Monday, September 22nd, 2025
Program Talk: Ruth Gregory
Time: 11:15 AM - 12:15 PM
Location: SCGP 313
Title: The effects of finite width on cosmic string spacetimes
Abstract: TBA
Title: The effects of finite width on cosmic string spacetimes
Abstract: TBA
Program Talk: David Maxwell
Time: 2:00 PM - 3:00 PM
Location: SCGP 313
Title: Einstein-Fluid Initial Data
Abstract: Fluids have been a standard matter source for gravitation since the beginning days of general relativity. Nevertheless, constructing initial data for this family of matter models is surprisingly nuanced. In this talk we describe a novel approach to building Einstein-fluid initial data based on a recently established phase-space technique for building non-vacuum initial data sets. Compared to prior approaches to working with fluids, the input parameters used here allow for more direct specification of physical quantities, such as the number of particles in any given region. We focus on perfect fluids but also discuss extensions of these ideas to viscous models.
Title: Einstein-Fluid Initial Data
Abstract: Fluids have been a standard matter source for gravitation since the beginning days of general relativity. Nevertheless, constructing initial data for this family of matter models is surprisingly nuanced. In this talk we describe a novel approach to building Einstein-fluid initial data based on a recently established phase-space technique for building non-vacuum initial data sets. Compared to prior approaches to working with fluids, the input parameters used here allow for more direct specification of physical quantities, such as the number of particles in any given region. We focus on perfect fluids but also discuss extensions of these ideas to viscous models.
Tuesday, September 23rd, 2025
Program Talk: Ivonne Zavala
Time: 11:15 AM - 12:15 PM
Location: SCGP 313
Title: Cosmological Dynamics of Scalar Fields in String-Theoretic Dark Sectors
Abstract: Scalar fields are a ubiquitous feature of string theory compactifications, typically appearing in axion–saxion pairs with non-trivial interactions. Motivated by this structure, I will use the dynamical systems approach to analyse how such scalars can play a role in the dark sector of the universe. First, I will present a model in which one scalar behaves as dark matter and the other as dark energy, providing a unified picture of the dark sector. I will then turn to multifield quintessence models, where accelerated expansion can arise even with steep potentials, thereby evading recent quantum gravity swampland constraints and offering a possible framework for evolving dark energy hinted at by observations.
Title: Cosmological Dynamics of Scalar Fields in String-Theoretic Dark Sectors
Abstract: Scalar fields are a ubiquitous feature of string theory compactifications, typically appearing in axion–saxion pairs with non-trivial interactions. Motivated by this structure, I will use the dynamical systems approach to analyse how such scalars can play a role in the dark sector of the universe. First, I will present a model in which one scalar behaves as dark matter and the other as dark energy, providing a unified picture of the dark sector. I will then turn to multifield quintessence models, where accelerated expansion can arise even with steep potentials, thereby evading recent quantum gravity swampland constraints and offering a possible framework for evolving dark energy hinted at by observations.
Wednesday, September 24th, 2025
Program Talk: Aghil Alaee
Time: 11:15 AM - 12:15 PM
Location: SCGP 313
Title: A comparison theorem for the total mass of AE, ALE, AF, and ALF Riemannian Toric 4-Manifolds
Abstract: One of the fundamental conjectures in mathematical relativity is the positivity of total mass for complete non-compact Riemannian manifolds assuming appropriate lower bounds on scalar curvature. This conjecture has been proved for AE manifolds using several techniques, starting with the celebrated results of Schoen-Yau and Witten. There are counterexamples to this conjecture in the AF, ALF, and ALE cases. In this talk, we will refine this conjecture and prove a comparison theorem for the mass of toric 4-manifolds. The proof is robust and can be extended to higher dimensions if additional assumptions are added. This is a joint work with Marcus Khuri and Hari Kunduri.
Title: A comparison theorem for the total mass of AE, ALE, AF, and ALF Riemannian Toric 4-Manifolds
Abstract: One of the fundamental conjectures in mathematical relativity is the positivity of total mass for complete non-compact Riemannian manifolds assuming appropriate lower bounds on scalar curvature. This conjecture has been proved for AE manifolds using several techniques, starting with the celebrated results of Schoen-Yau and Witten. There are counterexamples to this conjecture in the AF, ALF, and ALE cases. In this talk, we will refine this conjecture and prove a comparison theorem for the mass of toric 4-manifolds. The proof is robust and can be extended to higher dimensions if additional assumptions are added. This is a joint work with Marcus Khuri and Hari Kunduri.
Thursday, September 25th, 2025
Program Talk: Ghazal Geshnizjani
Time: 2:00 PM - 3:00 PM
Location: SCGP 313
Title: How much null-energy-condition breaking can the Universe endure?
Abstract: Quantum fields can notoriously violate the null energy condition (NEC). In a cosmological context, NEC violation can lead to, e.g., dark energy at late times with an equation-of-state parameter smaller than -1 and nonsingular bounces at early times. However, it is expected that there should still be a limit in semiclasssical gravity to how much “negative energy” can accumulate over time and in space as a result of quantum effects. In the course of formulating quantum-motivated energy conditions, the smeared null energy condition has emerged as a recent proposal. This condition conjectures the existence of a semilocal bound on negative energy along null geodesics, which is expected to hold in semiclassical gravity. In this work, we show how the smeared null energy condition translates into theoretical constraints on NEC-violating cosmologies. Specifically, we derive the implied bounds on dark energy equation-of-state parameters and an inequality between the duration of a bouncing phase and the growth rate of the Hubble parameter at the bounce. In the case of dark energy, we identify the parameter space over which the smeared null energy condition is consistent with the recent constraints from the Dark Energy Spectroscopic Instrument.
Title: How much null-energy-condition breaking can the Universe endure?
Abstract: Quantum fields can notoriously violate the null energy condition (NEC). In a cosmological context, NEC violation can lead to, e.g., dark energy at late times with an equation-of-state parameter smaller than -1 and nonsingular bounces at early times. However, it is expected that there should still be a limit in semiclasssical gravity to how much “negative energy” can accumulate over time and in space as a result of quantum effects. In the course of formulating quantum-motivated energy conditions, the smeared null energy condition has emerged as a recent proposal. This condition conjectures the existence of a semilocal bound on negative energy along null geodesics, which is expected to hold in semiclassical gravity. In this work, we show how the smeared null energy condition translates into theoretical constraints on NEC-violating cosmologies. Specifically, we derive the implied bounds on dark energy equation-of-state parameters and an inequality between the duration of a bouncing phase and the growth rate of the Hubble parameter at the bounce. In the case of dark energy, we identify the parameter space over which the smeared null energy condition is consistent with the recent constraints from the Dark Energy Spectroscopic Instrument.