General Relativity & Quantum Cosmology

As originally argued by Coleman, any breakdown of factorization arising from Euclidean wormholes may be reinterpreted as the result of having averaged over a hidden ensemble of theories. By directly categorifying Coleman's argument, we show that any breakdown of Hilbert space factorization arising from spatial wormholes (Einstein-Rosen bridges) may be reinterpreted as having projected to a subspace of neutral states in an underlying holographic quantum field theory. This talk is based on 2607.01322 with Zhencheng Wang.

The population study of stellar-mass black hole (sBH) binaries with ground-based gravitational-wave detection has achieved tremendous success in recent years. Future observations of extreme mass-ratio inspirals will similarly require proper population analyses that identify the formation channels, measure the branching ratio(s), and characterize the major properties within each channel. In this work, we propose that measurements of eccentricity, inclination, and component mass provide critical information for distinguishing different formation channels and probing detailed formation mechanisms. Focusing on dry and wet extreme mass-ratio inspirals, we establish the theoretical expectations for these observables in each formation channel. We also discuss how their distributions can be used to probe the lifetime and turbulence level of active galactic nucleus disks, the accretion patterns of supermassive black holes, and the population properties of sBHs within nuclear star clusters.

Asymptotic safety offers a compelling UV completion of quantum gravity through a non-trivial UV fixed point. Evidence for this has so far come predominantly from the functional renormalization group. In this talk, I explore a complementary perturbative approach, which offers manifest gauge invariance and allows for computations directly in Lorentzian signature. A crucial ingredient for this is the use of renormalization schemes that are not mass-independent but retain sensitivity to power-law effects. Two realizations of this idea are discussed: a variant of dimensional regularization that goes beyond minimal subtraction, and an adaptation of gradient-flow techniques that have proven successful in QCD. As a proof of concept, I show that in QCD these ideas capture effects that would otherwise be missed, such as the decoupling of heavy quarks. Applying the same methods to gravity, I find that a UV fixed point already emerges at one-loop, with indications that it lies within a perturbative domain. I close by comparing these findings with functional renormalization group results on asymptotic safety and discussing future directions.

Lorentz-violating theories modify the effective point-particle description of black holes through sensitivity parameters that describe their response to motion relative to a preferred frame. We calculate these sensitivities for Schwarzschild and Reissner–Nordström black holes in both Einstein-aether and khrono-metric gravity. Despite differences in black hole properties, theories, and parameter regimes, we find that the sensitivity approaches a common, small limiting behavior across most of the parameter space. I will discuss how this small sensitivity could manifest in gravitational-wave observations with future detectors such as LISA. I will also introduce the motivation for Lorentz-violating gravity, describe our method for calculating the sensitivity, and discuss directions for future work.

This talk is about the proposal that (1) quantum states that violate the momentum and Hamiltonian constraints represent instances of time, and (2) time evolution corresponds to the gradual collapse of an initial state toward a diffeomorphism-invariant state, driven by stochastic fluctuations of the lapse and shift. During the wavefunction collapse, the scale factor increases monotonically and thus acts as a clock. Scalar, vector, and tensor gravitons arise as physical excitations. In the long-time limit, the tensor gravitons exhibit emergent unitary dynamics, while the extra modes are damped by the non-unitary dynamics that suppress constraint-violating excitations. I will also comment on possible physical implications of these extra graviton modes.

Quasinormal-mode frequencies are characteristic frequencies of a black hole spacetime, and can be detected using gravitational waves. The measurement of these frequencies is used to test the nature of astrophysical black holes. However, the quasinormal-mode spectrum is unstable. The spectral instability of black holes refers to the large change in quasinormal-mode frequencies produced by small perturbations to the gravitational potential. Historically, this phenomenon has been studied primarily in the frequency domain. Here we investigate its effect on the time domain waveform using a simple model. We show that there is a time delay before any shift in the frequencies, related to the echo time between the perturbation and the primary potential. In a perturbative limit, the change in the frequencies after a single echo matches the usual frequency-domain perturbed frequencies. Outside of that limit, the change in the signal at intermediate times cannot be described by a constant shift in frequency.