General Relativity & Quantum Cosmology

Many cosmological scenarios beyond the Standard Model lead to the formation of a network of cosmic strings. In this talk, I will review how these models lead in a natural way to the production of a stochastic gravitational wave background and how this signal could account for the recently reported results from the NANOGrav collaboration. Finally, we will explain how future observations could allow us to confirm this interpretation of the NANOGrav data.

In this talk I introduce nonlocal (infinite derivative) field theories. First of all, I discuss how and which principles of quantum field theory are affected when higher-order derivative operators are taken into account in a Lagrangian. In particular, I focus on the issue of unitarity and on how to make higher-derivative theories healthy by means of non-polynomial differential operators. I extend the treatment to the gravity sector and consider nonlocal theories whose graviton propagators are ghost-free, and explore the possibility of regularizing singularities. Next, I discuss some recent progress in proving perturbative unitarity for a very general class of nonlocal field theories. Finally, I will make some remarks on nonlocality and quantum gravity.

The problem of time is often discussed as an obstacle in the canonical quantisation of gravitational systems: general covariance means there is no preferred time parameter with respect to which evolution could be defined. We can instead characterise dynamics in relational terms by defining one degree of freedom to play the role of an internal clock for the other variables; this leads to a multiple choice problem of which variable should play the role of clock. I will review recent results obtained in a quantum cosmological model with three dynamical degrees of freedom: a volume or scale factor variable for the geometry, a massless scalar matter field, and a perfect fluid. Each of these variables can be used as a clock for the other two. We obtain three different theories which, if we require them to have unitary time evolution with respect to the given clock, make very different statements about the fate of the Universe. Only one resolves the classical singularity, and only one leads to a quantum recollapse of the Universe at large volume. Nonclassical behaviour arises whenever a classical solution terminates in finite time so that reflecting boundary conditions are needed to make the theory unitary. We discuss general implications for a canonical quantisation of gravitational systems.

Zoom Link: https://pitp.zoom.us/j/95556457739?pwd=S0dZUVYwTTBOaFNpNGcrc2ladHZ5QT09

In this talk, I will present how gravitational-wave (GW) measurements can be utilized to search for ultralight bosons and primordial black holes (PBHs), which can be the keys to some of the unsolved questions about our Universe. In the first half, I will discuss the current constraints on excluding the mass of ultralight bosons around ~10^-13 eV, based on the mass-spin measurements of BBHs from the second GW catalog (GWTC-2). These constraints are driven by the phenomenology called “superradiance”, which extracts the angular momentum of a rapidly spinning black hole to form a slowly spinning black hole-boson cloud system. In the second half, I will illustrate how the distance measurements of very high-redshift BBHs made by the next-generation GW detectors may provide cleaner hints or constraints for the existence of PBHs in the future.

Zoom Link: https://pitp.zoom.us/j/95012157554?pwd=UWdBZ1FOTSt2dVlBaS81MjRLbVBvUT09

We will begin by explaining the black hole information paradox, starting from first principles. We will then explain how computations in string theory yield a resolution of this paradox. When we make a bound state of strings and branes, then this bound state is found to swell up into a horizon sized `fuzzball'; this fuzzball radiates like a normal body without any information loss. The existence of these fuzzballs implies a new picture for the quantum gravitational vacuum, where the virtual fluctuations resemble the scale free fluctuations at a second order phase transition, rather than being confined to within the planck scale.  We will see how this `vecro' picture of the vacuum might give a resolution to several puzzles we face in cosmology, like the origin of energy needed for inflation and the existence of a cosmological constant.

I will discuss the dynamical friction experienced by black holes moving through scalar or “wave-like” dark matter. This has been studied analytically and numerically in the non relativistic case, which is applicable to its effect in “fuzzy dark matter” halos where the scalar wavelength is on galactic scales. On smaller scales, dynamical friction from dark matter clouds formed from accretion or superradiance may cause a dephasing in the gravitational wave signal in LISA EMRIs. I will discuss our recent numerical work (https://arxiv.org/abs/2106.08280) to extend the description of dynamical friction to relativistic scalars, in order to treat this case.

Zoom Link: https://pitp.zoom.us/j/98815587081?pwd=a3FvcUtTaWFUejBpR1Q3QUhRRzF4dz09

The information loss paradox is usually stated as an incompatibility between general relativity and quantum mechanics. However, the assumptions leading to the problem are often overlooked and, in fact, a careful inspection of the main hypothesises suggests a radical reformulation of the problem. Indeed, I will present a thought experiment that shows the existence of an incompatibility between (i) the validity of the laws of general relativity to describe infalling matter far from the Planckian regime, and (ii) the so-called central dogma which states that as seen from an outside observer a black hole behaves like a quantum system whose number of degrees of freedom is proportional to the horizon area. We critically revise the standard arguments in support of the central dogma, and argue that they cannot hold true unless some new physics is invoked even before reaching Planck scales. This leads to the counterintuitive result that the information loss problem, in its current formulation, is not necessarily related to any loss of information or lack of unitarity. Semiclassical general relativity and quantum mechanics can be perfectly compatible before reaching the final stage of the black hole evaporation where, instead, a consistent theory of quantum gravity is needed to make any prediction.

Zoom Link: https://pitp.zoom.us/j/94637978136?pwd=VmlWa1BJU3I1UGVRNjliMEpUTlo2dz09

In the asymptotic safety approach to quantum gravity it is conjectured that a nonperturbative ultraviolet completion exists for Einstein's metric gravity and its perturbative effective description in four dimensions. Such completion would circumvent the notorious problems of renormalizability and predictivity at high energies of the quantum theory. I want do discuss the conjecture armed with pragmatism and avoiding preconceived ideologies. I also want to present some perturbative results in two dimensions, that can be used to argue the validity of the conjecture above two dimensions, and to discuss the implications for the current status of the approach.

Gravitational waves (GWs) are reshaping our understanding of the universe, and the more exciting discoveries are yet to come. In the next observing runs we will observe hundreds to thousands of events per year at increasingly higher redshifts, opening unique opportunities to test our cosmological model. In this talk I will focus on how this coming data could help probing gravity and dark energy, and what new information GW lensing will provide. In the first part of the talk I will describe how, without the need for electromagnetic counterparts or galaxy catalogs, the study of the binary black hole mass distribution can constrain LCDM and Einstein’s gravity. Moreover, I will show that black hole mergers are also promising laboratories to  bound GW interactions with other cosmological fields leading to waveform distortions and echoes. In the second part, I will discuss current searches for strongly lensed GWs and the importance of the GW phase measurement in this identification. Observing GW lensed events will allow us to probe the matter distribution in the universe and further constrain the laws of gravity. In both parts of the talk I will emphasize the important role of next generation ground- and space-based detectors.

The incorporation of classical general relativity into the framework of quantum field theory yielded a rather surprising result -- thermodynamic particle production. In short, for fundamental deformations in the structure of spacetime, quantum mechanics necessitates the creation of thermalized particles from the vacuum. One such phenomenon, known as the Unruh effect, causes empty space to effervesce a thermal bath of particles when viewed by an observer undergoing uniformly accelerated motion. These highly accelerated systems will also have an associated Rindler horizon which produces this Unruh radiation at the celebrated Fulling-Davies-Unruh temperature. For accelerated charges, the emission and absorption of this Unruh radiation will not only affect the associated Rindler horizon in accordance with the Bekenstein-Hawking area-entropy law, but will also imprint the FDU temperature on any photons emitted and subsequently detected in the laboratory. A recent series of high energy channeling experiments carried out by the NA63 collaboration at CERN have finally brought about the first observations and insights into the nature of the Unruh effect. In this presentation, I will discuss the various aspects of acceleration-induced thermality measured by these experiments at NA63.

Zoom Link: https://pitp.zoom.us/j/97257949405?pwd=Ung4TXVwbHJDdm9LbEVRSExQTzI4Zz09

At some length scale, Einstein's theory of general relativity (GR) must break down and be reconciled with quantum mechanics in a quantum theory of gravity. Binary black hole mergers probe the strong field, non-linear, highly dynamical regime of gravity, and thus gravitational waves from these systems could contain beyond-GR signatures. While LIGO presently performs model-independent and parametrized tests of GR, in order to perform model-dependent tests, we must have access to numerical relativity binary black hole waveform predictions in beyond-GR theories through full inspiral, merger, and ringdown. In this talk, I will discuss our results in producing full numerical relativity waveforms in beyond-GR theories, including dynamical Chern-Simons gravity and Einstein dilaton Gauss-Bonnet gravity, and performing gravitational wave data analysis on these waveforms.

Zoom Link: https://pitp.zoom.us/j/91782607606?pwd=SkpaYlF6a04zVDNXS2ZlWjJwdUpkQT09

The advent of black hole imaging has opened a new window into probing the horizon scale of black holes. An important question is whether string theory results for black hole physics can predict interesting and observable features that current and future experiments can probe.

I will discuss the physical properties of four-dimensional, string-theoretical, horizonless “fuzzball” geometries by means of imaging their shadows. Their microstructure traps light rays straying near the would-be horizon on long-lived, highly redshifted chaotic orbits. In fuzzballs sufficiently near the scaling

limit this creates a shadow much like that of a black hole, while avoiding the paradoxes associated with an event horizon.

Finally, I will consider comparing such fuzzball images to their black hole counterparts. In particular, detailed measurements of higher order photon rings have the potential to discriminate between fuzzballs and black holes in future observations.