History of Cosmology

A very brief history of Cosmology till Hubble

(XL BC - XX AD)

4000s-1000s BC

According to the Rig Vedas -- ancient Hindu texts -- the universe undergoes an endless cycle of fiery deaths and rebirths. This cycle of creation and destruction is a manifestation of the dance of Shiva. The time for each cycle is a "day of Brahma", which lasts 4.32 billion years. (This number is curiously close to the age of the Earth, 4.6 billion years, as determined from the age of the elements.) Among the ancient cultures, the Babylonians distinguished themselves as skilled astronomers who were able to measure the apparent motions of the moon and the stars and the planets and the Sun upon the sky, and could even predict eclipses. But it was the Greeks who were the first to build a cosmological model within which to interpret these motions.

400s BC - 200s AD

Plato, Pythagoras, Aristarchos, Hipparchos, and Aristotle influenced the first logically consistent cosmological worldview based on the idea that the universe is eternal and has never changed. The Earth is at the center, and the stars are located at the outer boundary, where all things (including space itself) fade into nothingness. (This idea foreshadows the modern concept of the "horizon" of the universe.) The ancient Greeks deduced, from their observations, that the Earth and Moon are spherical, that the distance from the Earth to the Sun was much greater than the distance from the Earth to the Moon, and that Mars was closer to the Earth than Jupiter, and that Jupiter was closer than Saturn. This model was further developed in the following centuries, culminating in the second century AD with Ptolemy's great system. Perfect motion should be in circles, so the stars and planets, being heavenly objects, moved in circles. However, to account for the complicated motion of the planets, which appear to periodically loop back upon themselves, epicycles had to be introduced so that the planets moved in circles upon circles about the fixed Earth.


Discussion: The Ptolemaic Geocentric Model

1500s - 1700s AD

Despite its complicated structure, Ptolemy produced a model so successful at reproducing the apparent motion of the planets that when, in the sixteenth century, Copernicus proposed a heliocentric system, he could not match the accuracy of Ptolemy's Earth-centred system. Copernicus constructed a model where the Earth rotated and, together with the other planets, moved in a circular orbit about the Sun (which was at the center of the universe). But the observational evidence of the time favoured the Ptolemaic system! Soon after Copernicus's death, Thomas Digges publishes "A Perfect Description of the Celestial Orbes", a variant of the theory of Copernicus, in which Digges claims that the universe is infinite in spatial extent and filled uniformly with stars. This is one of the first clear statements of the "Cosmological Principle".


There were other practical reasons why many astronomers of the time rejected the Copernican notion that the Earth orbited the Sun. Tycho Brahe was the greatest astronomer of his the sixteenth century. He realised that if the Earth was moving about the Sun, then the relative positions of the stars should change as viewed from different parts of the Earth's orbit. But there was no evidence of this shift, called parallax. Either the Earth was fixed, or else the stars would have to be fantastically far away.

It was only with the aid of the newly-invented telescope in the early seventeenth century that Galileo could deal a fatal blow to the notion that the Earth was at the centre of the Universe. He discovered moons orbiting the planet Jupiter. And if moons could orbit another planet, why could not the planets orbit the Sun?


Discussion: Observations of Brahe and Galileo

At the same time, Tycho Brahe's assistant Kepler discovered the key to building a heliocentric model. The planets moved in ellipses, not perfect circles, about the Sun. Newton later showed that elliptical motion could be explained by his inverse-square law for the gravitational force.

But the absence of any observable parallax in the apparent positions of the stars as the Earth rotated the Sun, then implied that the stars must be at a huge distance from the Sun. The cosmos seemed to be a vast sea of stars. With the aid of his telescope, Galileo could resolve thousands of new stars which were invisible to the naked eye. Newton concluded that the Universe must be an infinite and eternal sea of stars, each much like our own Sun (otherwise the universe would collapse under its own gravity). Kepler, however, had previosly pointed out a fundamental problem with an infinite universe populated with stars: the entire sky would be blazing hot.

1700s - 1900s AD

In 1823, the German astronomer Olbers once again discusses this problem of the night sky darkness, and suggests that the solution might be that the universe is filled with dust, which prevents the light from distant stars from reaching us. This problem, now known as "Olbers' paradox" was not resolved until 1920, with one of the key observations of modern cosmology.

In the late eighteenth century, William Herschel used the largest telescope on Earth at the time (1.2m diameter) to derive the first map of the known universe. He assumed all stars had the same luminosity as the Sun and counted more than 90,000 stars in 2,400 sample areas (directions in the sky). After nearly 20 years of observations, he concluded that stars in the universe were distributed over a flattened region of space spanning nearly 10 kpc long and 2 kpc thick with the Sun roughly located at the center.


Discussion: Herschel's observations

It was not until the nineteenth century that the astronomer and mathematician Bessel finally measured the distance to the stars by parallax. The nearest star (other than the Sun) turned out to be about 1.4 pc away. This provided the first conclusive demonstration that stars are at very large distances from the Sun, and that the universe extends over a huge volume of space.

In 1859, Le Verrier noted that the perihelion of Mercury advanced by 38'' --- a more precise measurement is 43''--- per century more than could be accounted for Newton's theory of gravitation. Many possible solutions were proposed (e.g., Venus was 10% heavier than was thought, Mercury had a moon, there was another planet inside Mercury's orbit, Nexton's law was incorrect...). However, these observations turned out to be key in demonstrating the validity of the General Relativity Theory proposed by Einstein in 1914. In addition, Le Verrier provided one of the most important theortical results in support of the Newtonian theory of gravitation: the discovery of Neptune, responsible for the observed irregularities in Uranus's orbit. Le Verrier calculated the position of the new planet which was actually observed by Galle (the discovery of Neptune was also made independently by Adams and Airy).

Early in the twentieth century, Harlow Shapley used observations of RR Lyrae stars to make two very important discoveries about the galactic globular cluster system. First, he showed that most globular clusters reside at great distances (many thousands of light years) from the Sun. Second, by measuring the direction and distance of each cluster, he was able to determine that their three-dimensional distribution in space is roughly spherical with about 30 kpc diameter. However, the center of the distribution is not the Sun, but it is located nearly 8 kpc away in the direction of Sagittarius.


Discussion: Shapley's observations

Most of the stars we can see, however, are contained in the Milky Way - the bright band of stars that stretches across our night sky. Kant and others proposed that our Milky Way was in fact a lens shaped "island universe" or galaxy, and that beyond our own Milky Way must be other galaxies. Since Messier's first catalog of non-stellar objects, astronomers had studied fuzzy patches of light on the night sky, which they called nebulae. Some astronomers thought these could be distant galaxies.

In 1920, at the meeting of the National Academy of Sciences, Harlow Shapley and Eric Curtis both gave talks under the title "The Scale of the Universe", which passed into the literature as "The Great Debate". Curtis argued that the Universe is composed of many galaxies like our own, which had been identified by astronomers of his time as "spiral nebulae". Shapley argued that these "spiral nebulae" were just nearby gas clouds, and that the Universe was composed of only one big Galaxy. In Shapley's model, our Sun was far from the center of this Great Universe/Galaxy. In contrast, Curtis placed our Sun near the center of our relatively small Galaxy. The Shapley-Curtis debate makes interesting reading even today. It is important, not only as a historical document, but also as a glimpse into the reasoning processes of eminent scientists engaged in a great controversy for which the evidence on both sides is fragmentary and partly faulty.


Discussion: The Great Debate (I)

Based in part on articles by: David Wands and Richard McCray