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Elliot Nelson

Portrait de Elliot Nelson

Quantum mechanics (QM) is the foundation of modern physics and has seen unparalleled success in predicting conditional probabilities for observations. But while QM can be used operationally, we don’t have a unique prescription for mapping the features of an evolving quantum state onto the observed world. Specifically, do the quantum state and its dynamics (e.g. for the Standard Model with semiclassical gravity) uniquely determine which conditional probabilities describe the stochastic dynamics of the world (both observed, as in lab experiments, and unobserved, as in the stochastic initial conditions for cosmological perturbations). In Everettian terminology, what are the branches of the wavefunction?

The early universe is a unique and realistic setting (with quantum fields in curved spacetime) in which branching of the wavefunction can be studied quantitatively. The accelerated expansion of an inflationary or de Sitter phase causes light or massless fields to decohere, and generates a growing number of redundant records of classical field configurations -- an essential feature of wavefunction branching. I have focused on understanding the important role in this process played by gravitational nonlinearities, which are universally present and couple degrees of freedom on different scales, allowing short-wavelength modes to act as a decohering environment or measuring device for long-wavelength modes.

I have also studied observable effects in astrophysics (pulsar timing) and cosmology (CMB power spectra) due to vacuum fluctuations of the energy-momentum tensor from heavy (TeV scale) fields, and helped to develop a numerical pipeline for accurately simulating weakly primordial non-Gaussianity from inflation in the formation of large-scale cosmological structures.
Previously, my PhD thesis focused on primordial non-Gaussianity of the local type, which can lead to a spatial variation of locally measured statistics (such as the primordial power spectrum or spectral index) due to a modulation from density fluctuations on very large scales.

Recent Publications

  • Elliot Nelson, "Quantum decoherence during Inflation from gravitational nonlinearities", JCAP, 03 (2016) 022, arXiv: 1601.03734