Cosmology Seminars

Scheduled Seminars

Spring Term 2025

• 15.1.2025 Denis Werth (Paris, Inst. Astrophys.), (Onsite)  
  Title: Spectral Representation of Cosmological Correlators
  Abstract: Cosmological correlators hold the key to high-energy physics as they probe the earliest moments of our Universe. However, even at tree-level, perturbative calculations are limited by technical difficulties absent in flat-space Feynman diagrammatic. In this talk, after a pedagogical introduction to observational, phenomenological and theoretical aspects of cosmological correlators meant for the broad audience, I will present a new off-shell perturbative method to study and compute cosmological correlators. Using a spectral representation of massive cosmological propagators that encodes spontaneous particle production through a suitable prescription, I will show in detail how such approach not only makes the origin of the correlator singularity structure and factorisation manifest, but also renders practical analytical computations more tractable. This approach explicitly shows that complex correlators are constructed by gluing lower-point off-shell correlators, and suggests that dispersive methods hold promise for developing cosmological recursion relations, further connecting techniques from modern scattering amplitudes to cosmology. This talk will be mainly based on [arXiv:2409.02072]. With this talk, my goal will be twofold: (i) to introduce you to the physics of cosmological correlators, and (ii) to show you what I believe is some nice complex analysis with things like poles, branch cuts and the residue theorem.
• 22.1.2025 Free
• 29.1.2025 Free
• 5.2.2025 Free
• 12.2.2025 Philippa S. Cole (Sussex U.), (Onsite)
  Title: TBA
  Abstract: TBA
• 19.2.2025 Free
• 26.2.2025 TBA
• 5.3.2025 Jorge Sanchez Almeida (IAC, La Laguna), (Onsite)
• 12.3.2025 Nordic Cosmology Meeting
• 19.3.2025 Free
• 26.3.2025 TBA
• 2.4.2025 Elina Keihänen (Helsinki Inst. of Phys.), (Onsite)
  Title: TBA (Euclid)
  Abstract: TBA
• 9.4.2025 David Cabo Almeida (ICC, Barcelona U.), (Onsite)
• 16.4.2025 Free
• 23.4.2025 Free
• 30.4.2025 Free
• 7.5.2025 Free
• 14.5.2025 Free
• 21.5.2025 Natalia Korsakova (Hannover, Max Planck Inst. Grav.), (Onsite)
  Title: TBA
  Abstract: TBA
• 28.5.2025 Free
• 4.6.2025 TBA

Autumn Term 2024

• 18.9.2024 Timo Kärkkäinen (NICPB, Tallin), (Onsite)
  Title: Neutrino phenomenology from uncertainty principle? [Video]
  Abstract: Generalized uncertainty principles are effective changes to the Heisenberg uncertainty principle that emerge in several quantum gravity models. We study the consequences of two classes of these modifications on neutrino phenomenology: oscillation probabilities, coherence lengths and nonstandard neutrino interactions. We find the constraints imposed by oscillation experiments and in particular the IceCube neutrino observatory to be as strict as the current bounds.
  
• 2.10.2024 Enrico Sessolo (Warsaw, Inst. Nucl. Studies), (Onsite)
  Title: Indications for particle physics from asymptotic safety [Video]
  Abstract: I will discuss some of the phenomenological aspects of embedding the Standard Model and/or BSM models in the framework of trans-Planckian asymptotic safety. In this setting, the presence of an interactive UV fixed point in the renormalization group flow of the gauge and Yukawa couplings imposes a set of boundary conditions at the Planck scale. The ensuing fixed-point analysis leads to specific predictions for the IR phenomenology and it can lead to the dynamical generation of arbitrarily small quantities, for example the Yukawa coupling of (Dirac) neutrinos. I will show that a small Dirac mass for the neutrinos may appear more naturally in the gauged B-L model compared to the Standard Model, when we factor in first-principle calculations of the UV completion based on quantum gravity. In a B-L scenario embedded in asymptotic safety potential model discrimination may emerge from non-traditional signatures, in particular the detection of gravitational waves from first-order phase transitions.
• 9.10.2024 Ville Vaskonen (Padua U. and NICPB, Tallinn), (Onsite)
  Title: Consistency of the JWST black hole observations with PTA gravitational wave measurements [Video]
  Abstract: Inspiralling SMBH binaries constitute a natural astrophysical explanation of the gravitational wave (GW) background discovered in pulsar timing array (PTA) data. In this seminar, I present a fast semianalytical computation of the expected GW background from SMBHs that allows for systematical studies of the effects of binary environments and eccentricities on the total GW background. I will discuss the fit of the SMBH model to the PTA data that shows evidence of environmental effects or binary eccentricities and identify signatures that can be used to distinguish between these effects and to confirm whether the signal comes from SMBH binaries. Finally, I will discuss how the PTA observations can be linked to the JWST observations of dual AGNs and little red dots and how these observations point towards strong environmental effects due to which the final-parsec problem is avoided.
• 23.10.2024 Bastián Reinoso (Helsinki U.), (Onsite)
  Title: The origin of Supermassive Black Holes: Formation, growth, and insights from JWST
  Abstract: The origin of the supermassive black holes that power the high redshift quasars (z>6) discovered in the last decades remains an important open problem in Astrophysics. The short time-span available from the formation of the first stars until the point at which these objects are observed implies the existence of processes capable of growing light BH seeds from 10^2 MSun up to 10^9 MSun in < 1Gyr, or alternatively, processes capable of producing heavy BH seeds with 10^4-10^5 MSun at birth. In this talk, I will review the current status of the field, focusing primarily on the theoretical channels proposed to form massive black hole seeds, their caveats, and prospects for testing their predictions. I will also briefly mention and discuss the light BH seed scenario and rapid BH growth. In the final part of the talk, I will present some recent JWST results and their implications for the current BH seeding models.
  
• 30.10.2024 Gianluca Calcagni (Madrid, Inst. Estructura Materia), (Onsite)
  Title: Imminent test of quantum gravity with gravitational waves
  Abstract: We present a model of the early Universe stemming directly from a UV-complete, nonlocal, unified theory of quantum gravity and matter. The problems of the hot big bang are solved by virtue of the Weyl invariance enjoyed by the theory without the need to invoke inflation. Primordial tensor and scalar spectra are naturally generated by, respectively, quantum and thermal fluctuations. Relying on very few assumptions, the theory predicts a blue-tilted tensor spectrum feeding a primordial stochastic background observable by DECIGO, as well as a lower bound for the tensor-to-scalar ratio detectable by BICEP Array by 2027. Based on arXiv:2206.07066, 2206.06384.
  
• 6.11.2024 Kuroda Tomotaka, (Onsite)
  Title: Stochastic Delta-N Formalism in a curvaton scenario
  Abstract: The stochastic formalism is an effective theory of long-wavelength modes that incorporates probabilistic noise from short-wavelength modes as they exit the horizon. This is expected to have non-perturbative properties, particularly useful in the context of primordial black hole (PBH) formation. Indeed, PBHs are formed from large perturbations exceeding a threshold when they re-enter the horizon. In this presentation, I will review the stochastic delta-N formalism, which combines stochastic inflation and the delta-N formalism to relate curvature perturbations to fluctuations in e-foldings. This framework allows us to calculate curvature perturbations from each stochastic realization using mathematical tools. Additionally, I will demonstrate the application of the stochastic formalism in the presence of a curvaton, and discuss the relationship between perturbations from the inflaton and curvaton and the possibilities of PBH formation in this scenario.  
• 8.11.2024 Two shorter talks, (Onsite) On Friday 14:15
  Kristjan Müürsepp (NICPB, Tallinn)
  Title: Can the QCD axion feed a dark energy component?
  Abstract: Coupling a pseudo-Nambu Goldstone boson (pNGB) to the gauge bosons of a non-abelian gauge group may result in a non-zero contribution to the pNGB mass through instanton effects. Before confinement, this mass is temperature dependent m2(T) ∝ T^{−n}. Thus if the pNGB particles come to dominate the energy density of the Universe in the non-relativistic regime they would accelerate the expansion of the Universe for n > 2, providing a dark energy (DE) component. In this presentation, we outline a scenario in which a pNGB ϕb presently undergoing confinement could realize such a scenario. Using energetic considerations we find that ϕb alone is not enough to produce the experimentally observed amount of DE. However, coupling ϕb to the QCD axion ϕa allows to convert a small fraction of the QCD axion number density into the number density of ϕb end thus enabling us to explain both the observed amount of dark matter and dark energy.
  Juan Urrutia (NICPB, Tallinn)
  Title: The dark timbre of gravitational waves
  Abstract: Gravitational wave timbre, the relative amplitude and phase of the
  different harmonics, can change due to interactions with low-mass halos.
  We focus on binaries in the LISA range and find that the integrated lens
  effect of cold dark matter structures can be used to probe the existence
  of Mv ≲ 10 M⊙ halos if a single binary with eccentricity e = 0.3−0.6 is
  detected with a signal-to-noise ratio 100 − 10^4.
• 13.11.2024 Henri Inchauspé (U. Heidelberg), (Onsite)
  Title: Observing Gravitational Non-Linear Memory with LISA [Video]
  Abstract: Gravitational Waves Astronomy has unlocked the access to direct observation of black hole dynamics, and from there, of gravity in the strong-field regime. Stellar-mass black hole mergers and their waveform signature observed by ground-based detectors since 2015 exhibit no evidence of a deviation from GR theory. The next generation of observatories, among which the space-borne detector LISA, are expected to yield orders of magnitude of SNR improvement, bringing to reach fainter and novel features of GR, witnessing its fundamental non-linear nature, such as GW memory.
  Interpreted as permanent deformation of the background spacetime after the GW perturbation has passed through the detector, signing the non-linear nature of gravity in GR, it offers a novel avenue to proof-test quantitatively Einstein’s gravitation theory. LISA is the most promising upcoming detector for its first detection. The mergers of massive black hole binaries observable by LISA at exceptionally high SNR and redshift, gives the promise of new insights on non-linear gravity and cosmology. Memory detection from individual events is probable with LISA, and possibly across a significant range of redshift, allowing to study not only Memory as a GR probe but also its behavior on cosmological scales.
  In this talk, I will present the detection prospects of LISA regarding GW memory, after reviewing in details LISA detection principle, as well as the physics and the expected signature of GW memory on the data-streams. I will confront LISA observation window to MBHBs population models and evaluate the odds and the expected accuracies regarding GW memory observations in LISA lifetime.
  
• 20.11.2024 Seshadri Nadathur (Portsmouth U., ICG), (Onsite)
  Title: Dark energy and cosmology from the first data release of DESI [Video]
  Abstract: The Dark Energy Spectroscopic Instrument (DESI) is the first of a new generation of “Stage-IV” cosmology survey experiments to be collecting data. I will describe the experiment and the cosmological results obtained from the first data release (DR1) – this includes redshifts for more than 6 million galaxies and quasars, a factor of 3x larger than for the previous biggest such survey! These data allow us to precisely measure the expansion rate of the Universe over the last 11 billion years. DESI results released in April 2024 were based on observations of the baryon acoustic oscillation feature, and showed tantalising evidence in favour of a time-varying dark energy equation of state, rejecting the cosmological constant at between 2.5 sigma and 3.9 sigma significance, depending on combinations with external datasets. I will describe these results as well as new ones based on the “full-shape” of the clustering power spectrum including redshift-space distortions, to be publicly released on November 19. Apart from dark energy, I will discuss the implications of DESI results for the Hubble constant, the neutrino mass scale and tests of general relativity.
• 27.11.2024 Swagat S. Mishra (U. Nottingham), (Remote)
  Title: Towards Preheating after inflation: Inflaton Fragmentation, Oscillon Formation and Decay
  Abstract: The transition period between the end of inflation and the onset of thermal radiation domination, known as ‘reheating,’ remains a key unresolved issue in cosmology. During the early stages of reheating, the inflaton condensate is expected to decay non-perturbatively via parametric resonance into lighter bosonic fields coupled to the inflaton, a phase referred to as ‘preheating.’ However, when the external couplings of the inflaton are sufficiently weak, the inflaton condensate may undergo fragmentation due to strong self-interactions, leading to the formation of quasi-stable solitonic objects known as ‘oscillons.’ In this seminar, we explore the conditions under which oscillons form during preheating, particularly in the presence of external couplings, within the framework of asymptotically-flat inflationary potentials—a class favored by the latest CMB observations. Using detailed numerical simulations carried out on the public lattice simulation platform CosmoLattice, we map the parameter space that supports the formation of long-lived oscillons. The analysis suggests that reheating could have proceeded through the channel of oscillon decay, along with the usual decay of the oscillating inflaton condensate into the offspring particles.
  
• 4.12.2024 Ander Urio (Basque U., Bilbao), (Onsite)
  Title: Non-linear dynamics of axion inflation
  Abstract: Axion inflation models offer a compelling approach to early-universe inflation, featuring a shift-symmetric inflaton that behaves like an axion-like particle. This symmetry preserves the inflaton from undesirable UV corrections and the coupling to other species allows for a rich phenomenology, including the production of gravitational waves. Previous studies have analyzed axion inflation models with a coupling between the inflaton and a U(1) gauge field, but they commonly assume a homogeneous inflaton, overlooking the role of spatial inhomogeneities. In this talk, I will present a lattice implementation of the full axion inflation model, capturing the inflaton’s fragmentation. I will show the results obtained, highlighting the differences from the homogeneous approximation and characterizing the substantial new insights we uncovered. These findings not only underline the critical influence of the non-linear dynamics, reviewing assumptions made in previous homogeneous studies, but also challenge the predictions done in those works.
• 11.12.2024 Alexander S. Belyaev (Southampton U. and Rutherford), (Onsite)
  Title: Towards decoding the nature of Dark Matter [Video]
  Abstract: The nature of Dark Matter (DM) remains one of the greatest puzzles in particle physics and cosmology. While overwhelming observational evidence across galactic and cosmological scales confirms its existence, decades of experiments have only verified its gravitational interaction. Key properties of DM — such as its spin, mass, non-gravitational interactions, stabilizing symmetry, number of associated states, and mediating particles linking DM to Standard Model interactions — remain unknown. To address these challenges, we propose a systematic classification of Dark Matter models based on mediator multiplets with varying spins and weak group charges. Additionally, we introduce a novel class of models — Fermionic Portal Vector Dark Matter (FPVDM) — that extends the Standard Model with an SU(2) dark gauge sector. FPVDM offers important implications for direct and indirect detection experiments, relic density, and collider searches. Examples of DM models from this classification will be discussed, alongside prospects for current and future experiments to test them. This talk will argue that a systematic classification of DM models and their experimental signatures provides a robust framework for discovering and identifying Dark Matter in the near future.
• 18.12.2024 Maciej Kierkla (Warsaw U.), (Onsite)
  Title: Finite-temperature (supercooled) bubble-nucleation with shifting scale hierarchies
  Abstract: We analyse the role of higher-order thermal corrections and gradient effects for supercooled phase transition within dimensionally reduced effective field theory (3D EFT). Usual computation of nucleation rate utilises the gradient expansion. It relies on the assumption that the fluctuating particle is heavier than the nucleating scalar field. However, the gauge boson’s mass is background dependent, thus the previous assumption will be generically broken by the “tail” of bounce solution. This is an example of “scale-shifters”. We omit the use of gradient expansion and instead evaluate the vector-goldstone determinant numerically. We compare our results to those obtained with gradient expansion. We find that the error from the scale-shifters is significant and affects quantities such as percolation temperature and thereby predictions of gravitational waves spectrum from (supercooled) transition.