Below you will find links to past seminars, including presentations (when available).

Past seminars from 2020 and onwards:

# 2023 seminars

### Autumn Term 2023

**13.12.2023 Kai Schmitz (Munster U., ITP and CERN) (Remote)**

Title: Probing New Physics at the Pulsar Timing Array Frontier

Abstract: Pulsar Timing Array (PTA) collaborations around the globe recently announced compelling evidence for low-frequency gravitational waves permeating our entire Universe, that is, a gravitational-wave background (GWB) reaching us from all directions and at all times. This breakthrough achievement has important implications for astrophysics, as the GWB signal, if genuine, is likely to originate from a cosmic population of supermassive black holes orbiting each other at the centers of galaxies. As I will illustrate in this talk, the new PTA data is, however, also of great interest to the high-energy physics community, as it allows to probe a broad range of particle physics models of the early Universe that predict the generation of a cosmological GWB in the Big Bang. In this sense, the PTA data opens a new window onto the very early Universe and enables particle physicists to constrain scenarios of new physics beyond the Standard Model at extremely high energies. In my talk, I will give an overview of these searches for new physics at the PTA frontier and highlight several cosmological scenarios that underline the relevance of PTA observations for fundamental problems such as dark matter, neutrino masses, and the matter-antimatter asymmetry of the Universe. Finally, I will conclude with a brief outlook on future measurements that may help in discriminating between a GWB signal of astrophysical origin and a GWB signal from the Big Bang.**29.11.2023 Verónica Errasti Díez (LMU Munich) (On-site)**

Title: Ghost killing versus ghost taming

Abstract: The generic appearance of negative energy modes, known as ghosts, is the first and most pressing challenge towards the postulation of well-defined field theories. As a remarkable example, recall the development of massive gravity theories: a multifaceted and popular effort spanning nearly two decades, driven by the goal to avoid ghosts. In this talk, I will present recent results in the context of ghost evading techniques. Additionally, we will discuss striking examples of what has been called benign ghosts. The latter strongly suggest we must reconsider physical viability criteria for field theories, opening an exciting scenario of possibilities in physics in general and in modified gravity in particular.**22.11.2023 Jorge Baeza-Ballesteros (Valencia U., IFIC) (On-site)**

Title: Simulating the early universe. Gravitational waves from global cosmic strings

Abstract: Early Universe phenomena are an open window to study particle physics at energy scales well above those that can be probe by ground-based experiments. These phenomena can usually be studied using classical field theory techniques, but the presence of large non-linearities in certain regimes limits the use of analytical techniques to obtain accurate predictions. One example is cosmic strings. These are one-dimensional topological defects that generate after phase transitions in some extensions of the Standard Model, such as in axion models. Typically, they are studied in the Nambu-Goto limit, that neglects their width. In this talk, I will briefly explain how lattice techniques can be used to directly study non-linear early-universe phenomena, with special interest on the evolution and decay of global string loops. I will present some recent results that indicate that the gravitational wave emission from global strings is very suppressed compared to their decay into particle radiation.**15.11.2023 John F. Donoghue (Massachusetts U., Amherst) (Remote)**

Title: Arrow of time, higher derivatives and Merlin modes

Abstract: Motivated by Quadratic Gravity – a renormalizable theory for quantum gravity – I have been exploring theories with higher derivatives. Such theories have aspects which differ from standard theories. In this talk I focus on the issues of causality and the time direction of propagation for fields. I will also briefly summarize the other issues for such theories, and the implications for Quadratic Gravity.**8.11.2023 Niko Koivunen (NICPB, Tallinn) (On-site)**

Title: Anatomy of single field inflationary models for primordial black holes [Video]

Abstract: I review the essential features and the phenomenology of single-field inflationary scenarios that can produce large spectral features resulting in the production of primordial black holes. I present a simple analytic set-up capable of capturing the features present in most models in the literature and discuss less common scenarios for PBHs such as inflation from double well potential.**1.11.2023 Emil Mottola (New Mexico U.) (Remote) at 16:15**

Title: Dark Energy & Gravitational Condensate Stars.[Video]

Abstract: The difficulties in reconciling Einstein’s classical General Relativity and quantum theory reach their apex in the twin puzzles of black holes and cosmological dark energy. Both of these problems are evidence that the gravitational sector of the Standard Model is incomplete at macroscopic distance scales. Indeed when General Relativity is treated as a low energy Effective Field Theory, it can be shown that it necessarily receives infrared relevant corrections from the fluctuations of massless or light quantum fields. The associated conformal anomaly implies the existence of a new massless scalar degree of freedom in gravity, which has long range gravitational effects, that in particular are significant in the vicinity of black hole event horizons. This scalar conformalon also allows the effective value of the vacuum energy, described as a condensate of an exact 4-form abelian gauge field strength F = dA, to change in space and time. The resulting EFT thus replaces the fixed constant Λ of classical gravity, and its unnaturally large sensitivity to UV physics, with a dynamical condensate whose ground state value vanishes identically in empty flat space. By also allowing Λeff to vary rapidly near the 2-surface of a black hole horizon, the proposed EFT of dynamical vacuum energy provides an effective Lagrangian framework for gravitational vacuum condensate stars, as the final non-singular state of complete gravitational collapse, consistent with quantum theory. A gravitational condensate star replaces the classical black hole event horizon by a surface and its interior by a vacuum condensate with dark energy eq. of state, p= – rho, with no entropy and no information paradox. The prospects for testing this EFT in gravitational wave and cosmological observations will be discussed.**18.10.2023 Fotis Koutroulis (Warsaw U.) (On-site)**

Title: Thermal effects in Ising Cosmology [Slides]

Abstract: We consider a real scalar field in de Sitter background and compute its thermal propagators. Starting from the Bunch-Davies, |in> vacuum, a Bogolyubov Transformation placed us in the interior of the finite temperature phase diagram in a thermal state |out;β>. The latter can be connected through holography to the vicinity of an interacting IR fixed point, in the universality class of the 3d Ising model. The system in this state is rather special, in the sense that the boundary operator that couples to the scalar curvature perturbations in the bulk has a classical scaling dimension and a critical exponent, η. The latter is the order parameter of the breaking of the scale invariant spectrum of curvature fluctuations and a simple argument from the dS/CFT correspondence fixes the parametric freedom in the dS scalar theory, yielding the prediction nS = 0.964. In the same context we also compute additional cosmological observables such as the first moment of the scalar spectral index and the non-Gaussianity bispectrum parameter fNL and evaluate them numerically. Our predicted values of these cosmological observables are well within current experimental bounds.**11.10.2023 Tomi Koivisto (Tartu U. and NICPB, Tallinn) (On-site)**

Title: Cosmology in the Lorentz gauge theory

Abstract: In the framework of Lorentz gauge theory, a novel action principle has been proposed as the possible foundation for a more fundamental theory of General Relativity and the Standard Model. It will be argued that the proposal improves the consistency of the field equations and the conserved currents. Phenomenological implications are pointed out and possible cosmological applications will be discussed. The emergence of spacetime is described by khronogenesis, a spontaneous symmetry breaking which might be relevant for the initial conditions of the universe. The Lorentz gauge theory predicts “dark shadow matter”. However, it is not clear how to reconcile this prediction with inflation.**20.9.2023 Jorinde van de Vis (Utrecht U.) (On-site)**

Title: GWs from cosmological phase transitions: sound speed and wall speed [Video]

Abstract: In many extensions of the Standard Model, the electroweak phase transition is first order. Such a phase transition proceeds via the formation and collision of bubbles. The bubble collisions can source a stochastic gravitational wave background signal, with characteristic frequency right in the sensitivity band of LISA. We can thus use data from gravitational wave experiments to probe physics beyond the standard model. In this talk, I will focus on the contribution to the gravitational wave signal from sound waves that get formed in the interactions between the plasma and the bubble walls. Predictions of the gravitational wave spectrum typically rely on hydrodynamic lattice simulations of the scalar-plasma system. Hydrodynamic solutions of a single expanding bubble provide a bridge between the particle physics model and the hydrodynamic lattice simulation. Two relevant quantities in this computation are the bubble expansion velocity and the kinetic energy budget. I will demonstrate how the speed of sound affects the kinetic energy budget, and discuss what values one expects for the speed of sound. I will then discuss the computation of the bubble wall velocity, and present a model-independent computation of the wall velocity, obtained in local thermal equilibrium.**13.9.2023 David A. Nichols (Virginia U.) (Remote) at 15:00**

Title: Gravitational-wave memory effects from binary-black-hole mergers

Abstract: Over ninety detections of binary-black-hole mergers were made during the first three observing runs of the LIGO and Virgo detectors. With this larger number of measurements of increasing accuracy, many of the remarkable predictions of general relativity for strongly curved, dynamical spacetimes will be able to be studied observationally. In this talk, I will discuss one class of strong-gravity phenomena, called gravitational-wave memory effects, which are predictions of general relativity that are most prominent in systems with high gravitational-wave luminosities, like binary black holes. Memory effects are characterized by changes in the gravitational-wave strain and its time integrals that persist after a transient signal passes by a detector. I will discuss the computation of these effects and the prospects for current and planned future gravitational-wave detectors to detect memory effects from black-hole mergers; in particular, there could be evidence for the memory effect in the population of binary mergers after the LIGO, Virgo, and KAGRA detectors finish their fifth observing run. I will also discuss what observing gravitational-wave memory effects can teach us about the low-frequency properties of gravitational theories.**6.9.2023 Rebeca Gonzalez Suarez (Uppsala U.) (On-site)**

Title: Dancing in the dark: exploring dark sectors at particle colliders, present and future.[Slides]

Abstract: Understanding the nature of DM is a pressing question in contemporary physics that connects particle physics and astronomy. Despite decades of searching for the preferred DM candidates: Weakly Interacting Massive Particles (WIMPs), no discovery has been made so far. Hidden sectors, consisting of new, invisible particles and forces that interact almost imperceptibly with the SM, are rapidly gaining popularity as an alternative that could hold the answer to the DM mystery and other open questions in the field. Searches for dark sectors connected to dark matter are ramping up at the LHC experiments at CERN, often in non-mainstream signatures. One of such searches, performed by my team in the ATLAS experiment using Run-2 data will be presented. The LHC at CERN is a state-of-the-art particle accelerator and collider built to probe the fundamental building blocks of the universe. But with the LHC Run-3 ongoing, the last running period before the High-Luminosity LHC, the global high energy physics community is working to define the future of collider particle physics. The European Strategy for Particle Physics identified in its latest update an e+e- Higgs factory as its top priority and the first step towards a future hadron collider with very high energy. This seminar will introduce the Future Circular Collider (FCC) proposed at CERN, along with its exciting potential for dark sector searches.

### Spring Term 2023

**14.6.2023 Sabir Ramazanov (Prague, Inst. Phys.) (On-site)**

Title: Phase transitions in the dark Universe

Abstract: Typically phase transitions are associated with a constant scale of

symmetry breaking. In cosmology, however, a different situation is also natural: the expectation value of a scalar field responsible for symmetry breaking can be induced by its interactions with hot primordial plasma, in which case the expectation value decreases as the Universe cools down. This picture can be realized in a simple renormalizable and approximately scale-invariant scenario. Non-trivial time-dependence of the symmetry breaking scale enables new mechanisms of particle production at phase transitions. These particles can be considered for the role of dark matter. Another phenomenologically interesting outcome of phase transitions is formation of topological defects, which serve as a source of potentially observable gravitational waves. The latter can be used to probe the underlying model in a very weakly coupled regime inaccessible by other experiments.

**7.6.2023 Isabella Masina (Ferrara U.) (Remote)**

Title: Dark matter and dark radiation from evaporating primordial black holes

Abstract: Primordial black holes might have existed in the early universe and, via

their evaporation mechanism (completed before Big Bang Nucleosynthesis),

they might have released stable particles beyond the Standard Model. We

review the possibility that such particles might contribute to dark matter

or dark radiation.**31.5.2023 Kimmo Kainulainen (Jyväskylä U.) (On-Site)**

Title: (P)reheating from renormalized CTP transport equations

Abstract: TBA**24.5.2023 Adriana G. Menkara (Chung-Ang U.) (Remote)**

Title: Inflationary and post-inflationary dynamics of Higgs-R^2 models [Video]

Abstract: The original proposal for Higgs inflation suffers from a unitarity problem due to its large non-minimal coupling to gravity. Lately, it has been shown that extending the model with extra scalar degrees of freedom recovers unitarity up to the Planck scale. We dub these models Higgs-sigma models.

In this talk, I will introduce the inflationary and post-inflationary dynamics in the Higgs-sigma models, taking a perturbative approach for reheating. Furthermore, we consider the possibility of Dark Matter production through a freeze-in mechanism.**17.5.2023 Francesco Costa (U. Southampton & Gottingen U.) (On-site)**

Title: FIMP @ direct detection and collider experiments [Video]

Abstract: I present a Spin 3/2 FIMP dark matter (DM) candidate. FIMP dark matter is produced via the freeze-in mechanism that generally implies tiny coupling between the DM and the standard model particles, making DM direct detection and collider searches almost hopeless. This is not the case for a spin 3/2 DM at low reheating temperature, where direct detection and collider bounds play a fundamental role in constraining the parameter space. We show the viability of the model and discuss the details of the production mechanism and future experiments that can test it.**10.5.2023 Catarina Cosme (Valencia U., IFIC) (On-site)**

Title: Neutrino Portal to FIMP Dark Matter with an Early Matter Era [Video]

Abstract: In this talk, I will discuss the freeze-in production of Feebly Interacting Massive Particle (FIMP) dark matter candidates through a neutrino portal, in the case where an early matter-dominated era took place for some period between inflation and Big Bang Nucleosynthesis. In this model, we consider a hidden sector comprised of a fermion and a complex scalar, with the lightest one regarded as a FIMP candidate, and three heavy neutrinos, responsible for mediating the interactions between the Standard Model and the dark matter sectors and for generating the masses of the Standard Model neutrinos. I will present the dynamics of the dark matter candidate throughout the modified cosmic history, evaluate the relevant constraints of the model, and discuss the consequences of the duration of the early matter-dominated era for dark matter production. Finally, I will show that, under some circumstances, this scenario becomes testable through indirect detection searches.**3.5.2023 Till Sawala (U. Helsinki) (On-site)**Abstract: Galaxies of different types are not equally distributed in the Local Universe. In particular, the supergalactic plane is prominent among the brightest ellipticals, but inconspicuous among the brightest disk galaxies. This striking difference provides a unique test for our understanding of galaxy and structure formation. Here we use a constrained simulation to confront the predictions of the standard ΛCDM model and standard galaxy formation theory with these observations. The simulation reproduces the spatial distributions of disks and ellipticals and, in particular, the observed excess of massive ellipticals near the supergalactic equator. We show that this follows directly from the local large-scale structure and from the standard galaxy formation paradigm, wherein disk galaxies evolve mostly in isolation, while giant ellipticals congregate in the massive clusters that define the supergalactic plane. Rather than an anomaly, the distributions of giant ellipticals and disks in the Local Universe and vis-a-vis the supergalactic plane are key predictions of the ΛCDM model.

Title: Distinct distributions of elliptical and disk galaxies across the Local Supercluster as a ΛCDM prediction [Video]**26.4.2023 Patrick Stengel (INFN, Ferrara) (On-site)**

Title: Mineral Detection of Neutrinos and Dark Matter

Abstract: Minerals are solid state nuclear track detectors – nuclear recoils in a mineral leave latent damage to the crystal structure. Depending on the mineral and its temperature, the damage features are retained in the material from minutes to timescales much larger than the age of the Solar System. The damage features from the fission fragments left by spontaneous fission of heavy unstable isotopes have long been used for fission track dating of geological samples. Laboratory studies have demonstrated the readout of defects caused by nuclear recoils with energies as small as ~1 keV. Using natural minerals, one could use the damage features accumulated over geological timescales to measure astrophysical neutrino fluxes (from the Sun, supernovae, or cosmic rays interacting with the atmosphere) as well as search for Dark Matter. Research groups in Europe, Asia, and America have started developing microscopy techniques to read out the nanoscale damage features in crystals left by keV nuclear recoils. The research program towards the realization of such mineral detectors is highly interdisciplinary, combining geoscience, material science, applied and fundamental physics with techniques from quantum information and Artificial Intelligence. In this talk, I will highlight the scientific potential for mineral detectors and briefly describe status and plans of the MDvDM community.**19.4.2023 M. Sten Delos (Garching, Max Planck Inst. Astrophys.), (Remote)**

Title: Prompt cusps of dark matter halos [Video]

Abstract: The onset of the formation of dark matter halos in the early universe was marked by the monolithic collapse of smooth peaks in the density field. This process creates prompt rho ~ r^-1.5 density cusps, which persist largely unaltered through the subsequent growth phases of their halos. Consequently, in the standard collisionless dark matter paradigm, a prompt cusp is expected to reside at the center of every halo and subhalo. In annihilating dark matter models, the abundance of these features and the high density inside them greatly influence the intensity and morphology of the annihilation signal. Moreover, the properties of prompt cusps are closely linked to details of the primordial density field. In warm dark matter models, prompt cusps are expected to be large enough to influence dwarf galaxy kinematics at detectable levels.**12.4.2023 Andreas Goudelis , (Remote)**

Title: FIMPy baryogenesis [Video]

Abstract: The nature of dark matter and the origin of the matter-antimatter asymmetry of the Universe are two important open puzzles in contemporary high energy physics and cosmology. In this seminar, after introducing some necessary background notions, I will present a mechanism which aims at simultaneously addressing both of these questions. In particular, I will show that out-of-equilibrium decay or scattering processes involving exotic heavy particles in the early Universe can produce dark matter along with an asymmetry between baryons and antibaryons. I will further show that, depending on the nature of the underlying interactions, both dark matter production and baryogenesis can take place at drastically different temperatures during the cosmological evolution. The mechanism can have interesting phenomenological implications for collider observations, structure formation and, eventually, inflationary observables.**5.4.2023 Anna Socha, (On-site)**

Title: Higgs boson-induced reheating and its implications for dark matter [Video]

Abstract: According to the standard model of cosmology, the Universe at its very beginning underwent a phase of rapid expansion, followed by a reheating period. During this epoch, the energy density, initially accumulated in the coherent oscillations of the inflaton field, was injected into the visible sector, eventually setting the initial conditions for the hot Big Bang. In this talk, I will discuss perturbative production of the Standard Model (SM) particles adopting a non standard post-inflationary scenario with a generic equation-of-state parameter $\bar{w}$. To specify the inflaton dynamics, I will employ the $\alpha$-attractor T-model of inflation, such that $\phi$ has a monomial potential $V(\phi) \propto \phi^{2n}$ about the minimum. Moreover, I will explore the Higgs boson-induced reheating, assuming that it is achieved through a cubic inflaton-Higgs coupling $\phi |\mathcal{H}|^2$. In the presence of such interaction, the Higgs field acquires a $\phi$-dependent mass which generates a vacuum expectation value that oscillates in time and breaks the electroweak gauge symmetry. Interestingly, the non-zero Higgs mass leads to a time-dependent inflaton decay rate and generates a phase space suppression of the reheating efficiency. This, in turn, has non-trivial consequences for the reheating dynamics, modifying the evolution of the SM radiation energy density or the duration of the reheating phase. Furthermore, the implications of the non-standard reheating for the dark sector will be discussed, exemplified by the UV freeze-in dark matter model.

**22.3.2023 Vadim Briaud (LPENS, Paris), (Remote)**

Title: Uphill inflation [Video]

Abstract: Primordial black holes (PBH) may form from large cosmological perturbations, produced during inflation when the inflaton’s velocity is sufficiently slowed down. This usually requires very flat regions in the inflationary potential. In this talk, I will discuss another possibility, namely that the inflaton climbs up its potential. When it turns back, its velocity crosses zero, which triggers a short phase of “uphill inflation” during which cosmological perturbations grow at a very fast rate. This naturally occurs in double-well potentials if the width of the well is close to the Planck scale. I will discuss the effect of quantum diffusion in this scenario, which plays a crucial role, through the stochastic-δN formalism. Finally, I will discuss the amount of PBH that is expected to be produced in such a model.**15.3.2023 Asta Heinesen (Lyon, ENS), (On-site)**

Title: Dimming of light in general relativity: On the possible emergence of dark energy effects in space-times with ordinary matter [Video]

Abstract: Can modern cosmological observations be reconciled with a general-relativistic Universe without an anti-gravitating energy source? Usually, the answer to this question by cosmologists is in the negative, and it is commonly believed that the observed excess dimming of supernovae relative to that in the Milne model is evidence for dark energy. This intuition can be shown to be correct within Friedman-Lemaitre-Robertson-Walker space-times. In this talk I will illustrate that there is no fundamental result in general relativity that prevents the excess dimming of light in space-times satisfying the strong energy condition, once the Friedman-Lemaitre-Robertson-Walker symmetries are broken. This opens up an avenue of research into general-relativistic space-time solutions without dark energy that may be competitive cosmological models. I will discuss the geometrical constraints that such space-times must satisfy in order to conform with cosmological observations.

**3.3.2023 Marco Taoso (INFN, Turin) , (On-site) Note: Friday at 12:15-13:15**

Title: Primordial black holes as dark matter [Video]

Abstract: Primordial black holes are a candidate for explaining the dark matter in our Universe. I will review the basics of this scenario and discuss the relevant signatures and the observational constraints. Then, I will present the conditions under which such black holes may have originated from a single-field model of inflation.

**1.3.2023 Valentina De Romeri (Valencia U., IFIC), (On-site)**

Title: Coherent elastic neutrino-nucleus scattering in the Standard Model and beyond [Video]

Abstract: I will present the physics potential of the coherent elastic neutrino-nucleus scattering (CEvNS) process. I will first briefly review the status of current observations. Then I will comment on their implications for both precision tests of the Standard Model and for new physics in the neutrino sector. Finally, I will discuss the relevance of these measurements for direct dark matter detection probes.

**22.2.2023 Lucien Heurtier (Durham U., IPPP), (On-site)**

Title: Mini Primordial Black Hole Archaeology [Video]

Abstract: Light primordial black holes – formed shortly after the end of cosmic inflation – contain crucial information regarding the evolution of the early Universe. Depending on their masses and spins, these black holes can evaporate via Hawking radiation at different times throughout cosmic history, leaving various traces in cosmological data that may become measurable soon. In this talk, I will introduce general aspects of primordial black hole phenomenology and discuss how one can search for imprints of their early evaporation using cosmic microwave background measurements, large-scale structure surveys, dark-matter direct detection, or gravitational wave observatories.**8.2.2023 Sebastian Bahamonde (Tokyo Inst. Tech.), (Remote)**

Title: Teleparallel Gravity: From Theory to Cosmology [Video]

Abstract: Teleparallel gravity has significantly increased in popularity in recent decades, bringing attention to Einstein’s other theory of gravity. These theories have the feature that the general curvature is vanishing while torsion or non-metricity can be non-zero. I will introduce Metric-Affine theories and show that one can describe the Einstein’s equations just with torsion/nonmetricity. Then, I will introduce some popular modified theories of gravity which are based on torsion Teleparallel gravity and explain some cosmological results within them.**1.2.2023 Thomas Konstandin (DESY), (On-site)**

Title: Gravitational waves from cosmological phase transitions [Slides]

Abstract: I will discuss the stochastic background of gravitational waves produced by cosmological first-order phase transitions. I focus on recent simulations of the dynamics of the primordial fluid after percolation and how to connect simulation results to observations and model building. In particular, I will talk about a recent simulation scheme where the Higgs field is not dynamical and only acts as a background. This allows for efficient and cheap simulations. Besides, I will discuss how to extrapolate results from models with the bag equation of state to arbitrary models.**18.1.2023 Giorgio Torrieri (Campinas State U.), (On-site)**

Title: The hidden gauge theory of relativistic hydrodynamics [Video]

Abstract: The observation, in hadronic collisions, of “ideal fluid” type behavior in systems of a comparatively small number of particles, presents a conceptual puzzle, since the way we usually derive hydrodynamics is via approximating “many” particles as a continuum. I will argue that making sense of this requires re-deriving relativistic hydrodynamics as a “bottom-up” theory, with no reference to microscopic physics except the local emergence of a thermalized system. We attempt to do this using basic statistical mechanics, and find the appearance of a gauge-like redundancy hidden within relativistic dissipative hydrodynamics, arising from the fluctuation-dissipation theorem. This might lead to the apparently counter-intuitive conclusion that in the small viscosity limit it might indeed be that smaller systems could thermalize faster.**11.1.2023 Daniel Blixt (SSM, Naples), (On-site)**

Title: Teleparallel gravity, theoretical issues and possible future directions [Video]

Abstract: In recent years there has been a growing interest in modified theories of gravity which could, among other things, give predictions that better fits cosmological data. In this talk I will focus on teleparallel theories of gravity which includes an equivalent formulation of GR where curvature is replaced by torsion in the action formulation. Furthermore, the presence of torsion allows to construct modified theories of gravity with antisymmetric field equations. I will give an overview of the theoretical viability of teleparallel theories and explain what could be two future directions of giving observational predictions to the (to date) most viable teleparallel theories. Included in this discussion I will present the Hamiltonian and give the total number of degrees of freedom for the most popular teleparallel theories. Finally, I will discuss symmetry breaking and guiding principles in modified theories of gravity.

# 2022 seminars

### Autumn Term 2022

**14.12.2022 Jiamin Hou (U. Florida), (Remote), Note: at 15:15 – 16:15**

Title: Recent Developments in Clustering Analysis of Large-Scale Structure [Video]

Abstract: Large-scale structure provides insights into the Universe’s expansion history and the growth rate of structure. While the 2-point statistics can capture all information in a Gaussian random field, higher-order statistics encode crucial information about gravitationally-induced nonlinearity and deviation from a Gaussian initial condition. Higher-order N-point correlation function or polyspectra as its Fourier counterpart thus allow us more precise knowledge about the energy density content of the Universe and provides unique windows into the primordial Universe. Recently, we showed that higher-order statistics could be used to test parity violation at cosmological scales. In this talk, I will present our analysis and the implication of the measurement. Finally, I will also present our recent analysis of the galaxy skew spectra as a compressed bispectrum estimator and a forecast of its cosmological information.**2.12.2022 Andrew Gow (Portsmouth U., ICG), (On-site) Note: Friday at 14:15**

Title: Primordial black holes: rapid transitions and non-Gaussianity [Video]

Abstract: Primordial black holes may form in the early universe and could explain some or all of the dark matter. In this talk, I will discuss aspects of PBH formation from inflation. In particular, I will demonstrate the effects from rapid transitions between inflationary phases and discuss whether these are realistic. I will also discuss a non-perturbative method of incorporating primordial non-Gaussianity into the PBH calculation.**30.11.2022 Ivan Rybak (Porto U., Astron. Dept.), (On-site)**

Title: Superconducting cosmic strings with zero thickness approximation [Video]

Abstract: Some early universe scenarios predict the formation of cosmic strings. Moreover, it is generic to anticipate that these one-dimensional topological defects are superconducting. We consider an effective action to describe the dynamics and evolution of current-carrying cosmic strings. We demonstrate how the presence of a current on cosmic strings can change observational predictions, particularly CMB anisotropies and gravitational wave radiation.

**23.11.2022 Oksana Iarygina (Nordita), (On-site)**

Title: Multi-field effects in the early universe [Video]

Abstract: Nowadays, the search for primordial gravitational waves is mainly focused on the parity-odd polarization pattern in the CMB the B-modes. A correct interpretation of B-mode measurements strongly relies on understanding their production mechanism. In the first part of my talk I will describe the viability of inflation with a spectator sector comprised of non-Abelian gauge fields coupled through a higher order operator. I will discuss the theoretical restrictions for the amplitude and tensor tilt for chiral gravitational waves as well as the maximum possible enhancement of the gravitational wave background with respect to the one coming from vacuum fluctuations. Gravitational waves can also be produced during reheating. The reheating era, that connects inflation and big-bang nucleosynthesis, is still very weakly constrained. Inefficient preheating can lead to a prolonged matter-dominated phase after inflation, changing the time during inflation when the Cosmic Microwave Background (CMB) modes exit the horizon. This shifts CMB predictions and thus can break the degeneracy of otherwise indistinguishable inflation models. In the second part of the talk I will demonstrate how physical mass scales of multi-field preheating control the dynamics and observable predictions of models of inflation with hyperbolic manifolds.

**16.11.2022 Volodymyr Takhistov (KEK, Tsukuba), (Remote)**

Title: Marvelous Manifestations of Primordial Black Holes: from Multiverse to Neutron Star Explosions

Abstract: Primordial black holes from the early Universe constitute an attractive non-particle dark matter candidate and appear in a multitude of theoretical models. I will discuss novel observational signatures for primordial black holes spanning orders of magnitude in mass-range, connecting them to recent breakthroughs in gravitational wave and multi-messenger astronomy as well as long-standing astrophysical puzzles such as the origin of heavy elements. Primordial black holes from intriguing scenario of bubble multiverse might have already been seen by Subaru HSC.

**9.11.2022 Sebastian von Hausegger (Oxford U.), (Remote)**

Title: The cosmic matter dipole [Video]

Abstract: The ever-growing quantity and quality of astronomical data today has underpinned both the LCDM model of cosmology and general relativity as effective working theories. Yet also tensions between theoretical prediction and observation, and even between different observations arose. I will centre my presentation on one such discrepancy — the excess amplitude of the cosmic matter dipole in the angular number counts of high-redshift galaxies. We established this discrepancy at high significance across frequencies by compiling high-redshift galaxy catalogues in the mid-IR and the radio. In particular, these observations represent a significant challenge to the Cosmological Principle, upon which the LCDM model is built, evocative of inhomogeneous and anisotropic cosmological models. I will present details of this analysis, and also provide an overview of the relevant literature and elaborate on consequences as well as future research directions. This talk is based on https://arxiv.org/abs/2009.14826 and https://arxiv.org/abs/2206.05624.

**26.10.2022 George Zahariade , (Remote)**

Title: Quantum Mechanics of Gravitational Waves [Video]

Abstract: We study the effect of a quantized gravitational wave on a LIGO-type gravitational wave detector. We find that the arm-length is subject to a stochastic tidal force whose properties depend on the exact quantum state of the gravitational field, if the gravitational field is quantized. The quantum nature of gravity may thus be detectable as an additional noise source at gravitational wave detectors.

**19.10.2022 Sarif Khan (Gottingen U.), (On-site)**

Title: WIMP and FIMP Dark Matter in Singlet-Triplet Fermionic Model [Video]

Abstract: In this talk, I will present an extension of the SM involving three triplet fermions, one triplet scalar and one singlet fermion, which can explain both neutrino masses and dark matter. One triplet of fermions and the singlet are odd under a $Z_2$ symmetry, thus the model features two possible dark matter candidates. The two remaining $Z_2$-even triplet fermions can reproduce the neutrino masses and oscillation parameters consistent with observations. We consider the case where the singlet has feeble couplings while the triplet is weakly interacting and investigate the different possibilities for reproducing the observed dark matter relic density. This includes production of the triplet WIMP from freeze-out and from decay of the singlet as well as freeze-in production of the singlet from decay of particles that belong to the thermal bath or are thermally decoupled. While freeze-in production is usually dominated by decay processes, we also show cases where the annihilation of bath particles give substantial contribution to the final relic density. This occurs when the new scalars are below the TeV scale, thus in the reach of the LHC. The next-to-lightest odd particle can be long-lived and can alter the successful BBN predictions for the abundance of light elements, these constraints are relevant in both the scenarios where the singlet or the triplet are the long-lived particle.In the case where the triplet is the DM, the model is subject to constraints from ongoing direct, indirect and collider experiments. When the singlet is the DM, the triplet which is the next-to-lightest odd particle can be long-lived and can be probed at the proposed MATHUSLA detector. Finally we also address the detection prospects of triplet fermions and scalars at the LHC.

**28.09.2022 Enrico Pajer (Cambridge U., DAMTP), (Remote)**

Title: Bootstrapping large graviton non-Gaussianities [Video]

Abstract: Gravitational interferometers and cosmological observations of the cosmic microwave background offer us the prospect to probe the laws of gravity in the primordial universe. To study and interpret these datasets we need to know the possible graviton non-Gaussianities. To this end, I’ll derive the most general tree-level three-point functions (bispectra) for a massless graviton to all orders in derivatives, assuming scale invariance. Instead of working with explicit Lagrangians, I’ll take a bootstrap approach and obtain our results using the recently derived constraints from unitarity, locality and the choice of vacuum, which will be reviewed during the talk. Since I’ll make no assumptions about de Sitter boosts, the results capture the phenomenology of large classes of models such as the effective field theory of inflation and solid inflation. I’ll present formulae for the infinite number of parity-even bispectra as well. Remarkably, for parity-odd bispectra, unitarity allows for only a handful of possible shapes: three for graviton-graviton-graviton, three for scalar-graviton-graviton and one for scalar-scalar-graviton, which we bootstrap explicitly. These parity-odd non-Gaussianities can be large, for example in solid inflation, and therefore constitute a concrete and well-motivated target for future observations.

**21.09.2022 Takashi Toma (Kanazawa U.), (On-site)**

Title: Ultra-violet completion of a pseudo-Nambu-Goldstone dark matter model [video]

Abstract: Dark matter direct detection experiments impose strong bounds on the elastic scattering cross section with nuclei. A pseudo-Nambu-Goldstone dark matter with a soft explicit breaking mass term suppresses the amplitude for the elastic scattering in non-relativistic limit, and thus can naturally avoid the strong bound of the dark matter direct detection experiments. In this talk, we build an ultra-violet complete model of the pseudo-Nambu-Goldstone dark matter based on a gauged U(1) B-L symmetry. Namely, the soft explicit breaking term can be derived from high energy. In addition, the gauge coupling unification is achieved by further embedding the model in SO(10).

**14.09.2022 Archie Cable (Imperial Coll., London), (On-site)**

Title: Second-order stochastic theory for self-interacting scalar fields in de Sitter spacetime [Slides]

Abstract: The study of scalar field theory in de Sitter spacetime is of great importance in many facets of inflationary cosmology; however current perturbative techniques in QFT contain IR divergences that seriously limit its computational power. Instead, we turn to effective theories. One such theory is the stochastic approach, where one utilises the inflationary expansion to write the scalar field equations in a stochastic framework. In this talk, I will introduce a second-order stochastic effective theory of spectator scalar fields in de Sitter in the light field limit, extending the stochastic approach beyond the massless limit and demonstrating how it can be used to compute long-distance correlation functions non-perturbatively.

**7.09.2022 Mikhail M. Ivanov (IAS, Princeton****), (Remote)**

Title: Fundamental Physics with Galaxy Surveys [video]

Abstract: The distribution of galaxies on large scales is a sensitive probe of cosmological physics. In particular, the structure of this distribution depends on properties of dark matter and the dynamics of the early universe. Understanding this dependence, however, is a challenging task because the observed galaxy distribution is modulated by a variety of non-linear effects. I will present the effective field theory of large-scale structure (EFT) – an innovative theoretical tool that has allowed for a systematic analytic description of these effects. The EFT plays a central role in a new program of extracting cosmological information from galaxy surveys. I will share some results of this program from my independent analysis of the public data from the Baryon acoustic Oscillation Spectroscopic Survey. These results include the measurement of the Hubble constant as well as constraints on new physics and the early Universe.

### Spring Term 2022

**11.05.2022 Daniel Carney (LBL, Berkeley)**

Title: Dark matter searches with mechanical quantum sensors [video]

Abstract: Mechanical objects read out by light in the quantum regime have recently enabled the detection of gravitational waves with LIGO. While the LIGO mirrors are huge, roughly 40kg, similar measurement techniques can be applied to devices at a wide variety of mass scales, from macro-scale objects down to single ions and electrons. I will outline a nascent set of experimental programs utilizing these opto-mechanical devices to search for a wide variety of potential dark matter candidates. In particular, I’ll discuss the long-term prospect of designing a detector that could detect heavy (m ~ m_planck) dark matter candidates directly, purely via their gravitational coupling to the visible sector.**27.04.2022 Dražen Glavan (Prague, Inst. Phys.)**

Title: Nonlinear dissipation and how to describe it for long times [video]

Abstract: Relaxation of systems to equilibrium is often described by linear physics. However, this need not be so and the relaxations does not need to be exponential. Furthermore, there are other transient phases where the system is not close to equilibrium. Reheating is one such example, where an oscillating inflaton is dissipating by producing other particles it couples to. Different kinds of interactions lead to qualitatively different ways in which the inflaton dissipates. These are missed by the time dependent perturbation theory that suffers from secular divergences and cannot describe the long time behaviour. I will present efficient and conceptually simple resummation methods appropriate to quantify the long time behaviour of the oscillating scalar condensate and compare them to the more known approaches. I will also draw some parallels to the dynamical renormalization group often utilized in inflation and point out its limitations.**20.04.2022 Glenn D. Starkman (Case Western Reserve U. and Imperial Coll., London)**

Title: Macroscopic dark matter, and how we might find it [video]

Abstract: To be dark, matter doesn’t have to have a low scattering cross-section, it can just be massive, and so have a very low abundance. Primordial black holes exploit this fact, but they are probably at least 10^15g, and so extremely rare — best searched for astrophysically/cosmologically. Dark matter more massive than about 50g can be much less dense and so we can hope that terrestrial processes will allow us to find them. Most models for such macroscopic dark matter envisions it being of approximately nuclear density. I will discuss what we know about the mass versus cross-section parameter space for these objects, and how we might make progress on searching it.**06.04.2022 Gongjun Choi (CERN)**

Title: Cosmic Birefringence and Electroweak Axion Dark Energy [video]

Abstract: In this talk, we discuss the electroweak (EW) axion quintessence dark energy in light of the recently reported non-zero rotation angle of the cosmic microwave background (CMB) linear polarization β=0.35±0.14deg. We further discuss the model’s prediction on the equation of state of EW axion dark energy with the aid of the cosmic birefringence.**23.03.2022 Mustafa A. Amin (Rice U.)**

Title: A Spin on Wave Dark Matter [video]

Abstract: Can the intrinsic spin of light dark matter manifest itself on macroscopic/astrophysical scales? I will first show that in the non-relativistic limit, bosonic spin-s dark fields are described by a 2s+1 component Schrodinger-Poisson (SP) system. I will then show that in this multi-component SP system, there exist solitons with macrosopic spin. Using 3+1 dimensional simulations of the SP system, I will discuss how the small scale structure in spin-1 (vector) dark matter can differ from the same in spin-0 (scalar) dark matter. Halo cores in vector dark matter are less dense and more extended than their scalar counterparts, they can have macroscopic spin, and the interference patterns in the halo are less pronounced — all leading to potential observational avenues to distinguish the two.**02.03.2022 Christian G. Boehmer (University Coll. London)**

Title: Modified Theories of Gravity — Foundations and Models [video]

Abstract: In the first part of the talk I am discussing General Relativity, in particular its basic ingredients and its mathematical structure. This will naturally lead the way to consider various modifications or extensions of General Relativity, many of which have been studied recently. Next I will discuss modified gravity models based on generalised geometries and on actions no longer linear in curvature. The main part of the talk will discuss how these many different theories can be studied using a single unified approach which also shows the equivalence of some of these models. Boundary terms in the action will play a crucial role in establishing the equivalence between different theories. The final part discusses the study of cosmological models using dynamical systems techniques.**16.02.2022 Philip J. Bull (Queen Mary, U. of London and Western Cape U.)**

Title: 21cm intensity mapping and the limits of cosmography [video]

Abstract: Line intensity mapping is a relatively new observational technique that seeks to map out the 3D structure of the Universe using low angular resolution spectroscopic instruments. Intensity mapping with the 21cm line of neutral hydrogen is a particularly popular approach, as it allows such maps to be made over a truly vast redshift range, spanning from the local Universe all the way back to the cosmic Dark Ages. In this talk I will try to appease both observers and theorists, by reviewing some of the exciting applications of forthcoming large 21cm intensity mapping surveys while also giving an overview of the systematics challenges they face. I will particularly focus on an ongoing “autocorrelation” survey with MeerKAT, a 64-dish radio telescope in South Africa that is a precursor to the Square Kilometre Array, before turning to future prospects for constraining dark energy models with gigantic arrays (10,000’s of dishes).**09.02.2022 Lukas Witkowski (Paris, Inst. Astrophys.)**

Title: Detecting primordial features with gravitational waves [video]

Abstract: The nascent field of gravitational wave astronomy can give us new insights about inflation where Cosmic Microwave Background data is not available. In particular, it allows for experimentally testing models of inflation that at some point depart from the single-field slow-roll paradigm, as motivated by ultraviolet completions of inflation, which can lead to a characteristic oscillation in the stochastic gravitational wave background (see also the previous seminar by Jacopo Fumagalli). I will explain what a detection of these oscillations can teach us about inflation and comment on the prospects of observing this oscillatory signal with upcoming gravitational wave observatories such as LISA.**02.02.2022 Jacopo Fumagalli (Madrid, IFT and Madrid, Autonoma U.)**

Title: Lighting the dark ages of inflation: features in the stochastic gravitational wave background [video]

Abstract: Observational constraints on inflation have so far concentrated on the CMB and Large Scale Structure surveys. After a broad introduction, I will show how phenomena arising during unconstrained stages of inflation, and motivated by high energy embeddings, lead to characteristic oscillatory patterns in the stochastic gravitational wave background. Probing these features could, for instance, establish the existence of heavy particles beyond the reach of terrestrial experiments, and even test the inflationary paradigm or point to alternatives to it. This provides a clear target for gravitational wave observatories as well as a challenge for developing dedicated data analysis techniques to look for this unique insight into the physics of the early universe.**26.01.2022 (at 16:15) Dillon Brout (Harvard-Smithsonian Ctr. Astrophys.)**

Title: The Recent Results From The Pantheon+ Sample and SH0ES Analysis [video]

Abstract: It has now been 5 years since the last main analyses of the Hubble constant and cosmic acceleration by the SH0ES and Pantheon teams. Since then, both the datasets of cepheid calibrators and Type Ia supernovae have more than doubled in size. Meanwhile there have also been critical leaps in our understanding of systematics and improvements to calibration. In this talk I will detail the progress made towards a 1km/s/Mpc measurement of the Hubble constant and measurements of dark energy.**19.01.2022 Pierre Sikivie (Florida U.)**

Title: Cold dark matter caustics [video]

Abstract: On a sunny breezy day, sharp lines of light dance on the bottom of a swimming pool. They are due to folds – sometimes called ‘caustics’, or ‘catastrophes’ – in the wavefront of light from the Sun. Caustics also arise naturally in the distribution of dark matter in space. The dark matter density is very large at the location of a caustic. I’ll show that the late infall of cold dark matter onto isolated galaxies, such as our own, produces discrete flows throughout the galactic halo, and associated caustics. One set of caustics are topological spheres surrounding the galaxy. Another set are rings in the galactic plane. Caustic rings are closed tubes whose cross-section is a D_{-4} catastrophe. I’ll argue on theoretical and observational grounds that the caustic ring radii a_n (n=1,2,3..) obey the approximate law: a_n goes like 1/n. There is evidence for these rings in the distribution of bumps in the rotation curves of spiral galaxies, including our own Milky Way. The implications for dark matter searches will be discussed.

# 2021 seminars

### Autumn Term

**15.12.2021 Luca Visinelli (Tsung-Dao Lee Inst., Shanghai and Shanghai Jiao Tong U.)**

Title: Axion Miniclusters in the Milky Way [video]

Abstract: Axion miniclusters (AMCs) are relatively dense, gravitationally bound clumps of dark matter (DM) QCD axions. AMCs have intriguing observational consequences for Earth-based axion detectors, for DM substructure searches with microlensing, and for radio signatures from AMC encounters with neutron stars (NSs). However, the properties of AMCs in the Milky Way may be drastically altered by tidal interactions with ordinary stars. We present Monte Carlo simulations following the evolution of AMCs orbiting in the Milky Way, which can be used to estimate the properties of AMCs throughout the Galaxy today and can be easily recast. We use this information as a key ingredient in estimating the rate, duration, flux and sky locations of radio signals from axion-photon conversion due to NS encounters with AMCs. The resulting radio transients are within reach of current and future radio telescopes, opening a new avenue for detecting QCD axion DM.Based on 2011.05377, 2011.05378 and upcoming material.

**08.12.2021 Ariel Megevand (Mar del Plata U.)**

Title: Gravitational waves from bubble walls [video]

Abstract: In a cosmological first-order phase transition, the propagation of phase transition fronts in the hot plasma provides several mechanisms for the formation of gravitational waves. The gravitational radiation can be produced either by the motion of the thin bubble walls or by the subsequent bulk fluid motions. The kinematics of the phase transition is usually modeled by spherical bubbles which nucleate with a specific rate and expand with a constant velocity. In this talk, I will review the general dynamics of the phase transition, including the possibility of hydrodynamic instabilities which cause wall corrugations and turbulent fluid perturbations. I will then discuss the generation of gravitational waves by bubble walls, and I will address the case of walls which are deformed from the spherical shape.**01.12.2021 Angelo Ricciardone (Padua U. and INFN, Padua)**

Title: Cosmology with Einstein Telescope [video]

Abstract: The Einstein Telescope (ET) has been recently included in the 2021 roadmap of the European Strategy Forum on Research Infrastructures, a concrete starting point for the construction of this third generation gravitational wave detector. The ET will make it possible, for the first time, to explore the Universe through gravitational waves along its cosmic history up to the cosmological dark ages, shedding light on open questions of fundamental physics and cosmology of the early and late universe. In this talk I will make an overview of the scientific capabilities and cosmological targets of ET, discussing the road map for building a cosmology community to develop the science of this detector.**24.11.2021 Tracy Slatyer (MIT)**

Title: Dark Matter Puzzles from Indirect Searches [video]

Abstract: The nature and origin of dark matter is one of the key unresolved questions of fundamental physics. Astrophysical and cosmological data provide powerful probes of dark matter properties, although to date no signal has been confirmed. I will outline current constraints before discussing a number of claimed possible signals of novel dark matter physics in astrophysical datasets, alternative explanations, and open questions.**17.11.2021 Jorinde van de Vis (DESY)**

Title: Constraining phase transitions with LISA – Prospects and degeneracies [video]

Abstract: Sound waves sourced by bubble collisions during a cosmological first order phase transition can constitute an observable gravitational wave signal. In the standard picture, the gravitational wave spectrum is described in terms of four parameters: the wall velocity and the strength, duration and temperature of the phase transition. These parameters set the amplitude and peak position of the broken power law that describes the gravitational wave spectrum, which can be measured by LISA.In this talk, I will refine this picture. The sound shell model and hybrid simulations have demonstrated that the gravitational wave spectrum has a more complicated structure than the simple broken power law. I will discuss LISA’s ability to extract this additional information. I will then point out that the gravitational wave spectrum is not just a function of four parameters, but that further details of the new physics model enter via the speed of sound. Lastly, I will demonstrate that density perturbations can enhance and deform the signal.

**03.11.2021 (at 17:15) Elisabeth Krause (Arizona U., DES)**

Title: Recent cosmology results and modeling challenges from Year 3 data of the Dark Energy Survey [video]

Abstract: I will give an overview of the cosmology analysis of weak lensing and clustering measurements from Year 3 data from the Dark Energy Survey. This analysis is detailed in 30 papers, available at https://www.darkenergysurvey.org/des-year-3-cosmology-results-papers/; I will focus here on the theoretical modeling of the mean signal and covariance, as well as metrics for assessing the agreement or tension between different measurements. I will summarize the cosmological results, and highlight outstanding modeling challenges, especially the clustering anomaly X_lens.**27.10.2021 (at 16:15) Malte Buschmann (Princeton U.)**

Title: Dark Matter from Axion Strings [video]

Abstract: Axions are hypothetical particles that may explain the observed dark matter (DM) density and the non-observation of a neutron electric dipole moment. An increasing number of axion laboratory searches are underway worldwide, but these efforts are made difficult by the fact that the axion mass is largely unconstrained. If the axion is generated after inflation there is a unique mass that gives rise to the observed DM abundance; due to nonlinearities and topological defects known as strings, computing this mass accurately has been a challenge for four decades. Recent works, making use of large static lattice simulations, have led to largely disparate predictions for the axion mass, spanning the range from 25 microelectronvolts to over 500 microelectronvolts. We show that adaptive mesh refinement (AMR) simulations are better suited for axion cosmology than the previously-used static lattice simulations because only the string cores require high spatial resolution. Using dedicated AMR simulations we obtain an over three order of magnitude leap in dynamic range and provide evidence that axion strings radiate their energy with a scale-invariant spectrum, to within ∼5% precision, leading to a mass prediction in the range (40,180) microelectronvolts.**20.10.2021 (at 16:15) Eleanor Hall (UC, Berkeley)**

Title: Non-perturbative methods for false vacuum decay [video]

Abstract: Gravitational waves from phase transitions in the early universe are one of our most promising signal channels of BSM physics; however, existing methods for predicting these signals are limited to weakly-coupled theories. In this talk, I present a new, non-perturbative formalism for false vacuum decay that integrates over local fluctuations in field space using the functional renormalization group. As I demonstrate for a simple model, this method opens the door to reliable calculation of gravitational wave signals and false vacuum decay rates for strongly-interacting theories.**13.10.2021 Graham Ross (Oxford U.)**

Title: R^2/Higgs inflation and the hierarchy problem [video]

Abstract: We analyse Starobinsky (R^2) inflation in the presence of the Brout Englert Higgs (BEH) boson with non-minimal coupling to the Ricci scalar, R. It is shown how contact terms are needed to determine the effective Lagrangian which has a new scalar field, the scaleron, associated with the R^2 term. The heavy scaleron couples to the BEH boson and radiatively generates a contribution to the BEH mass, resulting in an unacceptably high level of fine tuning, greater than 1 part in 10^10. This is significantly reduced to one part in 10^4 in a scale invariant version of the model. We analyse the consistency with electroweak vacuum stability both during and after inflation and discuss phenomenological aspects of the models.**06.10.2021 (at 16:15) Itamar Allali (Tufts U., Inst. of Cosmology)**

Title: Gravitational Decoherence with Applications to Dark Matter Phenomenology [video]

Abstract: Nontrivial quantum arrangements of matter, such as Schrodinger cat-like states, are sensitive to decoherence from their environment. However, matter that interacts only gravitationally is weakly coupled to its environment, and thus may exhibit slower rates of decoherence. Since dark matter (DM) may only interact via gravity, we explore the decoherence rate of a dark-matter-Schrodinger-cat-state (DMSCS). In the nonrelativistic approximation of gravity, we find that a superposition of distinct DM density profiles can undergo decoherence from the scattering of nearby standard model (SM) particles on observable timescales. In addition, when considering light bosonic DM like an axion, one can conceive of a superposition of the phase of oscillation of the scalar (axion) field, requiring a truly relativistic formalism of gravitational scattering. We derive such a formalism and find that for typical DM populations in the Milky Way, a DMSCS of the axion phase can maintain quantum coherence for exponentially long times, while exotic configurations including DM near a black hole and dense boson stars can experience rapid decoherence. This can have potential observable consequences for direct detection experiments that are sensitive to the axion’s phase, such as haloscopes which rely on resonant cavities to detect axions. This talk will be based on the work in Refs. 2005.12287, 2012.12903, and 2103.15892.**22.09.2021 Scott Melville (Cambridge U., DAMTP)**

Title: Positivity Bounds for Cosmology [video]

Abstract: “Positivity bounds” are a powerful tool in particle physics which use scattering amplitudes to connect large-scale phenomenology with the underlying fundamental physics on small scales. This talk will describe the recent progress in bringing this machinery to cosmology, and show how the basic requirements of unitarity, causality and locality on small scales can be used to place constraints on scalar-tensor theories of dark energy and on the non-Gaussianities in the CMB which can arise from single-field inflation.**15.09.2021 Kensuke Akita (IBS, Daejeon, CTPU)**

Title: Precise Capture Rate of the Cosmic Neutrino Background [video]

Abstract: The Cosmic Neutrino Background (CνB) is one of the most promising candidates, containing a wealth of information about the universe and neutrino physics. Although the direct detection of the CνB is still difficult, a precise understanding of the CνB and its direct observations would be important. In this talk, I will discuss the number density of the CνB and the capture rates of its direct observations, including sub-leading cosmological contributions, such as the neutrino spectral distortions from their decoupling and neutrino clustering on our Galaxy and nearby galaxies. In particular, I will focus on cosmic neutrino capture on tritium, which is the most discussed method so far.

### Spring Term

**19.05.2021 (at 16:15) James M. Cline (McGill U.)**

Title: A little theory of everything [video]

Abstract: I present a minimal model that attempts to address the main missing ingredients of the standard model: inflation, baryogenesis, dark matter, and the origin of neutrino masses. We introduce a complex inflaton that decays into three generations of GeV-scale heavy neutral leptons, creating a lepton asymmetry during inflation. One HNL is stable and provides (partially) asymmetric dark matter. A light scalar singlet is needed to suppress its symmetric relic density. Neutrino masses are generated by the usual seesaw mechanism, with heavy right-handed neutrinos above the inflation scale, and an MFV-like ansatz that relates neutrino masses to the HNL couplings, that are then linked to the light neutrino properties with only one adjustable parameter. The stability of dark matter implies the lightest neutrino is massless. The model is highly testable, and could explain excess events seen by KOTO.**12.05.2021 Jessica Turner (Durham U.)**

Title: Towards an all-orders calculation of the electroweak bubble wall velocity [video]

Abstract: In this talk I discuss recent work where we calculate the velocity of the Higgs condensate bubble wall during a first-order electroweak phase transition in the early Universe. The interaction of particles with the bubble wall can be accompanied by the emission of multiple soft gauge bosons. When computed at fixed order in perturbation theory, this process exhibits large logarithmic enhancements which must be resummed to all orders when the wall velocity is large. We perform this resummation both analytically and numerically at leading logarithmic accuracy. The numerical simulation is achieved by means of a particle shower in the broken phase of the electroweak theory. The two approaches agree to the 10% level. For fast-moving walls, we find the scaling of the thermal pressure exerted against the wall to be P~γ^2T^4. This is impactful for baryogenesis, gravitational wave radiation and generation of other cosmic relics.**28.04.2021 Susana Cebrián (Zaragoza U.)**

Title: Dark matter annual modulation results from 3 years exposure of the ANAIS-112 experiment [video]

Abstract: For dark matter particles in the Milky Way’s halo, an annual modulation in the interaction rate is expected by the revolution of the Earth around the Sun; an annual modulation signal compatible with expectations has been observed by the DAMA/LIBRA experiment for about twenty years, being one of the most puzzling experimental results in the field as it has not been confirmed by other dark matter direct detection experiments. The ANAIS experiment, based on the same target and technique using 112.5 kg of sodium iodide as target, is taking data smoothly since August 2017 at the Canfranc Underground Laboratory in Spain aiming at testing the observation by the DAMA/LIBRA experiment in a model independent way.The results on the search for modulation in the ANAIS-112 experiment were firstly derived applying a blind procedure for an exposure of 157.5 kg·y [1] and later updated from two years of data corresponding to 220.7 kg·y [2]. Before unblinding the data, the whole analysis procedure was fixed [3], the background of the experiment thoroughly studied [4] and the expected sensitivity evaluated [5]. Now, the analysis of three years of data for an expsoure of 313.95 kg·y has been presented, following the same method but improving the background modelling in the fitting of the rates in the region of interest [6]. The best fits obtained for the modulation amplitude in the [1-6] keV ([2-6] keV) energy regions are (-0.0034)±0.0042 cpd/kg/keV (0.0003±0.0037 cpd/kg/keV), supporting the absence of modulation in the data and being incompatible with the DAMA/LIBRA result at 3.3 (2.6) σ, for a sensitivity of 2.5 (2.7) σ. In addition, some complementary analyses (a phase-free annual modulation search and the exploration of the possible presence of a periodic signal at other frequencies) have been made together with several consistency checks. All the obtained results have confirmed the ANAIS-112 projection of reaching a 3σ sensitivity for the scheduled 5 years of operation.

In this seminar, after reminding the DAMA/LIBRA conundrum and the description of the ANAIS-112 experiment including the set-up, performance and analysis methods, the obtained results from the annual modulation analysis will be shown and their implications and the future prospects will be discussed.

[1] First results on dark matter annual modulation from ANAIS-112 experiment, J. Amaré et al, Phys. Rev. Lett. 123 (2019) 031301.

[2] ANAIS-112 status: two years results on annual modulation, J. Amaré et al, J. Phys.: Conf. Ser. 1468 (2020) 012014.

[3] Performance of ANAIS-112 experiment after the first year of data taking, J. Amaré et al, Eur. Phys. J. C (2019) 79:228.

[4] Analysis of backgrounds for the ANAIS-112 dark matter experiment, J. Amaré et al, Eur. Phys. J. C (2019) 79:412

[5] ANAIS-112 sensitivity in the search for dark matter annual modulation, I. Coarasa et al, Eur. Phys. J. C (2019) 79:223.

[6] Annual Modulation Results from Three Years Exposure of ANAIS-112, J. Amaré et al, arXiv:2103.01175 [astro-ph.IM]**21.04.2021 Deanna C. Hooper (Brussels U.)**

Title: Cracks in the CDM paradigm: NCDM as a way forward [video]

Abstract: Despite its remarkable success, the standard cosmological paradigm has been challenged lately by a growing tension in the Hubble Constant measurements, as well as a mismatch between simulations and observations on smaller scales. Combined with the lack of detection of dark matter in any experiment, this has called into question the standard cold dark matter paradigm, and reinvigorated interest in non-cold dark matter models. Such models are characterised by modified dark matter couplings or production mechanisms. In this talk I will review two such models: dark matter interacting with an additional dark sector, and non-cold dark matter produced by primordial black hole evaporation. These scenarios leave a clear imprint in the matter power spectrum on small scales, making them an ideal target to be constrained with Lyman-alpha data. I will discuss new ways of extracting information from Lyman-alpha data for non-cold dark matter scenarios, and present up-to-date constraints on these models.**14.04.2021 Constantinos Skordis (Prague, Inst. Phys.)**

Title: New gravitational degrees of freedom as a solution to the dark matter problem

Abstract: Cosmological and astronomical observations indicate that the majority of mass and energy density of fields in the universe are in a form which interacts extremely weakly, if at all, with light. The standard interpretation is the existence of dark matter, commonly thought to be in the form of particles not part of the standard model of particle physics. At present a firm detection of such a particle is lacking, and moreover, all these observations concern a mismatch between the observed dynamics of visible matter with its gravitational influence. Hence, a less explored interpretation is that the underlying theory of gravity may not be General Relativity. A hint that this may be the case is the observation by Milgrom that discrepancies concerning galaxies are controlled by a single, seemingly universal, acceleration scale.In this talk, I will discuss this possibility and focus on a particular relativistic realization constructed to reproduce Milgrom’s Modified Newtonian Dynamics law at the scale of galaxies. I will show that this proposal leads to (i) correct gravitational lensing on galactic scales, (ii) tensor modes propagating at the speed of light, and (iii) cosmological evolution in line with observations of the Cosmic Microwave Background anisotropies and the large-scale structure power spectrum. I will show that propagating fluctuations around Minkowski spacetime are healthy and discuss possibilities towards creating a more fundamental theory with these properties.

**07.04.2021 Enrico D. Schiappacasse (Jyväskylä U., HIP)**

Title: A novel way to detect the QCD Axion [video]

Abstract: Since the QCD axion is predicted to interact very weakly with Standard Model particles, probing the QCD axion dark matter (DM) hypothesis is quite challenging. As a result, it is the crucial importance to explore new avenues for the QCD detection including those set in the astrophysical context. We propose a new striking avenue to probe the QCD axion dark matter (DM) hypothesis via transient radio signatures coming from encounters between neutron stars (NSs) and axion minihalos around primordial black holes (dressed PBHs). Due to PBHs are local overdensities in the DM distribution, they will unavoidably acquire axion minihalos around them. Thus, we expect the presence of dressed PBHs today in the Milky Way halo. During NS-dressed PBH encounters, the resonant axion-photon conversion in the NS magnetosphere will give rise to a transient line-like emission of radio frequency photons. Based on the sensitivity of current and prospective radio telescopes, we show that this transit emission should be detectable on the Earth under suitable conditions.Papers: arXiv:2102.05680 [hep-ph], Phys.Lett.B 807 (2020) 135566

**31.03.2021 (at 13:15) Ilia Musco (Rome U.)**

Title: Primordial Black Holes: formation mechanism and cosmological impact [video]

Abstract: Primordial black holes (PBHs) formed in the radiation dominated Universe are possible candidates for the dark matter as well as for the seeds of supermassive black holes observed in the centre of galaxies. Numerical simulations of spherically symmetric collapse shows that a PBH is formed if a cosmological perturbation amplitude is larger than a threshold value depending on the specific shape of the perturbation. Recents studies have investigated how to link the initial conditions of numerical simulations with the power spectrum of cosmological perturbations: with simple analytic prescription it is possible to compute the threshold of PBHs from the shape of the peak of the power spectrum. This will allow to compute more precisely the cosmological impact (i.e. the abundance and the mass spectrum) of these objects.**17.03.2021 Yann Gouttenoire (Tel Aviv U.)**

Title: String Fragmentation in Supercooled Confinement and implications for Dark Matter [video]

Abstract: A strongly-coupled sector can feature a supercooled confinement transition in the early universe. When fundamental quanta of the strong sector are swept into expanding bubbles of the confined phase, the distance between them is large compared to the confinement scale. The flux linking the fundamental quanta then deforms and stretches towards the wall, producing an enhanced number of composite states upon string fragmentation. The composite states are highly boosted in the plasma frame, which leads to additional particle production through the subsequent deep inelastic scattering. I will discuss the modelling of these dynamics and introduce the consequences for the abundance and energetics of particles in the universe and for bubble-wall Lorentz factors. As a case of study, I will show that the composite dark matter relic density is affected by many orders of magnitude.**10.03.2021 (at 15:15) Suddhasattwa Brahma (McGill U.)**

Title: Quantum entanglement in the sky: Predictions for inflation [video]

Abstract: Cosmic inflation is widely accepted as the standard paradigm of the early universe since it not only solves the usual cosmological puzzles but also explains observed large scale inhomogeneities as originating from quantum vacuum fluctuations. However, much of its quantum properties remain obscure, and indeed, there have been recent arguments against (some models of) inflation having a consistent quantum embedding into a UV theory. In this talk, I shall explore an often-ignored aspect of this — the crucial role that quantum entanglement (between the quantum modes of the fluctuating field) plays in the dynamics, and observable predictions, of inflation. The same dissipative effects which are at the heart of decoherence (and how quantum fluctuations turn “classical”) will be shown to be responsible for this primordial entanglement, resulting from treating inflation as an open quantum system. I will demonstrate why our conclusions are inescapable, and provide universal upper bounds on the duration of inflation similar to those coming from quantum gravity considerations.**03.03.2021 (at 16:15) Jeff Dror (UC, Santa Cruz, Inst. Part. Phys.)**

Title: The Cosmic Axion Background [video]

Abstract: Existing searches for cosmic axions relics have relied heavily on the axion being non-relativistic and making up dark matter. However, light axions can be copiously produced in the early Universe and remain relativistic today, thereby constituting a Cosmic axion Background (CaB). In this talk, I will study the production and detection of a CaB. Prototypical examples of axion sources are thermal production, dark-matter decay, parametric resonance, and topological defect decay. Each of these has a characteristic frequency spectrum that can be searched for in axion direct detection experiments. I will focus on the axion-photon coupling and study the sensitivity of current and future versions of ADMX, HAYSTAC, DMRadio, and ABRACADABRA to a CaB, finding that the data collected in search of dark matter can be repurposed to detect axion energy densities well below limits set by measurements of the energy budget of the Universe. In this way, direct detection of relativistic relics offers a powerful new opportunity to learn about the early Universe and, potentially, discover the axion.**24.02.2021 (at 12:15) Kazunori Kohri (Tokyo U., IPMU and KEK, Tsukuba)**

Title: Formations of primordial black holes in the early matter-dominated Universe [video]

Abstract: After aLIGO detected the gravitational wave (GW) produced by mergers of binary black holes (BHs), researchers have aggressively studied the origin of the BHs with masses of the order of O(10) M_sun. In addition to astrophysical origins through evolutions of Pop.III/Pop.II stars, one of the attractive candidates of those BHs should be Primordial Black Holes (PBHs). The PBHs can be produced in the early radiation and matter dominated Universe due to spherical collapses of regions which have a large curvature perturbation produced by inflation. I will explain the mechanism of the PBH formations in the early matter dominated Universe in detail. Next, I will review the current status of cosmological and astrophysical constraints on PBHs with introducing my own latest bounds on PBHs in terms of polarization of Cosmic Microwave Background photons due to cosmological accretions onto PBHs (arXiv:1707.04206 [astro-ph.CO], arXiv:2002.10771 [astro-ph.CO]), CMB distortions (arXiv:1405.5999 [astro-ph.CO]), gamma-ray/cosmic-rays/BBN bounds on evaporating PBHs (arXiv:0912.5297 [astro-ph.CO], arXiv:2002.12778 [astro-ph.CO]), Higgs phenomenology (arXiv:1708.02138 [hep-ph]), stochastic GWs (arXiv:1903.05924 [astro-ph.CO], arXiv:1904.12879 [astro-ph.CO], arXiv:2009.11853 [astro-ph.CO]), PBH dark matter (arXiv:1802.06785 [astro-ph.CO]), ultra-compact mini halo formations (arXiv:1905.04477 [astro-ph.CO]) and so on. If time permitted, I will mention the current status of concrete inflation modes based on particle physics which can successfully produce PBHs (e.g., arXiv:0711.5006 [hep-ph], arXiv:0906.1398 [astro-ph.CO], arXiv:1211.2371 [hep-ph]).**17.02.2021 (at 12:15) Yuto Minami (Osaka U., Res. Ctr. Nucl. Phys.)**

Title: Search for parity-violating physics in the polarisation of the cosmic microwave background, so called “Cosmic Birefringence” [video]

Abstract: Polarised light of the cosmic microwave background, the remnant light of the Big Bang, is sensitive to parity-violating physics, cosmic birefringence. In this presentation we report on a new measurement of cosmic birefringence from polarisation data of the European Space Agency (ESA)’s Planck satellite. The statistical significance of the measured signal is 2.4 sigma. If confirmed with higher statistical significance in future, it would have important implications for the elusive nature of dark matter and dark energy.**10.02.2021 Vincent Vennin (APC, Paris)**

Title: Quantum diffusion during cosmic inflation [video]

Abstract: When inhomogeneities are produced with sufficiently large amplitude during inflation, they may subsequently collapse into primordial black holes. I will explain why the effect of quantum diffusion during inflation needs to be taken into account in such a case, and how the abundance of primordial black holes can be predicted within the formalism of stochastic inflation. I will also show that quantum diffusion at small scales (potentially leading to primordial black holes) affects the large-scale perturbations observed in the cosmic microwave background. The cosmic microwave background measurements can thus set explicit constraints on the entire inflationary potential down to the end of inflation.**03.02.2021 Alexey Koshelev (UBI, Covilha)**

Title: Analytic Infinite Derivative (AID) gravity theories [video]

Abstract: I will explain in brief the ideas of AID gravity pointing precisely how such models arise naturally and what questions they are aimed at. Examples of AID scalar field theories will be discussed for deeper clarity. Based on an explicit model of quadratic in curvature AID gravity I will explain how Starobinsky inflation stays in this framework as an exact solution and will discuss inflationary observables in regard to available data. If time permits, I will briefly outline what happens to centrally symmetric solutions for massive objects in this class of theories.

# 2020 seminars

### Autumn Term

**09.12.2020 Mindaugas Karciauskas (Complutense University of Madrid)***Title: Quintessential inflation with a trap*

Abstract: We study a new model of quintessential inflation which is inspired by supergravity and string theory. The model features a kinetic pole, which gives rise to the inflationary plateau, and a runaway quintessential tail. We envisage a coupling between the inflaton and the Peccei-Quinn (PQ) field which terminates the roll of the runaway inflaton and traps the latter at an enhanced symmetry point (ESP), thereby breaking the PQ symmetry. The kinetic density of the inflaton is transferred to the newly created thermal bath of the hot big bang due to the decay of PQ particles. The model successfully accounts for the observations of inflation and dark energy with natural values of the model parameters, while also resolving the strong CP problem of QCD and generating axionic dark matter, without isocurvature perturbations. Trapping the inflaton at the ESP ensures that the model does not suffer from the infamous 5th force problem, which typically plagues quintessence.**02.12.2020 Anastasiia Filimonova (Nikhef)**

Title: Long-lived particles: connecting early universe dynamics to collider signatures

Abstract: An appealing hypothesis of thermally produced dark matter frequently meets strong constraints from direct detection. Consequently, dark sectors with small couplings to the Standard Model are experimentally motivated. In the very weakly-interacting regime, new processes, such as co-scattering, set the relic abundance. A distinctive feature of this regime is the suppressed decays of mediators. Interestingly, they appear to be long-lived also at collider scales, leading to signatures with displaced particles. Using higgs portal as a benchmark scenario, I will discuss the phenomenology of this correspondence between decoupling regimes and experimental signals. I will also briefly talk about the potential of searches for displaced soft particles at the LHC and Belle II.**25.11.2020 Mairi Sakellariadou (King’s College London)**

Title: Hunting for the stochastic gravitational-wave background: implications for astrophysical and high energy physics models

Abstract: I will define the stochastic gravitational-wave background (SGWB) and briefly present the method we use to detect it. I will then present a new method we have proposed for detecting correlated magnetic noise and separating it from a true SGWB signal. The challenge of claiming a true detection will be immediately succeeded by the difficulty of relating the signal to the sources that contribute to it. I will thus consider backgrounds that comprise compact binary coalescences and additional cosmological sources and set simultaneous upper limits on these backgrounds. The recent Advanced LIGO and Advanced Virgo joint observing runs have not claimed a stochastic gravitational-wave background detection, I will thus use the non-detection to constrain theoretical models of astrophysical or cosmological origin. Finally, I will briefly discuss anisotropies in the SGWB.**18.11.2020 Amel Durakovic (Institute of Physics of the Czech Academy of Sciences)***Title: Reconstructing the EFT of inflation from cosmological data*

Abstract: We have developed a precise dictionary between the spectrum of primordial density fluctuations and the parameters of the effective field theory (EFT) of inflation that determine the primordial power spectrum (PPS). At lowest order the EFT contains two parameters: the slow-roll parameter, which acts as an order parameter, and the speed of sound. Applying second-order perturbation theory, we provide maps from the PPS to the EFT parameters that are precise up to the cube of the fractional change in the PPS, or less than 1% for spectral features that modulate the PPS by 20%. While such features are not required when the underlying cosmological model is assumed to be ΛCDM they are necessary for alternative models that have no cosmological constant/dark energy. We verify the dictionary numerically and find those excursions in the slow-roll parameter that reproduce the PPS needed to fit Planck data for cosmological models with and without a cosmological constant.**11.11.2020 Hayley Macpherson (University of Cambridge)***Title: An improved calculation of cosmological backreaction in simulations with numerical relativity*

Abstract: High-precision cosmological surveys are due to deliver measurements accurate to the percent level. In order to ensure we correctly interpret these data, we need to be sure that our cosmological model is accurate. The current standard model assumes that the Universe is homogeneous and isotropic. These assumptions are often justified by the fact that our Universe is homogeneous and isotropic on large scales, however, we are therefore smoothing over highly nonlinear structures on small scales. This smoothing feeds back onto the large-scale evolution of the Universe in the context of nonlinear General Relativity. The true size of this backreaction of small-scale structures has been debated for decades. I will present our cosmological simulations of large-scale structure formation using numerical relativity. These simulations allow us to directly calculate cosmological backreaction in a background-free, symmetry-free framework. I will present new results using a recent improved formalism for averaging in general fluid cosmologies (Buchert et. al 2020), providing a calculation of backreaction with minimal foliation dependence.**04.11.2020 Giorgio Arcadi (Roma Tre University)***Title: Collider prospects for vector Dark Matter coupled with the Higgs*

Abstract: Searches of invisible decays of the SM-like Higgs represent one of the most interesting strategies for collider detection of potential Dark Matter candidates. We provide a reappraisal of some theoretical and phenomenological aspects related to the case of a vectorial Dark Matter candidate.**29.10.2020 Neil Cornish (Montana State University)***Title: On the road to low frequency gravitational wave detection: NANOGrav at mile marker 12.5*

Abstract: The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) monitors a large array of milli-second pulsars that together form a galactic scale gravitational wave detector. Analysis of 12.5 years of data collected between July 2004 and June 2017 show strong evidence for a common red-noise process. This could be the harbinger of a stochastic background of gravitational waves, or something more mundane such as a heretofore unknown type of low frequency noise. The current analysis shows scant evidence for the tell-tale quadrupolar correlations expected for a gravitational wave signal, but if the red-noise is indeed due to gravitational waves, these correlations should become clearly visible with the addition of another 3-5 years of data. Moreover, forecasts show that when the correlations are detected, measurements of the amplitude and spectral slope will be sufficiently precise to distinguish between potential sources such as cosmic strings and supermassive black holes, and to constrain the underlying astrophysical models.**28.10.2020 Tessa Baker (Queen Mary University of London)***Title: Testing gravity with gravitational waves*

Abstract: Gravitational waves (GWs) have already proved immensely powerful for constraining cosmological extensions of GR, both from data-driven and theoretical perspectives. However, GWs really come into their own when used in combination with complementary electromagnetic data. I’ll start by reviewing some of the bounds on extended gravity theories from GW detections to date. I’ll introduce the formalism, the phenomenology, and the astrophysical pitfalls of these tests. Finally, we’ll explore the impact of future experiments like LISA and accompanying galaxy surveys on the remaining parameter space of modified gravity theories.**21.10.2020 Marco Drewes (University of Louvain, CP3)***Title: Sterile neutrinos as Dark Matter candidates*

Abstract: Massive sterile neutrinos appear in many extensions of the Standard Model of particle physics. For sufficiently small mixing angles with ordinary neutrinos, their lifetime can exceed the age of the universe, and they are viable Dark Matter candidates. We give a pedagogical introduction and review recent progress in the study of sterile neutrino Dark Matter. We mostly focus on the minimal, highly testable scenario known as nuMSM, and briefly comment on alternative production mechanisms, such as the decay of heavy particles.**07.10.2020 Clare Burrage (University of Nottingham)***Title: Testing dark energy models with atom interferometry*

Abstract: The accelerated expansion of the universe motivates a wide class of scalar field theories that modify gravity on large scales. In regions where the General Relativity has been confirmed by experiment, such theories need a screening mechanism to suppress the new force. I will describe how theories with screening mechanisms can be tested in the laboratory, in particular with atom-interferometry experiments. I will describe the results of a recent experiment in which we measured the acceleration of an atom toward a macroscopic test mass inside a high vacuum chamber, where the new force is unscreened in some theories. Our measurement shows that the attraction between atoms and the test mass does not differ appreciably from Newtonian gravity. This result places stringent limits on the free parameters in chameleon and symmetron theories of modified gravity.**23.09.2020 Juan S. Cruz (Technical University of Munich)***Title: Gradient effects on false vacuum decay in gauge theory*

Abstract: We study false vacuum decay for a gauged complex scalar field in a polynomial potential with nearly degenerate minima. Radiative corrections to the profile of the nucleated bubble as well as the full decay rate are computed in the planar thin-wall approximation using the effective action. This allows to account for the inhomogeneity of the bounce background and the radiative corrections in a self-consistent manner. In contrast to scalar or fermion loops, for gauge fields one must deal with a coupled system that mixes the Goldstone boson and the gauge fields, which considerably complicates the numerical calculation of Green’s functions. In addition to the renormalization of couplings, we employ a covariant gradient expansion in order to systematically construct the counterterm for the wave-function renormalization. The result for the full decay rate however does not rely on such an expansion and accounts for all gradient corrections at the chosen truncation of the loop expansion. The ensuing gradient effects are shown to be of the same order of magnitude as non-derivative one-loop corrections.**16.09.2020 Chris Shepherd (University of Manchester)***Title: A heatwave affair: mixed Higgs-R^2 preheating on the lattice*

Abstract: Nonminimal Higgs inflation occupies a special position amongst inflationary theories, as it solves the horizon and flatness problems without introducing new physics between the electroweak and Planck scales. However, the theory becomes strongly-coupled during reheating, which cannot be studied using conventional methods. This issue may be remedied by including an R^2 term in the model Lagrangian, and for certain parameters the theory is perturbative up to the Planck scale. We use a semiclassical lattice approach to perform the first nonlinear study of preheating in the R^2-healed theory, in the regime where the curvature-squared coupling β and nonminimal coupling ξ of the Higgs field contribute similarly to the CMB scalar perturbations. Preheating occurs first through tachyonic production of Higgs bosons, and later scattering off the homogeneous inflaton field. We generalise our results for the parameter range 1.1 × 10^9 < β < 1.8 × 10^9 to “Higgs-like” parameters with smaller β, where observables saturate the bound of instantaneous preheating. All predictions for the spectral index and tensor-to-scalar ratio lie within the 1σ region of measurements by the Planck satellite, but a future ground-based experiment optimised for 21~cm tomography may be able to discriminate the mixed Higgs-curvature inflation from the pure Higgs and R^2 theories.**09.09.2020 Lisa Glaser (University of Vienna)***Title: Spectral dimension and other ways to recover geometric information from spectral triples*

Abstract: A compact manifold can be described through a spectral triple, consisting of a Hilbert space H, an algebra of functions A and a Dirac operator D. But what if we are given a spectral triple? Then the situation is more complicated, it is not clear how a spectral triple, in particular one with a non-commutative algebra A, or a finite hilbert space H, relates back to manifolds, or geometry in a more general way. But these are questions one would like to ask if trying to use spectral triples to possibly quantize gravity. In this talk I will explore the spectral dimension of random spectral triples as a possible way to characterize them, and also show how we can recover metric information from a truncation of a spectral triple.**02.09.2020 Daniel Litim (University of Sussex)***Title: Asymptotic safety and physics beyond the Standard Model*

Abstract: Ultraviolet fixed points are key for a fundamental definition of quantum field theory. Fixed points can be free such as in asymptotic freedom, or interacting such as in asymptotic safety. Today, I discuss the status and prospects for asymptotic safety in particle physics. This includes an overview of rigorous results for interacting fixed points in general 4d quantum field theories at weak coupling, necessary conditions, no-go theorems, and links with conformal field theory. These results are then used to find asymptotically safe extensions of the Standard Model which can explain the muon and electron g-2 anomalies. If time permits, I will also comment on the status for asymptotic safety of gravity.

### Spring Term

**04.03.2020 Valentina De Romeri (IFIC)***Title: Searches for new physics with neutrino experiments*

Abstract: In this talk I will discuss the possibility of unveiling new physics with neutrino experiments. I will focus on two different cases. In the first part of the talk I will use the recent observation of coherent elastic neutrino-nucleus scattering (CEvNS) to probe neutrino generalised interactions. Then, I will investigate opportunities for detecting light dark matter at DUNE, a next generation long baseline neutrino experiment.**12.02.2020 Dimitrios Karamitros (NCBJ, Warsaw)***Title: Recalculating freeze-in*

Abstract: The freeze-in mechanism has become very popular recently, due to its ability to explain the dark matter relic abundance with suppressed couplings, and thus (usually) evading all experimental constraints. However, the production of dark matter often is not treated carefully, and so some aspects of frozen-in relics are not pointed-out. In this talk, I will try to summarize some of the ways freeze-in can be different than one expects. In sort, I will show as generally as possible the effects of thermal masses, quantum statistics, and potential non-standard cosmological expansion on freeze-in.