Quantum Cosmology with a Cosmological Constant
Some of my research concerns the role of quantum cosmology in our leveraging current physics to create a future theory of quantum gravity, especially in the wake of a positive cosmological constant. One difficult, open question here concerns the relationship that obtains between the ‘Cosmological Constant Problem’ and global spacetime structure in the semiclassical limit of quantum gravity — two topics that feature prominently in my research summarized here.
Spacetime emergence: an (in)effective story
[Under review]
Physicists (and philosophers!) are quick to regard our current theories as providing ‘effective’ descriptions of real-world systems. In the context of quantum gravity research, this has led to a common view that classical general relativity (GR) provides such effective descriptions wherever the theory has been successfully applied. And this, in turn, leads to an ‘effective’ understanding of spacetime emergence in quantum cosmology. But effective descriptions are specifically about a system’s bulk dynamics, whereas descriptions in GR irreducibly include details about a system’s boundary. This significantly hampers the common view of GR, and generally undermines our thinking about spacetime emergence ‘effectively’.
Empty space and the (positive) cosmological constant
[SHPS, preprint available here: http://philsci-archive.pitt.edu/21936/]
I discuss empty space as it appears in the physical foundations of relativistic field theories, as well as the popular idea that there is some freedom available for physicists to pick between two inequivalent spacetime representations of empty space, moving forward: de Sitter spacetime or its ‘elliptic' cousin. Against the view that there exists a correspondence between the two representations (what I take to be the standard view), I argue that the freedom named is rather a freedom to postpone justification for having picked either one spacetime setting over the other, in the course of ongoing research.
Betting on future physics
[BJPS, preprint available here: http://philsci-archive.pitt.edu/16410/]
The "Cosmological Constant Problem" (CCP) has historically been understood as describing a conflict between cosmological observations in the framework of general relativity (GR) and theoretical predictions from quantum field theory (QFT), which a future theory of quantum gravity ought to resolve. I argue that this view of the CCP is best understood in terms of a bet about future physics made on the basis of particular interpretational choices in GR and QFT respectively. Crucially, each of these choices must be taken as itself grounded in the success of the respective theory for this bet to be justified.
What’s the problem with the cosmological constant?
[Philosophy of Science, preprint available here: http://philsci-archive.pitt.edu/16016/]
The "Cosmological Constant Problem" (CCP) is widely considered a crisis in contemporary theoretical physics. Unfortunately, the search for its resolution is hampered by open disagreement about what is, strictly, the problem. This disagreement stems from the observation that the CCP is not a problem within any of our current theories, and nearly all of the details of those future theories for which the CCP could be made a problem are up for grabs. Given this state of affairs, I discuss how one ought to make sense of the role of the CCP in physics and generalize some lessons from it.
Would two dimensions be world enough for spacetime?
[SHPMP, with co-authors Samuel C. Fletcher, JB Manchak, and James Owen Weatherall, preprint available here: http://philsci-archive.pitt.edu/14248/]
We consider various curious features of general relativity, and relativistic field theory, in two spacetime dimensions. In particular, we discuss: the vanishing of the Einstein tensor; the failure of an initial-value formulation for vacuum spacetimes; the status of singularity theorems; the non-existence of a Newtonian limit; the status of the cosmological constant; and the character of matter fields, including perfect fluids and electromagnetic fields. We conclude with a discussion of what constrains our understanding of physics in different dimensions.