Cosmology

Cosmology is a branch of astronomy and of metaphysics committed to the study of the universe as a whole, of the contents, structure, and evolution of the universe from the beginning of time to the future.

Cosmology can also be viewed as the complement of particle physics, a field that has greatly enhanced research in cosmology. Particle physics can be summarised as the study of the very small while cosmology is the study of the very large.

As a branch of two very old endeavours of humanity, the study of the sky and the origins of the world, cosmology embraces numerous related inquiries concerned with the world we live in and, to the extent it could be imagined and observed, the universe. As a branch of metaphysics it attained some prominence when Christian von Wolffe published Discursus Praeliminaris de Philosophia in Genere (1728). Von Wolff placed cosmology in his classification scheme of the main areas of philosophy, distinguishing cosmology from ontology, theology and psychology, essentially making it a distinctive field of philosophy unto itself. As a philosophy, its nature has been disputed over time since von Wolff but it can be said that generally, it is thought to encompass humanity's experience in, and the nature of, the physical world. As a science, it encompasses the work of observational astronomy and theoretical physics as scholars in both fields attempt to describe and explain the physical universe. Cosmology, as a science, attempts to construct models of the physical universe from observational data which are then tested. Cosmology, as metaphysics, involves a priori investigations of a rational cosmology and the conceptual and categorical analyses of the speculative philosopher.

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Ancient cosmology

Any attempt to understand cosmology, or simply what humanity thought about the universe whenever that might have occurred to them prior to the last few millennia would be largely guess work. Their understanding would have been inextricably interwoven with their impressions and their imagination. Impressions would have been made in a facile manner, what ever they could see where ever they were, their immediate circumstance, their fears and attempts at explaining the mysterious, all would have come together in a variety of concepts about the nature of the universe.

What we consider cosmology today, if we are addressing explanations from observations and precise measurements that can be duplicated—the scientific approach—or at least its basic and most primitive roots, would bare only passing resemblance to the ancient cosmology which would have been fraught with almost anything but what we consider scientific. Simple deduction about how fire is made and food accessed and shelter constructed would have been within the reach of prehistoric and ancient humanity, but how close they came to an understanding, no matter how fragmented or trivial, of the universe as we know it today, is simply beyond us to ever know. That they observed that things seemed to be regular and predictable is evident from stone circles and other constructions that indicated they knew the sun would strike a certain point on the ground at a certain time every year, the moon would change appearance on a regular basis—any number of possible conclusions could be reached about these clues that have been left. Burial customs going back as far as the last twenty millennium might indicate a belief in the after life, possibly an eternal existence—but that too is reaching and can not be safely concluded.

Prehistoric people left only fragments of their impressions in relics, most of which have long since disappeared. The ancients left us only fragmented documents, and instruments both crude and ingenious. As a consequence, prehistoric and ancient humanity's understanding will always be a fragmented picture to us today. In fact, if the definition of cosmology would be constructed around what it was meant to achieve, if it was meant to explain things, it would not necessarily follow that those explanations of long ago would be tested as we expect scientific theories to be tested today. Their explanations were often the interactions of extraordinary people and monsters, imaginary places and incredible events which could not be verified, such as monsters at the edge of the world as mariners sometimes believed as late as the 15th century.

Creation stories

Creation stories represent some of the earliest attempts to explain the origins of the world.

Mesopotamia

Mesopotamia refers to the area between the Tigris and Euphrates rivers in what is approximately present day Iraq. It was host to numerous civilisations dating back as far as 5000 B.C.. The more historically significant Mesopotamian cultures include the Sumerians (2900 to 1800 B.C.) who may have made the greatest impact over time, Akkadians (2340 to 2125 B.C.), Amorites (1800 to 1530 B.C.), Hittites (1600 to 717 B.C.), Kassites (1530 to 1170 B.C.), Assyrians (1170 to 612 B.C.) and finally the Chaldeans (612 to 599 B.C.) who were in turn over come by the Persians. The Persians went on to build an empire that encompassed Asia Minor, Lydia, Judah, Mesopotamia, and Egypt and developed their own religion, Zorastrianism, with its distinctive cosmology.

Sumerian

One of the oldest of the civilisations (dated variously over periods during the 5th millennia B.C to 1750 B.C.), Sumeria began as a collection of city states in the Tigris-Euphrates valley and were evidently sporadically united by various rulers. The Sumerian King List includes what are obviously mythical kings who began their rule before a flood. At the end of the Sumerian civilisation, what remained of the Sumerians was eventually subsumed by other groups and a common end point given is their conquest by Babylon. Sumerian as a spoken language ceased to exist about 2,000 B.C.

Sumerian creation mythology is evidently preserved in one source. The account is an introduction to the story, The Huluppu-Tree. Hymns and myths also contain verses that provide some information. They believed their world, the universe (anki), gods, and people, was created or formed out of an ancient sea (abzu) which pre-existed anything else. The boundary between heaven and earth was a solid vault, the earth was a flat disc and within the vault lay the gas lil, or atmosphere. Brighter portions of this atmosphere formed the stars, planets and sun and moon. Their universe was ruled by anthropomorphic gods (human features and personalities) representing various natural phenomenon. e.g. Enki (god of water), Ki (god of earth), Enlil (god of air), and An (god of heaven). In the beginning heaven and earth were one and then separated. Heaven was carried off by the Sky God, An, and earth carried off by the Air God, Enlil. Enil was the son of the male sky god An and the female earth, Ki. Enil later became the primary god of the Sumerian pantheon. Enlil carried off his mother and took his father’s place, a story that seems to prefigure Kronos who later took over from his father, Ouranos. Heaven and earth (An and Ki) were descended from Nammu, the Sea, referred to as “the mother”.

Another deity, Ereshkigal, the queen of the underworld, was carried off to the underworld. Ereshkigal seems to be an earlier version of Persephone. After Ereshkigal was carried off, the water god, Enki set sail only to have his boat swamped. It is unclear if the water god was attempting to rescue Ereshkigal, but his voyage may be an early version of Inanna's successful voyage to the underworld.

After the gods had established cities for Enlil, he rapes a goddess, Ninlil, who begets the moon god, Nanna (also referred to by the name Sin). Enlil is run out of town by the other gods for his crime and Ninlil follows him as he heads to the underworld. However, Enlil does not want his son, the moon, to live in the underworld so he impersonates three other men and has consensual sex with Ninlil, impregnating her three times, resulting in three separate offspring who will live with Ninlil in the underworld, setting Nanna free to rise to the heavens.

Sumerians also believed the gods created the rules of Sumerian society and the reason for humanity’s existence was for the gratification of the gods.

Babylon

The Babylonian vision of the universe seems to begin as does the Sumerian, from water. Their creation myth describes a world of fresh water, oceans and mists. It was featureless otherwise without solid land. Each had a god, Apsu, Tiamat and Mummu repectively and these gods eventually begat other gods who begat other gods and so forth. Eventually the god Ea, the cleverest, came forth and became the most powerful ruling over all the other gods. As the population of gods increased Apsu grew tired of the other gods and plotted to slay their off spring. However, Ea beat him to it, killed him and built a palace on the fresh waters and with another god fathered Marmaduke or Marduk, a giant who became the god of rain and storms. Ea’s crime did not go unpunished and Tiamat raised up an army of monsters which was more than Ea could withstand so he called forth Marmaduke to fight. Marmaduke agreed, with the condition that upon winning he would assume absolute power over all the other gods. His condition was met and he prevailed over Tiamat’s host. After the ensuing carnage, Marmaduke took the body of Tiamat and used half for the sky and the other half for land. One of the defeated gods was later slain and his body was used to create humans.

The themes of conflict between supernatural beings, a universe originating from a seemingly homogenous source without distinct definition, employing the bodies of gods slain in battle or dead by other means to create additional features of the world are very common. As such there is no attempt to correlate the explanation with precise measurements, duplicated results, nothing even vaguely scientific. It was a world of imagination and evidently remained that way for millennium.

China

The Chinese creation myth from an anthology, The Classic of Mountains and Seas, collected in the first century B.C., contains some details vaguely reminiscent of how modern cosmology views the beginning of the universe today. At the beginning, heaven and earth were not separate and contained in an egg-shaped cloud wherein all matter existed in chaos. The giant Pan Gu, grew in the chaos, sleeping and developing for 18,000 years. When he awoke and stretched, the egg broke releasing the matter of the universe. The lighter purer elements rose to make the sky and heavens, and the heavier impure elements settled to make the earth. Pan Gu lived another 18,000 years holding the sky and the earth separate to a distance of 30,000 miles. When Pan Gu died his body became parts of the world: His arms and legs became the four directions and the mountains; his blood became the rivers; his sweat, the rain and dew.; his voice, the thunder; his breath, the winds; his hair, the grass; his veins, the roads and paths; teeth and bones, the minerals and rocks; his flesh became the soil; his left eye the sun, and his right the moon.

Sometime later a goddess named Nü Wa comes into the picture, and being lonely she made humans out of mud. Seeing that humans would not live forever, she divided them into male and female so they could procreate. Her first creations made by hand were the rulers and aristocrats, others she made from mud slung from a vine become the common folk.

Eventually there was some kind of geological upheaval and the heavens collapsed creating holes in the sky and a deluge ensued, then the earth cracked from which poured fire. Beasts appeared in the forests and attacked humans. She drove the beasts away, placed river stones to patch the holes in the sky and put a turtle in place to hold the earth and sky separate. She used molten rock to fill the holes in the earth. Upon her death, her body became more parts of the world

Many creation stories contain a vision of the universe as formless or without land and other features. People are often made from the soil itself.

Cyclical cosmology

Whereas the above creation stories can be classified as "linear" cosmologies. Other ancient traditions believed in a cyclical cosmology, holding that the world is repeatedly created and destroyed over enormous timescales. This is most well known in traditional Indian (Hindu and Buddhist) cosmology.

Physical cosmology

At various points throughout history, there is evidence of the attempt to provide a meaningful explanation of the origin of the physical world that corresponds to both observation and description of the physical world by measuring it and the idea that the explanation should predict what will happen next. Early attempts at discerning such things as the shape of Earth, the cycles of the Moon, and the distance from Earth to other celestial bodies have been recorded and passed down to us. They indicate a move toward a more precise understanding of the cosmos and in some cases show a surprising degree of insight that is still respected today.

The first solid evidence of a cosmological model that would explain observations come to us from the Greeks of the 4th century B.C.. Babylonians in the 4th millennium B.C. were making accurate observations of the planets, the moon, the stars and the Sun and were providing reasonable predictions of their motions, but they did not leave us with a model to explain these motions as the Greeks did.

The Greeks over time developed a cosmological perspective that the stars were placed firmly and unchangingly in the sky in a sphere that rotated around the Earth every 24 hours. Likewise, the planets, the Sun and the Moon, everything not on the Earth moved in a zone of aether between the Earth and the stars.

This model had many contributors and some detractors, unfortunately the records of their thoughts and work are often fragmentary at best and some we know of only through the comments of others. By the second century, Ptolemy of Alexandria (he may have been Egyptian or he may have been Greek) set down a system to account for the motion of the planets and the Sun and the Moon and the stars around the Earth, a model that was based on perfect circles and epicycles to explain loops observed in planetary motions, loops that were actually retrograde motion caused by the motion of the Earth's movement around the Sun along with the other planets. It was a very complicated system and it stood for a long time. Its demise was posited by quite a number of people over the centuries, even before Ptolemy. but it was not seriously rejected until Galileo. Even the concept of aether lasted until the late 19th century when the Michelson-Morley Experiment failed to account for the aether wind.

Hellenic cosmology

Cosmology in the Ancient Greek world was inherited in part from Egypt. Their thoughts on the origins of the universe involved speculations about the order of things on Earth and the order of the universe. Early speculations involved natural topics of the material world (mathematics and subjects common to physics) up till the time of Plato. One of the earliest works, Hesiod's Theogony, written about 725 B.C., reiterates the myths of gods and addresses the origins of things and the universe.

In the latter half of the 5th century their focus changed and the Sophists, an emerging school, focused on morality and society rather than natural philosophy. The philosopher Socrates addressed the same topics as the Sophists. Prior to that time there were a number of groups, usually referred to as schools, that are categorised as the Pre-Socratic Greek philosophers.

Pre-Socratic cosmology

Ionian School

In Milesia on the coast of Ionia, in the 6th century B.C. philosophers attempted to discover a primary material substance, the elemental foundation of all things, and the source of motion.

Thales (early 6th century) suggested water as a primary substance, Anaximander said it was "the unlimited" or "the infinite" (in Greek: apeiron απειρον), Anaximenes defined the fundamental substances as air.

Pythagoreans

Near the end of the 6th century in the Greek cities of southern Italy, those who adhered to Pythagoreanism demonstrated a greater influence of Egyptian thought. They turned to an intellectual foundation for religion, developing a more abstract, mathematical philosophy than the Ionians

Their foundation for all things in the universe was numbers—unity of the universe and everything in it lay in numbers and numerical relations

Xenophanes

Xenophones of Colophon was born in Ionia, settled in southern Italy and about lived 570 to 475 B.C. Xenophones viewed the traditional gods of Greece with little regard and believed in one great God. This one god was without physical form and was all mind (in Greek: nous νοuζ) This one god, without moving, moved things by the force of spirit.

Aristotelian cosmology

"ALL men by nature desire to know.

(Aristotle, Metaphysics, 350 B.C. )

Aristotle (384-322 B.C) represented an advanced paradigm at the time of his work. His epistemology contradicted his teacher Plato in a crucial manner. Both valued and emphasised reason and its use but Plato insisted that the most important truths, the objects of knowledge, must be attained through reason alone,

Aristotle on the other hand, emphasised observation, holding that the world and the mind were compatible in that understanding was possible. This may have been articulated earlier by someone else, we’ll probably never know. But it is crucial in any field of science that we believe that we can know. And for Aristotle that knowing was achieved through observing.

Most of Aristotle’s observations have been lost. His world was the world of Philip of Macedon and Alexander the Great. His association with the royal Macedonian house made it necessary to move around a great deal. In the years that followed his death, most of his works were lost and much of what remains are compilations made centuries later, collections of notes and original works. As the centuries continued, translations were made and then translations of those translations. In the end very little of his original work remains now, more than 2,300 years later

So, while his observations and his deductions for those observations were very important in the development of science that was to come later, it is fragmented and what remains is full of errors. He did however bestow the early seeds of systematic investigation into natural phenomena and to that extent can be credited at least as a midwife at the birth of empirical science if not actually the founder. It is a tragic irony that his observations and opinions were to stifle the very thing he pursued for so many years.

Aristotle treated knowledge as common property, not to be held in secret. He worked in the company of others and readily spoke and wrote of this thoughts. His attitude in this prefigures one of the foundations of modern science, in that he believed that one could not claim to know a subject unless capable and willing to transmit that knowledge to others. This attitude of openness was often lacking in some of the greatest thinkers of the 15th through the 17th century and was to cause no end of grief. Even up to this day the actual credit for some of the primary advances in science are still being debated due to a lack of cooperation and openness practiced by Artistotle nearly 2,000 years earlier.

Another of his contributions, Aristotle also made the divisions in knowledge we have today, theology and physics and mathematics, language, ethics and politics are all distinct separate fields. This too would have far reaching implications.

One of the most enduring works on the subject of cosmology was his On the heavens written about 350 B.C. Until it was seriously challenged in the early 16th century by Copernicus, amongst others, it was the considered authority on cosmology.

Aristotle asserted that all matter or bodies are made of only four elements: earth, water, air and fire. These elements in their pure form, had characteristic movement, fire and air would rise (fire was the lightest) and water and earth would sink, (earth was the heaviest). These elements also made up the bodies so that a composite body of more than one of these elements would be in conflict and the result was imperfection.

Aristotle perceived the states of gas, liquid and solid, and combustion, a chemical process, as elements. Aristotle also associated things of the Earth with the imperfect and this concept was embodied in his idea of what was the nature of the universe here on Earth and above the earth. His concept of movement was central to Aristotelian cosmology. All bodies, have a natural way of moving according to their very nature: Fire rises, earth sinks.

But, according to Aristotle, movement of any body was not the result of the influence of one body on another, movement of any body was an integral nature of the elements composing it. A modern understanding of the phenomenon of gravity makes it clear that the definitions of gravity later posited by Newton and Hooke and others were a clear departure from the Aristotelian tradition.

Movement was also linear on Earth, things moved in a straight line or they stayed where they were. But the movement was naturally one type.

Circular motion, while natural as well, was reserved for another place, the heavens. Only heavenly bodies moved in circles because circular movement was exalted and pure and only those things in the heavens were pure, bodies on Earth were not pure, ergo, straight lines.

Naturally this meant that the things of the heavens were not made of the same elements as things on Earth since their natural movement was very different. This perfection of movement was also reflected in their perfect shape—spherical. Oddly, the Earth, while holding such impure bodies was itself a perfect sphere since it was a heavenly body. So the cosmos was made of a spherical Earth (though not evidently a perfect body), surrounded by perfectly spherical bodies (the Moon, the Sun and the stars) that moved perfectly about the imperfect Earth in perfect circles. All of this combined, Aristotle called the World.

All of this was set in motion. In other words, the cosmos had a starting point. The motion had a Prime Mover. The motion acted on the outer spheres—the fixed stars—and the motion trickled down to the other spheres and dragged them along. This would seem to contradict the idea he posited that no movement is caused by the influence of another body.

While motion had a starting point, evidently the Earth did not have a beginning. Aristotle’s cosmos was a steady state cosmos: Eternal without beginning and without end. He also believed that the Earth was unique. His reasoning was that since the Earth is made of earth, and earth is the heaviest element and sinks, if there was another Earth it would have sunk as well and there would be two centers to the world. Ergo, the Earth is the one and only centre of the world, that is to say, the universe. It was not until Galileo spotted Jupiter with its own miniature satellite system that anyone had the evidence to refute this however. By that time, a great deal that Aristotle had said was falling by the way.

Medieval cosmology

The Medieval period is generally cast as that time when the Roman Empire withdrew from the West and the coming of the Renaissance, a period of approximately 476 to 1453 A.D. In this period the work of Ptolemy held sway, the universe was geocentric, it moved about the Earth which was the center of the universe.

Renaissance cosmology

A number of things had to develop to firmly establish the field of physical cosmology as distinct from the traditional cosmologies embracing philosophical and religious perspectives: the development of scientific enquiry and methodology; the attitude that would compel people to challenge the status quo; the means to make more precise observations and reveal the vast extent of the universe; and the means to measure and analyse data to arrive at rational, mathematically derived models. Eventually astronomers took issue with the problems Ptolemy's model presented when compared with the data they gathered.

William Gilbert

One of the first, if not the first scientist, was a man named William Gilbert (1544-1603). Gilbert's world was naturally infused with the mystical since the world had been seen through mystical perspectives long before Aristotle posited a "Prime Mover" to explain how the world was set in motion. Gilbert had the attitude, the drive to simply say, "Is that true?" and then test the idea to reveal its credibility. Galileo credited Gilbert as the first true scientist.

Georg Peurbach

Georg Peurbach (1423-1461) challenged errors in astronomy texts that predated Ptolemy and wrote a new textbook and guide to Ptolemy's Almagest. Peurbach's New Theory of the Planets (published 1454) addressed problems encountered in earlier models employing descriptive geometrics to predict planetary motions.

Johannes Regiomontanus

Johannes Regiomontanus (1436-1476), a student of Georg Peurbach, continued his work of observation and critique, improving translations of the ancient Greek works, and openly pointing out the discrepancies between observations and current astronomical theory.

Nicolaus Copernicus

Nicolaus Copernicus (1473-1543) established, at long last, the heliocentric theory putting the solar system in orbit around the sun and thereby resolving many of the problems that Ptolemy and others had striven to answer with increasingly complicated models of the universe.

Leonard Digges

Leonard Digges (1520-1559) invented the theodolite, and is credited with being one of the earliest inventors of the telescope, the reflecting telescope and possibly the refractive telescope eventually providing cosmology with the means to an end--practical and precise observations to serve the theoretical.

Thomas Digges

Thomas Digges (1543-1595), son of Leonard, continued his work, attempted to resolve questions with observations with the telescope and posited the infinite universe.

Galileo Galilei

Galileo Galilei (1564-1642) supported the heliocentric model, employed practical observations with telescopes of his own construction, made precise measurements and first introduced concepts of inertia and relativity of motion.

Johannes Kepler

Johannes Kepler (1571-1630) was instrumental in marrying the best observational data of the time (that of Tycho Brahe) and the most plausible cosmological model of the time (that of Copernicus). Kepler abandoned the Aristotelian perfection of circular orbits and posited the elliptical, and resolved the problems of period and area of orbits.

Robert Hooke

Robert Hooke (1635-1703) posited some major theories and invented or improved practical apparatus that would enhance methods of analyses of cosmological phenomena.

He developed more accurate time keeping devices by inventing a spring control for the balance wheel in watches, enhancing the accuracy of measuring movement of celestial bodies as well as improving navigation. He was possibly the first to stress the need for resolving power and point out the advantages of using hair lines in place of silk or metal wire. He built the one of first reflecting telescopes, observed and described the rotation of Mars, was the first to infer the rotation of Jupiter, and described one of the earliest examples of a double star.

Hooke’s wave theory of light was an essential step forward in spectrum analysis.

Hooke was the first to propose that the motions of astronomical bodies were a matter of dynamics. He published, Attempt to Prove the Motion of the Earth (1674), in which he offered a theory of planetary motion. employing the correct principle of inertia and a balance between an outward centrifugal force and an inward gravitational attraction to the Sun. He proffered three principles of gravity in a lecture entitled “System of the World,” given in 1674

• . . .all Coelestial Bodies whatsoever have an attraction or gravitating power towards their own Centers, whereby they attract not only their own parts, and keep them flying from them . . . but they do also attract all the other Coelestial Bodies that are within the sphere of their activity
• . . . all bodies whatsoever that are put into a direct and simple motion, will continue to move forward in a straight line, till they are by some other effectual powers deflected and bent into a Motion describing a Circle, Ellipses, or some other more compounded Curve Line.
• In that same lecture, he also posited the strength of the attraction but wrongly suggested that gravity decreases inversely with distance from the object.

${\frac {1}{X}}$

where X is the distance

He later corrected this in 1679, in a letter to Newton in which he suggested that this attraction would vary inversely as the square of the distance from the Sun.

${\frac {1}{X^{2}}}$

where X is the distance

Hooke's theory was qualitatively correct. However he did not have the mathematical skills to provide exact and quantitative definition.

Modern Cosmology

Modern Cosmologists

Albert Einstein

Modern scientific cosmology probably began in 1917 with Albert Einstein's publication of his work on general relativity in "Cosmological Considerations of the General Theory of Relativity." Einstein’s model was for a static universe. He proposed that the gravitational potential and the average density of matter remained constant, in a state of equilibrium.

However, initially he had the same problem as Newton, a collapsing universe or the other possibility, an expanding universe, either of which he felt was implied by his original work. So he introduced a cosmological constant lambda (λ), to take account of a static universe. This required that he alter his original equations. He moved to the position of a static universe for a number of reasons, including the fact that red shift—which had already been noted—was not understood and the apparent universe, at that time limited to The Milky Way, did not appear to be expanding or contracting.

Perhaps the biggest obstacle Einstein faced in this counterintuitive result was that the idea of an expanding universe would mean that the universe had a beginning, where it all started from a single point, an infinitely dense and infinitely small gravitational singularity. If this were so, there would be no conceivable way to study the universe at any moment before the gravitational singularity existed, an ultimate barrier to science (which it actually is to date). So, his misgivings about his equations were largely intuitive rather than scientific. In 1917, he stated, in a presentation to the Berlin Academy of Sciences: “that term is necessary only for the purpose of making possible a quasi-static distribution of matter, as required by the fact of the small velocity of the stars.”

Ironically, different values could be ascribed to lambda which would then provide for a static, expanding or a contracting universe. However, he was describing the universe as it was understood in 1917.

Willem de Sitter

Using Einstein's general theory of relativity de Sitter (1872-1934) published a series of papers (1916-1919) in astronomy. De Sitter’s interpretation of Einstein’s work differed in that unlike Einstein he believed the universe was expanding. De Sitter’s solution to Einstein’s equation described a universe that expanding exponentially: if the distance between particles were to double after a certain time, then it would quadruple in the next equal time interval and then increase eight times and so forth. De Sitter’s solution however had another attribute which Einstein could not accept, de Sitter’s model of space contained no matter. Einstein believed that matter defined space. Einstein’s and de Sitter’s models competed for the next decade.

Aleksander Friedmann

Aleksander Friedmann (1888-1925) used Einstein’s equations and found multiple solutions (1922) showing that in some cases the universe expanded and in others the universe contracted including a class of solutions that represented a uniform distribution of expanding matter.

In 1932, de Sitter and Einstein collaborated to obtain a specific solution to Friedmann’s solution—flat space, a cosmological constant of zero but expanding—known as Einstein-de Sitter space.

Georges Lemaître

Lemaître (1894-1966) was responsible in a large way with moving the modern view of cosmology to its present perspective. Lemaître published solutions similar to de Sitter and Freidman in 1927. He had conversed with Edwin Hubble and Einstein and by the early 1930s when Hubble and Humanson's work on redshifts was published it was clear that the observational data implied a universe of a much great size which was rapidly expanding, Lemaître was prepared to take modern cosmology much further. Lemaître developed the concept of the universe beginning from a Primeval Atom, a super nucleus which contained all matter in the universe. This Primeval Atom exploded and from there the universe began. This was only really accepted as part of mainstream astronomy in the 1940s.

COBE

A seminal project in the advancement of cosmology was COBE. With COBE, the extent and precision of the data gathered shifted the entire field of cosmology prompting the Nobel Foundation (Royal Swedish Academy of Sciences) to comment, "the COBE-project can also be regarded as the starting point for cosmology as a precision science: For the first time cosmological calculations (like those concerning the relationship between dark matter and ordinary, visible matter) could be compared with data from real measurements. This makes modern cosmology a true science (rather than a kind of philosophical speculation, like earlier cosmology)."

Leading theoretical focus

The Big Bang

The Big Bang, a label bestowed derisively by its leading critic, Fred Hoyle in the early part of the 20th century, is basically a theory of the universe with a beginning, and possibly an end. Until the Big Bang, cosmology was established on an immutable, everlasting and unchanging universe, the Aristotelian model.

Until Georges Lemaître, the idea of a beginning was hotly rejected, even by those whose evidence best supported this theory, people such as Einstein and Hubble.

In essence the Big Bang is about a moment--an extremely short moment--wherein all the matter and energy of the universe is condensed into a space smaller than the subatomic components of an atom, and then are released in a sudden moment. The problem with this model of the cosmos is that prior to the moment of the Big Bang, it is not possible to actually investigate what happened, to scientifically research the nature of things, a problem that nearly lead Einstein to ultimately reject this theory.

Cosmological curvature

One view of the geometry of the cosmos is that of three scenarios, a hyperbolic geometry (also referred to metaphorically as a saddle shaped curvature), the flat (or Euclidean) geometry, and a spherical geometry. In each case there is a corresponding mass measured by the ratio denoted Ω: density of the universe/critical density where the critical density is that which determines the amount of gravitational effect in the universe.

$\Omega _{total}=\Omega _{M}$

where

$\Omega _{tot}={\frac {density}{critical\,\,density}}$

and

$\Omega _{M}={matter\,\,density}$ (includes regular and dark matter)

• If Ω_tot < 1 then the geometry is open or saddle shaped, it will expand forever;
• If Ω_tot = 1 then it is flat, it will eventually glide to a halt and neither expand or contract;
• If Ω_tot > 1 then it is spherical, it is closed and will therefore eventually contract and collapse.

There are two developments in recent years from the cosmic microwave background measurements (COBE and WMAP), additional observations at various wavelengths and other sources that have changed this view: The universe is expanding at a increasing rate and the universe is flat.

A new equation for the geometry has been proposed to explain this phenomenon after the initial inflation:

Ω_tot=Ω_M + Ω_Λ = 1.0

where

• $\Omega _{tot}$ = density/critical density
• Ω_M = matter density (includes regular and dark matter)

Ω_tot = Ω_M = 0.27

Therefore

Ω_Λ = the cosmological constant or dark energy density = 0.73

In other words, there is theoretically a mass energy with negative pressure that actually repulses rather than attracts, reviving the cosmological constant first proposed by Einstein.

One possible explanation for this is that in the beginning stages of the universe, gravity dominated to form solid bodies: stars, galaxies, structures made from normal matter. As the universe expanded, the dark matter began to exert a repulsive force that had been overwhelmed by normal gravitation with the result that the expansion is not slowing but is accelerating.