We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.
Thesis D1: An initial singularity may or may not have occurred.
A start to the universe may have occurred a finite time ago, but a variety of alternatives are conceivable: eternal universes, or universes where time as we know it came into existence in one or another way. We do not know which actually happened, although quantum gravity ideas suggest a singularity might be avoided.
If we imagine, for a moment, running the clock of the universe backwards to earlier and earlier times, its size gets smaller and its density gets larger until we reach a moment—even earlier than the putative inflationary era—when classical physics at the macroscopic level no longer applies and some (as yet unknown) quantum physics must apply to everything—even gravity. Therein lies the problem, because if you run the clock backwards just 5.39 x 10-44 second from this time, you reach the purported moment of the Big Bang—the initial singularity. But whoa (or perhaps woe)! How can we say anything about the Big Bang—or even if it occurred at all—since the laws of known physics completely break down 5.39 x 10-44 second (the Planck time) after the Big Bang! See the problem?
Perhaps the universe came into existence through a process analogous to radioactive decay where an alpha particle leaves a nucleus through quantum tunneling. Perhaps our universe “tunneled” into existence from somewhere else, and thus our beginning isn’t really the beginning. This is just one of many possibilities.
This is a key issue in terms of the nature of the universe: a space-time singularity is a dramatic affair, where the universe (space, time, matter) has a beginning and all of physics breaks down and so the ability to understand what happens on a scientific basis comes to an end. However eternal existence is also problematic, leading for instance to the idea of Poincaré’s eternal return: everything that ever happened will recur an infinite number of times in the future and has already occurred an infinite number of times in the past. This is typical of the problems associated with the idea of infinity. It is not clear in the end which is philosophically preferable: a singularity or eternal existence. That decision will depend on what criteria of desirability one uses.
While infinity is a highly useful mathematical device, one can make a strong argument that infinities do not exist in the physical universe (or even multiverse). Quantum physics already gives us a possible clue about the infinitely small: we appear not to be able to subdivide space or time any further than the Planck length (1.616 x 10-35 meter) or the Planck time (5.39 x 10-44 second). We would not be able to distinguish between two points less than a Planck length apart, nor two moments in time less than a Planck time apart. While harder to envision, might not there also be an upper limit to size? And time?
Thesis D2: Testable physics cannot explain the initial state and hence specific nature of the universe.
A choice between different contingent possibilities has somehow occurred; the fundamental issue is what underlies this choice. Why does the universe have one specific form rather than another, when other forms consistent with physical laws seem perfectly possible? The reasons underlying the choice between different contingent possibilities for the universe (why one occurred rather than another) cannot be explored scientifically. It is an issue to be examined through philosophy or metaphysics.
Metaphysics is the part of philosophy that deals with existence, space, time, cause and effect, and the like. Metaphysics begins where physics necessarily ends due to observational limitations.
Did anything exist before the Big Bang?
Was there a Big Bang?
What are the physical properties of the very early universe, when energy densities existed that are far beyond our ability to recreate in the laboratory?
What lies beyond our particle horizon?
Are there other universes?
Why does anything exist at all?
Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.
Liddle, A.R. 2015, An Introduction to Modern Cosmology, 3rd ed., Wiley, ISBN: 978-1-118-50214-3.