In the beginning, the earth was flat. At least it appeared so to its
first observers, hunters and gatherers, and members of primitive
civilizations. 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 unconceivable even to the archaic
tribesman, 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 center of the
cosmos. This belief was later carried into the Babylonian and Greek
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 favor 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 indicates that the surface of
the ocean is curved. Second, he realized 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 center 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.
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 center
of the universe and that sun, 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, since the planets' motion, size and brightness
varied with time. In order to account for the observed derivations he
introduced the idea of epicycles, smaller circular orbits around imaginary
centers 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.
Ptolemy's model of the universe was that of an onion with the earth at
its center and stars arranged in layers around it. The outer layer was
thought to be like a crystal to which the fix stars were attached. The
hypothesis of epicycles accounted for the observable derivations.
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 center 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 magnitude, much to the discontent of the
Roman Catholic Church. Consequently, 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
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
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 domino stones 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
astronomy has finally 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 another 300
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 about 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 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.
If two intelligent life forms in two different galaxies look at each
other's galaxy, they perceive the same thing. The light of the other
galaxy appears redshifted in comparison to nearer objects. This is
caused by ballooning space that stretches the wavelength of emitted
light. The magnitude of this effect is proportional to the distance of
the observed galaxy.
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 homogenous and isotropic. There is overwhelming
observational 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
favor 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
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. Despite everything pointing 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 biggest open questions in cosmology. Another riddle is the
process that took place during the first second after the Big Bang. Neither
quantum theory nor relativity can explain these phenomena alone. It is
expected that that future research will eventually answer these questions
with the help of a higher-level theory that unifies quantum theory and
relativity. Until then, the field is open to speculation.