Cosmology Seminars

Scheduled Seminars

Autumn Term 2023

• 13.12.2023 Kai Schmitz (Munster U., ITP and CERN) (Remote)
  Title: TBA
  Abstract: TBA
• 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)
  Title: Distinct distributions of elliptical and disk galaxies across the Local Supercluster as a ΛCDM prediction [Video]
  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.
• 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.