The Model Universe And Other Stories

A Model World and Other Stories

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By traveling, an observer can come into contact with a greater region of spacetime than an observer who remains still. Nevertheless, even the most rapid traveler will not be able to interact with all of space. Typically, the observable universe is taken to mean the portion of the Universe that is observable from our vantage point in the Milky Way.

The distance the light from the edge of the observable universe has travelled is very close to the age of the Universe times the speed of light , Because we cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the Universe in its totality is finite or infinite. Astronomers calculate the age of the Universe by assuming that the Lambda-CDM model accurately describes the evolution of the Universe from a very uniform, hot, dense primordial state to its present state and measuring the cosmological parameters which constitute the model.

Over time, the Universe and its contents have evolved; for example, the relative population of quasars and galaxies has changed [55] and space itself has expanded. Due to this expansion, scientists on Earth can observe the light from a galaxy 30 billion light years away even though that light has traveled for only 13 billion years; the very space between them has expanded.

This expansion is consistent with the observation that the light from distant galaxies has been redshifted ; the photons emitted have been stretched to longer wavelengths and lower frequency during their journey. Analyses of Type Ia supernovae indicate that the spatial expansion is accelerating. The more matter there is in the Universe, the stronger the mutual gravitational pull of the matter. If the Universe were too dense then it would re-collapse into a gravitational singularity.

However, if the Universe contained too little matter then the self-gravity would be too weak for galaxies and hence planets to form. Since the Big Bang, the universe has expanded monotonically. Perhaps unsurprisingly , our universe has just the right mass-energy density equivalent to about 5 protons per cubic meter, which has allowed it to expand for the last There are dynamical forces acting on the particles in the Universe which affect the expansion rate.

Before , it was expected that the expansion rate would be decreasing as time went on due to the influence of gravitational interactions in the Universe; and thus there is an additional observable quantity in the Universe called the deceleration parameter , which most cosmologists expected to be positive and related to the matter density of the Universe. In , the deceleration parameter was measured by two different groups to be negative, approximately Spacetimes are the arenas in which all physical events take place.

The basic elements of spacetimes are events. In any given spacetime, an event is defined as a unique position at a unique time. A spacetime is the union of all events in the same way that a line is the union of all of its points , formally organized into a manifold. The Universe appears to be a smooth spacetime continuum consisting of three spatial dimensions and one temporal time dimension an event in the spacetime of the physical Universe can therefore be identified by a set of four coordinates: On the average, space is observed to be very nearly flat with a curvature close to zero , meaning that Euclidean geometry is empirically true with high accuracy throughout most of the Universe.

However, present observations cannot exclude the possibilities that the Universe has more dimensions which is postulated by theories such as the String theory and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or toroidal topologies of two-dimensional spaces.

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Spacetime events are not absolutely defined spatially and temporally but rather are known to be relative to the motion of an observer. Minkowski space approximates the Universe without gravity ; the pseudo-Riemannian manifolds of general relativity describe spacetime with matter and gravity. General relativity describes how spacetime is curved and bent by mass and energy gravity.

The topology or geometry of the Universe includes both local geometry in the observable universe and global geometry. Cosmologists often work with a given space-like slice of spacetime called the comoving coordinates. The section of spacetime which can be observed is the backward light cone , which delimits the cosmological horizon. The cosmological horizon also called the particle horizon or the light horizon is the maximum distance from which particles can have traveled to the observer in the age of the Universe.

This horizon represents the boundary between the observable and the unobservable regions of the Universe. These are called, respectively, the flat, open and closed universes. The Universe may be fine-tuned ; the Fine-tuned Universe hypothesis is the proposition that the conditions that allow the existence of observable life in the Universe can only occur when certain universal fundamental physical constants lie within a very narrow range of values, so that if any of several fundamental constants were only slightly different, the Universe would have been unlikely to be conducive to the establishment and development of matter , astronomical structures, elemental diversity, or life as it is understood.

The Universe is composed almost completely of dark energy, dark matter, and ordinary matter. Other contents are electromagnetic radiation estimated to constitute from 0. The proportions of all types of matter and energy have changed over the history of the Universe. Dark matter, a mysterious form of matter that has not yet been identified, accounts for Dark energy, which is the energy of empty space and is causing the expansion of the Universe to accelerate, accounts for the remaining Matter, dark matter, and dark energy are distributed homogeneously throughout the Universe over length scales longer than million light-years or so.

Between the larger structures are voids , which are typically 10— Mpc 33 million— million ly in diameter. The observable Universe is isotropic on scales significantly larger than superclusters, meaning that the statistical properties of the Universe are the same in all directions as observed from Earth. The Universe is bathed in highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2. An explanation for why the expansion of the Universe is accelerating remains elusive.

It is often attributed to "dark energy", an unknown form of energy that is hypothesized to permeate space. However, in the present dark-energy era, it dominates the mass—energy of the universe because it is uniform across space. Two proposed forms for dark energy are the cosmological constant , a constant energy density filling space homogeneously, [97] and scalar fields such as quintessence or moduli , dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant.

The cosmological constant can be formulated to be equivalent to vacuum energy. Scalar fields having only a slight amount of spatial inhomogeneity would be difficult to distinguish from a cosmological constant.

Dark matter is a hypothetical kind of matter that is invisible to the entire electromagnetic spectrum , but which accounts for most of the matter in the Universe. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the Universe. Other than neutrinos , a form of hot dark matter , dark matter has not been detected directly, making it one of the greatest mysteries in modern astrophysics.

Dark matter neither emits nor absorbs light or any other electromagnetic radiation at any significant level. Dark matter is estimated to constitute This matter includes stars , which produce nearly all of the light we see from galaxies, as well as interstellar gas in the interstellar and intergalactic media, planets , and all the objects from everyday life that we can bump into, touch or squeeze.

Ordinary matter commonly exists in four states or phases: However, advances in experimental techniques have revealed other previously theoretical phases, such as Bose—Einstein condensates and fermionic condensates. Ordinary matter is composed of two types of elementary particles: An atom consists of an atomic nucleus , made up of protons and neutrons, and electrons that orbit the nucleus. Because most of the mass of an atom is concentrated in its nucleus, which is made up of baryons , astronomers often use the term baryonic matter to describe ordinary matter, although a small fraction of this "baryonic matter" is electrons.

Soon after the Big Bang , primordial protons and neutrons formed from the quark—gluon plasma of the early Universe as it cooled below two trillion degrees. A few minutes later, in a process known as Big Bang nucleosynthesis , nuclei formed from the primordial protons and neutrons.

This nucleosynthesis formed lighter elements, those with small atomic numbers up to lithium and beryllium , but the abundance of heavier elements dropped off sharply with increasing atomic number. Some boron may have been formed at this time, but the next heavier element, carbon , was not formed in significant amounts. Big Bang nucleosynthesis shut down after about 20 minutes due to the rapid drop in temperature and density of the expanding Universe. Subsequent formation of heavier elements resulted from stellar nucleosynthesis and supernova nucleosynthesis.

Ordinary matter and the forces that act on matter can be described in terms of elementary particles. A true force-particle "theory of everything" has not been attained. A hadron is a composite particle made of quarks held together by the strong force. Hadrons are categorized into two families: Of the hadrons, protons are stable, and neutrons bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions and are thus insignificant constituents of the modern Universe. Most of the hadrons and anti-hadrons were then eliminated in particle-antiparticle annihilation reactions, leaving a small residual of hadrons by the time the Universe was about one second old.

A lepton is an elementary , half-integer spin particle that does not undergo strong interactions but is subject to the Pauli exclusion principle ; no two leptons of the same species can be in exactly the same state at the same time. Electrons are stable and the most common charged lepton in the Universe, whereas muons and taus are unstable particle that quickly decay after being produced in high energy collisions, such as those involving cosmic rays or carried out in particle accelerators.

The electron governs nearly all of chemistry , as it is found in atoms and is directly tied to all chemical properties. Neutrinos rarely interact with anything, and are consequently rarely observed. Neutrinos stream throughout the Universe but rarely interact with normal matter. The lepton epoch was the period in the evolution of the early Universe in which the leptons dominated the mass of the Universe. It started roughly 1 second after the Big Bang , after the majority of hadrons and anti-hadrons annihilated each other at the end of the hadron epoch.

The mass of the Universe was then dominated by photons as it entered the following photon epoch. A photon is the quantum of light and all other forms of electromagnetic radiation. It is the force carrier for the electromagnetic force , even when static via virtual photons. The effects of this force are easily observable at the microscopic and at the macroscopic level because the photon has zero rest mass ; this allows long distance interactions.

Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave—particle duality , exhibiting properties of waves and of particles. The photon epoch started after most leptons and anti-leptons were annihilated at the end of the lepton epoch, about 10 seconds after the Big Bang. Atomic nuclei were created in the process of nucleosynthesis which occurred during the first few minutes of the photon epoch. For the remainder of the photon epoch the Universe contained a hot dense plasma of nuclei, electrons and photons. About , years after the Big Bang, the temperature of the Universe fell to the point where nuclei could combine with electrons to create neutral atoms.

As a result, photons no longer interacted frequently with matter and the Universe became transparent. The highly redshifted photons from this period form the cosmic microwave background. Tiny variations in temperature and density detectable in the CMB were the early "seeds" from which all subsequent structure formation took place. General relativity is the geometric theory of gravitation published by Albert Einstein in and the current description of gravitation in modern physics. It is the basis of current cosmological models of the Universe.

General relativity generalizes special relativity and Newton's law of universal gravitation , providing a unified description of gravity as a geometric property of space and time , or spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present.

The relation is specified by the Einstein field equations , a system of partial differential equations. In general relativity, the distribution of matter and energy determines the geometry of spacetime, which in turn describes the acceleration of matter. Therefore, solutions of the Einstein field equations describe the evolution of the Universe. Combined with measurements of the amount, type, and distribution of matter in the Universe, the equations of general relativity describe the evolution of the Universe over time.

This metric has only two undetermined parameters. An overall dimensionless length scale factor R describes the size scale of the Universe as a function of time; an increase in R is the expansion of the Universe. The index k is defined so that it can take only one of three values: When R changes, all the spatial distances in the Universe change in tandem; there is an overall expansion or contraction of space itself.

This accounts for the observation that galaxies appear to be flying apart; the space between them is stretching. The stretching of space also accounts for the apparent paradox that two galaxies can be 40 billion light years apart, although they started from the same point Second, all solutions suggest that there was a gravitational singularity in the past, when R went to zero and matter and energy were infinitely dense.

It may seem that this conclusion is uncertain because it is based on the questionable assumptions of perfect homogeneity and isotropy the cosmological principle and that only the gravitational interaction is significant. However, the Penrose—Hawking singularity theorems show that a singularity should exist for very general conditions.

Hence, according to Einstein's field equations, R grew rapidly from an unimaginably hot, dense state that existed immediately following this singularity when R had a small, finite value ; this is the essence of the Big Bang model of the Universe. Understanding the singularity of the Big Bang likely requires a quantum theory of gravity , which has not yet been formulated. Third, the curvature index k determines the sign of the mean spatial curvature of spacetime [] averaged over sufficiently large length scales greater than about a billion light years.

Conversely, if k is zero or negative, the Universe has an infinite volume. By analogy, an infinite plane has zero curvature but infinite area, whereas an infinite cylinder is finite in one direction and a torus is finite in both. A toroidal Universe could behave like a normal Universe with periodic boundary conditions. Some speculative theories have proposed that our Universe is but one of a set of disconnected universes, collectively denoted as the multiverse , challenging or enhancing more limited definitions of the Universe.

Max Tegmark developed a four-part classification scheme for the different types of multiverses that scientists have suggested in response to various Physics problems. An example of such multiverses is the one resulting from the chaotic inflation model of the early universe. In this interpretation, parallel worlds are generated in a manner similar to quantum superposition and decoherence , with all states of the wave functions being realized in separate worlds.

Effectively, in the many-worlds interpretation the multiverse evolves as a universal wavefunction. If the Big Bang that created our multiverse created an ensemble of multiverses, the wave function of the ensemble would be entangled in this sense. The least controversial category of multiverse in Tegmark's scheme is Level I. The multiverses of this level are composed by distant spacetime events "in our own universe". If space is infinite, or sufficiently large and uniform, identical instances of the history of Earth's entire Hubble volume occur every so often, simply by chance.

However, this existence does follow as a fairly straightforward consequence from otherwise unrelated scientific observations and theories. It is possible to conceive of disconnected spacetimes, each existing but unable to interact with one another. The entire collection of these separate spacetimes is denoted as the multiverse.

Historically, there have been many ideas of the cosmos cosmologies and its origin cosmogonies. Theories of an impersonal Universe governed by physical laws were first proposed by the Greeks and Indians. The modern era of cosmology began with Albert Einstein 's general theory of relativity , which made it possible to quantitatively predict the origin, evolution, and conclusion of the Universe as a whole. Most modern, accepted theories of cosmology are based on general relativity and, more specifically, the predicted Big Bang. Many cultures have stories describing the origin of the world and universe.

Cultures generally regard these stories as having some truth. There are however many differing beliefs in how these stories apply amongst those believing in a supernatural origin, ranging from a god directly creating the Universe as it is now to a god just setting the "wheels in motion" for example via mechanisms such as the big bang and evolution. Ethnologists and anthropologists who study myths have developed various classification schemes for the various themes that appear in creation stories.

In related stories, the Universe is created by a single entity emanating or producing something by him- or herself, as in the Tibetan Buddhism concept of Adi-Buddha , the ancient Greek story of Gaia Mother Earth , the Aztec goddess Coatlicue myth, the ancient Egyptian god Atum story, and the Judeo-Christian Genesis creation narrative in which the Abrahamic God created the Universe. In another type of story, the Universe is created from the union of male and female deities, as in the Maori story of Rangi and Papa.

In other stories, the Universe emanates from fundamental principles, such as Brahman and Prakrti , the creation myth of the Serers , [] or the yin and yang of the Tao. The pre-Socratic Greek philosophers and Indian philosophers developed some of the earliest philosophical concepts of the Universe. In particular, they noted the ability of matter to change forms e. The first to do so was Thales , who proposed this material to be water.

Thales' student, Anaximander , proposed that everything came from the limitless apeiron. Anaximenes proposed the primordial material to be air on account of its perceived attractive and repulsive qualities that cause the arche to condense or dissociate into different forms. Anaxagoras proposed the principle of Nous Mind , while Heraclitus proposed fire and spoke of logos. Empedocles proposed the elements to be earth, water, air and fire.

His four-element model became very popular. Like Pythagoras , Plato believed that all things were composed of number , with Empedocles' elements taking the form of the Platonic solids. Democritus , and later philosophers—most notably Leucippus —proposed that the Universe is composed of indivisible atoms moving through a void vacuum , although Aristotle did not believe that to be feasible because air, like water, offers resistance to motion. Air will immediately rush in to fill a void, and moreover, without resistance, it would do so indefinitely fast.

Although Heraclitus argued for eternal change, his contemporary Parmenides made the radical suggestion that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature. Parmenides' idea seemed implausible to many Greeks, but his student Zeno of Elea challenged them with several famous paradoxes.

Aristotle responded to these paradoxes by developing the notion of a potential countable infinity, as well as the infinitely divisible continuum. Unlike the eternal and unchanging cycles of time, he believed that the world is bounded by the celestial spheres and that cumulative stellar magnitude is only finitely multiplicative. The Indian philosopher Kanada , founder of the Vaisheshika school, developed a notion of atomism and proposed that light and heat were varieties of the same substance. They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.

The notion of temporal finitism was inspired by the doctrine of creation shared by the three Abrahamic religions: Judaism , Christianity and Islam. The Christian philosopher , John Philoponus , presented the philosophical arguments against the ancient Greek notion of an infinite past and future.

Astronomical models of the Universe were proposed soon after astronomy began with the Babylonian astronomers , who viewed the Universe as a flat disk floating in the ocean, and this forms the premise for early Greek maps like those of Anaximander and Hecataeus of Miletus. Later Greek philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the Universe-based more profoundly on empirical evidence. The first coherent model was proposed by Eudoxus of Cnidos. According to Aristotle's physical interpretation of the model, celestial spheres eternally rotate with uniform motion around a stationary Earth.

Normal matter is entirely contained within the terrestrial sphere. De Mundo composed before BC or between and BC , stated, "Five elements, situated in spheres in five regions, the less being in each case surrounded by the greater—namely, earth surrounded by water, water by air, air by fire, and fire by ether—make up the whole Universe". This model was also refined by Callippus and after concentric spheres were abandoned, it was brought into nearly perfect agreement with astronomical observations by Ptolemy.

The success of such a model is largely due to the mathematical fact that any function such as the position of a planet can be decomposed into a set of circular functions the Fourier modes.

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Other Greek scientists, such as the Pythagorean philosopher Philolaus , postulated according to Stobaeus account that at the center of the Universe was a "central fire" around which the Earth , Sun , Moon and Planets revolved in uniform circular motion. The Greek astronomer Aristarchus of Samos was the first known individual to propose a heliocentric model of the Universe.

Though the original text has been lost, a reference in Archimedes ' book The Sand Reckoner describes Aristarchus's heliocentric model. You, King Gelon, are aware the Universe is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth.

The Heat Death of the Universe and Other Stories

This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the Universe is many times greater than the Universe just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.

Aristarchus thus believed the stars to be very far away, and saw this as the reason why stellar parallax had not been observed, that is, the stars had not been observed to move relative each other as the Earth moved around the Sun. The stars are in fact much farther away than the distance that was generally assumed in ancient times, which is why stellar parallax is only detectable with precision instruments. The geocentric model, consistent with planetary parallax, was assumed to be an explanation for the unobservability of the parallel phenomenon, stellar parallax.

The rejection of the heliocentric view was apparently quite strong, as the following passage from Plutarch suggests On the Apparent Face in the Orb of the Moon:. Cleanthes [a contemporary of Aristarchus and head of the Stoics ] thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the Universe [i. The only other astronomer from antiquity known by name who supported Aristarchus's heliocentric model was Seleucus of Seleucia , a Hellenistic astronomer who lived a century after Aristarchus. Seleucus' arguments for a heliocentric cosmology were probably related to the phenomenon of tides.

The Aristotelian model was accepted in the Western world for roughly two millennia, until Copernicus revived Aristarchus's perspective that the astronomical data could be explained more plausibly if the Earth rotated on its axis and if the Sun were placed at the center of the Universe.

In the center rests the Sun. For who would place this lamp of a very beautiful temple in another or better place than this wherefrom it can illuminate everything at the same time? As noted by Copernicus himself, the notion that the Earth rotates is very old, dating at least to Philolaus c. Roughly a century before Copernicus, the Christian scholar Nicholas of Cusa also proposed that the Earth rotates on its axis in his book, On Learned Ignorance Empirical evidence for the Earth's rotation on its axis, using the phenomenon of comets , was given by Tusi — and Ali Qushji — This cosmology was accepted by Isaac Newton , Christiaan Huygens and later scientists.

In , when Hooker Telescope was completed, the prevailing view still was that the Universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope, Edwin Hubble identified Cepheid variables in several spiral nebulae and in — proved conclusively that Andromeda Nebula and Triangulum among others, were entire galaxies outside our own, thus proving that Universe consists of multitude of galaxies. The modern era of physical cosmology began in , when Albert Einstein first applied his general theory of relativity to model the structure and dynamics of the Universe.

Kanada, founder of the Vaisheshika philosophy, held that the world is composed of atoms as many in kind as the various elements. The Jains more nearly approximated to Democritus by teaching that all atoms were of the same kind, producing different effects by diverse modes of combinations.

Kanada believed light and heat to be varieties of the same substance; Udayana taught that all heat comes from the Sun; and Vachaspati , like Newton , interpreted light as composed of minute particles emitted by substances and striking the eye. Movement consists for them of moments, it is a staccato movement, momentary flashes of a stream of energy They are called "qualities" guna-dharma in both systems in the sense of absolute qualities, a kind of atomic, or intra-atomic, energies of which the empirical things are composed.

Both systems, therefore, agree in denying the objective reality of the categories of Substance and Quality, What we call quality is but a particular manifestation of a subtle entity. To every new unit of quality corresponds a subtle quantum of matter which is called guna , "quality", but represents a subtle substantive entity. The same applies to early Buddhism where all qualities are substantive From Wikipedia, the free encyclopedia.

For other uses, see Universe disambiguation. The Hubble Ultra-Deep Field image shows some of the most remote galaxies visible with present technology, each consisting of billions of stars. Discovery of cosmic microwave background radiation. Religious interpretations of the Big Bang theory. Big Bang and Chronology of the universe. Human timeline and Life timeline. Observable universe , Age of the Universe , and Metric expansion of space. Observable universe and Observational cosmology. Age of the universe and Metric expansion of space.

Spacetime and World line. Shape of the universe. Galaxy formation and evolution , Galaxy cluster , Illustris project , and Nebula. Timeline of the Big Bang. Big Crunch Big Rip Heat death of the universe. 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 , a few months after my first singularity result, with the discovery of a faint background of microwaves throughout space.

The Model Universe - Scribbles and Tunes

These microwaves are the same as those in your microwave oven, but very much less powerful. They would heat your pizza only to minus 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. 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.

A Storm in Tormay

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 , years. Despite having had some great successes, not everything is solved.

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