Time in Cosmology

Abstract

Modern cosmology apparently answers many longstanding questions regarding the nature of time. 1 For example, is the universe temporally fi nite or eternal, and is there a unique global sense of time? The Standard Model of cosmology renders the following verdicts. The universe is temporally fi nite, and approximately 13.7 billion years old. Events in the universe can be ordered according to a "cosmic time," which corresponds to time as measured by a particular class of fundamental observers since the "Big Bang." Many questions that remain unanswered seem to be answerable by empirical methods, if only in principle. Whether the universe will also be temporally fi nite to the future can be resolved, in principle, by measuring the actual matter and energy density in the universe. If it exceeds the so-called critical value, then the universe will be temporally fi nite to the future as well, collapsing back into a "Big Crunch."Taking relativistic cosmology to provide direct answers to such questions is, however, misleading in (at least) two different senses. First, it neglects the extent to which the questions have not been so much answered as transformed into, or replaced with, new questions. A similar transformation occurred with regard to the central cosmological question of the sixteenth and seventeenth centuries: does the Sun or the Earth move? Posing this question suffi ciently clearly for astronomical evidence to provide a decisive resolution of it required reformulating the concepts of space, time, and motion. Similarly, aspects of time presumed in posing the questions above pre-theoretically are signifi cantly revised in relativistic cosmology. Second, even taking these conceptual changes into account, the answers to the questions are subtler than is usually acknowledged. The answers summarized above hold for a class of particularly simple, idealized cosmological models. But in what sense do these answers apply to the real universe? And in what sense do these answers address the initial questions regarding the nature of time?This chapter aims to provide a self-contained introduction to time in relativistic cosmology that clarifi es both how questions about the nature of time should be posed 13 A Companion to the Philosophy of Time, First Edition. Edited by Heather Dyke and Adrian Bardon.

Neither Presentism nor Eternalism

“…But this or similar constructions are conventional and generally fail in the details (two galaxies can meet when their proper time from the big-bang differs, so under this definition a single event could be in the future of itself). See [7]. 1 P is said to be simultaneous to P ∈ γ with respect to a freefalling worldline γ if a light ray emitted at E ∈ γ reaches P and a light ray emitted at P reaches R ∈ γ and the proper time between E and P is equal to the proper time between P and R. Alternatively, we can give up detectability with known physics.…”

“…But this or similar constructions are conventional and generally fail in the details (two galaxies can meet when their proper time from the big-bang differs, so under this definition a single event could be in the future of itself). See [7].…”

String theory, loop quantum gravity and eternalism

“…For a general overview and why "whatever presentism remains compatible with empirical facts and our best physics is metaphysically repugnant", see . For an explanation of why the parameter called cosmological time to be found in the standard model of big bang cosmology, based on general relativity, does not help recovering a global foliation of spacetime that the presentist may rely on, see Wüthrich (2013, section 5) and Smeenk (2013). 14 No-futurism may sound appealing as a third way.…”

Quantum Mechanics, Time, and Theology: Indefinite Causal Order and a New Approach to Salvation

“…1919For a discussion of whether such foliation is provided by GR, see (Smeenk 2013; Wüthrich 2013; Read and Qureshi‐Hurst 2020). …”