A Potted Prehistory of Cosmology

A few years ago I was asked to provide a short description of the history of cosmology, from the dawn of civilisation up to the establishment of the Big Bang model, in less than 1200 words. This is what I came up with. Who and what have I left out that you would have included?

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 Is the Universe infinite? What is it made of? Has it been around forever?  Will it all come to an end? Since prehistoric times, humans have sought to build some kind of conceptual framework for answering questions such as these. The first such theories were myths. But however naïve or meaningless they may seem to us now, these speculations demonstrate the importance that we as a species have always attached to thinking about life, the Universe and everything.

Cosmology began to emerge as a recognisable scientific discipline with the Greeks, notably Thales (625-547 BC) and Anaximander (610-540 BC). The word itself is derived from the Greek “cosmos”, meaning the world as an ordered system or whole. In Greek, the opposite of “cosmos” is “chaos”. The Pythagoreans of the 6th century BC regarded numbers and geometry as the basis of all natural things. The advent of mathematical reasoning, and the idea that one can learn about the physical world using logic and reason marked the beginning of the scientific era. Plato (427-348 BC) expounded a complete account of the creation of the Universe, in which a divine Demiurge creates, in the physical world, imperfect representations of the structures of pure being that exist only in the world of ideas. The physical world is subject to change, whereas the world of ideas is eternal and immutable. Aristotle (384-322 BC), a pupil of Plato, built on these ideas to present a picture of the world in which the distant stars and planets execute perfect circular motions, circles being a manifestation of “divine” geometry. Aristotle’s Universe is a sphere centred on the Earth. The part of this sphere that extends as far as the Moon is the domain of change, the imperfect reality of Plato, but beyond this the heavenly bodies execute their idealised circular motions. This view of the Universe was to dominate western European thought throughout the Middle Ages, but its perfect circular motions did not match the growing quantities of astronomical data being gathered by the Greeks from the astronomical archives made by the Babylonians and Egyptians. Although Aristotle had emphasised the possibility of learning about the Universe by observation as well as pure thought, it was not until Ptolemy’s Almagest, compiled in the 2nd Century AD, that a complete mathematical model for the Universe was assembled that agreed with all the data available.

Much of the knowledge acquired by the Greeks was lost to Christian culture during the dark ages, but it survived in the Islamic world. As a result, cosmological thinking during the Middle Ages of Europe was rather backward. Thomas Aquinas (1225-74) seized on Aristotle’s ideas, which were available in Latin translation at the time while the Almagest was not, to forge a synthesis of pagan cosmology with Christian theology which was to dominated Western thought until the 16th and 17th centuries.

The dismantling of the Aristotelian world view is usually credited to Nicolaus Copernicus (1473-1543).  Ptolemy’s Almagest  was a complete theory, but it involved applying a different mathematical formula for the motion of each planet and therefore did not really represent an overall unifying system. In a sense, it described the phenomena of heavenly motion but did not explain them. Copernicus wanted to derive a single universal theory that treated everything on the same footing. He achieved this only partially, but did succeed in displacing the Earth from the centre of the scheme of things. It was not until Johannes Kepler (1571-1630) that a completely successful demolition of the Aristotelian system was achieved. Driven by the need to explain the highly accurate observations of planetary motion made by Tycho Brahe (1546-1601), Kepler replaced Aristotle’s divine circular orbits with more mundane ellipses.

The next great development on the road to modern cosmological thinking was the arrival on the scene of Isaac Newton (1642-1727). Newton was able to show, in his monumental Principia (1687), that the elliptical motions devised by Kepler were the natural outcome of a universal law of gravitation. Newton therefore re-established a kind of Platonic level on reality, the idealised world of universal laws of motion. The Universe, in Newton’s picture, behaves as a giant machine, enacting the regular motions demanded by the divine Creator and both time and space are absolute manifestations of an internal and omnipresent God.

Newton’s ideas dominated scientific thinking until the beginning of the 20th century, but by the 19th century the cosmic machine had developed imperfections. The mechanistic world-view had emerged alongside the first stirrings of technology. During the subsequent Industrial Revolution scientists had become preoccupied with the theory of engines and heat. These laws of thermodynamics had shown that no engine could work perfectly forever without running down. In this time there arose a widespread belief in the “Heat Death of the Universe”, the idea that the cosmos as a whole would eventually fizzle out just as a bouncing ball gradually dissipates its energy and comes to rest.

Another spanner was thrown into the works of Newton’s cosmic engine by Heinrich Olbers (1758-1840), who formulated in 1826 a paradox that still bears his name, although it was discussed by many before him, including Kepler. Olbers’ Paradox emerges from considering why the night sky is dark. In an infinite and unchanging Universe, every line of sight from an observer should hit a star, in much the same way as a line of sight through an infinite forest will eventually hit a tree. The consequence of this is that the night sky should be as bright as a typical star. The observed darkness at night is sufficient to prove the Universe cannot both infinite and eternal.

Whether the Universe is infinite or not, the part of it accessible to rational explanation has steadily increased. For Aristotle, the Moon’s orbit (a mere 400,000 km) marked a fundamental barrier, to Copernicus and Kepler the limit was the edge of the Solar System (billions of kilometres away). In the 18th and 19th centuries, it was being suggested that the Milky Way (a structure now known to be at least a billion times larger than the Solar System) to be was the entire Universe. Now it is known, thanks largely to Edwin Hubble (1889-1953), that the Milky Way is only one among hundreds of billions of similar galaxies.

The modern era of cosmology began in the early years of the 20th century, with a complete re-write of the laws of Nature. Albert Einstein (1879-1955) introduced the principle of relativity in 1905 and thus demolished Newton’s conception of space and time. Later, his general theory of relativity, also supplanted Newton’s law of universal gravitation. The first great works on relativistic cosmology by Alexander Friedmann (1888-1925), George Lemaître (1894-1966) and Wilhem de Sitter (1872-1934) formulated a new and complex language for the mathematical description of the Universe.

But while these conceptual developments paved the way, the final steps towards the modern era were taken by observers, not theorists. In 1929, Edwin Hubble, who had only recently shown that the Universe contained many galaxies like the Milky way, published the observations that led to the realisation that our Universe is expanding. That left the field open for two rival theories, one (“The Steady State”, with no beginning and no end)  in which matter is continuously created to fill in the gaps caused by the cosmic expansion and the other in which the whole shebang was created, in one go, in a primordial fireball we now call the Big Bang.

Eventually, in 1965, Arno Penzias and Robert  Wilson discovered the cosmic microwave background radiation, proof (or as near to proof as you’re likely to see) that our Universe began in a  Big Bang…

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21 Responses to “A Potted Prehistory of Cosmology”

  1. John Peacock Says:

    Only objection is the common attribution of discovery of expansion to Hubble alone. He combined his distance estimates (which were hugely flawed in two distinct ways) with redshifts measured by Slipher (which were all correct). Velocities are surely more important than distances in suggesting that the universe is expanding. How about

    “By 1917, Vesto Slipher (1875-1969) had shown that all galaxies tended to recede from each other. Edwin Hubble showed in 1924 that galaxies were distant systems of stars and in 1929 that the galaxies with the largest recessional velocities were the most distant. This combination of velocities and distances suggested a uniformly expanding universe.”

    • telescoper Says:

      Yes, Hubble’s synthesis of the data undoubtedly grabbed the headlines but Slipher did the really hard work. I was going to say the “donkey work”, but that was in fact Humason – who was a mule driver earlier in his career.

      I did phrase that bit in a way that was intentionally slightly vague.

    • Check out Virginia Trimble’s article on this in the latest issue of The Observatory.

  2. Some comments:
    1) The Pythagoreans believed everything consisted of numbers but not geometry

    2) Cosmic circular motion comes from Empedocles via Plato

    3) Ptolemy’s model does not agree with all the available data

    4) The mediaeval Aristotelean cosmological model was largely created by Albertus Magnus Thomas’ teacher

    5) Ptolemy had been translated in the 12th century

    6) The expanding universe model including so-called Hubble constant was delivered by Lemaître before Hubble delivered his data. Also Hubble himself rejected the expanding universe model right up to his death.

    However on the whole given the self imposed limitations not a bad précis. ;)

    • telescoper Says:

      1) OK. I’m not an expert on Pythagorean philosophy, but my understanding is that they viewed numbers and geometry as different aspects of the same thing; hence why the relationship between triangles and Pythagorean triples.

      2) Ok

      3) That depends how many epicycles you add!

      4) OK – I didn’t know that.

      5) I don’t know, but if it was how widely available was it?

      6) That’s what the article says. I’m not sure Hubble “rejected” the model either. I’ve seen too many people misquote bits of his writings about this issue. I think he was a (typical?) hard-nosed observer, not particularly interested in theoretical interpretations, in other words a sceptic.

      • I attended a lecture last October by Hilmar Duerbeck, who unfortunately died in January, and he clearly showed Hubble’s rejection of the expanding universe model through Hubble interview quotes and no he wasn’t quote mining.

        On Ptolemy’s models: several of them did not fit the data no mater how many epicycles you add! In particular as Copernicus pointed out according to Ptolemy’s lunar model the moon’s apparent diameter must change by a factor of four during its orbit!

        The Ptolemy translation was known and available but the general level of mathematical knowledge was so low that nobody could really do anything with it.

  3. Not a task I’d fancy! Well done! My quibbles would be that it was Newtonianism rather than Newton that described the universe as machine-like. Newton’s universe had God much more directly and intimately involved, constantly enacting and tinkering.

    It’s also Intresting to think about how long it took to confirm many of these steps – it took until the mid-18th-century for Newton’s 1670s ideas. Should more space be given to observation as confirmation, as well as the thing – pace Tycho and Kepler – that prompts new theories?

    • Newton was actually attacked by Leibniz and the Cartesians because his theory of gravity wasn’t mechanical but relied on “occult forces”.

      • telescoper Says:

        Yes, this is because Newton was aware that the Solar System should be unstable, and therefore required some sort of intervention to keep it going forever.

  4. I should add again, despite all my criticism, an excellent effort.

  5. Bryn Jones Says:

    That’s a really nice summary. I’m reluctant to suggest any changes given the word limit. However, a slightly longer article would allow a mention of Aristarchus who correctly deduced that the Earth was not motionless at the centre of the Universe, even if he was largely ignored.

    William Herschel demonstrated that the physical laws within the Solar System apply (at some level) across the Milky Way (from the orbital motions of binary stars). He also attempted to map the distribution of stars in the Milky Way, showing that they lie in a flattened system.

    I also like a mention of Slipher.

    Would a mention of the discovery of the relative abundances of the chemical elements be relevant – that hydrogen and some helium are dominant? Should Fritz Zwicky’s discovery of dark matter be included?

    • “He also attempted to map the distribution of stars in the Milky Way, showing that they lie in a flattened system. ”

      I’m sure peter would prefer to give some of the credit for the suggested shape of the Milky Way to a “proper” northerner (and an early cosmologist to boot): Thomas Wright.

      in fact, i wonder if Herschel’s taste for astronomy was strengthened by his time in the “proper” north (in the decade before Wright’s death).

    • Bryn Jones Says:

      Yes, Thomas Wright of Durham first suggested that the Milky Way is flattened. Herschel confirmed this model through detailed observations and analysis … from Slough.

  6. telescoper Says:

    Of course the real anomaly is that there’s no mention of Brian Cox.

  7. Bryn Jones Says:

    Another thought: lower limits on the age of the Earth – and therefore of the Universe – came in the 19th century from geology. Astronomical constraints on the ages of stars came later, first for the Sun, then for other stars, with the ages of globular clusters providing a firm lower limit on the age of the Universe before the Big Bang model.

  8. Anton Garrett Says:

    “Much of the knowledge acquired by the Greeks was lost to Christian culture during the dark ages, but it survived in the Islamic world.”

    True, but not only there. The Dark Ages were specifically in Western Europe, the part in which the intelligentsia communicated in Latin. The Greek-speaking eastern portions of the old Roman Empire never fell into a dark age and the learning survived there too. Even at the height of the papacy in the 13th century travellers from Rome to Constantinople, the capital of the eastern half (the ‘Byzantine Empire’), were awed by its opulence. Western Europe relearned the Greek achievement from Byzantine culture to its east as well as from Moorish Spain.

    In the 15th century everything changed. Spanish Catholics expelled the Muslims back across the Mediterranean and Constantinople fell to Islamic forces. But by then Western Europe had picked up the ball of Greek learning and was running with it in the Renaissance.

    • I highly recomend the book The Fourth Part of the World. The fourth part is America. However, like Isaac Asimov’s wonderful book on the neutrino, the title character does not make his real appearance until halfway through, in both cases because of the long setup. In the case of neutrinos etc any physicist knows about conservation laws, early atomic models etc but I suspect that most people who know about America have absolutely no idea about things relating to the mediaeval conception of maps. It is impossible to understand a mediaeval or early Renaissance map unless you can put it in context.

      I won’t write a review since there is a nice one here: http://www.telegraph.co.uk/culture/books/6551122/The-Fourth-Part-of-the-World-by-Toby-Lester-review.html

  9. I think that such accounts should mention that the Ancient Greeks knew the distance to the Sun (and hence, trivially, its size), based on the size of the Earth, the distance to the Moon etc.

    Galileo has to be mentioned somewhere.

    It is Willem de Sitter

    As I mentioned in another thread, even though the discovery of the CMB was a huge support for the BB, wasn’t the fact that the observed number counts of radio sources differed from the SS prediction the real death knell?

  10. [...] long ago I posted a short piece about the history of cosmology which got some interesting comments, so I thought I’d try [...]

  11. Ah! I take a small time out from the blogging universe and come back to find that you
    have become awesome?! Wow, times have changed! Keep up the superb work!

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