Eratosthenes (c.270 - c.190 B.C.) was the first to accurately measure the
radius of the Earth in 196 B.C. by determining the minimum angle between the
Sun's direction and the vertical at Alexandria on the day of the summer solstice. He knew
that a zero angle occurs approximately when the Sun was at its highest point at the city of
Syene (now Aswan), and he knew the base of the triangle, i.e., the distance from Alexandria
to Syene.
Copernicus (1473-1543), a Polish astronomer, placed the Sun at the center of the solar
system with the Earth orbiting around the Sun, thus proposing a heliocentric cosmology
(De revolutionibus orbium coelestium, 1543). This entirely new basis of cosmological
consideration did not fit the observations much better than the Aristotelian-Ptolemaic
system but was justified by the "divine" principle of simplicity in comparison to the rather
complicated construction using epicycles as employed in Ptolemy's cosmology. However, like
Aristotle and Ptolemy, Copernicus retained the conventional idea that the planets moved in
perfectly circular orbits and continued to believe that the stars were fixed and unchanging.
Kepler (1571-1630), a German astronomer and physicist, adopted the Copernician system but
introduced the concept of planets with elliptical orbits. He still believed that the Sun was
the centre of the universe. The cosmologies of Aristotle and Ptolemy had nevertheless been
abandoned.
The Italian philosopher Bruno (1548-1600) laid the groundwork for Newtonian cosmology by
emphasizing that the universe is infinite and stars are scattered outward through infinite
space (see, however, Figure 4).
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Bruno even went so far as to say that stars are Suns, perhaps with orbiting planets and life on them (De l'infinito, universo e mondi, 1584). This far-reaching statement signaled a transfer of attention away from the planets in the solar system to the stars in the Milky Way. The laws describing the behavior of planets are the same laws which describe the behavior of the stars. That the physical laws are of universal nature, and can be applied on Earth as well as in the heavens, was discussed on a philosophical basis by the French philosopher and mathematician Descartes (1596-1650). He compared the universe with a giant clock, obeying mechanical laws which later on had a major influence on Newton's (1643-1727) thinking. It was his belief that the universe was infinite and that the primary qualities of the universe were mathematical in nature. A major step in observational techniques was achieved through the invention of the reflecting telescope between, say, 1545 and 1559 by the Britain Leonhard Digges (c.1520-1559). But it was not until 1609 that the Italian astronomer and physicist Galileo (1564-1642) realized observationally that the Milky Way is actually a collection of individual stars. Galileo also observed mountains on the Moon and discovered four satellites around Jupiter. He had taken the first step toward deducing the structure of the Milky Way. The discovery of heavenly bodies that evidently did not circle the Earth, and his support for the Copernician heliocentric cosmology were described in his work Dialogo sopra i due massimi sistemi del mondo, Tolemaico e Copernicano (1632). Cassini (1625-1712), an Italian-French astronomer, used the telescope to make accurate measurements of the "dimensions of the universe", particularly determining the distance of the Earth from the planet Mars. A rigorous mathematical foundation of Descartes' notion of the universe as a giant mechanical clock was provided by Newton's theory of gravity and his laws of motion (Philosophiae naturalis principia mathematica, 1687). From these he explained Galileo's results on falling bodies, Kepler's three laws of planetary motion and the motion of the Moon, Earth and tides. Newton clearly realized that gravity is the dominant force for understanding the structure of the universe; however, he argued that the universe must be static in a famous letter he sent to the theologian Richard Bentley (1662-1742) in 1692. Mainly for religious reasons, at this time, constancy and stability were associated with the perfection of God and change with friction and decay. Philosophical and religious ideas served as the scaffolding upon which scientific systems of thought developed. It was Bentley who derived for the first time, based on Newton's gravitational theory, what is still considered to be one of the fundamental constants of nature, the gravitational constant.
In Kant's (1724-1804) cosmology the gravitational attraction of stars for each other was
exactly balanced by the orbital motions of the stars and he argued that forces can act at a
distance without the necessity for a transmitting medium. In 1755 Kant proposed the nebular
hypothesis for the formation of the solar system. In 1788 Laplace (1749-1827) attempted a
mathematical proof of the stability of the solar system (Système du monde,
1796). Stability and order of the universe were considered as eternal principles in the
heavens and on Earth. The cosmology of Copernicus, as refined by Kepler, is now believed to
be essentially correct.
In 1915 Einstein (1879-1955) put forth his general relativity theory (a new theory of
gravity); he applied this theory to cosmology in 1917. The theory describes gravity as a
distortion of the geometry of space and time. Unlike Newton's theory of gravity, general
relativity was consistent with special relativity, which Einstein had introduced in 1905.
Cosmology, based on general relativity, broadened the problem into one of finding a model of
the space-time structure of the universe. Einstein's original solutions of his gravitational
field equations left the universe in a stable state of static equilibrium and he provided
physical conditions required to maintain such static (time-independent) equilibrium (the
"cosmological constant"). Only in 1922 Friedmann (1888-1925) succeeded in finding solutions
of Einstein's field equations that evolved in time describing an expanding (or contracting,
if one cares to reverse the sense of time) universe.
Persuasive observational evidence that the universe is indeed expanding and changing in
time was found by Hubble (1889-1953) in 1929 while employing the technique known as Doppler
shift for measuring the red shift of colors in the spectrum of nebulae. "Modern cosmology"
may be said to have began with Einstein (1917) and Friedmann (1922, 1924), based on
observations of cosmological relevance made by Slipher (1875-1969) in 1914 and Hubble in
1929. Slipher discovered the redshift of nebulae, later on identified by Hubble to be entire
galaxies similar to the Milky Way; however, until 1929 their cosmological significance
remained obscure. In 1929 Hubble had counted a great number of galaxies (to determine their
distribution throughout the observable universe), and plotted the galaxie's redshifts
against magnitudes for the brightest E-type cluster galaxies (Hubble diagram). Hubble found
evidence that the outward speed of a galaxy is directly proportional to its distance away
from the observer (Hubble law). This observational fact was exactly what would be expected
if the universe is expanding, as discussed in a paper of Lemaître (1894-1966) in 1927.
Hubble's diagram reveals a linear increase of the magnitude of galaxies with increasing
redshift. In 1956, Hoyle and Sandage developed the q0
criterion which could be used to distinguish one cosmological model from others.
Lemaître's model and Hoyle's steady state' model were ruled out by predicting
q0 = - 1, whereas Hubble's linearity gave
q0 = + 1. Cosmology focused on the search for two numbers:
H0, the rate of expansion of the universe at the position
of the Milky Way; and q0, the deceleration parameter,
characterizing the change of the rate of universal expansion. The value of
q0is believed to lie in the range between 0 and 0.5; the
value of H0 is thought to be uncertain by a factor of about
2 (H0 = 100 h km s-1
Mpc-1, where 0.5
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2.6 Big Bang
Astrophysical arguments were introduced into the cosmological models with Gamow's (1946) prediction that helium (and possibly heavier elements) were generated at an early stage of the evolution of the universe. At this time it was believed that the relative abundance of cosmic nuclei represents truly cosmic abundances. Based on Gamow's arguments the cosmological criterion of the origin of chemical elements in the primeval fireball was substantiated by Alpher and Herman (1949, 1950) and Alpher et al. (1953). However, after Burbidge et al. (1957) it became evident that the bulk of the chemical elements beyond helium where not synthesized at the early stages of the expansion of the universe but in the stars. Only the synthesis of helium (Hoyle and Tayler, 1964), which is not produced in sufficient quantities in stars, and deuterium (Peebles, 1966), which is destroyed during galactic evolution, continued to need Gamow's primordial nucleosynthesis to arrive at reasonable abundances as observed in the universe. Later on Wagoner et al. (1967) were able to show that in addition to D, 3He, and 4He, the only other cosmologically significant element was 7Li. In 1965 the microwave background radiation of 3 degrees Kelvin was discovered by Penzias and Wilson (1965), as predicted by Alpher and Herman in 1949 based on Gamow's considerations of a hot and dense origin of the universe. |