Does thermodynamics rely on Lorentz invariance?

• Francesco Del Porro (Istituto Nazionale di Fisica Nucleare)

Room: Aula Magna 'Fratelli Pontecorvo' It is a well-known fact that, in General Relativity, there exists a strong connection between gravity, acceleration and thermality. Indeed, computations of quantum field theory in a curved background show that the vacuum state for a matter field is described as a thermal spectrum both in the presence of a Black Hole (Hawking Radiation) and in an accelerated framework (Unruh effect). Previous investigations have shown that, if matter is given with a Lorentz Violating dispersion relation, this connection breaks down. On the other side, Lorentz Violating theories of gravity, such as Horava Gravity, represent good candidates for an UV completion of GR. Therefore, in this talk I will investigate thermodynamical properties in such a background, showing that it is actually possible to recover them, even if in a different fashion with respect to the GR case.

Causality Constraints on Mergers beyond General Relativity

• Francesco Serra (Istituto Nazionale di Fisica Nucleare)

Room: Aula Magna 'Fratelli Pontecorvo' Gravitational waves emitted during a black-hole merger might differ from what General Relativity predicts due to the presence of yet unknown fields coupled only to gravity. A simple example is that of a single shift-symmetric scalar field that produces black hole hair, affecting both the inspiral and ringdown phases of black hole mergers. We can constrain such scenarios from the point of view of fundamental principles: can black holes be altered by a scalar field in an effective field theory (EFT) consistent with causality, unitarity and locality? I will consider the requirement that the EFT produces no measurable time-advances in the propagation of scalar and graviton probes. This forces the scalar-graviton interaction to be much weaker than what would be allowed by perturbative control, making deviations from General Relativity unobservable in the next gravitational-wave detectors. Further constraints may arise from unitarity and causality in the form of dispersion relations for S-matrix elements. I will discuss the interplay between these constraints and the requirement of macroscopic causality.

Regular black holes, universes without singularities, and phantom-scalar field transitions

• Aleksandr Kamenshchik (Università di Bologna)

Room: Aula Magna 'Fratelli Pontecorvo' We consider a procedure of elimination of cosmological singularities similar to that suggested in the recent paper by Simpson and Visser for the construction of regular black holes. It is shown that by imposing a non-singular cosmological evolution with a bounce in a flat Friedmann universe filled with a minimally coupled scalar field, we obtain a transition between the standard scalar field and its phantom counterpart. The potential of the scalar field has in this case a non-analyticity of the cusp type. This result is readily reproduced also in the case of an anisotropic Bianchi I universe. We obtain also a new regular black hole solution sustained by a scalar field.

Dissipative processes at acoustic horizon

• Silvia Trabucco (Istituto Nazionale di Fisica Nucleare)

Room: Aula Magna 'Fratelli Pontecorvo' transonic fluid flow generates an acoustic hole that is the hydrodynamic analogue of a gravitational black hole. Acoustic holes emit a detectable thermal radiation of phonons at a characteristic Hawking temperature. The crucial concept is that the spontaneous phonon emission at the horizon produces an irreversible heat increase at the expenses of the bulk fluid kinetic energy. We show that such process can be described in terms of effective shear and bulk viscosities that are defined close to the horizon. We analyze this quantum friction process by resorting to a general kinetic theory approach as well as by the specific description of phonon emission as a tunneling process. The celebrated Kovtun, Son and Starinets (KSS) universal lower bound η/s = 1/4π of the shear viscosity coefficient to entropy density ratio, readily follows, and is extended to the longitudinal bulk viscosity at the horizon. We come to the same saturation of the KSS bound after considering the shear viscosity arising from a perturbation of the background metric at the acoustic horizon providing a – in principle testable – realization of the so called black hole membrane paradigm.

Fermion soliton stars

• Loris Del Grosso (La Sapienza University of Rome)

Room: Aula Magna 'Fratelli Pontecorvo' A real scalar field coupled to a fermion via a Yukawa term can evade no-go theorems preventing solitonic solutions. For the first time, we study this model within General Relativity without approximations, finding static and spherically symmetric solutions describing fermion soliton stars. The Yukawa coupling provides an effective mass for the fermions, which is key to the existence of self-gravitating solutions. I will present this novel family of star solutions and describe their mass-radius diagram and maximum compactness. Besides, I will discuss the ranges of the parameters of the fundamental theory in which the latter might have interesting astrophysical implications. Finally, I will highlight how these results may hint towards the existence of classical gravitational solitons in the standard model.

Perturbative treatment of particle interactions in Kinetic Field Theory

• Christophe Pixius (Heidelberg University)

Room: Aula Magna 'Fratelli Pontecorvo' Finding an analytic theory to describe non-linear cosmic structure formation has proven to be a challenge that is yet to be solved in a satisfactory way. What makes the matter so difficult is that we are looking at a system of classical particles far from equilibrium with long-range interactions. One theory that has been developed in recent years to tackle such systems is Kinetic Field Theory (KFT), which uses the microscopic phase-space dynamics of individual particles to predict collective quantities of the entire system, such as correlation functions. In contrast to more established analytical methods such as Eulerian- or Lagrangian perturbation theory KFT, in principle, offers a complete description of the system and does not suffer from inconsistencies arising due to the crossing particle streams. I want to present recent progress in the description of particle interactions in the framework of KFT and how this affects our predictions for measurable quantities such as the non-linear dark matter power spectrum.

The impact of General Relativity on Galactic Dynamics

• Matteo Luca Ruggiero

Room: Aula Magna 'Fratelli Pontecorvo' The observation of flatness of rotation curves is one element supporting the existence of dark matter, since their behaviour cannot be explained in terms of Newtonian gravity. Far from the galactic center, the gravitational field is supposed to be weak enough so we expect to be able to use Newtonian Gravity; however, even in the weak-field approximation, there are general relativistic effects without a Newtonian counterpart, such as the gravitomagnetic effects originating from mass currents. We suggest the surprising result that gravitomagnetic effects may have a relevant role in better understanding the impact of Dark Matter on galactic dynamics.

Primordial Black Holes as dark matter and source of gravitational waves

• Encieh Erfani

Room: Aula Magna 'Fratelli Pontecorvo' With the discovery of gravitational waves from merging pairs of massive black holes, the interest in the question of whether Primordial Black Holes (PBHs) could constitute Dark Matter (DM) has recently been revived. In this talk, I will review the different mechanisms for (DM) PBH formation by focusing on the excursion set theory. I will also explain our recent work on the merger history of DM PBHs in a core of typical dwarf galaxies. In this work, we investigate the possibility of a sequence of collisions and the final mass spectrum in such clusters. Our study shows that the merging process results in the transfer of about 40% of the total mass of the core to the merger products regardless of the initial mass of PBHs, with about 5% of energy radiated out in the form of gravitational waves.

A new paradigm for a Quantum Big Bounce in the Wheeler-DeWitt framework

• Eleonora Giovannetti

Room: Aula Magna 'Fratelli Pontecorvo' We introduce a quantum scenario for a non-singular Bianchi I model in the Wheeler-DeWitt approach, in order to go beyond the pure semiclassical description of the bouncing dynamics that can be lacking in many relevant cosmological models near the singularity. First of all, we study the Bianchi I wave packet by adopting the isotropic Misner variable as an internal clock and we show its spreading behavior towards the singularity, with the consequence that the semiclassical description of the cosmological dynamics loses its predictability in the Planckian region. Also, we highlight the isomorphism between the Wheeler-DeWitt equation in this particular Minisuperspace and the Klein-Gordon one for a relativistic scalar field. This comparison allows to identify the positive and negative frequency solutions with the collapsing and expanding Universe respectively. Then, we consider an ekpyrotic phase in the primordial Bianchi I evolutin and compute the transition amplitude from a collapsing to an expanding Universe, thanks to the standard techniques of the relativistic scattering theory. Actually, the ekpyrotic-like matter component mimics a time-dependent potential term and breaks the frequency separation, making it possible for a quantum resolution of the initial singularity in the Wheleer-DeWitt framework from a probabilistic point of view. In particular, the probability density associated to this \textquotedblleft Quantum Big Bounce\textquotedblright results to be well-defined in the mean values of the momenta conjugate to the anisotropies and peaked when their mean values in the collapsing Universe are close enough to the corresponding ones in the expanding solution, depending on the variances of the in-going and out-going Universe wave packets. This relation between the pre-Bounce and post-Bounce mean values in the transition probability reflects the behavior of the typical bouncing solution, which symmetrically reconnect the two singular branches at a semiclassical level.

Quintessential Inflation in Palatini Modified Gravity

• Samuel Sanchez Lopez (Lancaster University)

Room: Aula Magna 'Fratelli Pontecorvo' In this talk, I will give a brief introduction to quintessential inflation, a theoretical framework that aims to explain both inflation and dark energy observations to alleviate the incredible fine-tuning of ΛCDM. Furthermore, I will show how adding both an R^2 Starobinsky term and a non-minimal coupling to the inflaton/quintessence field term in the action in the Palatini formalism can rescue the exponential potential, which is well known not to be valid for either inflation or dark energy in the canonical setup. Since the full equations of motion in the Jordan frame are numerically solved, and a parameter scan of the theory is performed, we are able to obtain specific testable predictions, such as the barotropic parameter of dark energy and its running, which will be testable in the near future, as well as constraints on the theory, e.g., on the value of the running of the non-minimal coupling term.

Primordial Black Holes from Stochastic Tunneling

• Chiara Animali (LPENS, Paris)

Room: Aula Magna 'Fratelli Pontecorvo' If the inflaton gets trapped in a local minimum of its potential shortly before the end of inflation, it escapes by building up quantum fluctuations in a process known as stochastic tunnelling. In this work we study cosmological fluctuations produced in such a scenario, and how likely they are to form Primordial Black Holes (PBHs). This is done by using the stochastic-$δN$ formalism, which allows us to reconstruct the highly non-Gaussian tails of the distribution function of the number of $e$-folds spent in the false-vacuum state. We explore two different toy models, both analytically and numerically, in order to identify which properties do or do not depend on the details of the false-vacuum profile. We find that when the potential barrier is small enough compared to its width, $ΔV/V<Δϕ^2/M_{Pl}^2$, the potential can be approximated as being flat between its two local extrema, so results previously obtained in a "flat quantum well'' apply. Otherwise, when $ΔV/V<V/M_{Pl}^4$, the PBH abundance depends exponentially on the height of the potential barrier, and when $ΔV/V>V/M_{Pl}^4$ it depends super-exponentially ( i.e. as the exponential of an exponential) on the barrier height. In that later case PBHs are massively produced. This allows us to quantify how much flat inflection points need to be fine-tuned. In a deep false vacuum, we also find that slow-roll violations are typically encountered unless the potential is close to linear. This motivates further investigations to generalise our approach to non-slow-roll setups.

Relativistic cosmology on the linearized past light-cone

• Matheus Rodrigues Medeiros Silva (University of Pisa and INFN Pisa)

Room: Aula Magna 'Fratelli Pontecorvo' One of the main challenges of theoretical physics in high-precision cosmology is to provide predictions with at least the same level of accuracy from the next surveys. In this regard, I will present a cosmological perturbation theory directly adapted to the past light-cone. Due to its adapted light-cone decomposition in scalars and pseudo-scalars (SPS), the relation to the standard scalar-vector-tensor (SVT) becomes involved, notwithstanding, I will present how one can build operators to extract the SVT information from the SPS perturbations. Moreover, I will show how the SPS perturbations provide a simple decomposition of the perturbations in E- and B- modes highlighting the adaptability of this framework to describe cosmological observables. As an application, I will show how the SPS perturbations on the light-cone provides a recipe to compute gauge invariant observables. Finally, I will apply this recipe to obtain an expression for the angular distance-redshift relation.

Effects of CMB lensing beyond the Born approximation

• Simony Santos Da Costa (Istituto Nazionale di Fisica Nucleare)

Room: Aula Magna 'Fratelli Pontecorvo' In this talk, I will briefly review the lensing corrections to the cosmic microwave background (CMB) temperature anisotropies considering effects beyond the Born approximation. The small deflection angle approximation is used to connect lensed and unlensed power spectrum, up to third order in the gravitational potential. This approach shows that the non-Gaussian nature of deflection angle at higher order is an important correction to the CMB lensing effect. This work has been done previously in literature and the idea here is to implement those modifications on a Boltzmann solver code in order to consider properly all the corrections of CMB lensing aiming the future CMB-IV experiments.

Scale-invariant inflation

• Massimiliano Rinaldi (Istituto Nazionale di Fisica Nucleare)

Room: Aula Magna 'Fratelli Pontecorvo' We consider an exactly scale-invariant model of scalar-tensor gravity and we show that it leads to a consistent model of cosmic inflation. We compute the spectral indices and we show that they fit with the most recent data. From a fundamental level, and when viewed in the Jordan frame, the model displays a classical breaking of scale-invariance and the natural emergence of a mass scale at the end of inflation.

Understanding the non-Gaussianities in the Hubble-Lemaitre diagram

• Giuseppe Fanizza (IA & FCUL)

Room: Aula Magna 'Fratelli Pontecorvo' I will present the theoretical framework to understand the non-Gaussianities in the Hubble-Lemaître diagram, namely the distance-redshift relation, emerging from relativistic cosmological simulations, such as gevolution. With these analytic results, I will discuss which kind of non-Gaussianities can be addressed to intrinsic non-linear effects, such as post-Born corrections and higher-order statistic, against spurious effects introduced by the binning in redshift along the data analysis. Moreover, the numerical shortcuts introduced to account for the matter bispectrum will be discussed, especially in regard of the choice of the appropriate UV-cutoff to be chosen for a well-posed comparison with the above-mentioned relativistic simulations.

Model independent approach for gravitational waves and curvature perturbations

• Antonio Enea Romano (UDEA/LIGO)

Room: Aula Magna 'Fratelli Pontecorvo' Model independent equations can be derived for gravitational waves (GW) and curvature perturbations in terms of an appropriately defined effective sound speed. The effective sound speed encodes the effects of the source terms in the equations of motion associated to the interactions terms in the Lagrangian, related to the coupling with other fields. The effects of the interaction terms can be modeled by introducing an effective metric, in terms of which the equations of motion reduce to the covariant d'Álembert operator. For GWs the effective sound speed depends on the transverse-traceless anisotropic part of the stress-energy tensor, and for curvature perturbation on the entropy and anisotropy. The effective action is derived in both the Einstein's and Jordan's frame. We show applications to multi-fields inflationary models, and GW propagation in presence of dark energy or dark matter. For GW the effective speed can be polarization and frequency dependent, and we obtain a model independent relation between electromagnetic and gravitational luminosity distance, $d_L^{GW}(z)=\tilde{c}_T(z)\, d_L^{EM}(z)$. We discuss the implications for constraining dark energy and inflation models with gravitational waves, CMB, large structure observations and other cosmological probes.

Axion Free-kick Misalignment Mechanism

• Lingxiao Xu (Istituto Nazionale di Fisica Nucleare)

Room: Aula Magna 'Fratelli Pontecorvo' We propose an alternative scenario of axion misalignment mechanism based on nontrivial interplay between axion and a light dilaton in the early universe. Dark matter abundance is still sourced by the initial misalignment of axion field, whose motion along the potential kicks the dilaton field away from its minimum, and dilaton starts to oscillate later with a delayed onset time for oscillation and a relatively large misalignment value due to kick, eventually the dilaton dominates over axion in their energy densities and dilaton is identified as dark matter. The kick effect due to axion motion is the most significant if the initial field value of dilaton is near its minimum, therefore we call this scenario axion “free-kick” misalignment mechanism, where axion plays the role similar to a football player. Dark matter abundance can be obtained with a lower axion decay constant compared to the conventional misalignment mechanism.

The axion-flavour connection

• Clemente Smarra (Sissa)

Room: Aula Magna 'Fratelli Pontecorvo' A local flavour symmetry acting on the quarks of the Standard Model can automatically give rise to an accidental global $U(1)$ which remains preserved from sources of explicit breaking up to a large operator dimension, while it gets spontaneously broken together with the flavour symmetry. Such non-fundamental symmetries are often endowed with a mixed QCD anomaly, so that the strong CP problem is automatically solved via the axion mechanism. We illustrate the general features required to realise this scenario, and we discuss a simple construction based on the flavour group $SU(3)×SU(2)×U(1)_F$ to illustrate how mass hierarchies can arise while ensuring at the same time a high quality Peccei-Quinn symmetry.

Parker Bound and Monopole Production from Primordial Magnetic Fields

• Daniele Perri (INFN)

Room: Aula Magna 'Fratelli Pontecorvo' Monopoles are inevitable predictions of GUT theories. They are produced during phase transitions in the early universe, but also mechanisms like Schwinger effect in strong magnetic fields could give relevant contributions to the monopole number density. I will show that from the detection of intergalactic magnetic fields of primordial origin we can infer additional bounds on the magnetic monopole flux at present time. I will also discuss the implications of these bounds for millicharged monopoles and for magnetic monopole pair production in primordial magnetic fields.

Model Agnostic Probes at Neutrino Experiments

• Marco Costa (Istituto Nazionale di Fisica Nucleare)

Room: Aula Magna 'Fratelli Pontecorvo' Present and upcoming neutrino experiments can have considerable sensitivity to dark sectors that interact feebly with the Standard Model. We consider dark sectors interacting with the SM through irrelevant portals that are motivated on general principles. We derive bounds on such scenarios by considering decays of dark sector excitations inside the neutrino detector, placed downstream from the target. Our approach is model agnostic and applies to a wide range of dark sector models, both strongly and weakly coupled. In this approach, the dark sector is characterized by two scales: $\Lambda_\text{UV}$ (mass of mediators generating the portals) and $\Lambda_\text{IR}$ (mass gap of the dark sector). At intermediate energies, far away from these scales, the theory is approximately scale-invariant. This allows the calculation of production rates independent of the threshold corrections, although some mild model- dependent assumptions are needed. We look at various dark sector production processes relevant at neutrino experiments such as meson decays, direct partonic production, and dark bremsstrahlung. We consider representative experiments from past (CHARM), present (ICARUS, NOvA, Micro- BooNE), and upcoming future (DUNE-MPD), and compare their reach to existing bounds from high energy experiments (LHC and LEP) and dedicated future LLP experiments (SHiP). We find that the upcoming DUNE-MPD can probe $\Lambda_\text{UV}$ in the TeV range, and $\Lambda_\text{IR}$ in the 0.1-1 GeV range, covering parts of parameter space currently inaccessible in high energy experiments and fixed- target/beam-dump experiments, and is comparable to future LLP experiments. In general, future neutrino experiments can be an efficient probe of dark sectors, providing complementary as well as new reach in parameter space.

Neutrino non-standard self-interactions and their impact on sterile neutrino dark matter production and detection

• Cristina Benso (Max Planck Institute for Nuclear Physics, Heidelberg)

Room: Aula Magna 'Fratelli Pontecorvo' Sterile neutrinos with keV-scale masses are popular candidates for warm dark matter. In the most straightforward case they are produced via oscillations with active neutrinos. We introduce effective self-interactions of active neutrinos and investigate the effect on the parameter space of sterile neutrino mass and mixing. Our focus is on mixing with electron neutrinos, which is subject to constraints from several upcoming or running experiments like TRISTAN, ECHo, BeEST and HUNTER. Depending on the size of the self-interaction, the available region of the parameter space moves closer to, or further away from the one testable by those future experiments. In particular, phase 3 of the HUNTER experiment will test a larger region of parameter space in the presence of self-interactions than without them. We report also the effect of the self-interactions on the free-streaming length of the sterile neutrino dark matter, important for structure formation observables.

Ultra-relativistic bubbles from the simplest Higgs portal and their cosmological consequences

• Giulio Barni (SISSA)

Room: Aula Magna 'Fratelli Pontecorvo' We analyze phase transitions in the minimal extension of the SM with a real singlet scalar field. The novelty of our study is that we identify and analyze in detail the region of parameter space where the first order phase transition can occur and in particular when the bubbles with true vacuum can reach relativistic velocities. This region is interesting since it can lead to the new recently discussed baryogenesis and Dark Matter production mechanisms. We fully analyze different models for the production of Dark Matter and baryogenesis as well as the possibilities of discovery at the current and future experiments.

The GUT way for composite dark matter

• Sonali Verma (Istituto Nazionale di Fisica Nucleare)

Room: Aula Magna 'Fratelli Pontecorvo' In this talk, I will try to address the following question: *Can we find an $SU(5)-$ GUT completion for composite dark matter models giving an accidentally stable dark matter?* In this work, we consider extending the Standard Model (SM) gauge group by a new confining gauge group *dark color*. We do this by adding new *dark fermions* transforming as fundamentals under *dark colour* and as vector-like representations under the SM. These dark fermions are *light*: their mass $m$ is lighter than or close to the dark confinement scale, $m < \Lambda_{DC}$. The bound states in the dark sector would then be dark pions and dark baryons with mass $\sim \Lambda_{DC}$. The dark baryons are considered to be robust dark matter (DM) candidates stable due to a dark baryon number broken by dimension-6 operators. For the DM to be a thermal relic, $\Lambda_{DC} $ must be of $\mathcal{O}(100 \ \mathrm{TeV})$. In our work, we consider the dark fermions as low-energy remnants of a grand unified-GUT theory by embedding them in $SU(5)$ GUT multiplets such that each viable model has a GUT counterpart. We find that the dark fermions cannot come in almost degenerate GUT multiplets. We further find that cosmological constraints from Big Bang Nucleosynthesis in addition to unification requirements allow for only certain values of masses for these GUT fermions. However, these mass values give a too large contribution to the DM relic abundance. To evade this, the mass of the GUT states must be lower than the reheating temperature. In general, we find that the heavy dark GUT states impact both cosmological evolution and grand unification. Our study clarifies under which conditions both aspects of the theory are realistic.