Establishing Branch Libraries

published or submitted for publicatio

In What Sense Is the Early Universe Fine-Tuned?

It is commonplace in discussions of modern cosmology to assert that the early universe began in a special state. Conventionally, cosmologists characterize this fine-tuning in terms of the horizon and flatness problems. I argue that the fine-tuning is real, but these problems aren't the best way to think about it: causal disconnection of separated regions isn't the real problem, and flatness isn't a problem at all. Fine-tuning is better understood in terms of a measure on the space of trajectories: given reasonable conditions in the late universe, the fraction of cosmological histories that were smooth at early times is incredibly tiny. This discussion helps clarify what is required by a complete theory of cosmological initial conditions.Comment: 28 pages. Prepared for a volume of essays commemorating David Albert's Time and Chance, B. Loewer, E. Winsberg and B. Weslake, ed

Spoken word classification in children and adults

Purpose: Preschool children often have difficulties in word classification, despite good speech perception and production. Some researchers suggest they represent words using phonetic features rather than phonemes. We examine whether there is a progression from feature based to phoneme based processing across age groups, and whether responses are consistent across tasks and stimuli. Method: In Study 1, 120 3 to 5 year old children completed three tasks assessing use of phonetic features in classification, with an additional 58 older children completing one of the three tasks. In Study 2, all of the children, together with an additional adult sample, completed a nonword learning task. Results: In all four tasks, children classified words sharing phonemes as similar. In addition, children regarded words as similar if they shared manner of articulation, particularly word-finally. Adults also showed this sensitivity to manner, but across the tasks there was a pattern of increasing use of phonemic information with age. Conclusions: Children tend to classify words as similar if they share phonemes or share manner of articulation word finally. Use of phonemic information becomes more common with age. These findings are in line with the theory that phonological representations become more detailed in the preschool years

Aether compactification

We propose a new way to hide large extra dimensions without invoking branes, based on Lorentz-violating tensor fields with expectation values along the extra directions. We investigate the case of a single vector aether field on a compact circle. In such a background, interactions of other fields with the aether can lead to modified dispersion relations, increasing the mass of the Kaluza-Klein excitations. The mass scale characterizing each Kaluza-Klein tower can be chosen independently for each species of scalar, fermion, or gauge boson. No small-scale deviations from the inverse square law for gravity are predicted, although light graviton modes may exist

Bulk Entanglement Gravity without a Boundary: Towards Finding Einstein's Equation in Hilbert Space

We consider the emergence from quantum entanglement of spacetime geometry in a bulk region. For certain classes of quantum states in an appropriately factorized Hilbert space, a spatial geometry can be defined by associating areas along codimension-one surfaces with the entanglement entropy between either side. We show how Radon transforms can be used to convert this data into a spatial metric. Under a particular set of assumptions, the time evolution of such a state traces out a four-dimensional spacetime geometry, and we argue using a modified version of Jacobson's "entanglement equilibrium" that the geometry should obey Einstein's equation in the weak-field limit. We also discuss how entanglement equilibrium is related to a generalization of the Ryu-Takayanagi formula in more general settings, and how quantum error correction can help specify the emergence map between the full quantum-gravity Hilbert space and the semiclassical limit of quantum fields propagating on a classical spacetime.Comment: 29 pages, 2 figure