The Universe in which we live is a huge and almost empty place. Bright stars, like our Sun, huddle together in groups called galaxies, which may contain a trillion (million million) stars. Some idea of how distant the stars are from each other can be gained by looking up at the dark night sky, and realizing that each of the tiny pinpricks of light we see represents a star, and that each of these stars is intrinsically about as bright as our Sun. When we look up at the night sky on the darkest, moonless night, far away from the city lights, we can see no more than two thousand stars with the naked eye. The faint band of light which we call the Milky Way is all that marks the rest of our island in space, the combined glow of millions upon millions of stars too faint and far away to be seen separately except with the aid of a telescope.
And yet, that Milky Way Galaxy of stars is itself no more than an island in space, a dot on the cosmic landscape. Just as there are millions upon millions of stars in our Milky Way Galaxy, so there are many millions of other islands in space, other galaxies, scattered across the Universe, separated from each other by distances hundreds or thousands of times greater than the size of the whole Milky Way.
To a cosmologist, someone who studies the nature and evolution of the whole Universe, a galaxy is just about the smallest thing worthy of consideration. To a human being, living on one small planet circling an ordinary star in the backwoods of a run-of-the-mill galaxy, a galaxy—our Milky Way—is just about the largest thing we can have knowledge of with our own senses, and then only with one of them, sight.
The best modern scientific understanding of the Universe reveals that it was born in fire, a Big Bang of creation some fifteen billion years ago. Cosmologists can now explain how the Universe got a superhot, superdense fireball into the state we see today, with island galaxies separated from one another by vast gulfs of space. They can, thanks to the very latest work by researchers such as Stephen Hawking, in Cambridge, at least suggest how and why the Big Bang itself occurred. And they can provide us with at least an outline guide to the ultimate fate of the Universe. This understanding of the origin and fate of the entire Universe, of everything that exists and of which we can have knowledge, is the theme of my book. But none of this understanding could have been achieved without the discovery that our Milky Way is just one, ordinary galaxy amongst millions. The scientific search for an understanding of the origin of our Universe—the search for the Big Bang—really began when other galaxies, beyond our Milky Way, were first unequivocally identified as comparable collections of stars to our own Galaxy. And that firm identification was made only in the 1920s.
Cosmology is very much a science of the twentieth century. But like all of twentieth-century science, its roots go back to the speculations of natural philosophers and metaphysicians of old.
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To the ancient Greeks and Romans, the Earth was both the centre of the Universe and its most important constituent. Although Greek philosophers had a fair grasp of the distance to the Moon, it was only with the advent of the telescope in the seventeenth century that anyone began to comprehend the great remoteness of the stars. Galileo was the first person to use a telescope for astronomical observations, and he was surprised to discover that even with the aid of his telescope’s magnifying power the stars still appeared only as points of light, not as spheres like the Sun and planets. This could only mean that they were very much further away than the Sun and planets. He also found a multitude of stars visible through the telescope but unseen to the unaided human eye, and his telescope revealed the Milky Way itself to be made up of swarms of individual stars. At the same time, in the early seventeenth century, that Galileo was opening a new observational window on the Universe, Johannes Kepler was developing the basis of a theoretical understanding of our own backyard, the Solar System. His discovery of a relationship between the time it takes for a planet to orbit once around the Sun and the average distance of that planet from the Sun led, by the 1670s, to a reasonably accurate estimate of the distance from the Earth to the Sun, which we now know to be about 150 million kilometres. Kepler’s observations also provided one of the foundations for Isaac Newton’s study of gravity.
It took another 150 years for astronomers to refine and improve both their observations and their theories to the point where accurate distances to a few stars were first estimated, in the late 1830s. Such estimates, and those of the twentieth century, provide a crucial stepping stone in measuring the scale of the Universe, right out to the most distant galaxies. But even before the distances to the stars were known accurately, the revolutionary discoveries of the seventeenth century provided a new view of the Universe, on a vastly greater scale than the old vision of a series of crystal spheres surrounding the Earth and extending out a little beyond the orbit of Saturn. In the eighteenth century, a few philosophers interpreted these new discoveries in terms of a picture, an imaginary model, of the Milky Way and its place in the Universe. That model is surprisingly close to modern thinking, and fuelled debate among astronomers and philosophers for the best part of two centuries.
Credit for the new theory of the Universe—the first modern cosmological theory—belongs to Thomas Wright of Durham, England. Wright was an English philosopher, born in 1711, who like most thinkers of his era spread his interests over a wide variety of subjects, including astronomy. He was the son of a carpenter, and his interest in astronomy was kindled by his childhood teacher. But his formal education was curtailed by a serious speech impediment and for a time he ran wild, becoming, he tells us in his journal, ‘much addicted to sport’. At thirteen he was apprenticed to a clock- and watchmaker, where he stayed for four years but spent all of his spare time studying astronomy, encouraged by his mother but violently opposed by his father, who did everything he could to prevent these studies, including burning young Thomas’s books. During his turbulent early adult years, Wright tried his hand at the sailor’s life, quitting after a violent storm during his first voyage, set up as a tutor of mathematics at Sunderland, was involved with a scandal concerning a clergyman’s daughter, taught navigation to seamen, and then in the 1730s began to achieve success and prosperity (after some initial trials and tribulations with dishonest publishers and a failed attempt to produce an almanac) as a tutor and consultant to the aristocracy. The speech impediment, if it still existed, was no longer a handicap to this confident and self-assured young man. He would survey a grand estate (or a modest one), hold private classes in natural philosophy, mathematics or navigation, and along the way began to publish successful books and broadsheets. By 1742, his reputation was such that Wright was invited to become Professor of Navigation at the Imperial Academy in St Petersburg, at a salary of £300 a year; he declined the post after failing to get the proposed salary increased to £500 per annum. So it was as a successful, educated (if largely self-educated) and reasonably well-known philosopher of his day that Thomas Wright published, in 1750, a work entitled An Original Theory or New Hypothesis of the Universe. This is the work for which he is remembered today, with a place in the history of science important enough for the book to have been reprinted, in facsimile form, in 1971.
~~In Search of The Big Bang -by- John Gribbin
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