Panspermia

An idea, with ancient roots, according to which life arrives, ready-made, on the surface of planets from space. Anaxagoras is said to have spoken of the "seeds of life" from which all organisms derive. Panspermia began to assume a more scientific form through the proposals of Berzelius (1834), Richter (1865), Thomson (Lord Kelvin) (1871), and Helmholtz (1871), finally reaching the level of a detailed, widely-discussed hypothesis through the efforts of the Swedish chemist Svante Arrhenius. Originally in 1903, but then to a wider audience through a popular book in 1908,³ Arrhenius urged that life in the form of spores could survive in space and be spread from one planetary system to another by means of radiation pressure. He generally avoided the problem of how life came about in the first place by suggesting that it might be eternal, though he did not exclude the possibility of living things generating from simpler substances somewhere in the universe. In Arrhenius's view, spores escape by random movement from the atmosphere of a planet that has already been colonized and are then launched into interstellar space by the pressure of starlight ("radiopanspermia"). Eventually, some of the spores fall upon another planet, such as the Earth, where they inoculate the virgin world with new life or, perhaps, compete with any life-forms that are already present.

Arrhenius's ideas prompted a variety of experimental work, such as that of Paul Becquerel, to test whether spores and bacteria could survive in conditions approximating those in space. A majority of scientists reached the conclusion that stellar ultraviolet would probably prove deadly to any organisms in the inner reaches of a planetary system and, principally for this reason, panspermia quietly faded from view-only to be revived some four decades later.


Sagan's analysis

In the early 1960s, Carl Sagan analyzed in detail both the physical and biological aspects of the Arrhenius scenario. The dynamics of a microorganism in space depend on the ratio p/g, where p is the repulsive force due to the radiation pressure of a star and g is the attractive force due to the star's gravitation. If p > g, a microbe that has drifted into space will move away from the star; if p less then g, the microbe will fall toward the star. For a microbe to escape into interstellar space from the vicinity of a star like the Sun, the organism would have to be between 0.2 and 0.6 microns across. Though small, this is within the range of some terrestrial bacterial spores and viruses. The ratio p/g increases for more luminous stars, enabling the ejection of larger microbes. However, main sequence stars brighter than the Sun are also hotter, so that they emit more ultraviolet radiation which would pose an increased threat to space-borne organisms. Additionally, such stars have a shorter main sequence lifespan, so that they provide less opportunity for life to take hold on any worlds that might orbit around them. These considerations, argued Sagan, constrain "donor" stars for Arrhenius-style panspermia to spectral types G5 (Sun-like) to A0. Stars less luminous than the Sun would be unable to eject even the smallest of known living particles. "Acceptor" stars, on the other hand, must have lower p/g ratios in order to allow microbes, approaching from interstellar space, to enter their planetary systems. The most likely acceptor worlds, Sagan concluded, are those circling around red dwarfs (dwarf M stars), or in more distant orbits around G stars and K stars. In the case of the solar system, he surmised, the best place to look for life of extrasolar origin would be the moons of the outer planets, in particular Triton.


Life-carrying rocks?

Many variations on the panspermia theme have been put forward. William Thomson (Lord Kelvin) proposed that spores might travel aboard meteorites ("lithopanspermia"), thus affording them better protection from high-energy radiation in space. Whether events violent enough to hurl rocks from the surface of a biologically active planet into interstellar space ever occur is not clear. But there is now overwhelming evidence that ballistic panspermia occasionally operates between worlds of the same planetary system. This follows the discovery of meteorites on Earth that have almost certainly come from the surface of Mars (see SNC meteorites) and the Moon. There is also controversial evidence for fossil remains aboard some carbonaceous chondrites, including the Orgueil meteorite.


Contamination

In the 1960s, Thomas Gold pointed out another way in which life might travel from world to world (see "garbage theory," of the origin of life). A team of explorers from an advanced, interstellar-faring race might land on the planet of a foreign star and, unwittingly, leave behind "bugs" which then adapt to the local conditions. He imagined, for example, the visitors having a picnic and not clearing up afterward. What effect microscopic alien fauna and flora might have on the indigenous species is impossible to predict, but such considerations were foremost in the minds of scientists receiving the first samples of rock and soil from the Moon. Precautions against alien contamination will be even more important when the first spacecraft return from Mars or Europa where the possibility of extant life is far greater ( back-contamination). And there is the reverse problem (forward-contamination). The remarkable case of Surveyor 3 makes it clear that some terrestrial microbes can survive for significant periods in hostile conditions on other worlds. What if such a world (like Mars) had life-forms of its own? What chaos might the "alien" microbes from Earth wreak? It would be tragic indeed if the very means of discovering the first examples of extraterrestrial life were also to be the vehicle of its extinction. On the other hand, as Carl Sagan pointed out, if Gold's "picnic scenario" had actually happened in the Earth's past "some microbial resident of a primordial cookie crumb may be the ancestor of us all." Just as the chance of accidental contamination arising from intelligent activity cannot be ruled out, there is the complimentary possibility of intentional or directed panspermia.

Life from space

Today, the panspermia hypothesis has finally achieved some measure of scientific respectability. Although it remains the orthodox view that life evolved in situ on this world and, possibly, many others, there is mounting evidence of at least some extraterrestrial input to the formative stages of planet-based biology. Prebiotic chemicals have been detected in interstellar clouds (similar to that from which the Solar System formed), comets, and meteorites (see astrochemistry). At the very least, it seems that some of the raw ingredients for life, such as amino acids, may have fallen from the sky in addition to being manufactured here on Earth. But some researchers have gone much further in their speculations. Most notably, Fred Hoyle and Chandra Wickramasinghe have argued persistently since the 1970s that complex organic substances, and perhaps even primitive organisms, might have evolved on the surface of cosmic dust grains in space and then been transported to the Earth's surface by comets and meteorites (see life, in space). The extraordinary durability of some extremophiles, bacterial spores, and even exposed DNA, lends credence to the view that simple life-forms may have originated between the stars or been capable of surviving long interstellar journeys.

excerpt taken from http://www.daviddarling.info/encyclopedia/P/panspermia.html

Carl Gustav Jung

Amid all the talk about the "Collective Unconscious" and other sexy issues, most readers are likely to miss the fact that C.G. Jung was a good Kantian. His famous theory of Synchronicity, "an acausal connecting principle," is based on Kant's distinction between phenomena and things-in-themselves and on Kant's theory that causality will not operate among thing-in-themselves the way it does in phenomena. Thus, Kant could allow for free will (unconditioned causes) among things-in-themselves, as Jung allows for synchronicity ("meaningful coincidences"). Next to Kant, Jung is close to Schopenhauer, praising him as the first philosopher he had read, "who had the courage to see that all was not for the best in the fundaments of the universe" [Memories, Dreams, Reflections, p. 69]. Jung was probably unaware of the Friesian background of Otto's term "numinosity" when he began to use it for his Archetypes, but it is unlikely that he would object to the way in which Otto's theory, through Fries, fits into Kantian epistemology and metaphysics.

Jung's place in the Kant-Friesian tradition is on a side that would have been distasteful to Kant, Fries, and Nelson, whose systems were basically rationalistic. Thus Kant saw religion as properly a rational expression of morality, and Fries and Nelson, although allowing an aesthetic content to religion different from morality, nevertheless did not expect religion to embody much more than good morality and good art. Schopenhauer, Otto, and Jung all represent an awareness that more exists to religion and to human psychological life than this. The terrifying, uncanny, and fascinating elements of religion and ordinary life are beneath the notice of Kant, Fries, and Nelson, while they are indisputable and irreducible elements of life, for which there must be an account, with Schopenhauer, Otto, and Jung. As Jung again said of Schopenhauer: "He was the first to speak of the suffering of the world, which visibly and glaringly surrounds us, and of confusion, passion, evil -- all those things which the others hardly seemed to notice and always tried to resolve into all-embracing harmony and comprehensibility" [ibid. p. 69]. It is an awareness of this aspect of the world that renders the religious ideas of "salvation" meaningful; yet "salvation" as such is always missing from moralistic or aesthetic renderings of religion. Only Jung could have written his Answer to Job.

Jung's great Answer to Job, indeed, represents an approach to religion that is all but unique. Placing God in the Unconscious might strike most people as reducing him to a mere psychological object; but that is to overlook Jung's Kantianism. The Unconscious, and especially the Collective Unconscious, belongs to Kantian things-in-themselves, or to the transcendent Will of Schopenhauer. Jung was often at pains not to complicate his theory of the Archetypes by committing himself to a metaphysical theory -- he wanted the theory to work whether he was talking about the brain or about the Transcendent -- but that was merely a concession to the materialistic bias of contemporary science. He had no materialistic commitment himself and, when it came down to it, was not going to accept such naive reductionism. Instead, he was willing to rethink how the Transcendent might operate. Thus, he says about Schopenhauer:

I felt sure that by "Will" he really meant God, the Creator, and that he was saying that God was blind. Since I knew from experience that God was not offended by any blasphemy, that on the contrary He could even encourage it because He wished to evoke not only man's bright and positive side but also his darkness and ungodliness, Schopenhauer's view did not distress me. [ibid. pp. 69-70]

The Problem of Evil, which for so many people simply denuminizes religion, and which Schopenhauer used to reject the value of the world, became a challenge for Jung in the psychoanalysis of God. The God of the Bible is indeed a personality, and seemingly not always the same one. God as a morally evolving personality is the extraordinary conception of Answer to Job. What Otto saw as the evolution of human moral consciousness, Jung turns right around on the basis of the principle that the human unconscious, expressed spontaneously in religious practice and literature, transcends mere human subjectivity. But the transcendent reality in the unconscious is different in kind from consciousness. As Jung said in Memories, Dreams, Reflections again:

If the Creator were conscious of Himself, He would not need conscious creatures; nor is it probable that the extremely indirect methods of creation, which squander millions of years upon the development of countless species and creatures, are the outcome of purposeful intention. Natural history tells us of a haphazard and casual transformation of species over hundreds of millions of years of devouring and being devoured. The biological and political history of man is an elaborate repetition of the same thing. But the history of the mind offers a different picture. Here the miracle of reflecting consciousness intervenes -- the second cosmogony [ed. note: what Teilhard de Chardin called the origin of the "noosphere," the layer of "mind"]. The importance of consciousness is so great that one cannot help suspecting the element of meaning to be concealed somewhere within all the monstrous, apparently senseless biological turmoil, and that the road to its manifestation was ultimately found on the level of warm-blooded vertebrates possessed of a differentiated brain -- found as if by chance, unintended and unforeseen, and yet somehow sensed, felt and groped for out of some dark urge. [p. 339]

In other words, a "meaningful coincidence." Jung also says,

As far as we can discern, the sole purpose of human existence is to kindle a light in the darkness of mere being. It may even be assumed that just as the unconscious affects us, so the increase in our consciousness affects the unconscious. [p. 326]

However, Jung has missed something there. If consciousness is "the light in the darkness of mere being," consciousness alone cannot be the "sole purpose of human existence," since consciousness as such could appear as just a place of "mere being" and so would easily become an empty, absurd, and meaningless Existentialist existence. Instead, consciousness allows for the meaningful instantiation of existence, both through Jung's process of Individuation, by which the Archetypes are given unique expression in a specific human life, and from the historic process that Jung examines in Answer to Job, by which interaction with the unconscious alters in turn the Archetypes that come to be instantiated. While Otto could understand Job's reaction to God, as the incomprehensible Numen, Jung thinks of God's reaction to Job, as an innocent and righteous man jerked around by God's unconsciousness. Jung's idea that the Incarnation then is the means by which God redeems Himself from His morally false position in Job is an extraordinary reversal (I hesitate to say "deconstruction") of the consciously expressed dogma that the Incarnation is to redeem humanity.

It is not too difficult to see this turn in other religions. The compassion of the Buddhas in Mahâyâna Buddhism, especially when the Buddha Shakyamuni comes to be seen as the expression of a cosmic and eternal Dharma Body, is a hand of salvation stretched out from the Transcendent, without, however, the complication that the Buddha is ever thought responsible for the nature of the world and its evils as their Creator. That complication, however, does occur with Hindu views of the divine Incarnations of Vishnu. Closer to a Jungian synthesis, on the other hand, is the Bahá'í theory that divine contact is though "Manifestations," which are neither wholly human nor wholly divine: merely human in relation to God, but entirely divine in relation to other humans. Such a theory must appear Christianizing in comparison to Islam, but it avoids the uniqueness of Christ as the only Incarnation in Christianity itself. This is conformable to the Jungian proposition that the unconscious is both a side of the human mind and a door into the Transcendent. When that door opens, the expression of the Transcendent is then conditioned by the person through which it is expressed, possessing that person, but it is also genuinely Transcendent and reflecting the ongoing interaction that the person historically embodies. The possible "mere being" even of consciousness then becomes the place of meaning and value.

Whether "psychoanalysis" as practiced by Freud or Jung is to be taken seriously anymore is a good question; but both men will survive as philosophers long after their claims to science or medicine may be discounted. Jung's Kantianism enables him to avoid the materialism and reductionism of Freud ("all of civilization is a substitute for incest") and, with a great breadth of learning, employs principles from Kant, Schopenhauer, and Otto that are easily conformable to the Kant-Friesian tradition. The Answer to Job, indeed, represents a considerable advance beyond Otto, into the real paradoxes that are the only way we can conceive transcendent reality.

excerpt taken from http://www.friesian.com/jung.htm

Quasars

In the 1960s it was observed that certain objects emitting radio waves but thought to be stars had very unusual optical spectra. It was finally realized that the reason the spectra were so unusual is that the lines were Doppler shifted by a very large amount, corresponding to velocities away from us that were significant fractions of the speed of light. The reason that it took some time to come to this conclusion is that, because these objects were thought to be relatively nearby stars, no one had any reason to believe they should be receding from us at such velocities.

Quasars and QSOs

These objects were named Quasistellar Radio Sources (meaning "star-like radio sources") which was soon contracted to quasars. Later, it was found that many similar objects did not emit radio waves. These were termed Quasistellar Objects or QSOs. Now, all of these are often termed quasars (Only about 1% of the quasars discovered to date have detectable radio emission).

Quasars Are Related to Active Galaxies

The quasars were deemed to be strange new phenomena, and initially there was considerable speculation that new laws of physics might have to be invented to account for the amount of energy that they produced. However, subsequent research has shown that the quasars are closely related to the active galaxies that have been studied at closer distances. We now believe quasars and active galaxies to be related phenomena, and that their energy output can be explained using the theory of general relativity. In that sense, the quasars are certainly strange, but perhaps are not completely new phenomena.

Quasar Redshifts Imply Enormous Distance and Energy Output

The quasars have very large redshifts, indicating by the Hubble law that they are at great distances. The fact that they are visible at such distances implies that they emit enormous amounts of energy and are certainly not stars.

The Energy Source of Quasars is Extremely Compact

Quasars are extremely luminous at all wavelengths and exhibit variability on timescales as little as hours, indicating that their enormous energy output originates in a very compact source. Here are some light curves at different wavelengths illustrating the variability in intensity of some quasars and other active galaxies. Here is an explanation of these light curves. In all cases, the timescale for variability of the light from an active galaxy sets an upper limit on the size of the compact energy source that powers the active galaxy. These limits are typically the size of the Solar System or smaller.

Some quasars emit radio frequency, but most (99%) are radio quiet. Careful observation shows faint jets coming from some quasars. The above images of the quasar 3C273 illustrate both a jet in the optical image on the left and radio frequency emission associated with the jet on the right. Here are some spectra of quasars and other active galaxies - see the following description.

Relationship of Quasars and Active Galaxies

The quasars are thought to be powered by supermassive rotating black holes at their centers. Because they are the most luminous objects known in the universe, they are the objects that have been observed at the greatest distances from us. The most distant are so far away that the light we see coming from them was produced when the Universe was only one tenth of its present age.

The present belief is that quasars are actually closely related to active galaxies such as Seyfert Galaxies or BL Lac objects in that they are very active galaxies with bright nuclei powered by enormous rotating black holes. However, because the quasars are at such large distances, it is difficult to see anything other than the bright nucleus of the active galaxy in their case. As we have noted above, modern observations have begun to detect around some quasars jets and evidence for the surrounding faint nebulosity of a galaxy-like object.

Evolution of Quasars

The standard theory is that quasars turn on when there is matter to feed their supermassive black hole engines at the center and turn off when there is no longer fuel for the black hole. Recent Hubble Space Telescope observations indicate that quasars can occur in galaxies that are interacting with each other. This suggests the possibility that quasars that have turned off because they have consumed the fuel available in the original galaxy may turn back on if the galaxy hosting the quasar interacts with another galaxy in such a way to make more matter available to the black hole. Here is a recent survey of quasar host galaxies that sheds light on this issue.

Abundance of Quasars in the Early Universe

Looking at large distances in the Universe is equivalent to looking back in time because of the finite speed of light. Thus, the observation of quasars at large distances and their scarcity nearby implies that they were much more common in the early Universe than they are now.

This is one piece of evidence that argues against the steady state theory of the Universe but would be consistent with the big bang theory. We shall discuss this further below.

Hungry Black Holes

Notice that the greater abundance of quasars early in the Universe would be consistent with the mechanism discussed above whereby a quasar shuts off when its black hole engine has consumed the fuel available in the host galaxy. We would expect that generally in the early Universe there may have been more mass easily accessible to the black hole than later, after much of it had been consumed. Perhaps later quasars are more dependent on interactions between galaxies to disturb mass distributions and cause galaxies to begin to feed the hungry black hole.

excerpt taken from http://csep10.phys.utk.edu/astr162/lect/active/quasars.html

Dark Energy

The discovery in 1998 that the Universe is actually speeding up its expansion was a total shock to astronomers. It just seems so counter-intuitive, so against common sense. But the evidence has become convincing.

The evidence came from studying distant type Ia supernovae. This type of supernova results from a white dwarf star in binary system. Matter transfers from the normal star to the white dwarf until the white dwarf attains a critical mass (the Chandrasekhar limit) and undergoes a thermonuclear explosion. Because all white dwarfs achieve the same mass before exploding, they all achieve the same luminosity and can be used by astronomers as "standard candles." Thus by observing their apparent brightness, astronomers can determine their distance using the 1/r² law.

By knowing the distance to the supernova, we know how long ago it occurred. In addition, the light from the supernova has been red-shifted by the expansion of the universe. By measuring this redshift from the spectrum of the supernova, astronomers can determine how much the universe has expanded since the explosion. By studying many supernovae at different distances, astronomers can piece together a history of the expansion of the universe.

In the 1990's two teams of astronomers, the Supernova Cosmology Project and the High-Z Supernova Search, were looking for distant type Ia supernovae in order to measure the expansion rate of the universe with time. They expected that the expansion would be slowing, which would be indicated by the supernovae being brighter than their redshifts would indicate. Instead, they found the supernovae to be fainter than expected. Hence, the expansion of the universe was accelerating!

In addition, measurements of the cosmic microwave background indicate that the universe has a flat geometry on large scales. Because there is not enough matter in the universe - either ordinary or dark matter - to produce this flatness, the difference must be attributed to a "dark energy". This same dark energy causes the acceleration of the expansion of the universe. In addition, the effect of dark energy seems to vary, with the expansion of the Universe slowing down and speeding up over different times.

Astronomers know dark matter is there by its gravitational effect on the matter that we see and there are ideas about the kinds of particles it must be made of. By contrast, dark energy remains a complete mystery. The name "dark energy" refers to the fact that some kind of "stuff" must fill the vast reaches of mostly empty space in the Universe in order to be able to make space accelerate in its expansion. In this sense, it is a "field" just like an electric field or a magnetic field, both of which are produced by electromagnetic energy. But this analogy can only be taken so far because we can readily observe electromagnetic energy via the particle that carries it, the photon.

Some astronomers identify dark energy with Einstein's Cosmological Constant. Einstein introduced this constant into his general relativity when he saw that his theory was predicting an expanding universe, which was contrary to the evidence for a static universe that he and other physicists had in the early 20th century. This constant balanced the expansion and made the universe static. With Edwin Hubble's discovery of the expansion of the Universe, Einstein dismissed his constant. It later became identified with what quantum theory calls the energy of the vacuum.

In the context of dark energy, the cosmological constant is a reservoir which stores energy. Its energy scales as the universe expands. Applied to the supernova data, it would distinguish effects due to the matter in the universe from those due to the dark energy. Unfortunately, the amount of this stored energy required is far more than observed, and would result in very rapid acceleration (so much so that the stars and galaxies would not form). Physicists have suggested a new type of matter, "quintessence," which would fill the universe like a fluid which has a negative gravitational mass. However, new constraints imposed on cosmological parameters by Hubble Space Telescope data rule out at least simple models of quintessence.

Other possibilities being explored are topological defects, time varying forms of dark energy, or a dark energy that does not scale uniformly with the expansion of the universe.

excerpt taken from http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html

Quantum Cosmology

The physical laws that govern the universe prescribe how an initial state evolves with time. In classical physics, if the initial state of a system is specified exactly then the subsequent motion will be completely predictable. In quantum physics, specifying the initial state of a system allows one to calculate the probability that it will be found in any other state at a later time. Cosmology attempts to describe the behaviour of the entire universe using these physical laws. In applying these laws to the universe one immediately encounters a problem. What is the initial state that the laws should be applied to? In practice, cosmologists tend to work backwards by using the observed properties of the universe now to understand what it was like at earlier times. This approach has proved very successful. However it has led cosmologists back to the question of the initial conditions.

Inflation (a period of accelerating expansion in the very early universe) is now accepted as the standard explanation of several cosmological problems. In order for inflation to have occurred, the universe must have been formed containing some matter in a highly excited state. Inflationary theory does not address the question of why this matter was in such an excited state. Answering this demands a theory of the pre-inflationary initial conditions. There are two serious candidates for such a theory. The first, proposed by Andrei Linde of Stanford University, is called chaotic inflation. According to chaotic inflation, the universe starts off in a completely random state. In some regions matter will be more energetic than in others and inflation could ensue, producing the observable universe.

The second contender for a theory of initial conditions is quantum cosmology, the application of quantum theory to the entire universe. At first this sounds absurd because typically large systems (such as the universe) obey classical, not quantum, laws. Einstein's theory of general relativity is a classical theory that accurately describes the evolution of the universe from the first fraction of a second of its existence to now. However it is known that general relativity is inconsistent with the principles of quantum theory and is therefore not an appropriate description of physical processes that occur at very small length scales or over very short times. To describe such processes one requires a theory of quantum gravity.

In non-gravitational physics the approach to quantum theory that has proved most successful involves mathematical objects known as path integrals. Path integrals were introduced by the Nobel prizewinner Richard Feynman, of CalTech. In the path integral approach, the probability that a system in an initial state A will evolve to a final state B is given by adding up a contribution from every possible history of the system that starts in A and ends in B. For this reason a path integral is often referred to as a `sum over histories'. For large systems, contributions from similar histories cancel each other in the sum and only one history is important. This history is the history that classical physics would predict.

For mathematical reasons, path integrals are formulated in a background with four spatial dimensions rather than three spatial dimensions and one time dimension. There is a procedure known as `analytic continuation' which can be used to convert results expressed in terms of four spatial dimensions into results expressed in terms of three spatial dimensions and one time dimension. This effectively converts one of the spatial dimensions into the time dimension. This spatial dimension is sometimes referred to as `imaginary' time because it involves the use of so-called imaginary numbers, which are well defined mathematical objects used every day by electrical engineers.

The success of path integrals in describing non-gravitational physics naturally led to attempts to describe gravity using path integrals. Gravity is rather different from the other physical forces, whose classical description involves fields (e.g. electric or magnetic fields) propagating in spacetime. The classical description of gravity is given by general relativity, which says that the gravitational force is related to the curvature of spacetime itself i.e. to its geometry. Unlike for non-gravitational physics, spacetime is not just the arena in which physical processes take place but it is a dynamical field. Therefore a sum over histories of the gravitational field in quantum gravity is really a sum over possible geometries for spacetime.

The gravitational field at a fixed time can be described by the geometry of the three spatial dimensions at that time. The history of the gravitational field is described by the four dimensional spacetime that these three spatial dimensions sweep out in time. Therefore the path integral is a sum over all four dimensional spacetime geometries that interpolate between the initial and final three dimensional geometries. In other words it is a sum over all four dimensional spacetimes with two three dimensional boundaries which match the initial and final conditions. Once again, mathematical subtleties require that the path integral be formulated in four spatial dimensions rather than three spatial dimensions and one time dimension.

The path integral formulation of quantum gravity has many mathematical problems. It is also not clear how it relates to more modern attempts at constructing a theory of quantum gravity such as string/M-theory. However it can be used to correctly calculate quantities that can be calculated independently in other ways e.g. black hole temperatures and entropies.

We can now return to cosmology. At any moment, the universe is described by the geometry of the three spatial dimensions as well as by any matter fields that may be present. Given this data one can, in principle, use the path integral to calculate the probability of evolving to any other prescribed state at a later time. However this still requires a knowledge of the initial state, it does not explain it.

Quantum cosmology is a possible solution to this problem. In 1983, Stephen Hawking and James Hartle developed a theory of quantum cosmology which has become known as the `No Boundary Proposal'. Recall that the path integral involves a sum over four dimensional geometries that have boundaries matching onto the initial and final three geometries. The Hartle-Hawking proposal is to simply do away with the initial three geometry i.e. to only include four dimensional geometries that match onto the final three geometry. The path integral is interpreted as giving the probability of a universe with certain properties (i.e. those of the boundary three geometry) being created from nothing.

In practice, calculating probabilities in quantum cosmology using the full path integral is formidably difficult and an approximation has to be used. This is known as the semiclassical approximation because its validity lies somewhere between that of classical and quantum physics. In the semiclassical approximation one argues that most of the four dimensional geometries occuring in the path integral will give very small contributions to the path integral and hence these can be neglected. The path integral can be calculated by just considering a few geometries that give a particularly large contribution. These are known as instantons. Instantons don't exist for all choices of boundary three geometry; however those three geometries that do admit the existence of instantons are more probable than those that don't. Therefore attention is usually restricted to three geometries close to these.

Remember that the path integral is a sum over geometries with four spatial dimensions. Therefore an instanton has four spatial dimensions and a boundary that matches the three geometry whose probability we wish to compute. Typical instantons resemble (four dimensional) surfaces of spheres with the three geometry slicing the sphere in half. They can be used to calculate the quantum process of universe creation, which cannot be described using classical general relativity. They only usually exist for small three geometries, corresponding to the creation of a small universe. Note that the concept of time does not arise in this process. Universe creation is not something that takes place inside some bigger spacetime arena - the instanton describes the spontaneous appearance of a universe from literally nothing. Once the universe exists, quantum cosmology can be approximated by general relativity so time appears.

People have found different types of instantons that can provide the initial conditions for realistic universes. The first attempt to find an instanton that describes the creation of a universe within the context of the `no boundary' proposal was made by Stephen Hawking and Ian Moss. The Hawking-Moss instanton describes the creation of an eternally inflating universe with `closed' spatial three-geometries.

It is presently an unsolved question whether our universe contains closed, flat or open spatial three-geometries. In a flat universe, the large-scale spatial geometry looks like the ordinary three-dimensional space we experience around us. In contrast to this, the spatial sections of a realistic closed universe would look like three-dimensional (surfaces of) spheres with a very large but finite radius. An open geometry would look like an infinite hyperboloid. Only a closed universe would therefore be finite. There is, however, nowadays strong evidence from cosmological observations in favour of an infinite open universe. It is therefore an important question whether there exist instantons that describe the creation of open universes.

The idea behind the Coleman-De Luccia instanton, discovered in 1987, is that the matter in the early universe is initially in a state known as a false vacuum. A false vacuum is a classically stable excited state which is quantum mechanically unstable. In the quantum theory, matter which is in a false vacuum may `tunnel' to its true vacuum state. The quantum tunnelling of the matter in the early universe was described by Coleman and De Luccia. They showed that false vacuum decay proceeds via the nucleation of bubbles in the false vacuum. Inside each bubble the matter has tunnelled. Surprisingly, the interior of such a bubble is an infinite open universe in which inflation may occur. The cosmological instanton describing the creation of an open universe via this bubble nucleation is known as a Coleman-De Luccia instanton.

The Coleman-De Luccia Instanton

Remember that this scenario requires the existence of a false vacuum for the matter in the early universe. Moreover, the condition for inflation to occur once the universe has been created strongly constrains the way the matter decays to its true vacuum. Therefore the creation of open inflating universes appears to be rather contrived in the absence of any explanation of these specific pre-inflationary initial conditions.

Recently, Stephen Hawking and Neil Turok have proposed a bold solution to this problem. They constructed a class of instantons that give rise to open universes in a similar way to the instantons of Coleman and De Luccia. However, they did not require the existence of a false vacuum or other very specific properties of the excited matter state. The price they pay for this is that their instantons have singularities: places where the curvature becomes infinite. Since singularities are usually regarded as places where the theory breaks down and must be replaced by a more fundamental theory, this is a quite controversial feature of their work.

The Hawking-Turok Instanton

The question of course arises which of these instantons describes correctly the creation of our own universe. The way one might hope to distinguish between different theories of quantum cosmology is by considering quantum fluctuations about these instantons. The Heisenberg uncertainty principle in quantum mechanics implies that vacuum fluctuations are present in every quantum theory. In the full quantum picture therefore, an instanton provides us just with a background geometry in the path integral with respect to which quantum fluctuations need to be considered.

During inflation, these quantum mechanical vacuum fluctuations are amplified and due to the accelerating expansion of the universe they are stretched to macroscopic length scales. Later on, when the universe has cooled, they seed the growth of large scale structures (e.g. galaxies) like those we see today. One sees the imprint of these primordial fluctuations as small temperature perturbations in the cosmic microwave background radiation.

Since different types of instantons predict slightly different fluctuation spectra, the temperature perturbations in the cosmic microwave background radiation will depend on the instanton from which the universe was created. In the next decade the satellites MAP and PLANCK will be launched to measure the temperature of the microwave background radiation in different directions on the sky to a very high accuracy. The observations will not only provide us with a very important test of inflation itself but may also be the first possibility to observationally distinguish between different theories for quantum cosmology.

excerpt taken from http://www.damtp.cam.ac.uk/user/gr/public/qg_qc.html

Why is the sky blue?

It is easy to see that the sky is blue. Have you ever wondered why? A lot of other smart people have, too. And it took a long time to figure it out!

The light from the Sun looks white. But it is really made up of all the colors of the rainbow.

A prism separates white light into the colors of the rainbow.

A prism is a specially shaped crystal. When white light shines through a prism, the light is separated into all its colors.

If you visited The Land of the Magic Windows, you learned that the light you see is just one tiny bit of all the kinds of light energy beaming around the Universe--and around you!

Like energy passing through the ocean, light energy travels in waves, too. Some light travels in short, "choppy" waves. Other light travels in long, lazy waves. Blue light waves are shorter than red light waves.

Different colors of light have different wavelengths.

All light travels in a straight line unless something gets in the way to--

* reflect it (like a mirror)

* bend it (like a prism)

* or scatter it (like molecules of the gases in the atmosphere)

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

Atmosphere scatters blue light more than other colors.

Closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead. As the sunlight has passed through all this air, the air molecules have scattered and rescattered the blue light many times in many directions. Also, the surface of Earth has reflected and scattered the light. All this scattering mixes the colors together again so we see more white and less blue.

What Makes a Red Sunset?

As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.

Red sky at sunset

Sometimes the whole western sky seems to glow. The sky appears red because larger particles of dust, pollution, and water vapor in the atmosphere reflect and scatter more of the reds and yellows.

Red sun at sunset.
Why Does Scattering Matter?

How much of the Sun's light gets bounced around in Earth's atmosphere and how much gets reflected back into space? How much light gets soaked up by land and water, asphalt freeways and sunburned surfers? How much light do water and clouds reflect back into space? And why do we care?

Sunlight carries the energy that heats Earth and powers all life on Earth. Our climate is affected by how sunlight is scattered by forests, deserts, snow- and ice-covered surfaces, different types of clouds, smoke from forest fires, and other pollutants in the air.

excerpt taken from http://spaceplace.nasa.gov/en/kids/misrsky/misr_sky.shtml