The Role of Philosophy in Cosmological Development
As it has been observed throughout this course, philosophical thought has a lot to do with cosmology. It can be said that philosophy played a role in the progression of cosmology from the early 1900s to the present-day cosmology. Quantum mechanics, for example, is a part of science that is subject to philosophical thought simply because our abstract way of understanding it.
One of the classical philosophical problems is simply whether the concept of the universe has any physical meaning or if it is just an idea, and this problem obviously depends on another one, namely, what is to be understood by the “universe.” I started by mentioning that “universe” normally means the totality of physical things, but many scientists would add a further clause, namely, that these things have to be causally connected; since cosmologists happen to be situated on earth, this means that the universe, according to this view, is that part of spacetime and its physical constituents that are accessible in principle to observers on earth. This is an empirically reasonable definition which limits the universe to a spatial radius of the order of cT, or about 10 billion light years, but it is clearly a definition that ignores the multitude of things outside this radius. According to most cosmological models there is a cosmic horizon outside which galaxies recede from us with velocities larger than that of light, and which are therefore unobservable even in principle. Yet these objects too belong to our universe, in the wider sense, and we therefore have to accept that cosmology deals with objects that are unobservable in principle, which of course is a philosophically controversial claim, especially to empiricists.
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Cosmological knowledge seems to be conditioned by principles or assumptions that are themselves completely unverifiable insofar as they can only have a limited inductive support. The most important of these assumptions is the Cosmological Principle which includes the postulates of spatial homogeneity and isotropy and which lies at the heart of all models satisfying a Robertson-Walker metric. This principle, together with the relativistic field equations, determines models of the complete universe, which is a satisfying feature, but it also makes rough predictions about regions of the universe which are beyond observation. These predictions or knowledge-claims are postulates that can never be verified and so their reliability must remain a matter a faith. They are consequences of the Cosmological Principle, but this principle can only be verified in the observable part of the universe and its extrapolation beyond this part rests on faith, not knowledge. Still, there are different kinds of faith, and to say that a claim is a matter of faith does not imply that it is either irrational or completely arbitrary.
If the universe is the totality of things, then, as a consequence, it is also unique: there exists only one universe, not because we have empirical evidence for it, but because the concept of the universe is defined as it is. Yet many physicists speak of this or that universe, or about many universes, as though a universe was the same kind of object as an electron or a bottle of wine. In some cases such parlor is innocent: when the scientists speak of “universes” they often really mean “models of the universe,” and there are obviously many cosmological models seeking to describe the one and only universe. However, in other cases physicists refer to the idea of multiple universes, either so-called bubble-universes or other theories involving causally isolated regions of spacetime. The idea of multiple universes has its own philosophical problems, but I just want to mention that the way in which scientists use the term “universe” is often loose and confusing: There is nothing wrong with theories of multiple universes, but if one insists on calling such spacetime-regions “universes” one should invent a new word for the totality of things, which is the ultimate domain of cosmology.
GIVING UP ON CERTAINTY The quest for certainty has gotten epistemology into a lot of hot water, and I propose we give it up as a mistake. We should freely admit we can't be certain of anything, and move on. It is, of course, a reasonable question whether we can consistently get along without certainty, and even if it is possible, whether there is some terrible price to be paid if we do. I will ...
In any case, whether adopting the wider or more narrow point of view, from an empirical point of view the universe is unique. There is only one universe about which we can have empirical knowledge, and this, as I said, makes the domain of cosmology different in principle from other domains of science. To speak philosophically, science is nomological, meaning that it normally deals with objects or events which can be generalized or repeated and in this way be subjected to explanations by law; but there is only one universe, the big bang is a non-repeatable event, and it makes no sense to generalize cosmological knowledge which is supposed to be valid for the entire universe. In other words, cosmology seems to be non-nomological and therefore, according to some scientists, in need of its own methods of inquiry. This kind of philosophical argument was for example discussed by Hermann Bondi in the 1960s, when he argued that there are no, and can be none, cosmological laws and neither can the most general features of the universe be explained. For to explain something amounts to demonstrating that it is a special instance of some general class, and there is no general class of universes, and for this reason Bondi argued that all that cosmology can possibly do is to describe or record the universe.
I just mention this as an example of how philosophical reasoning may enter cosmology and I would like briefly to mention another example from the same period. The basic epistemological question in cosmology is if it is possible to have reliable knowledge af the universe. In other words, why should we believe that the cosmologists’ mathematical models represent the real universe, most of which is forever hidden from us? There have been periods of optimism and pessimism in the more recent history of cosmology, and in the years about 1960 it seemed to some cosmologists that pessimism was warranted by the incapability to decide observationally between the big bang and steady state models. William McCrea, a leading British cosmologist, argued that even in principle we cannot predict the behaviour of a remote part of the universe and that, the farther away the region, the less precise will our predictions be. He formulated a cosmological uncertainty principle of the same fundamental nature as the one known from quantum mechanics, namely, that there will be an uncertainty in cosmological knowledge proportional to the redshift, which meant that it would be meaningless to try distinguishing between cosmological models which only differ at very large distances. If the universe is fundamentally unpredictable and unexplainable it would seem that cosmology is not really a science, and this was indeed the conclusion drawn by McCrea and some other prominent cosmologists of the period.
Big Bang Theory Fifteen billion years ago, give or take five billion years, the entirety of our universe was compressed into the confines of an atomic nucleus. Known as a singularity, this is the moment before creation when space and time did not exist. According to the prevailing cosmological models that explain our universe, an ineffable explosion, trillions of degrees in temperature on any ...
There are many more interesting philosophical problems in cosmology, such as the questions of the anthropic principle and the problem of creation in big bang theory. These have been much discussed during the last couple of decades, both by philosophers and cosmologists, but — as one would rather expect — without any satisfactory solution. As far as the anthropic principle is concerned there seems to be a growing consensus that this is not, after all, a scientific meta-principle of any real worth. And in spite of much discussion and many ingenious suggestions, the ultimate question of the creation of the universe seems as far from a solution as ever. But time does not allow me to go into these problems and I will leave the matter with these loose remarks and go on to another philosophical problem related to modern astrophysics.