Current student collaborators
Matthew Muscat (B.S. '17)
Previous Student Projects
Constraining cosmic string abundance with CMB distortion data
students: Madeleine Anthonisen (M.S. '15), Alex Laguë (B.S. '17), Daixi Xia (B.S. '17)
Cosmic strings are topological defects – field configurations in which stress-energy is concentrated in a string-like structure – predicted by many models of particle physics beyond the Standard Model. Cosmic strings can form cusps, much like a kink in a garden hose; these cusps decay by emitting Standard Model particles, particularly photons. If cosmic strings are present in the early universe, the photons they emit through cusp annihilation are out of equilibrium with the photon background and unable to thermalize prior to reionization; as a result, they impart on the cosmic microwave background potentially observable deviations from a black body spectrum. Using simple scaling laws and back-of-the-envelope bounds, we were able to show that the planned PIXIE experiment could rule out cosmic strings with values for the characteristic string tension spanning 3 orders of magnitude. Because the string tension is directly related to the energy scale of the underlying particle physics model, our findings indicate that future experiments would rule out a class of such models.
Higher-spin symmetries in the early universe
student: Renato Costa (Ph.D. '16)
Quantum field theories are difficult to solve. While this statement is true in almost any setting, QFTs are particularly challenging on curved spacetimes such as those which describe the early universe. A QFT becomes more tractable when it contains heightened symmetries -- i.e. symmetries not realized in nature -- whose presence provides additional handles with which to grasp the theory. A classic example of heightened symmetry is higher-spin symmetry. Typically, these symmetries are so constraining that their presence ensures that at least some aspects of the theory may be computed exactly.
In this project we considered the consequences of higher-spin symmetry in QFTs in a de Sitter spacetime. De Sitter space is our simplest (i.e., most symmetric!) model of an inflating universe. We found that, when certain technical conditions are met, the presence of higher-spin symmetries forces the QFT to be essentially "free" (non-interacting) at late times. Such theories yield trivial cosmological spectra. This result may be viewed as an analogue for inflationary cosmology of the celebrated Coleman-Mandula theorem. Cases where our technical conditions are not met open the door to integrable models of interacting QFTs on curved spacetime, particularly for low-dimensional theories.
Exploring big-bounce cosmologies with AdS/CFT
student: Elisa Ferreira (Ph.D. '17)
In a cosmological "big bounce" the universe undergoes an epoch of contraction followed by an epoch of expansion, with a highly quantum interlude at the nadir. Big bounce cosmology could potentially describe the initial conditions generating inflation in our early universe, or even an alternative to the inflation hypothesis. Unfortunately, due to the highly quantum bounce regime, big bounces are notoriously difficult to describe quantitatively.
In this project we analyzed simple bouncing cosmologies within the framework of the AdS/CFT correspondence. Through the AdS/CFT dictionary, certain quantitative details of the bounce region -- which contains strong spacetime curvature and highly quantum-mechanical fields -- may be tackled through its dual description in the gauge theory. Our investigation, while exploratory, reveals the strengths and limits of gauge/gravity duality as a tool to explore the spacelike singularities present in classical bounce geometries.