General Relativity & Quantum Cosmology

The detection of gravitational waves from mergers of compact binaries in the first two runs of the Advanced LIGO-Virgo have brought in valuable insights into fundamental physics and astrophysics. The coalescence process sweeping the components through a range of frequencies at highly relativistic velocities, have enabled some of the first tests of general relativity in its highly dynamical and extremely strong field regime. The recent detection of the binary neutron star merger has shed first light on the elusive neutron star equation of state. Furthermore, with its coincident electromagnetic counterpart, a first "standard-siren" measurement of the Hubble parameter has been made independent of a cosmic distance ladder. Subsequent detections are expected to give us a more direct understanding of the nature of the black holes and the composition of neutron stars, as well as bring in a plethora of results in astrophysics and possibly new insights into cosmology. I will go over some of these exciting new results and talk about future prospects of fundamental physics and cosmology from gravitational-wave observations.

Over the last few years gravitational wave (GW) detections have marked
the beginning of a new era of astrophysical observations. When the
emitters include a compact object like a neutron star, the GW signal
is accompanied by emissions in different bands, e.g. X-rays,
gamma-rays, optical and neutrinos. The interpretation of such
multimessenger signals allows us to gain a deeper understanding of the
interiors of compact objects. One main challenge is to link our
knowledge of nuclear interactions to macroscopic properties of dense
objects in the Universe. In this talk I will discuss selected aspects
along the interface of nuclear physics and astrophysics

We present a consistent theory of classical gravity coupled to quantum field theory. The dynamics is linear in the density matrix, completely positive and trace-preserving, and reduces to Einstein's equations in the classical limit. Several no-go theorems are evaded since the assumption that gravity is classical necessarily modifies the dynamical laws of quantum mechanics -- the theory must be fundamentally stochastic involving finite sized and probabilistic jumps in space-time and in the quantum field. Nonetheless the quantum state of the system can remain pure conditioned on the classical degrees of freedom. The measurement postulate of quantum mechanics is not needed since the interaction of the quantum degrees of freedom with classical space-time necessarily causes collapse of the wave-function. More generally, we derive a form of classical-quantum dynamics using a non-commuting divergence which has as its limit deterministic classical Hamiltonian evolution, and which doesn't suffer from the pathologies of the semi-classical theory. Details at http://arxiv.org/abs/1811.03116

The path integral approach to quantum gravity has given us many interesting results in the semiclassical regime. However, the canonical path integral for General Relativity still poses a variety of conceptual and computational challenges. To probe the deep quantum regime, Path Integral Monte Carlo (PIMC) techniques are being used successfully in approaches like Causal Dynamical Triangulations and Causal Set Theory. 

In this talk I will discuss how we can apply PIMC techniques to make the canonical Path Integral amenable to computation using matter-time. I will present results from PIMC simulations of homogeneous & isotropic cosmologies, and discuss the extension of this technique to anisotropic as well as inhomogeneous settings. 

The recent gravitational-wave detections of binary mergers 
have opened a new and exciting window into observing the Universe. 
Persistent sources of gravitational waves must also exist; searches for 
continuous gravitational-wave signals generally look for signals from 
non-axisymmetric neutron stars, but are also suitable for continuous 
gravitational waves from axion annihilations in clouds around black 
holes. I will discuss the expected axion annihilations signal from an 
astrophysical standpoint, starting from the Galactic isolated black hole 
population.

I show that Unruh effect can be associated with the partitioning of the real line, and derived from the basic representation theory of the group of affine transformations in one dimension. This result shows that thermal distributions naturally emerge in connecting quantum
states belonging to representations related to distinct notions of translational symmetry.

Neutron stars host the densest stable matter in the universe. Accurately modeling their multi-messenger astrophysics relies on a detailed description of the equation of state above nuclear density, and astronomical observations of neutron stars can in turn be used to constrain the properties of such dense matter. On August 17, 2017 the Advanced LIGO and Advanced Virgo detectors discovered the first gravitational-wave signal consistent with a binary neutron star inspiral. The three-dimensional localization of GW170817’s source using LIGO and Virgo data enabled a successful electromagnetic follow-up campaign. We are also able to constrain the equation of state of dense matter in neutron stars using the signature of tidal interaction in the gravitational-wave signal. I will outline these constraints, the methods by which they are made, how the gravitational-wave information connects with other observations of neutron stars, and outline future prospects for gravitational-wave astronomy with neutron stars.

I will present a new method to constrain the quantum effective action of gravity, inspired by recent results from causal dynamical triangulations (CDT). After a short introduction to CDT, I will start from a parameterisation of the effective action to calculate correlation functions for observable quantities. Matching these correlators to CDT data allows to reverse-engineer the couplings which describe the effective dynamics. As a concrete example, the autocorrelation of spatial volume fluctuations is considered. The comparison to CDT data suggests the existence of non-local interaction terms, which might have interesting phenomenological consequences for the interpretation of dark energy.

I will address a long-standing problem of describing the thermodynamics of an accelerating black hole, concentrating on the special case of slowly accelerating black holes in AdS. The key
ingredient of consistent thermodynamics is to ensure that the system is not over-constrained by including the possibility of varying the string tensions that are responsible for the acceleration of the black hole. The first law assumes the standard form, with the entropy given by one quarter of the horizon area and other quantities identified by standard methods. The dual
energy-momentum tensor can be written as a three-dimensional perfect fluid plus a non-hydrodynamic contribution. Some novel thermodynamic phase behavior related to the black hole acceleration will also be discussed.

The ultra-strong magnetic fields of magnetars have profound implications for their radiative phenomena. We studied the dynamics of strong magnetic fields inside and outside magnetars. Inside the magnetar, the strong magnetic stress can break the crust and trigger plastic failures. The interaction between magnetic fields and plastic failures is studied in two scenarios: 1. Internal Hall waves launched from the core-crust interface can initiate plastic failures and lead to X-ray outbursts. 2. External Alfven waves produced by giant flares can also initiate crustal plastic failures which dissipate the waves and give rise to delayed thermal afterglow. The crustal dissipation of Alfven waves is competed by the magnetospheric dissipation outside the magnetar. Using a high order simulation of Force-Free Electrodynamics (FFE), we found that the magnetospheric dissipation of Alfven waves is generally slow and most wave energy will dissipate inside the magnetar.

We consider a natural extension of general relativity, by the addition of a term which is a topological invariant when the cosmological constant is in fact constant.  Allowing the cosmological constant to vary, we discover it is endowed with a natural dynamics, which also includes a mechanism to suppress its value.   A key role in hiding the consequences of a dynamical cosmological constant is played by the torsion of the spacetime connection.

As a dynamical variable, the varying cosmological constant turns out to be canonically conjugate to a measure of intrinsic, cosmological time, given by the Chern-Simons invariant of the Ashtekar connection (which we originally studied with Chopin Soo).  As a result, a small quantum universe that "knows what time it is" does not know the value of the cosmological constant, and visa versa.   This leads to some novel scenarios for the early universe.

This is joint work in progress with Stephon Alexander, Joao Magueijo and Robert Sims.

Since the first detection two years ago, gravitational waves have promised to revolutionize the physics and astrophysics of compact objects. But to understand what these gravitational waves are telling us, we need to understand how these relativistic binary systems form in the first place.  In this talk, I will describe the various astrophysical pathways for forming the binary mergers detected by LIGO/Virgo, and how specific features of the gravitational waves (such as the eccentricities and spins) can illuminate the formation histories of these exotic objects.  In particular, I will discuss how black holes can undergo multiple mergers in dense star clusters, creating a second generation of black holes more massive (and with potentially greater spins) than those formed through the collapse of isolated stars.