Welcome to the first part of our MOOC, Philosophy and the Sciences. In the next three lectures we will explore a variety of philosophical issues arising in contemporary cosmology. In today's lecture, the Cosmologist John Peacock and I will briefly review the history of cosmology from the 18th century to modern days. And we'll highlight three main philosophical problems that stood in the way of cosmology becoming a science. In the next lecture, we focus on two entities at the very heart of contemporary cosmology: dark energy and dark matter. We explain in a simple, non technical way what they are, and we address the philosophical problem of whether there are any rivals to dark energy and dark matter, and how scientists go about making rational decisions about which theory to endorse in the lack of available evidence. In the fourth lecture, we finally turn our attention to anthropic reasoning in cosmology, and to the philosophical and physical debate surrounding the so-called anthropic principle. So why cosmology? The experimental discovery that our universe is accelerating in its expansion, for which the 2011 Nobel Prize was awarded, has sparked important philosophical debates about cosmology. This comes at a time when new data coming from large galaxy surface are trying to give an answer to pressing questions about the existence of dark matter and dark energy. But for very long time, cosmology was not even regarded as a science. Indeed, it was regarded as closer to philosophy, than to physics. Consider Newton's 'Mathematical Principles of Natural Philosophy'. This extraordinary book laid the foundations of modern physics by providing testable laws of nature. That could explain a variety of observable phenomena from free fall, to planetary motion. Yet Newton's mathematical physics didn't explain what set planets in motion at the origin of our universe, or in general, explaining how planets and stars formed, how they evolved, what set them in motion, was beyond the scope of Newton's mathematical principles of natural philosophy. And the task was taken up in the 18th century by philosophers in the context of other discussion of Newton's natural philosophy, or in the context of metaphysics. For example, the German philosopher, Alexander Baumgarten in his 1739 book 'Metaphysics', claimed that, since cosmology contained the first principles of psychology, physics, theology, teleology, and practical philosophy, cosmology belonged to metaphysics. Thus cosmology in the works of Baumgarten and Christian Wolf, became a rational exercise of examining the concept of the world or of the universe. The necessity of supplementing Newton's physics with metaphysical foundations found its most influential expression in the works of Immanuel Kant. One of the first attempts at the modern metaphysical explanation of the origin of our universe according to Newtonian Principals can be found in Kant's 1755 'Universal Natural History Interior of the Heavens', whose subtitle reads, 'Essay on the Constitution and the Mechanical Origins of the Whole Universe according to Newtonian Principles'. In this text Kant claimed that at the origin of the universe, space was not empty as Newton suggested, but was filled with what he called the fine matter on which two fundamental forces acted. The first force was attraction as a force acting at a distance and capable of lumping matter into what became planets and stars. The second force was repulsion as a force counterbalancing attraction and causing matter to whirl in vertices that would eventually become planets and stars. While both attraction and repulsion can be found in Newton as fundamental forces of nature both in the 'Principia' and in the 'Optics', Kant's original take on Newton consisted in using those two forces to provide an explanation of the presumed mechanisms at work in the constitution of the universe. In so doing, Kant laid the foundation of what became known as the Kant-Laplace Nebular Hypothesis, one of the very first attempts at a scientific explanation of the origin of our universe. And here we see exemplified an interesting connection between philosophy and the sciences. Kant's 1755 'Cosmogony' effectively provided the basis for the development of some seminal ideas in cosmology. Yet, Kant himself was skeptical about the possibility of developing cosmology as a science, because the very metaphysical idea of a universe having a beginning in space and time seemed fraught with contradictions, something that Kant in his mature work called the Antinomies of Reason. Thus at the end of the 18th century, the prospects of developing cosmology as a branch of metaphysics looked dim and the path to cosmology as a science was still very long. Why did it take so long for cosmology to become a science? It wasn't just that the metaphysical foundations of cosmology proved fraught with contradictions as Kant said. Three main problems stood in the way of cosmology to become a science. Scientific theories allow scientists to make inferences based on laws of nature. For example, given Newton's Law of Gravity, scientists can make inferences about planetary motions, as well as the fall of an apple or the times of the tides. A specific problem arises in cosmology when we try to use the laws of nature to make inferences about the origins of our universe. How do we know that the laws of nature that we know and love today apply to the origin of our universe? Didn't our laws come into existence with our universe? And how can we extrapolate from the present physics and its laws, to the origins of our universe? For cosmology to have the status of an experimental science, it should be possible to run experiments to test hypothesis. But running an experiment typically involves being able to repeat the test more than once, and on several different samples of the same object. If repeating test on multiple samples and in different circumstances is key to experimenting, then the prospects for cosmology as an experimental science look unpromising. We have only one universe to observe and to experiment upon, ours. A third problem with cosmology concerns the extent to which we can extrapolate information from our current vantage point, our planet earth, to the universe as a whole. The amount of information we can access from our current vantage point, considering the speed of light limit, which restricts how far back into the history of our universe we can, so to speak, observe, is restricted to events in the so called past light cones, parts of the universe that have been able to send information to us. This is known as the horizon problem. Objects at a distance more than about ct away, where c is the speed of light, can not be seen before time, t. This, per say, is not a huge problem since we might assume that our horizon will grow and then any object will eventually be seen, however distant. But in an accelerating universe like ours, there exists an event horizon. Points sufficiently far apart from each other, will never be in contact. That means there are bound to be vast regions of our universe that will remain unobservable to us forever. Despite these three methodological problems, cosmologists come a long way from the time of the Kant-LaPlace Nebular Hypothesis and has established itself as a science in its own right in just over a century. The path that led cosmology from a branch of metaphysics to a proper science, has not been without lively philosophical discussions. Still in the 1950s, McCreer complained that cosmology was a highly unsatisfactory subject and with Rolament the many physicists that found cosmology baffling, compared to other branches of physics. In the next section, we review some of the milestones in the history of observational and theoretical cosmology of the past century. We return at the end of this lecture to the three issues of laws of nature, uniqueness, and unobservability, and we draw some philosophical conclusions about the history of cosmology.
