would become a meaningless question, because there is nothing south of the South Pole.

Time, as measured in degrees of latitude, would have a beginning at the South Pole, but the South Pole is much like any other point, at least so I have been told. I have been to Antarctica, but not to the South Pole. The same laws of Nature hold at the South Pole as in other places. This would remove the age-old objection to the universe having a beginning; that it would be a place where the normal laws broke down. The beginning of the universe would be governed by the laws of science. The picture Jim Hartle and I developed of the spontaneous quantum creation of the universe would be a bit like the formation of bubbles of steam in boiling water.

The idea is that the most probable histories of the universe would be like the surfaces of the bubbles. Many small bubbles would appear, and then disappear again. These would correspond to mini universes that would expand but would collapse again while still of microscopic size. They are possible alternative universes but they are not of much interest since they do not last long enough to develop galaxies and stars, let alone intelligent life. A few of the little bubbles, however, grow to a certain size at which they are safe from recollapse. They will continue to expand at an ever increasing rate, and will form the bubbles we see. They will correspond to universes that would start off expanding at an ever increasing rate. This is called inflation, like the way prices go up every year.

The world record for inflation was in Germany after the First World War. Prices rose by a factor of ten million in a period of 18 months. But that was nothing compared to inflation in the early universe. The universe expanded by a factor of million trillion trillion in a tiny fraction of a second. Unlike inflation in prices, inflation in the early universe was a very good thing. It produced a very large and uniform universe, just as we observe. However, it would not be completely uniform. In the sum over histories, histories that are very slightly irregular will have almost as high probabilities as the completely uniform and regular history. The theory therefore predicts that the early universe is likely to be slightly non-uniform. These irregularities would produce small variations in the intensity of the microwave background from different directions. The microwave background has been observed by the Map satellite, and was found to have exactly the kind of variations predicted. So we know we are on the right lines.

The irregularities in the early universe will mean that some regions will have slightly higher density than others. The gravitational attraction of the extra density will slow the expansion of the region, and can eventually cause the region to collapse to form galaxies and stars. So look well at the map of the microwave sky. It is the blue print for all the structure in the universe. We are the product of quantum fluctuations in the very early universe. God really does play dice.

We have made tremendous progress in cosmology in the last hundred years. The General Theory of Relativity and the discovery of the expansion of the universe shattered the old picture of an ever existing and ever lasting universe. Instead, general relativity predicted that the universe, and time itself, would begin in the big bang. It also predicted that time would come to an end in black holes. The discovery of the cosmic microwave background and observations of black holes support these conclusions. This is a profound change in our picture of the universe and of reality itself. Although the General Theory of Relativity predicted that the universe must have come from a period of high curvature in the past, it could not predict how the universe would emerge from the big bang. Thus general relativity on its own cannot answer the central question in cosmology: Why is the universe the way it is? However, if general relativity is combined with quantum theory, it may be possible to predict how the universe would start. It would initially expand at an ever increasing rate.

During this so called inflationary period, the marriage of the two theories predicted that small fluctuations would develop and lead to the formation of galaxies, stars, and all the other structure in the universe. This is confirmed by observations of small non uniformities in the cosmic microwave background, with exactly the predicted properties. So it seems we are on our way to understanding the origin of the universe, though much more work will be needed. A new window on the very early universe will be opened when we can detect gravitational waves by accurately measuring the distances between space craft. Gravitational waves propagate freely to us from earliest times, unimpeded by any intervening material. By contrast, light is scattered many times by free electrons. The scattering goes on until the electrons freeze out, after 300,000 years.

Despite having had some great successes, not everything is solved. We do not yet have a good theoretical understanding of the observations that the expansion of the universe is accelerating again, after a long period of slowing down. Without such an understanding, we cannot be sure of the future of the universe. Will it continue to expand forever? Is inflation a law of Nature? Or will the universe eventually collapse again? New observational results and theoretical advances are coming in rapidly. Cosmology is a very exciting and active subject. We are getting close to answering the age old questions. Why are we here? Where did we come from?

Age and size of the universe :

Astronomers use the distance to galaxies and the speed of light to calculate that the light has been traveling for billions of years. The expansion of the universe gives an age for the universe as a whole: 13.7 billion years old.The Universe is a vast, seemingly unending marvel of existence. Over the past century, we’ve learned that the Universe stretches out beyond the billions of stars in our Milky Way, out across billions of light years, containing close to a trillion galaxies all told.The best estimate of the age of the universe as of 2013 is 13.798 ± 0.037 billion years but due to the expansion of space humans are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological proper distance, which is equal to the comoving distance at the present time) than a static 13.8 billion light-years distance.It is estimated that the diameter of the observable universe is about 28 billion parsecs (93 billion light-years),putting the edge of the observable universe at about 46–47 billion light-years away.

Chronology of the universe :

This chronology of the universe describes the history and future of the universe according to Big Bang cosmology, the prevailing scientific model of how the universe .developed over time from the Planck epoch, using the cosmological time parameter of comoving coordinates. The instant in which the universe is thought to have begun rapidly expanding from a singularity is known as the Big Bang. As of 2013, this expansion is estimated to have begun 13.798 ± 0.037 billion years ago.It is convenient to divide the evolution of the universe so far into three phases.The very earliest universe was so hot, or energetic, that initially no (matter) particles existed or could exist (except perhaps in the most fleeting sense), and the forces we see around us today were believed to be merged into one unified force. Space-time itself expanded during an inflationary epoch due to the immensity of the energies involved. Gradually the immense energies cooled – still to a temperature inconceivably hot compared to any we see around us now, but sufficiently to allow

forces to gradually undergo symmetry breaking, a kind of repeated condensation from one status quo to another, leading finally to the separation of the strong force from the electroweak force and the first particles.History of the Universe - gravitational waves are hypothesized to arise from cosmic inflation, a faster-than-light expansion just after the Big Bang (17 March 2014).

In the second phase, this quark–gluon plasma universe then cooled further, the current fundamental forces we know take their present forms through further symmetry breaking – notably the breaking of electroweak symmetry – and the full range of complex and composite particles we see around us today became possible, leading to a gravitationally dominated universe, the first neutral atoms

(~ 80% hydrogen), and the cosmic microwave background radiation we can detect today. Modern high energy

particle physics theories are satisfactory at these energy levels, and so physicists believe they have a good understanding of this and subsequent development of the fundamental universe around us. Because of these changes, space had also become largely transparent to light and other electromagnetic energy, rather than "foggy", by the end of this phase.

The third phase started after a short dark age with a universe whose fundamental particles and forces were as we know them, and witnessed the emergence of large scale stable structures, such as the earliest stars, quasars, galaxies, clusters of galaxies and superclusters, and the development of these to create the kind of universe we see today. Some researchers call the development of all this physical structure over billions of years "cosmic evolution". Other, more interdisciplinary, researchers refer to "cosmic evolution" as the entire scenario of growing complexity from big bang to humankind, thereby incorporating biology and culture into a grand unified view of all complex systems in the universe to date.

Beyond the present day, scientists anticipate that the Earth will cease to be able to support life in about a billion years, and will be drawn into the Sun in about 5 billion years. On a far longer timescale, the Stelliferous Era will end as stars eventually die and fewer are born to replace them, leading to a darkening universe. Various theories suggest a number of subsequent possibilities. If particles such as protons are unstable then eventually matter may evaporate into low level energy in a kind of entropy related heat death. Alternatively the universe may collapse in a big crunch, although current data shows the rate of expansion is still increasing. If this is correct then it may end in a "big freeze" as matter and energy become very thinly spread and cool down. Alternative suggestions include a false vacuum catastrophe or a Big Rip as possible ends to the universe.

History of the Big Bang theory

One of the most persistently asked questions has been: How was the universe created? Many once believed that the universe had no beginning or end and was truly infinite. Through the inception of the Big Bang theory, however,no longer could the universe be considered infinite. The universe was forced to take on the properties of a finite phenomenon, possessing a history and a beginning.

About 15 billion years ago a tremendous explosion started the expansion of the universe. This explosion is known as the Big Bang. At the point of this event all of the matter and energy of space was contained at one point. What exisisted prior to this event is completely unknown and is a matter of pure speculation. This occurance was not a conventional explosion but rather an event filling all of space with all of the particles of the embryonic universe rushing away from each other. The Big Bang actually consisted of an explosion of space within itself unlike an explosion of a bomb were fragments are thrown outward. The galaxies were not all clumped together,

but rather the Big Bang lay the foundations for the universe. The origin of the Big Bang theory can be credited to Edwin Hubble. Hubble made the observation that the universe is continuously expanding. He discovered that a galaxys velocity is proportional to its distance. Galaxies that are twice as far from us move twice as fast. Another consequence is that the universe is expanding in every direction. This observation means that it has taken every galaxy the same amount of time to move from a common starting position to its current position. Just as the Big Bang provided for the foundation of the universe, Hubbles observations provided for the foundation of the Big Bang theory.

Since the Big Bang, the universe has been continuously expanding and, thus, there has been more and more distance between clusters of galaxies. This phenomenon of galaxies moving farther away from each other is known as the red shift. As light from distant galaxies approach earth there is an increase of space between earth and the galaxy, which leads to wavelengths being stretched.

In addition to the understanding of the velocity of galaxies emanating from a single point, there is further evidence for the Big Bang. In 1964, two astronomers, Arno Penzias and Robert Wilson, in an attempt to detect microwaves from outer space, inadvertently discovered a noise of extraterrestrial origin. The noise did not seem to emanate from one location but instead, it came from all directions at once. It became obvious that what they heard was radiation from the farthest reaches of the universe which had been left over from the Big Bang. This discovery of the radioactive aftermath of the initial explosion lent much credence to the Big Bang theory.

Even more recently, NASAs COBE satellite was able to detect cosmic microwaves eminating from the outer reaches of the universe. These microwaves were remarkably uniform which illustrated the homogenity of the early stages of the universe. However, the satillite also discovered that as the universe began to cool and was still expanding, small fluctuations began to exist due to temperature differences. These flucuatuations verified prior calculations of the possible cooling and development of the universe just fractions of a second after its creation. These fluctuations in the universe provided a more detailed description of the first moments after the

Big Bang. They also helped to tell the story of the formation of galaxies which will be discussed in the next chapter. The Big Bang theory provides a viable solution to one of the most pressing questions of all time. It is important to understand, however, that the theory itself is constantly being revised. As more observations are made and more research conducted, the Big Bang theory becomes more complete and our knowledge of the origins of the universe more substantial.

Discovery of penzias and Wilson discover cosmic microwave radiation (1965 ) :

Bell Labs built a giant antenna in Holmdel, New Jersey, in 1960. It was part of a very early satellite transmission system called Echo. By collecting and amplifying weak radio signals bounced off large metallic balloons high in the atmosphere, it could send signals across long distances. Within a few years, the Telstar satellite was launched. It had built-in transponders and made the Echo system obsolete.Meanwhile, two employees of Bell Labs had had their eye on the antenna. Arno Penzias (b. 1933), a German-born radio astronomer, joined Bell Labs in 1958. He had done his PhD on using masers (microwave amplification by stimulated emission of radiation) to amplify and measure radio signals from the spaces between galaxies. He knew the Holmdel antenna would also make a great radio telescope and was dying to use it to continue his observations, but he pursued other research while the antenna was booked for commercial use. Another radio astronomer came to Bell Labs in 1962 with the same idea. Robert Wilson (b. 1936) had also used masers to amplify weak signals in mapping radio signals from the Milky Way.

The launch of Telstar in 1962 gave both researchers what they wanted: the Holmdel antenna was freed up for pure research. When they began to use it as a telescope they found there was a background "noise" (like static in a radio). This annoyance was a uniform signal in the microwave range, seeming to come from all directions. Everyone assumed it came from the telescope itself, which was not unusual. It hadn't interfered with the Echo system but Penzias and Wilson had to get rid of it to make the observations they planned. They checked everything to rule out the source of the excess radiation.

They pointed the antenna right at New York City -- it wasn't urban interference. It wasn't radiation from our galaxy or extraterrestrial radio sources. It wasn't even the pigeons living in the big, horn-shaped antenna. Penzias and Wilson kicked them out and swept out all their droppings. The source remained the same through four seasons, so it couldn't have come from the solar system or even from a 1962 above-ground nuclear test, because in a year that fallout would have shown a decrease. They had to conclude it was not the machine and it was not random noise causing the radiation.

Penzias and Wilson began looking for theoretical explanations. Around the same time, Robert Dicke (1916Ð1997) at nearby Princeton University had been pursuing theories about the big bang. He had elaborated on existing theory to suggest that if there had been a big bang, the residue of the explosion should by now take the form of a low-level background radiation throughout the universe. Dicke was looking for evidence of this theory when Penzias and Wilson got in touch with his lab. He shared his theoretical work with them, even as he resignedly said to his fellow-researchers, "We've been scooped." Ironically, Robert Wilson had been trained in steady state theory (which suggested the universe was without beginning or end, unlike big bang theory), and he felt uncomfortable with the big bang explanation of their radio noise. When he and Penzias jointly published their research with Dicke, the Bell Lab researchers stuck to

"just the facts" -- simply reporting their recorded observations.

It is ironic, too, that many researchers -- both theoretical and experimental -- had stumbled on this phenomenon before, but either discounted it or never put it all together. This was partly because, as Steven Weinberg wrote, "in the 1950s, the study of the early universe was widely regarded as not the sort of thing to which a respectable scientist would devote his time." Since Penzias, Wilson, and Dicke's work, all that has changed. The measurement of cosmic background radiation (as the Holmdel telescope's noise is now called), combined with Edwin Hubble's much earlier finding that the galaxies are rushing away, makes a strong case for the big bang.

By the mid 1970s, astronomers called it "the standard model." Arno Penzias and Robert Wilson received the Nobel Prize in physics in 1978.

•Future of an expanding universe

Overview

There are basically two possible futures of the universe. The first is that the universe will eventually collapse in on itself in the reverse of the Big Bang - a process called the "Big Crunch." This ending would result if there is enough matter in the universe to counteract the force of the expansion.The second possible end is where the universe would continue to expand forever: Everything will eventually disappear, and the temperature of the universe will be absolute zero (0 K, -459.688 °F). This second process has many names, but the most common is the "Big Freeze;" it would happen in the reverse situation of the Big Crunch - if the universe does not have enough matter for the collective gravity to counteract the expansion.

A Geometry LessonFlat Geometry :

The future of the universe ultimately depends upon its overall geometry: Flat, Spherical, or Hyperbolic (diagrams of these geometries lie throughout this page). The universe's geometry is determined by the average density of everything in it - a function of the mass. There is one magic number, called the "critical density" which is represented by ?c, that determines which fate the universe will take.

Big Crunch

Spherical GeometryThe Big Crunch is one scenario for the end of the universe, and it will result if the universe has a spherical geometry. This "spherical geometry" is not an abstract idea: It actually relates to what the shape of the universe would be if one could observe it from the "outside."

In this case, the universe contains enough mass - it is above the critical density - to stop its expansion. Once it stops expanding, it will start to contract. Slowly at first, and then faster and faster, the universe will contract and galaxies will come closer to each other. Eventually, everything will merge, for the universe will no longer be large enough for separate galaxies or stars. As it continues to shrink, the universe will heat to huge temperatures, and everything will be compacted into a black hole. Finally, at the end, the universe will be as it began - an infinitely small, infinitely dense, and infinitely hot point. No one knows what, if anything, would happen after that *.

An easy way to think of this is by throwing a ball; you throw a ball up into the air. Your release is like the Big Bang, and starts the ball's acceleration. As the ball climbs skyward, it slows its ascent because the Earth has enough gravity to slow it down and pull it back to it. This is like the mass of the universe being enough to overcome its expansion. As the ball reaches its maximum height, it stops, which is what the universe will do if it is over the critical density. Then, ever so slowly, the ball begins to fall back down, faster and faster, until it reaches your hand again (unless you miss). This is the end of the ball's throw, and is like the end of the universe.

*A popular idea is that the universe will then be re-born and it would continue to oscillate between Big Bangs and Big Crunches forever.

The Big Freeze

This scenario for the universe's future will result from either hyperbolic or flat geometry. As with spherical geometry discussed in the above section on the Big Crunch, these geometries are not abstract terms that only weird astrophysicists with thick glasses and poofy white hair use, but rather they are real shapes. A flat geometry is like a sheet of paper: It is flat; there is no curvature. Hyperbolic geometry is usually pictured with a saddle, and is depicted below to the right.Hyperbolic Geometry

Either one of these geometries will result in a universe that effectively expands forever. If the universe is hyperbolic - the density is lower than the critical density - then it will eventually reach a fixed rate of expansion, and continue to expand at that rate forever. If the universe is flat - the density is exactly the critical density - then it will asymptotically reach an expansion rate of 0.

Both of these pose the future of a never-ending universe. After enough time, all galaxies beyond our Local Group will have disappeared beyond the edge of the observable universe**. After a longer time, all the stars in all the galaxies will have died, and there will be nothing left to make new ones. The universe will be a dark and cold place. Eventually, there will be nothing left but a vast, frozen emptiness.

**We can only know a small bit of what the universe contains due to the finite speed of light (300,000 kmps; 186,000 miles per second). Because the universe is a certain age, we can only see that many light-years out; for any part of the universe beyond that, the light has not had enough time to reach us.

Recent and Current Research

Since 1992, there have been many different projects to determine the overall geometry of the universe. The only successful way to determine this so far has been to study the cosmic microwave background radiation*** (CMB). The first such was COBE, which stands for COsmic Background Explorer. It presented the first all-sky picture of the CMB, but its resolution was too poor to accurately determine the geometry (temperature resolution was about 0.002 K; angular resolution was 7° - 14 times the size of the full moon). It did show that the actual density of the universe is very close to the critical density.

The most recent and complete research is from the Wilkinson Microwave Anisotropy Probe (WMAP for short), sponsored mainly by NASA. It has made the highest-resolution image of the CMB: The angular resolution of WMAP was 0.3° and the temperature resolution is 20 µK. The WMAP results show that the universe is flat, meaning that the the universe will expand forever at an ever decelerating rate. Other results from the WMAP mission are:

    The universe is 13.7 billion years old with an uncertainty of ±1%.
    The first stars ignited 200 million years after the Big Bang.
    The CMB is from 380,000 years after the Big Bang.
    The content of the universe is 4% atoms, 23% cold dark matter, and 73% dark energy.
    The expansion rate (Hubble constant) value: H0 = 71 km/sec/Mpc with an uncertainty of 5%.

Early universe

Inflation :
In physical cosmology, cosmic inflation, cosmological inflation, or just inflation is the exponential expansion of space in the early universe. The inflationary epoch lasted from 10-36 seconds after the Big Bang to sometime between 10-33 and 10-32 seconds. Following the inflationary period, the universe continues to expand, but at a less accelerated rate. The inflationary hypothesis was proposed in 1980 by American physicist Alan Guth.

Inflation explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the universe Many physicists also believe that inflation explains why the Universe appears to be the same in all directions (isotropic), why the cosmic microwave background radiation is distributed evenly, why the universe is flat, and why no magnetic monopoles have been observed.While the detailed particle physics mechanism responsible for inflation is not known, the basic picture makes a number of predictions that have been confirmed by observation.The hypothetical field thought to be responsible for inflation is called the inflaton.On 17 March 2014, astrophysicists of the BICEP2 collaboration announced the detection of inflationary gravitational waves in the B-mode power spectrum, which if confirmed would provide clear experimental evidence for the theory of inflation.

Nucleosynthesis
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of the lightest isotope of hydrogen during the early phases of the universe. In physical cosmology, Big Bang nucleosynthesis (abbreviated BBN, also known as primordial nucleosynthesis) refers to the production of nuclei other than those of the lightest isotope of hydrogen during the early phases of the universe. Primordial nucleosynthesis is believed by most cosmologists to have taken place from 10 seconds to 20 minutes after the Big Bang, and is calculated to be responsible for the formation of most of the universe's helium as isotope He-4, along with small amounts of deuterium (H-2 or D), the helium isotope He-3, and a very small amount of the lithium isotope Li-7. In addition to these stable nuclei, two unstable or radioactive isotopes were also produced: tritium or H-3; and beryllium-7 (Be-7); but these unstable isotopes later decayed into He-3 and Li-7, as above.

Essentially all of the elements that are heavier than lithium were created much later, by stellar nucleosynthesis in evolving and exploding stars.

Neutrino background :

The spectrum of the Cosmic Microwave Background follows Planck's black body radiation formula and shows a remarkable constant temperature of T = 2.7. About 380 000 years after the Big Bang at a temperature of T = 3000 Kelvin in the matter dominated era the electrons combine with the protons and 4He and the photons move freely in the neutral universe. So the temperature and distribution of the photons give us information of the universe 380 000 years after the Big Bang. Information about earlier times can, in principle, be derived from the Cosmic Neutrino Background (relic neutrinos). The neutrinos decouple already about 1 second after the Big Bang at a temperature of around 1 MeV or 10^{10} Kelvin. Today their temperature is about 1.95 Kelvin. Registration of these neutrinos is an extremely challenging experimental problem, which can hardly be solved with the present technologies. On the other hand it represents a tempting opportunity to check one of key elements of the Big Bang Cosmology and to probe the early stages of the universe evolution. The search for the Cosmic Neutrino Background with the induced beta decay: relic neutrino + 3H --> 3He + e-, is the topic of this contribution. The signal would show up as a peak in the electron spectrum by an energy with the neutrino mass above the Q value. We discuss the prospects of this approach and argue that it is able to set limits on the Cosmic Neutrino density in our vicinity. We also discuss critically ways to increase with modifications of the present KATRIN spectrometer the Tritium source intensity by a factor 100, which would yield about 170 counts of relic neutrino captures per year. Presently such an increase of the Tritium source intensity seems not to be possible. But one should be able to find an upper limit for the local density of the relic neutrinos in our galaxy.

Expanding universe

Redshift :

The light from distant stars and more distant galaxies is not featureless, but has distinct spectral features characteristic of the atoms in the gases around the stars. When these spectra are examined, they are found to be shifted toward the red end of the spectrum. This shift is apparently a Doppler shift and indicates that essentially all of the galaxies are moving away from us. Using the results from the nearer ones, it becomes evident that the more distant galaxies are moving away from us faster. This is the kind of result one would expect for an expanding universe. The building up of methods for measuring distance to stars and galaxies led Hubble to the fact that the red shift (recession speed) is proportional to distance. If this proportionality (called Hubble's Law) holds true, it can be used as a distance measuring tool itself.The measured red shifts are usually stated in terms of a z parameter. The largest measured z values are associated with the quasars.
Red Shift of Galaxy 8C1435+635. Reported in November 1994 in Monthly Notices of the Royal Astronomical Society is a galaxy with a measured red shift of z=4.25 , a new record. This value for the z parameter corresponds to a recession speed of .93c. The galaxy 8C1435+635 was observed in a systematic search for faint, radio-emitting galaxies carried out by a team at Leiden Observatory led by George Miley. After discovery, the optical spectra was observed by the William Hershel Telescope in La Palma, Canary Islands. Two emission lines of ionized carbon and hydrogen were measured to obtain the red shift. This red shift corresponds to a distance of about 13 billion light years if one uses the current WMAP value of 71km/s/mpc for the Hubble parameter is used.

•Hubble's law :

Hubble's law is the name for the observation in physical cosmology that: objects observed in deep space are found to have a Doppler shift interpretable as relative velocity away from the Earth; and that .The Hubble constant H is one of the most important numbers in cosmology because it may be used to estimate the size and age of the Universe. It indicates the rate at which the universe is expanding. Although the Hubble "constant" is not really constant because it changes with time (and therefore should probably more properly be called the "Hubble parameter"). The Hubble constant is often written with a subscript "0" to denote explicitly that it is the value at the present time, but we shall not do so.
The Hubble Expansion Law
In 1929, Edwin Hubble announced that almost all galaxies appeared to be moving away from us. This phenomenon was observed as a redshift of a galaxy's spectrum. This redshift appeared to have a larger displacement for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth. The Hubble constant is given by H = v/d
where v is the galaxy's radial outward velocity, d is the galaxy's distance from earth, and H is the current value of the Hubble constant.
Determining the Hubble Constant
Obtaining a true value for H is complicated. Two measurements are required. First, spectroscopic observations reveal the galaxy's redshift, indicating its radial velocity. The second measurement, the most difficult value to determine, is the galaxy's precise distance from Earth. The value of H itself must be derived from a sample of galaxies that are far enough away that motions due to local gravitational influences are negligibly small (these are called peculiar motion, and they represent deviations from the Hubble Law).
Units for Hubble's Constant
The units of the Hubble constant are "kilometers per second per megaparsec." In other words, for each megaparsec of distance, the velocity of a distant object appears to increase by some value. For example, if the Hubble constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc would have a redshift corresponding to a radial velocity of 500 km/s.
Current Value of the Hubble Constant
The value of the Hubble constant initially obtained by Hubble was around 500 km/s/Mpc, and has since been radically revised because initial assumptions about stars yielded underestimated distances. For the past three decades, there have been two major lines of investigation into the Hubble constant. One team, associated with Allan Sandage of the Carnegie Institutions, has derived a value for H around 50 km/s/Mpc. The other team, associated with Gerard DeVaucouleurs of the University of Texas, has obtained values that indicate H to be around 100 km/s/Mpc

Metric expansion of space :

The metric expansion of space is the increase of the distance between two distant parts of the universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The metric expansion of space is a key part of science's current understanding of the universe, whereby spacetime itself is described by a metric which changes over time in such a way that the spatial dimensions appear to grow or stretch as the universe gets older. It explains how the universe expands in the Big Bang model, a feature of our universe supported by all cosmological experiments, astrophysics calculations, and measurements to date.The expansion of space is conceptually different from other kinds of expansions and explosions that are seen in nature. Our understanding of the "fabric of the universe" ( spacetime) requires that what we see normally as "space", "time", and " distance" are not absolutes, but are determined by a metric that can change. In the metric expansion of space, rather than objects in a fixed "space" moving apart into "emptiness", it is the space that contains the objects which is itself changing. It is as if without objects themselves moving, space is somehow "growing" in between them.Because it is the metric defining distance that is changing rather than objects moving in space, this expansion (and the resultant movement apart of objects) is not restricted by the speed of light upper bound that results from special relativity.Theory and observations suggest that very early in the history of the universe, there was an "inflationary" phase where this metric changed very rapidly, and that the remaining time-dependence of this metric is what we observe as the so-called Hubble expansion, the moving apart of all gravitationally unbound objects in the universe.The expanding universe is therefore a fundamental feature of the universe we inhabit—a universe fundamentally different from the static universe Albert Einstein first considered when he developed his gravitational theory.

Structure formation

Shape of the universe :

One of the most profound insights of General Relativity was the conclusion that mass caused space to curve, and objects travelling in that curved space have their paths deflected, exactly as if a force had acted on them. If space itself is curved, there are three general possibilities for the geometry of the universe. Each of these possibilites is tied to the amount of mass (and thus to the total strength of gravitation) in the universe, and each implies a different past and future for the universe.First, let's look at shapes and curvatures for a two-dimensional surface. Mathematicians distinguish 3 qualitatively different classes of curvature, as The flat surface at the left is said to have zero curvature, the spherical surface is said to have positive curvature, and the saddle-shaped surface is said to have

negative curvature.The preceding is not too difficult to visualize, but General Relativity asserts that space itself (not just an object in space) can be curved, and furthermore, the space of General Relativity has 3 space-like dimensions and one time dimension, not just two as in our example above. This IS difficult to visualize! Nevertheless, it can be described mathematically by the same methods that mathematicians use to describe the 2-dimensional surfaces. So what do the three types of curvature - zero, positive, and negative -mean to the universe? If space has negative curvature, there is insufficient mass to cause the expansion of the universe to stop. In such a case, the universe has no bounds, and will expand forever. This is called an open universe.

If space has no curvature (i.e, it is flat), there is exactly enough mass to cause the expansion to stop, but only after an infinite amount of time. Thus, the universe has no bounds and will also expand forever, but with the rate of expansion gradually approaching zero after an infinite amount of time. This is termed a flat universe or a Euclidian universe (because the usual geometry of non-curved surfaces that we learn in high school is called Euclidian geometry).if space has positive curvature, there is more than enough mass to stop the present expansion of the universe. The universe in this case is not infinite, but it has noend (just as the area on the surface of a sphere is not infinite but there is no point on the sphere that could be called the "end"). The expansion will eventually stop and turn into a contraction. Thus, at some point in the future the galaxies will stop receding from each other and begin approaching each other as the universe collapses on itself. This is called a closed universe.

Reionization :

In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the "dark ages", and is the second of two major phase transitions of gas in the universe. Reionization was complete about 1 billion years after the Big Bang, corresponding to a redshift of about 6.5 .Even with the quasar data roughly in agreement with the CMB anisotropy data, there are still a number of questions, especially concerning the energy sources of reionization and the effects on, and role of, structure formation during reionization.

Galaxy formation :

Galaxy Formation. We really don't know how various galaxies formed and took the many shapes that we see today. But we do have some ideas about their origins and evolution. Shortly after the big bang about 14 billion years ago, collapsing gas and dust clouds might have lead to the formation of galaxies.The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change. Theoretical investigations indicate that galaxies formed from a diluted but lumpy mixture of hydrogen and helium gas - the primordial elements forged in the Big Bang. They also indicate that two vastly different scales of mass prevailed less than 100 million years after the Big Bang, which ultimately affected the formation of galaxies. (See the later discussion of dark matter and the formation of structure.)
Two Scales of Matter ,Matter either was clumped into vast collections more than a million times the mass of the Milky Way, or into small clumps one million times smaller than the mass of

our Milky Way. Superclusters of galaxies may have evolved from the former. Globular clusters such as M15 in the adjacent image may have evolved from the latter.Results from Recent Observations .As we look deeper into the Universe and therefore back in time, galaxies appear to emit more of their light in the blue part of the visible spectrum. This blue light

is a sign that very young, massive and luminous stars are forming (see the discussion of the spiral arms in spiral galaxies, for example). Since we see these galaxies as they were between 5 and 10 billion years ago, we appear to be witnessing events that occurred within a few billion years after these galaxies were formed.Astronomers also have noticed that as they examine the images of these distant blue galaxies, the images are frequently distorted or contain what appear to be multiple nuclei. The Milky Way seen at a similar great distance would look like a uniformed flattened disk, with a single bright nucleus -- the galactic center. Nearby "multiple-nuclei" galaxies that have been studied show the cores of individual galaxies colliding and merging into one single system of stars and gas. These collisions are violent, and take millions of years to play out. But in at least some instances, such as NGC 1275, recently observed with the Hubble Space Telescope, galaxy collisions can actually trigger the formation of massive stars.
Cosmic Cannibalism ,In the depths of space, we may be witnessing collisions between smaller galaxies triggering the formation of massive luminous stars. The images, rich in blue light, gives tantalizing evidence that "environment" may have been more important than cosmic "genetics." Galactic cannibalism was far more common in the ancient past. Galaxies may have grown to their current size by consuming their neighbors. The ultimate building blocks may indeed have been the paltry million-solar-mass clumps that theoreticians believe were abundant before the Universe was a few million years old.

Large-scale structure :

In physical cosmology, the term large-scale structure refers to the characterization of observable distributions of matter and light on the largest scales (typically on the order of billions of light-years).Sky surveys and mappings of the various wavelength bands of electromagnetic radiation (in particular 21-cm emission) have yielded much information on the content and character of the universe's structure.The organization of structure arguably begins at the stellar level, though most cosmologists rarely address astrophysics on that scale.Stars are organised into galaxies, which in turn form clusters and superclusters that are separated by immense voids.Prior to 1989, it was commonly assumed that virialized galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction.However, based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the "Great Wall," a sheet of galaxies more than 500 million light-years long and 200 million wide, but only 15 million light-years thick.The existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from redshifts.In April 2003, another large-scale structure was discovered, the Sloan Great Wall.However, technically it is not a 'structure', since the objects in it are not gravitationally related with each other but only appear this way, caused by the distance measurement that was used.One of the biggest voids in space is the Capricornus void, with an est. diameter of 230 million light years.However in August 2007 a new supervoid was confirmed in the constellation Eridanus, which is nearly a billion light years across.In more recent studies the universe appears as a collection of giant bubble-like voids separated by sheets and filaments of galaxies, with the superclusters appearing as occasional relatively dense nodes.

Galaxy filament :

In physical cosmology, galaxy filaments, also called supercluster complexes, great walls, or "great attractors", are amongst the largest known cosmic structures in the universe.The filament is the first structure of its kind spied in a critical era of cosmic buildup when colossal collections of galaxies called superclusters began to take shape.In physical cosmology, galaxy filaments, also called supercluster complexes, great walls, or "great attractors", are amongst the largest known cosmic structures in the universe. They are massive, thread-like formations, with a typical length of 50 to 80 megaparsecs h-1, that form the boundaries between large voids in the universe.Filaments consist of gravitationally bound galaxies; parts where a large number of galaxies are very close to each other (in cosmic terms) are called superclusters.In the standard model of the evolution of the universe, galactic filaments form along and follow web-like strings of dark matter.It is thought that this dark matter dictates the structure of the Universe on the grandest of scales. Dark matter gravitationally attracts baryonic matter, and it is this "normal" matter that

astronomers see forming long, thin walls of super-galactic clusters.Discovery of structures larger than superclusters began in the 1980s. In 1987, astronomer R. Brent Tully of the University of Hawaii's Institute of Astronomy identified what he called the Pisces–Cetus Supercluster Complex. In 1989, the CfA2 Great Wall was discovered,followed by the Sloan Great Wall in 2003 .On January 11, 2013, researchers led by Roger Clowes of the University of Central Lancashire announced the discovery of a large quasar group, the Huge-LQG, which dwarfs previously discovered galaxy filaments in size.In November 2013, using gamma-ray bursts as reference

points, astronomers discovered the Hercules–Corona Borealis Great Wall, an extremely huge filament measuring more than 10 billion light-years across.In 2006, scientists announced the discovery of three filaments aligned to form one of the largest structures known to humanity,composed of densely packed galaxies and enormous blobs of gas known as Lyman-alpha blobs.