Cosmology Seminars

Scheduled Seminars

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.
• 25.9.2024 – no seminar.
  
• 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.
• 16.10.2024 – 
• 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.

Spring Term 2024

• 12.6.2024 John T. Giblin (Kenyon Coll.) (Onsite)
• Title: (P)reheating, Nonlinear Gravity and Primordial Black Holes
• Abstract: We have no direct evidence that general relativity is wrong; every precision test is a resounding confirmation of this elegant and powerful mathematical model. At the same time, we are just starting to learn how general relativity—beyond perturbation theory—requires us to revisit assumptions in cosmological contexts. In this talk, I will describe recent progress we have made in simulating different models of preheating using full numerical relativity. I will talk about how nonlinear gravity helps to resolve the breakdown of linearized gravity and comment on the possibilities of black holes form preheating. I will also discuss progress toward understanding primordial black hole formation in the presence of cosmic fluctuations.
• 5.6.2024 Alina Mierna (Remote)
• Title: The Nature of Initial Conditions for the Cosmological Gravitational Wave Background
• Abstract: The recent detection of a stochastic gravitational wave background (SGWB) by the PTA collaboration opens an exciting era for the study of early universe cosmology. Given the better angular resolution of future GW detectors, anisotropies in the energy density of the SGWB are expected to be an effective tool to distinguish among various sources of GWs in the early universe. The non-thermal nature of the cosmological background of GWs makes it difficult to define the initial conditions for the overdensity of gravitons. Specifically, the adiabatic initial condition, which holds for Cosmic Microwave Background (CMB) photons, is not guaranteed a priori for the primordial GWs. In this talk, I will present the result we obtained for the initial conditions of the CGWB generated by quantum fluctuations of the metric during inflation from the energy-momentum tensor. A direct consequence of our finding is that these initial conditions enhance the total CGWB angular power spectrum by an order of magnitude compared to the standard adiabatic case.
• 22.5.2024 Simona Procacci (U. Geneva) (Onsite)
• Title: Refining the PT –> GWs pipeline
• Abstract: The possible contribution of a would-be EW first-order phase transition to the gravitational wave background in the LISA frequency range has raised significant interest in the cosmology community. With the goal of improving the accuracy of templates, we revisit the derivations of the gravitational wave spectrum to single out generic features from first-principles analytical derivations. In particular, we show how the velocity spectrum of growing bubbles is determined asymptotically by the analyticity properties of the single-bubble velocity profiles. The superposition of self-similar radial profiles yields a statistically homogeneous and isotropic velocity field, and therefore no gravitational waves. We comment on how this differs from results obtained in the literature. For the sound-wave regime, the choice of initial conditions is examined with respect to causality. The gravitational wave spectrum is finally derived for an expanding universe. This influences the spectral amplitude and shape, along with their dependency on the duration of the sourcing process.
• 17.5.2024 Frank Jia Qu (Cambridge U.) (Onsite) (NOTE: Friday at 12:15)
• Title: Do we understand structure growth? A CMB lensing perspective
• Abstract: The cosmic microwave background serves as a unique backlight for illuminating the growth of structures in our universe. By measuring arc-minute deflections experienced by CMB photons, we can map matter distributions to high redshifts. This lensing signal not only offers a clean window for constraining fundamental physics, such as the sum of neutrino masses, but also facilitates powerful consistency tests of the standard cosmology model through comparisons with the predicted growth of large-scale structure. In this talk, I will provide an overview of the recent and forthcoming advances in this rapidly advancing field. In particular, I will highlight the recent CMB lensing results from the Atacama Cosmology Telescope (ACT). I will also show ongoing measurements with ACT and the opportunities that the soon-to-be-operational Simons Observatory (SO) will enable. New cross-correlations of CMB lensing with galaxy surveys also allow us to probe the amplitude of density fluctuations tomographically. I will discuss the implications of these lensing measurements in the context of cosmic structure growth and the S_8 tension (a previously reported discrepancy between CMB and low-z measurements).
• 15.5.2024 Teerthal Patel (Arizona State U.) (Postponed)
• 8.5.2024 Jun’ya Kume (Padua U.) (Onsite)
  Title: How are local cosmic strings constrained by Pulsar Timing Arrays?
  Abstract: Since its first exciting report of a common stochastic process in 2020, Pulsar Timing Arrays (PTAs) continue attracting our attention with the increasing significance of the gravitational wave background (GWB) signature. While GWB from cosmic strings has been actively investigated as a possible explanation for the observed signature, most studies do not accommodate results from cosmic string simulations in an underlying field theory. Therefore, we revisit the Bayesian inference analyses with the NANOGrav 15yrs data taking into account the indication that not all the loops emit GWs. Based on the result of our analyses, I will discuss how cosmic strings can really be constrained by PTAs.
• 24.4.2024 Simon Cléry (IJCLab, Orsay) (Remote)
  Title: Gravitational production of matter and radiation during reheating
  Abstract: I present new results on particle production during the reheating after inflation. We focus on the late-time evolution of the scalar inflationary field, during its coherent oscillation regime. We specifically consider the production of dark matter and radiation, restricting our attention to gravitational interactions, including minimal and non-minimal coupling to gravity. We also consider leptogenesis through the gravitational production of heavy right-handed neutrinos during reheating.

• 17.4.2024 Andreas Ringwald (Desy) (Onsite)
  Title: Prospects to scrutinise or smash SM*A*S*H [Slides]
  Abstract: SM*A*S*H is an extension of the Standard Model of particle physics which has just the minimal number of fields in order to solve six puzzles of particle physics and cosmology in one smash: vacuum stability, inflation, baryon asymmetry, neutrino masses, strong CP, and dark matter. The parameters of SM*A*S*H are constrained by symmetries and requirements to solve these puzzles. This provides various firm predictions for observables which can be confronted with experiments. We discuss the prospects and timeline to scrutinise or smash SM*A*S*H by cosmic microwave background polarisation experiments, axion haloscopes, and future space-borne gravitational wave detectors.

• 3.4.2024 Emilie Despontin (Brussels U.) (Onsite)
  Title: Baryogenesis from aborted primordial black holes
  Abstract: In this talk, I will introduce a novel mechanism of electroweak baryogenesis based only on Standard Model physics plus gravity. In our scenario, large curvature fluctuations slightly below the threshold for Primordial Black Hole (PBH) formation locally reheat the plasma above the sphaleron barrier, occurring at Hubble re-entry during the Quantum Chromodynamics crossover, particularly at cosmic temperatures between 20 MeV and 50 MeV. We show that the baryon-to photon ratio is comparable to the relative abundance of PBHs formed at the time, and its averaged value consistent with observations. 

• 27.3.2024 Subodh P. Patil (Leiden U.) (Onsite)
  Title: Primordial gravitational waves and N_{eff} bounds revisited [Slides]
  Abstract: In this talk I will present on some recent and ongoing work that follows through on the renormalization process on cosmological backgrounds to completion. Doing so by adopting established techniques is clarifying to the point of novelty. Among the results we’ll arrive at include — Clarifying the roll played by the (UV and IR) scales corresponding to the beginning and end of inflation relative to the UV and IR scales corresponding to the unknown completion of the theory and its observables; Showing how certain IR divergences are an artifact of assuming a past infinite de Sitter phase as opposed to finite duration inflation; Finally, deriving a stress tensor for gravitational wave that does not presume a prior scale separation (as with the standard Isaacson form), and is therefore fit for the purposes of renormalization, highlighting how any attempts to extract N_{eff} bounds is inextricable from the process of renormalization.

• 20.3.2024 Jérôme Martin (Paris, Inst. Astrophys.) (Onsite)
  Title: Can we show that the galaxies are of quantum-mechanical origin? [Slides]
  Abstract: According to the theory of cosmic inflation, all the structures in our Universe (CMB anisotropies, clusters of galaxies etc …) are of quantum-mechanical origin. They are nothing but vacuum fluctuations, amplified by gravitational instability and stretched to cosmological distances by cosmic expansion. This scenario is well supported by the most recent astrophysical data. In this talk, I discuss whether we can go beyond and obtain a direct proof of the quantum-mechanical origin of primordial fluctuations.

• 28.2.2024 Syksy Räsänen
  Title: Primordial black holes and stochastic inflation
  Abstract: Primordial black holes are an interesting dark matter candidate. I discuss how to generate the required large density perturbations from inflation, why stochastic effects are important, and how to model them consistently. I also discuss new results on stochasticity in black hole collapse.

• 21.2.2024 Giorgio Arcadi (Messina U.) (Remote)
  Title: Update on the 2HDM+a
  Abstract: I will illustrate the outcome of some recent studies on the 2HDM+a model. I will show how this model can accommodate viable Dark Matter phenomenology as well as Gravitational Wave signals from First Order Phase Transitions in the Early Universe. Furthermore it can provide an interpretation of some recent experimental anomalies.
  

• 7.2.2024 Mark Hindmarsh (On-site)
  Title: The AB transition in superfluid 3He and cosmological phase transitions
  Abstract: First order phase transitions in the very early universe are a prediction of many extensions of the Standard Model of particle physics and could provide the departure from equilibrium needed for a dynamical explanation of the baryon asymmetry of the Universe. They could also produce gravitational waves of a frequency observable by future space-based detectors such as the Laser Interferometer Space Antenna (LISA). All calculations of the gravitational wave power spectrum rely on a relativistic version of the classical nucleation theory of Cahn-Hilliard and Langer, due to Coleman and Linde. The high purity and precise control of pressure and temperature achievable in the laboratory make the superfluid 3He AB transition ideal for testing the theory. Yet when the A phase of superfluid 3He is supercooled, the B phase appears far faster than classical nucleation theory would predict. If the appearance of B phase is due to a new rapid intrinsic mechanism, gravitational wave production could be rendered negligible. Here we discuss studies of the AB phase transition dynamics in 3He, both experimental and theoretical, and show how the computational technology for cosmological phase transition can be used to simulate the dynamics of the AB transition, and to support the experimental investigation of the AB transition.

• 1.2.2024 Tomo Takahashi (Saga U.) (On-site) (NOTE: on Thursday at 11:15-12:15)
  Title: Quantum nature of primordial fluctuations and inflationary models
  Abstract: Primordial fluctuations are considered to be generated quantum mechanically, and its verification of the quantum nature would be of great importance for a deeper understanding of the origin of structure of the Universe. Several quantum measures have been discussed to characterize the quantumness of primordial fluctuations, among which we focus on quantum discord in this talk. We investigate its inflationary model dependence and argue that such a detailed study may provide a further insight into the quantum nature of primordial fluctuations, and also a test of inflationary models from a new perspective.

• 31.1.2024  Ingunn Kathrine Wehus (Oslo U. and Caltech) (Remote)
  Title: Cosmoglobe – mapping the sky from the Milky Way to the Big Bang
  Abstract: The cosmic microwave background (CMB) gives us information about the earliest history of the Universe, close after the Big Bang. After half a century of more and more sensitive CMB observations, from ground, space and balloons, we now have dozens of valuable data sets available. Each of these has their own strengths and weaknesses, including sensitivity, resolution, frequency bands, sky fraction and systematics. Traditionally each experiment has been analyzed separately, which means that one is blind to the modes not observed by that particular instrument. When instead analyzing them jointly, they will break each other’s degeneracies. Another benefit of joint analysis is that more data allows you to model and constrain the CMB and the foreground emissions from our own galaxy at the same time, which is needed to separate the different components and get the best constraint for the cosmological parameters. This type of joint global analysis is what the Cosmoglobe effort is all about.

• 24.1.2024 Giorgio Mentasti (Imperial Coll., London) (Remote)
  Title: Observing gravitational waves with solar system astrometry
  Abstract: We propose a novel technique to probe the Pulsar Timing Array frequency band of the gravitational waves spectrum by cross-correlating the astrometric deflection of solar-system objects. We will explain how this technique is complementary to the Pulsar Timing Array, by showing its observational perspectives and limits. At the end of the talk, we will consider current and future electromagnetic surveys (such as LSST or GAIA) and we will show to what extent those missions can put constraints on the detectability of a stochastic gravitational wave background in that frequency range.
• 17.1.2024 Antonio Racioppi (NICPB, Tallinn) (On-site)
  Title: Slow-roll inflation in Palatini F(R) gravity (and beyond)
  Abstract: We study single field slow-roll inflation embedded in Palatini F(R) gravity. In contrast to metric F(R), when rewritten in terms of an auxiliary field and moved to the Einstein frame, Palatini F(R) does not develop a new dynamical degree of freedom. However, it is not possible to analytically solve the constraint equation of the auxiliary field for a general F(R). We propose a method that allows us to circumvent this issue and compute the inflationary observables. Moreover, we prove that Palatini F(R)’s which, for infinite curvature, diverge faster than R^2, have a universal limit described by a Palatini quadratic gravity where the Einstein-Hilbert term has the wrong sign. Unfortunately, such configurations, even though a powerful tool in order to realize hilltop inflaton potentials, imply higher-order inflaton kinetic terms in the Einstein frame that might jeopardize the evolution of the system out of the slow-roll regime. We prove that a F(R+X) gravity, where X is the inflaton kinetic term, solves the issue, leaving the inflationary predictions unaffected.