Speakers 

More on Quantum decoherence signatures from inflation

Cosmic Inflation is a building block of the current standard cosmological model. It is able to explain the generation of primordial perturbation seeds in the Early Universe from which all the structures we see today in the Universe formed and from which a stochastic background of gravitational waves is unavoidable predicted. Such seeds are quantum mechanically generated, arising from quantum vacuum fluctuations. However what we observe in the sky (Cosmic Microwave Background radiation and/or the large-scale distribution of galaxies) are classical objects, implying a quantum-to-classical transition of the primordial perturbations on cosmological scales. Quantum decoherence in the Early Universe can realize such a transition and it has has been extensively re-discovered in recent years with new and fresh ideas and approaches in a cosmological  context. It could be interesting to discuss how quantum decoherence can leave some signatures on inflationary observables, including primordial non-Gaussianity and primordial gravitational waves and how one can try to "dig" into future observables in search for some clear   signatures of the quantum origin of cosmological perturbations. Also, some techniques that have been employed to compute quantum-decoherence seem to have some nice features that could  make them useful tools to be applied also to other problems in cosmology. 

Dynamical de Sitter black holes in a quasi-stationary expansion

We revisit and improve the analytic study arXiv:1804.03462 of spherically symmetric but dynamical black holes in Einstein's gravity coupled to a real scalar field. We introduce a series expansion in a small parameter ϵ that implements slow time dependence. At the leading order, the generic solution is a quasi-stationary Schwarzschild-de Sitter (SdS) metric, i.e. one where time-dependence enters only through the mass and cosmological constant parameters of SdS. The two coupled ODEs describing the leading order time dependence are solved up to quadrature for an arbitrary scalar potential. Higher order corrections can be consistently computed, as we show by explicitly solving the Einstein equations at the next to leading order as well. We comment on how the quasi-stationary expansion we introduce here is equivalent to the non-relativistic 1/c expansion.

Collisionless and collisional relaxation processes in Modified Newtonian Dynamics (MOND)

Modified Newtonian Dynamics (MOND) has been introduced (Milgrom 1983, Bekenstein & Milgrom 1984) in galactic dynamics to interpret the breakdown of Newtonian gravity in the weak field regime below a scale acceleration $a_0=10^{-10}$ m/s$^2$, without resorting to introducing a missing mass in the form of Dark Matter (DM). So far, MOND has been proved as a valid alternative, despite some challenges, in modelling several gravitational systems ranging from small galactic satellites to galaxy clusters. In Newtonian gravity with DM and

 MOND the kinematics of  a test particle in a given fixed potential is degenerate. However, the dynamics of particles in  a self-consistent and evolving potential may differ significantly in the two paradigms. In particular, due to the nonlinearity of the MOND field equation substituting the classical Poisson equation, one expects the collisionless processes of phase-mixing and violent relaxation, typically happening in a scenario of monolithic collapse of post merger, to work rather differently in a MONDian context. Moreover, the absence of the superposition principle also possibly affects the collisional mechanisms of two body relaxation and dynamical friction. 

In this talk I will review some results of numerical and analytical investigations on said processes in MOND versus Newtonian gravity with DM. 

Path integrals in the sky 

With the advent of fast radio bursts, pulsars and gravitational waves, it has become increasingly important to consider wave phenomena in astrophysics. However, unfortunately, interference phenomena are often delicate to define and expensive to evaluate as they involve the evaluation of highly oscillatory multi-dimensional conditionally convergent integrals. They are, in fact, a Feynman path integral in the sky. Using Picard-Lefschetz theory in complex analysis, I will demonstrate a way to unambiguously define and efficiently evaluate the Kirchhoff-Fresnel integral for lensing in wave optics. I will demonstrate the method in several astrophysical settings. Finally, I will elude to its connection with the theory of real-time Path Integrals, Resurgence, and Hyperasmptotics. 

Proper time path integrals for gravitational waves: an improved wave optics framework

An intriguing aspect of gravitational wave lensing is the emergence wave-effects: interference and diffraction patterns in the waveforms due to finite size effects, occurring when the wave’s wavelength is comparable to the Schwarzschild radius of the lens. 

These phenomena are particularly interesting because they induce frequency dependent modifications in the waveforms, allowing for a better lens’ parameter estimation, especially if the lensing event has an electromagnetic counterpart in the opposite optical regime.

Despite the promising potential of wave-optics effects, our current theoretical tools, based on the diffraction integral, rely on two main assumptions that limit their effectiveness: the eikonal and paraxial approximations on one hand, and the neglect of spin effects on the other.

In this talk I will present our new formalism, based on the established proper time technique in field theory, illustrating its robustness as the generalization of the diffraction integral, going beyond all of the limitations mentioned. 

“Scalar-Induced” Gravitational Waves as a probe for Modified Gravity

Due to their weak interaction with matter, we hope to observe Primordial Gravitational Waves (GWs) among the various signals that are expected from current and future-generation detectors. This offers a new and exciting opportunity to explore the physics of the early Universe. An open question to answer is if we can use Primordial GWs to probe beyond General Relativity?

Primordial GWs come in the form of a stochastic background (SGWB) due to the quantum nature of the fluctuations that generate them. Among the various sources, one contribution is the “scalar-induced” GWs (SIGWs). These are produced as a second-order effect due to the coupling of first-order scalar fluctuations. The contribution from anisotropic stress is usually neglected due to its minimal contribution and as a result the scalar potentials are set to be equal. In modified gravity theories there is a modification in the geometry of Einstein’s field equations and we see a mismatch between the potentials, subsequently a modification in the source term of SIGW. 

In this talk, I present computations of the source term of SIGW both in standard cosmology and beyond General Relativity, specifically considering f(R) modified gravity model. I concentrate on understanding the effect of the first-order correction to the source term due to the mismatch of the scalar potentials. The observability of this signature is studied by comparing the results of the modification w.r.t. standard general relativity

Quantum Signatures and Decoherence during Inflation

In order to shed light on the quantum to classical transition of the primordial perturbations during inflation, we investigated the decoherence of a system of scalar curvature perturbations induced by an unobservable environment of deep subhorizon tensorial modes.

We computed the associated corrections to the cosmological correlation functions, looking for distinguishable signatures which could, in future observations, prove the quantum origin of primordial perturbations. In doing this, we commented on proposed techniques to deal with non- Markovianity (i.e. memory effects in the environment, which drastically complicates calculations), which seems to be ubiquitous in an inflationary framework.

The making of a dynamical  cosmological constant

Dark energy is the most abundant component of our universe and drives its present acceleration. The simplest explanation is still Einstein biggest "blunder": the cosmological constant. The size of the cosmological constant when considered as the result of zero point-energy of standard model quantum fields gives rise to the most phenomenal failure of dimensional analysis in physics. Replacing the cosmological constant by some dynamical entity is challenging given the peculiar relation between its pressure p and energy density rho: p = - rho. The nature of the difficulties that are faced when one tries to implement dynamically the equation of state  p = - rho is discussed. It turns out that linear stability cannot be achieved  with a positive Hamiltonian. A relaxed version of stability can be obtained featuring strictly positive propagation velocities  for linear  perturbations. It is interesting that such a system resembles a spinning top and the Pais-Uhlenbeck oscillator.  

Quantum field theory of scalar dark matter: a simple model

We build up a simple Lorentz invariant model of dark matter consisting of a massive, real, scalar field coupled to its massless, real scalar mediator field. From a mathematical point of view the model belongs to the Klein-Gordon-Wave family of models. The dynamics is ruled by a single parameter, containing the mass of the dark particles and their initial number. It is shown that if the latter number is set to one, then the stationary states of the model exist only for values of the particle mass that are quantized, the minimum possible value being at the Planck scale, whereas no upper bound exists. Such states can be tentatively interpreted, within all the limits of the theory, as primordial black holes. On the other hand, allowing for a very large number of particles and plugging into the model a value of the dark particle mass of the order of $10^{-24}eV$, we get, by a perturbative procedure, the Schroedinger-Wave model. The spherically symmetric, stationary states of the latter model are those first interpreted by Sin (1994) as "cold" BEC clumps of galactic size. Thus, the simple model considered here allows for two components of dark matter (primordial black holes and ultralight scalars) whose mass ranges are completely far apart. The important issue concerning the stability of the two families of stationary states is also discussed. 

Joint work with L. Zanelli and G. Marangon 

Statistical mechanics of non additive systems and the Thirring model 

The standard formulation of thermodynamics relies on additivity. However, there are numerous examples of macroscopic systems that are non additive, due to long-range interactions among the constituents. These systems can attain equilibrium states under completely open conditions, for which energy, volume and number of particles simultaneously fluctuate. The unconstrained ensemble is the statistical ensemble describing completely open systems; temperature, pressure and chemical potential constitute a set of independent control parameters, thus violating Gibbs-Duhem relation. I will illustrate the properties of this ensemble by considering a modified version of the Thirring model with both attractive and repulsive long-range interactions. 

Interaction between a gravitational wave and a trapped Bose-Einstein condensate

In light of recent proposals for detecting gravitational waves by using Bose-Einstein condensates ,

we investigate the interplay between these two phenomena. As already established in the literature ,

we demonstrate how a gravitational wave induces a phase shift in the macroscopic wavefunction

of the Bose-Einstein condensate. Building upon existing work , we then examine this phenomenon in the case of Bose condensates confined by an anisotropic harmonic potential.  

Loop Corrections To The Power Spectrum

The current inflationary paradigm is based on the idea that early quantum fluctuations become the seeds of structure formation that we observe today. Different inflationary models predict different inflationary scenarios of how those seeds came out to be. Non-Gaussianity may allow us to say something about the underlying inflationary dynamics and hence serves as a direct probe for early universe physics. As such, we present an analytic approach of computing the loop corrections to the matter power spectrum based on a certain parametrization of non-Gaussianity, the so-called local type. Using this parametrization we show that we can compute a compact form of the loop correction that agrees with the numerical result up to 10% within the region of interest, and also reduces to more familiar forms present in the literature in certain limits.

Toward a Minkowskian formulation of lattice QCD 

Nonperturbative simulations of Quantum Chromodynamics (QCD) are currently feasible only in Euclidean space-time through the aggregate of techniques known as "Lattice QCD", achieving notable successes in calculating the hadron spectrum, understanding confinement, determining form factors and matrix elements, and studying many facets of QCD thermodynamics. However, real-time dynamics and non-equilibrium phenomena, such as short-lived resonances and relativistic scattering processes, remain problematic to study in Euclidean spacetime.

A Minkowskian lattice formulation of QCD could address these (and other) challenges by allowing to directly compute time-dependent processes.

Symplectic Quantization (SQ) is a promising new approach that allows sampling of quantum fluctuations directly in Minkowski spacetime. Recently, the equivalence of SQ with the Feynman functional integral for scalar fields has been proven, and the shape of  the Feynman propagator

has been numerically computed on a 1+1 lattice for a weakly interacting \lambda \phi^4 theory, showing good agreement with theoretical expectations. Current efforts focus on extending these results to gauge theory and developing an associated Minkowskian simulation code.

Gravitational Duals From Equation Of State

Holography relates gravitational theories in five dimensions to four-dimensional quantum field theories in flat space. Under this map, the equation of state of the field theory is encoded in the black hole solutions of the gravitational theory. Solving the five-dimensional Einstein's equations to determine the equation of state is an algorithmic, direct problem. Determining the gravitational theory that gives rise to a prescribed equation of state is a much more challenging, inverse problem. We present a novel approach to solve this problem based on physics-informed neural networks. The resulting algorithm is not only data-driven but also informed by the physics of the Einstein's equations. We successfully apply it to theories with crossovers, first- and second-order phase transitions.

An Entangled Universe

We propose a possible quantum signature of the early Universe that could lead to observational imprints of the quantum nature of the inflationary period. We suggest a setup in the context of one-field, slow-roll inflation where a Bell experiment can be performed through an entangled state of gravitons entangled in their polarization states. At horizon crossing, interactions between the gravitons and inflatons, together with the gathering of “which-path information” from the cosmological horizon, perform the required Bell experiments leading to a definitive measure, which can be imprinted in the scalar correlation four-point function. We hint how this signature could be measured in the high-order correlation function of galaxies, in particular on the halo bias and the intrinsic alignment.

Dynamics and Stability of Solitonic Galactic Core in Ultralight Dark Matter

A key feature of dark matter consisting of ultralight bosonic particles is the emergence of superfluid Bose-Einstein condensate (BEC) structures on galactic scales. We discuss stability of the galactic BEC cores, focusing on both fundamental and vortex solitons under various parameters of ultralight bosonic dark matter particles. We present our recent results on the oscillatory behavior of solitonic dark matter structures within the central galactic region, derived from numerical solutions of the Bogoliubov-de Gennes (BdG) equations, which incorporate perturbations in the gravitational potential and local self-interactions. Our analysis reveals that the central solitonic core, maintained by the interplay of gravitational attraction, quantum pressure, and repulsive interactions, exhibits pronounced oscillatory dynamics. These oscillations, characterized by distinct eigenmodes, offer significant insights into the dynamical properties of solitonic dark matter structures and their observational implications, particularly in the context of galactic structure formation and evolution.