The largest quantum gravity phenomenon

[Work in progress, accepted for symposium presentation at the 2022 PSA]

I argue that efforts to mine the standard model of cosmology for empirical clues — cosmic imprints — of quantum gravity should be understood as the latest installment of a longstanding effort to leverage empirical claims about large-scale cosmic structure in the service of ongoing theorizing in fundamental physics. In this light, I then pull apart (and critically assess) what I take to be two distinct recent programs of research in quantum gravity phenomenology. The first program treats the dynamics of standard model cosmology as an autonomous structural theory of our present-day observable universe at the largest accessible scales, to look for discrepant traces of quantum gravity in our cosmogenic record. The second program treats standard model cosmology as a low-energy effective description of a future model of quantum cosmology, so as to consider whether a sustained commitment to the former’s descriptive accuracy may ultimately be constraining on the topic of how to quantize gravity.

On efforts to decouple early universe cosmology and quantum gravity phenomenology

[Foundations of Physics, preprint available here: https://arxiv.org/abs/2206.07783v4]

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.

Stabs in the dark sector

[Currently undergoing substantial changes; please ask about previous circulated drafts]

How do our current empirical commitments in fundamental physics constrain the future of theory in the discipline? I argue that this question is key in establishing the continuity of scientific knowledge across (potentially radical) theory change in fundamental physics, and also that contemporary dark sector research can help illustrate a compelling answer. That answer includes a distinction drawn between expected empirical constraints (i.e. on a future theory) and empirical constraints on the decisions-laden activity of theory development, itself. Finally, a notion of ‘signposts' is introduced, by which means our current empirical commitments are rendered empirical constraints of the latter kind.

Quantum gravity in a laboratory?

[Monograph with Nick Huggett and Niels Linnemann, Cambridge Elements in the Foundations of Contemporary Physics series, earlier preprint available here: http://philsci-archive.pitt.edu/20623/]

It has long been thought that observing distinctive traces of quantum gravity in a laboratory setting is effectively impossible, since gravity is so much weaker than all the other familiar forces in particle physics. But the quantum gravity phenomenology community today seeks to do the (effectively) impossible, using a challenging novel class of `tabletop' Gravitationally Induced Entanglement (GIE) experiments, surveyed here. The hypothesized outcomes of the GIE experiments are claimed by some (but disputed by others) to provide a `witness' of the underlying quantum nature of gravity in the non-relativistic limit, using superpositions of Planck-mass bodies. We inspect what sort of achievement it would possibly be to perform GIE experiments, as proposed, ultimately arguing that the positive claim of witness is equivocal. Despite various sweeping arguments to the contrary in the vicinity of quantum information theory or given low-energy quantum gravity, whether or not one can claim to witness the quantum nature of the gravitational field in these experiments decisively depends on which out of two legitimate modelling paradigms one finds oneself in. However, by situating GIE experiments in a tradition of existing experiments aimed at making gravity interestingly quantum in the laboratory, we argue that, independently of witnessing or paradigms, there are powerful reasons to perform the experiments, and that their successful undertaking would indeed be a major advance in physics.