Mathematical and Theoretical Physics Seminar: Sarah Geller (UC Santa Cruz), "Making Black Holes at the Big Bang: multi-field inflation, spectator fields, and fine-tuning."
- Event Type
- Seminar/Symposium
- Sponsor
- Department of Physics
- Location
- Loomis 464
- Date
- Nov 13, 2025 1:00 - 2:00 pm
- Speaker
- Sarah Geller
- Contact
- Thierry Ramais
- ramais@illinois.edu
- Phone
- 217-300-2044
- Views
- 81
Abstract: Primordial black holes, or PBHs, are black holes that formed in the early universe by some means other than stellar collapse. : Primordial Black Holes (PBHs) are fascinating cosmological objects for a number of reasons: they are one of the most compelling candidates to explain some or all of the missing matter (dark matter) in our universe; they are one of only a couple potential probes of the first few seconds of the universe’s infancy, and they may also one day furnish an observational probe of quantum gravity through detection of Hawking radiation. As such, it is crucial to understand the mechanisms by which PBHs form. In this talk, I go beyond the already-described single-field inflationary models that typically produce PBHs through a period of ultra-slow-roll (USR), which are usually highly fine-tuned (the “fine-tuning” problem). I will first discuss the formation of PBHs in the context of two natural high-energy ingredients: multiple scalar fields and non-minimal gravitational couplings. I will reveal what we have learned by performing the first Markov Chain Monte Carlo (MCMC) analysis of parameter space for such models, and how these models can produce all of the dark matter while also satisfying the most recent Planck constraints, and even have natural UV completions in, for example, supergravity (SUGRA). I’ll then present a new generic mechanism in which a single-field inflationary model with an additional scalar spectator generates large power spectrum enhancements while still reducing the severity of the fine-tuning of single-field models (sans spectator) by orders of magnitude. In this mechanism, the system undergoes two turns in field space bracketing a phase where tachyonic isocurvature modes grow exponentially before transferring power to curvature perturbations. Crucially, this occurs without entering a USR phase. I will explicitly demonstrate the remarkable resilience of this inflationary mechanism to parameter variations, which may provide a template for addressing the longstanding “fine-tuning” problem.