Stranger things: multiverse, string cosmology, physical eschatology

The period (roughly) 1990-today is characterized by a big watershed and branching of cosmology into multiple and hitherto unexpected directions. On one side, the generic chaotic/eternal inflation has provided physical grounds for rather wild speculative ideas about the multiverse: the possibly infinite set of cosmological domains (‘universes’). In order to determine how observed features of our universe are (im)probable in the multiverse context requires application of anthropic reasoning which is still controversial in many circles. On the other side, we encounter applications of other speculative physical theories, like the string/M-theory to cosmology, resulting in unusual hypotheses like those of the pre-Big Bang cosmologies. In this period we have also witnessed the birth of physical eschatology as the true ‘cosmology of the future’. This chapter will attempt a survey of these and related developments, with necessary qualifications which accompany any ongoing, evolving research activity.

Philosophical aspects of cosmology

The development of cosmological theories has been accompanied by philosophical debates inspired by the contrasts between cosmology and other areas of physics. This chapter reviews aspects of these debates from a historical perspective, beginning with debates about whether the uniqueness of the universe implies that cosmology needs a distinctive methodology. Underdetermination of theory by the evidence is particularly challenging due to horizons and inaccessible physics. Theories of the ‘origin’ of the universe do not have the same structure as other physical theories. Recent debates have focused on how to evaluate theories that predict a multiverse, in particular regarding the significance of fine-tuning and how to conduct anthropic reasoning.

Fine-Tuning Fine-Tuning

This chapter argues that the fine-tuning argument for the existence of God is a straightforwardly legitimate argument. The fine-tuning argument takes certain features of fundamental physics to confirm the existence of God because these features of fundamental physics are more likely given the existence of God than they are given the non-existence of God. And any such argument is straightforwardly legitimate, as such arguments follow a canonically legitimate form of empirical argumentation. The chapter explores various objections to the fine-tuning argument: that it requires an ill-defined notion of small changes in the laws of physics, that it over-generalizes, that it requires implausible presuppositions about divine intentions, and that it is debunked by anthropic reasoning. In each case it finds either that the putatively objectionable feature of the fine-tuning argument is inessential to it or that the putatively objectionable feature of the fine-tuning argument is not actually objectionable.

The Quantum World

Scientific measurements of fine-tuning factors, especially the cosmological constant, have forced non-theists to fall back on anthropic reasoning and multiverse theories to try to explain away the implications of a theistically-designed universe. Whatever its other uses, employing anthropic reasoning in this way is questionable. It is unscientific to posit trillions upon trillions of universes--as many multiverse proponents and string theorists do--in order to try to explain away the fine-tuned existence of our own. Albert Einstein would likely dismiss many current multiverse theories. Yet, might we still live in a multiversal reality? This essay posits such a reality--a Triverse--as a more parsimonious view over popular multiverse theories. The proposed Triverse has some similarity to, but is distinct from, Roger Penrose’s “three worlds” in his Shadows of the Mind. A multiversal Triverse reality might also eventually be reconciled with some of the evidence and indicators that support quantum mechanics, and thus help define a more deterministic universe.