In the beginning, the Earth was flat. At least it appeared so to its first observers, hunters and gatherers, and members of early civilisations. Not totally unreasonable, one would think, because the curvature of our planet's surface is not immediately apparent. Yet we know, and it must have been not totally inconceivable even to the archaic tribesmen, that our senses occasionally deceive us. The Earth being flat brings about the problem that it must end somewhere, unless we imagine it to extend infinitely. Infinity is a rather unfathomable conception and, hence, right down to the Middle Ages people were afraid of the possibility of falling off the Earth's boundaries.
What lies beyond these boundaries was largely unknown and open to speculation. The starry heavens were a source of endless wonder and inspiration. Peoples from all parts of the world created their own myths, inspired by the skies and the celestial bodies. Their cosmogonies can be seen as an attempt to explain their own place in the universe. Six thousand years ago, the Sumerians believed that the Earth is at the centre of the cosmos. This belief was later carried into the Babylonian and Greek civilisations.
According to the history books, it was the Greeks who first put forward the idea that our planet is a sphere. Around 340 BC, the Greek philosopher Aristotle made a few good points in favour of this theory in On the Heavens. First, he argued that one always sees the sails of a ship coming over the horizon first and only later its hull, which suggests that the surface of the ocean is curved. Second, he realised that the eclipses of the Moon were caused by the Earth casting its shadow on the moon. Obviously, the shadow would not always appear round, if the Earth was a flat disk, unless the Sun was directly under the centre of the disk. Third, from their travels to foreign countries, the Greeks knew that the North Star appears higher on the northern firmament and lower in the south. Aristotle explained this correctly with the parallactic shift that occurs when moving between two observation points on a spherical object. Among the Greeks, the heliocentric system was proposed by the Pythagoreans and by Aristarchus of Samos (ca. 270 BC). However, Aristotle dismissed the case for heliocentrism.
Ptolemy's geocentric model of the cosmos.
The influence of Aristotle was significant. Around 150 AD, Claudius Ptolemaeus (Ptolemy) elaborated Aristotle's ideas into a complete cosmological model. He thought that the Earth was stationary at the centre of the universe and that the Sun, the stars, and all planets revolve around it in circular orbits, hence, the model is sometimes referred to as the geocentric system. Ptolemy was aware that the postulation of perfect circular orbits contradicted observation, because the planets' motion, size and brightness varied with time. To account for the observed deviations, he introduced the idea of epicycles, smaller circular orbits around imaginary centres on which planets were supposed to move while describing a revolution around Earth. This enabled astronomers to make reasonably accurate predictions about the movement of the celestial bodies, and consequently the Ptolemaic model was a great success. The system was later adopted by the Christian Church and became the dominant cosmology until the 16th century.
In 1514 the Polish astronomer Nicolaus Copernicus (1473-1543) put forward an alternative model, referred to as the heliocentric system, in which the Sun is at the centre of the universe, and all planets, including Earth, revolve around it. The further apart a planet is from the Sun, the longer it takes to complete a revolution. Copernicus said that the ostensible movement of the Sun is caused by the Earth rotating around its north-to-south axis. The heliocentric system got rid of Ptolemy's obscure epicycles, whose main weakness was that they did neither account for the observed backward motion of Mars, Jupiter, and Saturn, nor for the fact that Mercury and Venus never moved more than a certain distance from the Sun. Unfortunately, the Copernican system was not inherently simpler than the geocentric system; and it did not immediately render more accurate calculations of the planet's motion.
The end of the Ptolemaic theory came with the invention of the telescope. With the help of this device, Galileo Galilei (1564-1642) discovered the four largest Jupiter moons. The existence of these moons demonstrated beyond doubt that not all celestial bodies revolve around the Earth, contrary to Ptolemy’s theory. Galileo confirmed the Copernican model and thus initiated a scientific revolution of great importance, much to the discontent of the Roman Catholic Church. Unsurprisingly, Galileo struggled with church authorities during much of his lifetime. In 1594 the German astronomer Johannes Kepler (1571-1630) refined the heliocentric model in his book Mysterium Cosmographicum by showing that planets move on elliptical, rather than circular orbits. Kepler also prepared the idea of gravity by explaining that the Sun exerts a force on planets that diminishes inversely with distance and causes them to move faster on their orbits, the closer they come to the Sun. This theory finally allowed predictions that matched observations.
Kepler and Newton: The paradox of the collapsing universe.
Kepler’s model became the accepted 17th century cosmology, until Isaac Newton further refined Kepler's notion of the forces between celestial bodies. Newton postulated the law of universal gravitation that applied to all bodies, whether in space or on Earth, and he supplied the mathematical foundation for it. According to Newton, bodies attract each other proportionally with their size and inverse proportionally with the square of the distance between them. He went on to demonstrate that according to this law, planets move on elliptical orbits, as previously assumed by Kepler. Unfortunately, one consequence of this theory is that the stars of the universe attract each other and thus must eventually collapse onto each other. Newton was not able to give a plausible explanation for why this did not happen.
To counter this paradox, it was inferred that the universe is infinite in space, and thus contains an infinite number of evenly distributed stars, which would on the whole create a gravitational equilibrium. This assumption, however, would still imply instability. If the balance is disturbed in one region of space, the nearest stars collapse and the gravitational pull of the resulting more massive body draws in the next cluster of stars. Clusters would collapse like a house of cards and eventually draw in the entire universe. Today we know that this is not the case, because the universe is not static as Newton thought. The cosmos is in a state of expansion and therefore, gravitational collapse is prevented.
Is the universe infinite in space and time?
The question of whether the universe has boundaries in time and space has captivated the imagination of mankind since early times. Some would say the universe had existed forever, while others would say that the universe was created and thus had a beginning in time and space. The second thesis immediately raises the question what exists beyond its temporal and spatial bounds. Could it be nothingness? But then, what is nothingness? The absence of matter, or the absence of space and time itself? The German philosopher Immanuel Kant (1724-1804) dealt intensively with this question. In his book Critique of Pure Reason he came to the conclusion that the question cannot be answered reliably within the limits of human knowledge, since thesis and antithesis are equally valid. Kant thought instead of time and space as fundamental aspects of human perception.
Big Bang - the birth of our universe.
Fast forward: Despite Kant's doubts thereto, it appears that modern cosmology has answered the above question. The universe we can observe is finite. It has a beginning in space and time, before which the concept of space and time has no meaning, because spacetime itself is a property of the universe. According to the Big Bang theory, the universe began about twelve to fifteen billion years ago in a violent explosion. For an incomprehensibly small fraction of a second, the universe was an infinitely dense and infinitely hot fireball. A peculiar form of energy that we don't know yet, suddenly pushed out the fabric of spacetime in a process called "inflation", which lasted for only one millionth of a second. Thereafter, the universe continued to expand but not nearly as quickly. The process of phase transition formed out the most basic forces in nature: first gravity, then the strong nuclear force, followed by the weak nuclear and electromagnetic forces. After the first second, the universe was made up of fundamental energy and particles like quarks, electrons, photons, neutrinos and other less familiar particles.
About 3 seconds after the Big Bang, nucleosynthesis set in with protons and neutrons beginning to form the nuclei of simple elements, predominantly hydrogen and helium, yet for the first 100,000 years after the initial hot explosion there was no matter of the form we know today. Instead, radiation (light, X rays, and radio waves) dominated the early universe. Following the radiation era, atoms were formed by nuclei linking up with free electrons and thus matter slowly became dominant over energy. It took 200 million years until irregularities in the primordial gas began to form galaxies and early stars out of pockets of gas condensing by virtue of gravity. The Sun of our solar system was formed out of such a pocket of gas in a spiral arm of the Milky Way galaxy roughly five billion years ago. A vast disk of gas and debris swirling around the early Sun gave birth to the planets, including Earth, which is between 4.6 and 4.5 billion years old. This is -in short- the history of our universe according to the Big Bang theory, which constitutes today's most widely accepted cosmological viewpoint.
What speaks in favor of the Big Bang theory?
A number of different observations corroborate the Big Bang theory. Edwin Hubble (1889-1953) discovered that galaxies are receding from us in all directions. He observed shifts in the spectra of light from different galaxies, which are proportional to their distance from us. The farther away the galaxy, the more its spectrum is shifted towards the low (red) end of the spectrum, which is in some way comparable to the Doppler effect. This redshift indicates recession of objects in space, or better: the ballooning of space itself. Today, there is convincing evidence for Hubble's observations. Projecting galaxy trajectories backward in time means that they converge to a high-density state, i.e. the initial fireball.
According to the Copernican cosmological principle, the universe appears the same in every direction from every point in space, or in more scientific terms: The universe is homogeneous and isotropic. There is overwhelming evidence for this assertion. The best evidence is provided by the almost perfect uniformity of the cosmic background radiation. This observed radiation is isotropic to a very high degree and is thought to be a remnant of the initial Big Bang explosion. The background radiation originates from an era of a few hundred thousand years after the Big Bang, when the first atoms where formed. Another piece of evidence speaking in favour of Big Bang is the abundance of light elements, like hydrogen, deuterium (heavy hydrogen), helium, and lithium. Big Bang nucleosynthesis predicts that about a quarter of the mass of the universe should be helium-4, which is in good agreement with what is observed.
Will the universe expand forever?
On basis of our understanding of the past and present universe, we can speculate about its future. The prime question is whether gravitational attraction between galaxies will one day slow the expansion and ultimately force the universe into contraction, or whether it will continue to expand and cool forever. The current rate of expansion (Hubble Constant) and the average density of the universe determine whether the gravitational force is strong enough to halt expansion. The density required to halt expansion (=critical density) is 1.1 * 10^-26 kg per cubic meter, or six hydrogen atoms per cubic meter; the relation "actual density" / "critical density" is called Omega. With Omega less than 1, the universe is called "open", i.e. forever expanding. If Omega is greater than 1 the universe is called "closed", which means that it will contract and eventually collapse in a Big Crunch. In the unlikely event that Omega = 1, the expansion of the universe will asymptotically slow down until it becomes virtually imperceptible, but it won't collapse.
Big Bang - Big Crunch?
Some scientists think it not impossible that the universe is oscillating between eras of expansion and contraction, where every Big Bang is followed by a Big Crunch. Stephen Hawking (born 1942) pointed out the possibility that such an oscillating universe must not necessarily start and end in singularities, i.e. questionable points in spacetime where physical theories, such as General Relativity, break down while energy and density levels approximate infinity. Although everything points towards Big Bang, the future reversal and contraction of the universe is rather uncertain. Big Crunch is at most a hypothesis, because only about 1/100th of the matter needed for Omega=1 can be observed.
In spite of this, galaxies and star clusters behave as if they would contain more matter than we can see. It is almost as if these objects were engulfed by invisible matter. This "dark matter" that cannot be accounted for is one of the open questions in cosmology. Dark matter makes is thought to make up 23% of the universe.
Today, most cosmologists believe there is not enough matter in the universe to halt and revert expansion. Robert Caldwell of Dartmouth University has recently suggested a third alternative for the fate of the universe. His Big Rip scenario is based on astronomical observations made in the late 1990s according to which a mysterious force, labelled dark energy, is responsible for the expansion of the universe. Dark energy makes up 73% of the universe. If the rate of acceleration increases, there will be a point in time at which the repulsive force becomes so strong that it overwhelms gravity and the other fundamental forces. According to Caldwell, this will happen in 20 billion years. "The expansion becomes so fast that it literally rips apart all bound objects," Caldwell explains. "It rips apart clusters of galaxies. It rips apart stars. It rips apart planets and solar systems. And it eventually rips apart all matter." Even atoms would be torn apart in the last 10-19 seconds before the end of time. –Whether or not this scenario will become true is to be decided by future research. Until then, the field is open to speculation.