General Relativity & Quantum Cosmology

If dark matter is composed at least partially of primordial black holes (PBHs) then structure formation occurs very differently than in standard particle dark matter scenarios.  PBH binaries, halos and other structures can form at very early redshifts and the resulting nonlinear dynamics can change constraints on the abundance of PBHs.  In this talk I will describe this structure formation history using results from cosmological simulations and discuss constraints on solar mass and heavier PBHs from LIGO and the CMB.  Lastly, I will discuss how the baryons may be impacted by such nonlinear structures.

Asymptotically flat spacetimes are invariant under an infinite-dimensional symmetry group comprised of superrotations and supertranslations. These symmetries are spontaneously broken, leading to an infinite degeneracy of gravitational vacua in asymptotically flat spacetimes. Starting from an analysis of four-dimensional asymptotically flat gravity in first order formulation, I will describe how superrotation parametrization modes labelling distinct superrotation vacua are governed by an Alekseev–Shatashvili action on the celestial sphere. I will also comment on a recent construction of a two-dimensional effective action for the Goldstone modes of broken supertranslation invariance.

We introduce the basic elements of tensorial group field theories (TGFTs) for quantum gravity, emphasizing how they encode quantum geometry and their relation with canonical loop quantum gravity and spin foam models. Next, we discuss briefly the issue of continuum limit and how it could be understood in this framework.

In the bulk of the talk, we overview the work on the extraction of an effective cosmological dynamics from TGFTs, inspired by the idea of the universe as a quantum gravity condensate. In this context, we emphasize: the need for appropriately coarse-grained states capturing collective dynamics and the role of relational observables and their construction. We discuss what the theory says (so far) about the fate of the big bang singularity at the beginning of our universe and how it suggests a quantum gravity origin for (phantom) dark energy.

In the last couple of years it has been demonstrated that black hole spacetimes contain many more marginally outer trapped surfaces (MOTS) than had been previously recognized. For example, there is an infinite family of axially symmetric MOTS even in the Schwarzschild solution, of which the apparent horizon is only the first element. In a recent series of papers (arXiv: 2104.10265, 2104.11343, 2104.11343) we demonstrated that these exotic new MOTS play a key role in black hole mergers and, in fact, are the missing pieces needed to complete the apparent horizon “pair of pants” diagram. This merger turns out to be far richer than that of event horizons and, staying with clothing analogies, is better represented by a rococo ball gown than a pair of pants. 

In this talk, I will overview the techniques that we used to find these surfaces, explain the role played by the exotic MOTS in resolving the merger of apparent horizons during a (head-on, non-rotating) black hole merger and also show how the stability operator brings order to what initially appears to be a chaotic melee of (often self-intersecting!) MOTS as they create, annihilate and weave through time. The stability operator was initially introduced by Andersson, Mars and Simon to characterize when a MOTS will smoothly evolve in time, but I will show that it can also be fruitfully understood as the Jacobi operator for MOTS.

We discuss how we can use numerical-relativity simulations to derive gravitational-wave and electromagnetic models describing the binary  neutron star coalescence. We show how these models can be used within a multi-messenger framework to derive new constraints on the neutron-star equation of state and the Hubble constant. For this purpose, we analyze the gravitational wave signal GW170817 and its electromagnetic counterparts AT2017gfo and GRB170817A, together with X-ray observations by NICER, radio observations of massive pulsars, and nuclear theory computations. Similarly, we also discuss that a non-detection of a kilonova for the second binary neutron-star merger detection GW190425 placed constraints on the properties of the system.

Zoom Link: https://pitp.zoom.us/j/99911822549?pwd=NXNNMWJVOUhMTGJSSGYwWUN2NEcxQT09

An extension of general relativity (GR) obtained by adding local quadratic terms to the action will be considered. Such theory can be a viable UV completion of GR. The additional terms soften gravity above a certain energy scale and render gravity renormalizable. The presence of 4 derivatives implies via the Ostrogradsky theorem that the classical Hamiltonian is unbounded from below. Nevertheless, I will argue that the relevant solutions are not unstable, but metastable: when the energies are much below a threshold (that is high enough to describe the whole cosmology) runaways are avoided. Remarkably, the chaotic inflation theory of initial conditions ensures that such bound is satisfied and testable implications for the early universe will be discussed. I will also argue that the basic unitarity condition is satisfied when the theory is correctly formulated at the quantum level. Moreover, thanks to the UV softening of gravity, sufficiently light objects must be horizonless and I will discuss explicit analytic examples of horizonless ultracompact objects, which have interesting physical implications.

Gravitational wave observations are beginning to reveal the nature of the dark side of our universe. The Advanced LIGO and Virgo detectors have observed dozens of binary black hole mergers during the recent third observing run and, with planned sensitivity improvements, expect to observe significantly more binary black hole mergers in future observing runs. The combination of the increased number of detections  and the sheer volume of data associated with each detection provides a significant data analysis challenge. In recent years, various machine learning approaches such as convolutional neural networks have been explored as a basis for rapid analyses for gravitational wave data. This seminar will give a brief introduction to current transient gravitational wave data analysis methodology and highlight novel applications of machine learning for rapid detection of binary black holes and rapid inference of their astrophysical properties. The use of generative machine learning algorithms for transient gravitational wave signal generation will also be discussed.

A fundamental problem of quantum gravity is to understand the quantum evolution of black holes.  While aspects of their evolution are understood asymptotically, a more detailed description of their evolving wavefunction can be provided.  This gives a possible foundation for studying effects that unitarize this evolution, which in turn may provide important clues regarding the quantum nature of gravity.

The issue of whether quantum effects can affect gravity at cosmological distances still lacks a fundamental understanding, but there are indications of a non-trivial gravitational infrared dynamics. This possibility is appealing for building alternatives to the standard cosmological model and explaining the accelerated expansion of the Universe. In this talk I will discuss some large scale modifications of general relativity due to nonlocal terms, which are assumed to arise at the level of quantum effective action. Nonlocality is a general feature of quantum effective actions for theories with massless degrees of freedom and dynamical mass generation is a typical non-perturbative IR effect. Among several models, cosmological requirements select a single structure of the nonlocal term describing a mass for the conformal mode of the metric. The model fits very well cosmological data and has strong signatures in the tensor sector that could be tested in the future by gravitational-wave detections.

Since their first discovery in 2015, gravitational-wave observations yielded several "surprises." The LIGO and Virgo observatories detected more and heavier black holes than anticipated; the first object in the lower mass gap was found; and LIGO announced the discovery of a particularly heavy black hole that could have not come from stellar core collapse. The surprises point to the possibility that some of LIGO/Virgo's black hole mergers occurred in the dense accretion disks of active galactic nuclei (AGNs). AGNs act like black hole assembly lines, resulting in multiple consecutive mergers that create heavier and faster-spinning black holes. I will discuss what we currently know about AGN-assisted mergers and which of LIGO/Virgo's events are suspects. I will finally discuss the prospects of multi-messenger observations from AGN assisted mergers.

Zoom Link: https://pitp.zoom.us/j/93121526365?pwd=c1VCTjBEZnlXYk5HVTFObVBadHlxQT09

The model of Causal Dynamical Triangulations (CDT) is a background-independent and diffeomorphism-invariant approach to quantum gravity,

which provides a lattice regularization of the formal gravitational path integral. The framework does not involve any coordinate system and employs only geometric invariants. For a Universe with toroidal spatial topology, we can introduce coordinates using classical scalar fields with periodic boundary conditions with a jump. The field configurations reveal pictures of cosmic voids and filaments surprisingly similar to the ones observed in the present-day Universe. I will discuss the impact of dynamical matter fields on the geometry of a typical quantum universe in the four-dimensional CDT model and explain several observed phenomena. In particular, a phase transition is triggered by the change of the scalar field jump amplitude. This discovery may have important consequences for quantum universes with non-trivial topology since the phase transition can change the topology to a simply connected one.

Gravitational waves provide a unique observational handle on the properties of strong, dynamical gravity. Ringdowns, in particular, cleanly encode information about the structure of black holes, allowing us to test fundamental principles like the no-hair theorem and the area law. In this talk, I will review the status of this effort, including recent observational results and remaining challenges.