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On Efforts to Decouple Early Universe Cosmology and Quantum Gravity Phenomenology

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Abstract

The Big Bang singularity in standard model cosmology suggests a program of study in ‘early universe’ quantum gravity phenomenology. Inflation is usually thought to undermine this program’s prospects by means of a dynamical diluting argument, but such a view has recently been disputed within inflationary cosmology, in the form of a ‘trans-Planckian censorship’ conjecture. Meanwhile, trans-Planckian censorship has been used outside of inflationary cosmology to motivate alternative early universe scenarios that are tightly linked to ongoing theorizing in quantum gravity. Against the resulting trend toward early universe quantum gravity phenomenology within and without inflation, Ijjas and Steindhardt suggest a further alternative: a ‘generalized cosmic censorship’ principle. I contrast the generalized cosmic censorship principle with the logic of its namesake, the cosmic censorship conjectures. I also remark on foundational concerns in the effective field theory approach to cosmology beyond the standard model, which would be based on that principle.

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Notes

  1. Certain models of inflation—famously Starobinsky inflation [42]—may be motivated by UV quantum gravity. And the onset of inflation may itself be studied as a dynamical consequence of UV quantum gravity in the very early universe. But I take ‘inflationary cosmology’ to be the research program that approaches any one such model in quantum cosmology from an effective field theory perspective, thereby ignoring underlying UV quantum gravity degrees of freedom as physical causes of low-energy dynamical behavior.

  2. See, e.g., the reasoning on display in [5, 24]. For philosophical discussion on the underlying logic of trans-Planckian censorship, including its relationship to the swampland familiar in string theory, see [36, 37].

  3. Note that here and throughout, I have in mind effective field theory relative to some laboratory frame transported along a comoving or cosmic-stationary worldline within the standard model, as opposed to relative to, e.g., a choice of conformal frame defined over the entire conformal cosmos (see discussion in [20] about the matter Lagrangian being what ultimately disambiguates a ‘physical’ reference scale). As a matter of formalism, effective field theory in the context of general relativistic reasoning within theoretical cosmology faces difficulties [27]. But I understand the issues at play to be equally troubling for proponents of inflationary cosmology, the dominant approach in the study of the early universe. On the other hand, like I have already mentioned within inflationary cosmology, trans-Planckian censorship (and also the swampland program) applies pressure to the free employ of the effective field theory framework in quantum cosmology specifically as providing low-energy descriptions of a quantum cosmos These developments in quantum gravity research therefore apply some pressure on a ‘cosmology done as effective field theory’ approach, as well: underlying facts about UV quantum gravity may undermine the empirical adequacy of at least some models of an effective cosmos, when the latter are regarded as low-energy approximations of the quantum cosmos around us. (Though, for a pessimistic take on the pressure that is ultimately applied, see [38].)

  4. A modification of the BVG result [8] is used in [26] to argue that the new cyclic cosmology must be past-incomplete with respect to at least some non-comoving geodesic. Often, geodesic incompleteness is treated as something pathological in the classical description, and therefore a signal that quantum gravitational physics comes to dominate over classical somewhere in the vicinity. But signals can be noisy: more work is needed to show that, in the specific case of the past-incomplete geodesics within these cyclic models, it is likely due to some distinctive feature of the underlying quantum gravitational physics (i.e. not to be explained at the level of the effective high-energy dynamics) that the geodesic is rendered incomplete. In §3 below, I will return to this general topic of the interpretation of incomplete geodesics in cosmology given the generalized cosmic censorship principle, i.e., on a ‘cosmology done as effective field theory’ approach.

  5. Introducing that diagrammatic tool for use in showcasing the merits of a “classical (non-singular) bounce” is the raison d’être for [21]—interested readers are encouraged to look at the details of the diagrams on their own.

  6. This view is closely connected to the black hole evaporation paradox [18, 44]. As is pointed out in the “Open Issues” section of [44], if quantum gravity is not unitary, classical gravitational singularities (that witness violations of cosmic censorship) could represent absolute information loss scenarios—surely an important clue in ongoing quantum gravity research.

  7. In fact, motivation for trans-Planckian censorship has been tied directly to familiar thinking about cosmic censorship [9].

  8. A formal point also illustrates the conceptual gap between cosmic censorship and generalized cosmic censorship. The strongest cosmic censorship conjecture is that spacetime is necessarily globally hyperbolic, validating (by conjecture) 3+1 interpretations of classical general relativity that we might suppose are to be recovered in a suitable limit of the underlying quantum gravity theory. By contrast, it is unclear what interpreted sub-theory (or, indeed, sub-class of effective field theories) we would expect to stand in that same limit, were we to insist on the viability of generalized cosmic censorship in theorizing about the cosmos.

  9. I set aside, for the time being, a question as to whether finding leading-order terms dominating within an effective field theory approach to spacetime theories is indeed tantamount to preserving classical spacetime—thanks to Guilherme Franzmann for flagging this. It may be, of course, that talk of quantum corrections to classical dynamics is simply inappropriate in a quantum cosmology setting.

  10. ‘Local’ in the sense of neighborhoods; ‘quasi’ because the neighborhoods in question must be, by setup, carefully chosen as opposed to arbitrary.

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Acknowledgements

I am grateful for comments and conversations with Nick Huggett and Guilherme Franzmann at various stages, and for correspondence with Anna Ijjas near the end. My reviewers were also helpful, pushing me to clarify several key moments in my exposition and argument. This article was initially drafted and revised while I was a postdoctoral researcher at the University of Illinois Chicago on the Beyond Spacetime project, funded by a ‘Cosmology Beyond Spacetime’ grant from the John Templeton Foundation (No. 61387).

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    Mike D. Schneider

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Schneider, M.D. On Efforts to Decouple Early Universe Cosmology and Quantum Gravity Phenomenology. Found Phys 53, 77 (2023). https://doi.org/10.1007/s10701-023-00720-y

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