Introduction
to Cosmology
How
the universe began and what will happen to it in the future are questions
that have occupied the mind of man since antiquity. However it is only
in the 20th century that we now seem close to constructing a plausible
explanation of how the universe was created and what its eventual destiny
might be. By combining astrophysics with particle physics and using key
observational data, cosmologists have developed a model of the universe
which explains its origin and ultimate fate.
What
is Cosmology?
The
Sumerians and Babylonians
The Babylonians (c
2000-500 BC) began regular observations of the stars and planets using
much of the knowledge of the Sumerians and we know that in 1600 BC they
had produced star catalogues and were even able to predict planetary motions
from their records of daily, monthly and yearly cycles of celestial motions.
Greek Cosmology
Using simple trigonometry
Eratosthenes
in about 270 BC, worked out that the circumference of the Earth was about
40,000 km which is remarkably close to the correct value.
Aristarchus of Samos
(310-230 BC) actually proposed that the Earth went around the sun in one
year and in doing this proposed the first sun centered or heliocentric
model for the cosmos as opposed to a geocentric or Earth centered
one, but unfortunately Aristarchus's ideas never caught on, and Greek cosmology
reverted to a geocentric model.
The
Copernician Revolution
Tycho Brahe (1546-1601)
was a Danish astronomer who kept careful observations of planetary positions.
When he died he left his observations to his assistant, the German Johannes
Kepler(1571-1630). Galileo Galilei (1565-1642)
was a firm believer in Copernicus's heliocentric model and was one of the
first to use a telescope to to make astronomical observations. Galileo
found that the moon had mountains and craters and that the Milky Way could
Neither Copernicus, Kepler
nor Galileo were able to demonstrate a physical reason why planets
remained in orbit around the Sun. Newton, with his laws of
motion supplied the answer.
The
study of the structure and development of the universe as a whole is called
cosmology and scientists who study such things are called cosmologists.
The task of a cosmologist is to understand how different phenomena of nature
from small elementary particles right up to very large-scale structures
in the universe, such as clusters of galaxies, all fit together.
The
earliest civilisation was that of the Sumerians (c 7000 BC) who
lived in the region of the Middle East known as Mesopotamia, in what is
now called Iraq. The Sumerians recorded the positions of the stars and
planets and gave names to some of the constellations we know today.
Greek
cosmology was based on mathematics and the Greeks applied geometry and
algebra to develop models of the universe. The word 'cosmos' comes from
the Greek word kosmos which linguistically has connotations of symmetry
and harmony.
The Greeks made an important
step forward when they discovered the Earth was a sphere. Aristotle
(384-325 BC) advanced arguments that this was so by observing that the
pole star Polaris remains stationary while all other stars revolve
around it.
It was Claudius Ptolemy
in 125 AD who developed a systematic cosmological model that endured for
over 1400 years. Ptolemy formulated a geocentric model of the universe
based upon the properties of circles. Particularly puzzling was the movement
of some of the planets which at times, seemed to stop and move backwards
before resuming their normal paths. Ptolemy, like all Greeks, was imbued
with a sense of 'form' or harmony but this retrograde motion of the planets
did not fit the ides of a uniform circle which was considered to be the
perfect form. Ptolemy devised a geocentric system based on a series of
concentric circles with the Earth at the center and the stars remaining
fixed on the outermost celestial sphere. A system of smaller circles called
epicycles
described the path of a planet around the Earth. The center of epicycle
called the deferent also moved around the Earth in a circle. The
Ptolemaic model was able to account for many of the observed motions of
the planets including predicting their positions with some accuracy and
this is one of the main reasons it lasted as long as it did, despite its
complicated use of the geometry of circles.
Nicolaus Copernicus
(1473-1543), after studying Ptolemy's model, came to the conclusion that
it could be greatly simplified if the sun was placed at the center with
all the planets revolving around. Copernicus knew that this theory would
put him at odds with the teaching of the Roman Catholic Church which at
the time maintained that the Earth was the center of the universe. Copernicus
still believed that the planets went round the sun in perfect cycles. He
even had to bring back epicycles, but his important contribution was to
place the Sun at the center of motion.
Kepler believed in Copernicus's model and used Tycho's planetary
observations to construct a heliocentric model based not on circles but
on ellipses. From, this Kepler formulated his three laws of planetary
motion. Kepler found that his model predicted planetary positions to a
far greater accuracy than the Copernican and Ptolemaic systems.
be resolved into vast numbers of stars. He discovered that the planet Venus
displayed phases similar to that of the moon and that the planet Jupiter
possessed four moons which changed position nightly as they revolved around
it. In 1632, Galileo published a book called the Dialogue on the Two
Chief World Systems which compared and contrasted the Ptolemaic and
Copernican systems. Beyond the orbit of Saturn lay the spherical shell
of stars envisaged by the Greeks except that Galileo allowed for the possibility
that the stars might extent to infinity which was a significant departure
from the 'closed' universe of Greek thinking. It was also Galileo who founded
the science of mechanics and developed concepts of force, inertia and motion
under gravity.
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