Astrophysics

"I describe a candidate for a fundamental physical theory called the causal theory of views. This describes a world constructed by a continual creation of events; where an event is a transition at which a small portion of the possible becomes actual. I first recall older results which includes the emergence of space and, with space, a non-relativistic N-body quantum dynamics. I next describe recent progress on this model including, in a different limit, a formulation of a cut off quantum field theory, which we describe in terms of an S-Matrix formulation of amplitudes. The dynamics is specified by an action principle consisting of a kinetic energy and potential energy term. The former are based on measures of how quickly components of causal change do so with respect to averaged notions. The potential energy terms measure how much local moves alter an observer's ""view"" of the universe, as seen from their perspective. These results show that quantum dynamics is restored in an N to infinity limit. Measurable non-linear corrections to quantum dynamics emerge to higher order in 1/sqrt{N}. "

Analogical reasoning has a long and distinguished history as a method for making discoveries in physics. I will discuss novel uses of formal analogies in twentieth century particle physics and condensed matter physics. I will then offer some reflections on how methodological lessons from these cases could inform the use of analogies in discoveries related to quantum gravity.

In modern cosmology there is an agreement that the seeds of structure formations resides in the quantum fluctuation of the geometry in the early universe, but there is no agreement about how these could be derived from a quantum theory of gravity. In this talk I present a proposal based on the covariant formulation of Loop Quantum Gravity. I describe how to define a wavefunction of the universe in this context, and a how we can study fluctuations and correlations between spacial regions. The results obtained so far has been made possible by recent progress in numerical computations. I discuss the current state of this research program and the possible implications for modeling the early universe.

Quantum gravitational phenomena dominate the physics of a black hole in the past of a high curvature spacelike surface. Because of the backreaction of the evaporation, this surface crosses the horizon. I describe a recent line of investigation on the possible evolution compatible with nonperturbative quantum gravity, and in particular I illustrate what can be predicted using loop quantum gravity.

I will discuss a number of theoretical, observational, and conceptual aspects of the Everpresent Λ cosmological model arising from fundamental principles in causal set theory and unimodular gravity. In this framework the value of the cosmological constant (Λ) fluctuates, in magnitude and in sign, over cosmic history. At each epoch, Λ stays statistically close to the inverse square root of the spacetime volume. Since the latter is of the order of H^2 today, this provides a way out of the cosmological constant puzzle without fine tuning. I will review the theoretical background of this idea. I will also describe a phenomenological implementation of this model, and discuss recent results on the statistics of its simulations and observational tests of it.

The nonlocality of causal sets gives us hope of solving the cosmological constant puzzle (``why is the universe so smooth, big and old if there is only one scale---the discreteness scale---in the theory?’’) On the other hand locality, , GR and local QFT, must be recovered from quantum gravity in the continuum approximation at large scales, which is a challenge. If we are lucky though (like Goldilocks, the universe gets the nonlocality ``just right’’) nonlocality may be a rich source of phenomenology. Yasaman Yazdi’s talk will be on cosmological models based on the nonlocality of causal sets. I will give a couple of examples of more astrophysical phenomenological models based on simple assumptions---randomness due to spacetime uncertainty and Lorentz invariance---and pose a research question: is there a model of ``quantum swerves’’?

In addition to the now-well known “Hubble tension”, in recent years a second tension has emerged: the $S_8$ tension. This is a measure of the homogeneity of the Universe. Specifically, $S_8$ is defined as $(\Omega_{\mathrm matter}/0.3)^{0.5} \sigma_8$ where $\sigma_8$ is the standard deviation of the density fluctuation in an 8 $h^{-1}$ Mpc radius sphere. As with the Hubble tension, there is disagreement, at greater than 4 $\sigma$ significance between what is predicted by extrapolating the fluctuations in the Cosmic Microwave Background forward to the present day, and what is measured by multiple probes of the inhomogeneity in the nearby Universe. I will discuss the diverse lines of evidence for the tension, showing it is not restricted to one probe, but is seen in weak gravitational lensing, peculiar velocities and redshift-space distortions and cluster abundances. I will conclude by discussing prospects for future measurements.

Local measurements of the expansion rate of the local universe differ from predictions of simple models fitted to large-scale cosmological measurements, at a statistically significant level. Sample variance (often called cosmic variance) is a key component of errors placed on measurements made from a small data set. For the Hubble constant, which parametrises the expansion rate, the size of the patch of the Universe covered by recent supernovae observations has a radius of 300Mpc. The smaller the patch, the larger the patch-to-patch fluctuations and the larger the error on the measured value of H0 from sample variance. Using the H0 measurement from supernovae as an example, I will consider a number of different ways to estimate sample variance using techniques developed for multiple uses, and show that they all approximately agree. The sample variance error on H0 measurements from the recent Pantheon supernovae sample is +/-1 kms^-1Mpc^-1, insufficient to explain the Hubble tension in a standard Lambda-CDM universe. This will demonstrate methods for comparing variations in expansion rate in the universe and what we mean by saying the universe is expanding (on average), or that galaxies move apart with particular velocities.

I present an unprecedented template-based search for stimulated emission of Hawking radiation (or Boltzmann echoes) by combining the gravitational wave data from 65 binary black hole merger events observed by the LIGO/Virgo collaboration. With a careful Bayesian inference approach, I found no statistically significant evidence for this signal in either of the 3 Gravitational Wave Transient Catalogs GWTC-1, GWTC-2 and GWTC-3. However, the data cannot yet conclusively rule out the presence of Boltzmann echoes either, with the Bayesian evidence ranging within 0.3-1.6 for most events, and a common (non-vanishing) echo amplitude for all mergers being disfavoured at only 2:5 odds. The only exception is GW190521, the most massive and confidently detected event ever observed, which shows a positive evidence of 9.2 for stimulated Hawking radiation. An optimal combination of posteriors yields an upper limit of A<0.42 (at 90% confidence level) for a universal echo amplitude, whereas A∼1 was predicted in the canonical model. The next generation of gravitational wave detectors such as LISA, Einstein Telescope, and Cosmic Explorer can draw a definitive conclusion on the quantum nature of black hole horizons.

It has been proposed that quantum-gravitational effects may change the near-horizon structure of black holes, e.g. firewalls or ultra-compact objects mimicking black holes. Also, a Lorentz-violating theory as a candidate of quantum gravity, e.g. the Horava-Lifshitz theory, changes the causal structure of black holes due to the superluminal propagation of excited modes. The late-time part of the gravitational wave ringdown from a black hole is significantly affected by those effects, and the emission of gravitational wave echoes may be induced. The black hole quasi-normal (QN) modes are affected by the change of the horizon structure, which results in the drastic modification of the late-time signal of the gravitational wave. In this talk, I will discuss how the gravitational wave echo can be modeled and how the echo model is reasonable from an entropic point of view by counting QN modes to estimate the black hole entropy.