Majorana-Raychaudhuri Seminars

Title: BelleII excess & Muon g-2 illuminating Light DM with Higgs Portal

Abstract: The Belle II collaboration recently announced that they observed the B→ Kνν decay process for the first time. However, their result encounters a 2.7σ deviation from the Standard Model calculation. Additionally, Fermilab released new data on muon g -2 away from the SM expectation with 5.1σ. In this talk, I would like to talk about the simplest UV-complete U(1)Lµ−Lτ-charged complex scalar Dark Matter model. Thanks to the existence of light dark Higgs boson and light dark photon, I can explain the observed relic density of DM and resolve the results reported by both Belle II and Fermilab experiments simultaneously. As a byproduct, the Hubble tension can be alleviated.

Title: Origin of cosmic rays in our galaxy

Abstract: The origin of cosmic rays remains unclear, with particles up to the second knee or ankle of cosmic ray energy spectra believed to be of galactic origin. As cosmic rays are charged particles, they are deflected by galactic/intergalactic magnetic field and hence are not directly point back to their sources. An alternate approach to searching for cosmic ray origin is through the detection of gamma-rays and neutrinos which are produced in the interaction of cosmic rays with the ambient matter/radiation in the vicinity of the sources. Supernova remnants are potential galactic objects capable of accelerating cosmic rays up to the knee or ankle energy, but key issues in the SNR origin model remain unresolved. Supernova explosions, on the other hand, tend to cluster in space and time because big OB stars develop in clusters and have limited lifetimes. As a result, overlapping shocks from SNRs and huge star winds created by OB associations can accelerate cosmic rays to PeV energy. In this presentation, our understanding on these issues will be addressed. Possibility  of cosmic ray acceleration at star atmosphere to high energies will be explored.

Title: The interactions and detection of neutrinos from sub-TeV to beyond EeV

Abstract: High-energy (HE; ~100 GeV to 100 PeV) and ultrahigh-energy (UHE; >~ 100 PeV) neutrinos are essential for advancing both particle physics and astrophysics, offering significant opportunities to study neutrino interactions and test the Standard Model and beyond. On the other hand, neutrino interactions are the cornerstone of all neutrino measurements, as neutrinos are always detected via particles from their interactions. Studying neutrino interactions also reveals new event classes (e.g., dimuons), offering novel measurement opportunities. Neutrino interaction theory is both interesting and challenging: although neutrinos only participate in gravitational and weak interactions, neutrino interaction studies relevant to neutrino experiments also significantly involve strong interactions and quantum electrodynamics. For sub-TeV–EeV neutrinos, while the dominant interaction, deep-inelastic scattering (DIS), is well understood, the increasing data from current and future experiments demands studying subdominant interactions, which remain poorly understood.

In this talk, I will talk about the studies of the sub-TeV--EeV neutrino interactions and detection, including W-boson and trident production, dimuons, and final state radiation (a QED radiative correction as large as 25% while completely overlooked by current experiments).

Title: Constraining the bispectrum from bouncing cosmologies with Planck

Abstract: Bouncing models followed by an inflationary phase not only yield nearly scale-invariant fluctuation spectra but also offer a means to mitigate the large-scale anomalies present in the Cosmic Microwave Background (CMB). This mitigation is contingent upon the introduction of substantial non-Gaussianity on very large scales, which decays exponentially and becomes small deep within the horizon. For this reason, this non-Gaussianity was thought to be invisble in observational data, as cosmological observations are subject to cosmic variance. In this talk I will explain how such models can be excluded with high signifcances by the Planck data if their non-Gaussianity is enough to appreciably alleviate the CMB large-scale anomalies. Furthermore, this result underscores the sensitivity of the Planck data to scales beyond the pivot scale.

Title: Bounds on sterile neutrino lifetime and mixing angle with active neutrinos by global 21 cm signal

Abstract: Sterile neutrinos can be a possible candidate for dark matter. Sterile neutrinos are radiatively unstable and can inject photon energy into the intergalactic medium (IGM). The injection of photon energy into IGM can modify the temperature and ionization history of IGM gas during cosmic dawn. Theoretical models based on the ΛCDM framework predict an absorption profile in the 21 cm line during the cosmic dawn era. Recently, the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) collaboration confirmed such an adsorption signal. Injection of energy into IGM can modify the absorption amplitude in the 21 cm signal. Considering the 21 cm absorption signal at cosmic dawn, we constrain the lifetime of sterile neutrinos and the mixing angle of sterile neutrinos with active neutrinos. We also compare these bounds with other astrophysical observational bounds.

Title: Inflationary Models in String Theory

Abstract: In this talk, I will discuss a phenomenon called cosmic inflation in which the Universe went through accelerated exponential expansion to solve the horizon problem of Cosmological Microwave Background within a billionth of a trillionth of a trillionth of a second, in the very early Universe. This accelerated expansion, in its minimal form, is driven by a scalar field (inflaton) and it takes place when this scalar field slowly rolls down a potential well. However, the origin of this scalar field and the correct form of the scalar potential remains an open question in cosmology. I will present a string theory motivated model where the inflaton is connected to the geometry of the internal space -- the overall volume of it drives the inflation. In particular, I will present a construction where the overall volume modulus (scalar field) is dynamically stabilized to an exponentially large value only via perturbative corrections, also known as perturbative large volume scenario (LVS). In this framework, the robustness of the single-field inflationary model is checked against possible sub-leading corrections. In the later part of my talk, I will focus on the global embedding of the fibre inflation in perturbative LVS and show how our constructions pose less challenge in realizing a successful period of inflation.

Title: Primordial Black Holes from Null Energy Condition Violation in Inflation

Abstract: We propose a novel approach to generate Primordial Black Holes (PBHs) in inflationary cosmology via an intermediate stage which violates the Null Energy Condition (NEC). The formation of PBHs in the early universe is vastly studied due to its relevance to astrophysical and cosmological phenomena such as dark matters and supermassive black holes. The production of abundant amount of PBHs relies on the amplification of primordial curvature power spectrum P_ζ∝H^2/ϵ on small scales, which is conventionally achieved by an intermediate stage where the slow-roll parameter ϵ is suppressed. In our work, we present a new approach by enhancing the curvature perturbations through the growth of Hubble parameter H in an intermediate stage in inflation. The growth of H implies the violation of NEC, and by making use of the recent development in modified gravity theory, we successfully construct pathology-free models where NEC violation takes place in inflationary cosmology. We successfully generate sizable PBHs of astrophysical interests. Additionally, our scenario simultaneously predicts a peaked scalar-induced gravitational wave signal and a nearly scale-invariant primordial gravitational waves in different frequency band, admitting our model to be falsified by joint observations of gravitational waves in near future.

Title: Unraveling the origins of magnetic fields in the early universe

Abstract: Magnetic fields permeate the entire universe, extending from​ the smallest to the largest observable length scales. A popular​ explanation for the origin of the magnetic fields observed in galaxies,​ clusters of galaxies, and the intergalactic medium is that seed fields​ generated due to quantum fluctuations in the primordial universe is​ amplified later by astrophysical processes. According to the standard​ paradigm of magnetogenesis, the seed magnetic fields on cosmological​ scales are generated during inflation by breaking the conformal

invariance of the standard electromagnetic action. This is usually​ achieved through a non-conformal coupling of the electromagnetic field​ to the scalar field that drives inflation.

I will begin the talk with a brief introduction to the essential idea of​ inflation. Thereafter, I will discuss the challenges in generating​ primordial magnetic fields in non-trivial inflationary scenarios,​ particularly focusing on how deviations from slow-roll inflation impact​ the spectra of non-helical and helical electromagnetic fields across​ large and small scales in single-field models of inflation. To​ circumvent the challenges that arise in single-field models, in the​ second part of the talk, I will discuss the generation of magnetic​ fields in two-field models. In two field models with suitably chosen​ non-conformal coupling functions, we can obtain spectra of magnetic​ fields of the required strength and shape even in situations involving​ strong departures from a slow roll. I will also discuss the imprints of​ the primordial magnetic fields generated in certain two-field models on​ the anisotropies in the cosmic microwave background (CMB). Finally, I​ will discuss an alternative method for addressing the challenges in​ magnetogenesis in single-field models, using a non-conformal coupling​ function that depends on the kinetic energy of the inflaton and I will​ highlight the corresponding behaviour of the spectra of electromagnetic​ fields.

Title: Some properties of Gravitational Waves in Modified Theories of Gravity

Abstract: In this talk, at first, I'll briefly discuss polarisation modes of Gravitational Waves (GWs) in modified theories of gravity mainly focusing on f(R) gravity metric formalism and Palatini formalism. After that I'll discuss quasinormal modes of black holes or ring-down GWs in some selected modified theories of gravity. These two properties may be helpful in the near future to constrain different modified theories of gravity.

Title: Smooth reheating via non-Abelian dark sector

Abstract: We consider a model, where a single inflaton interacts weakly as an axion with Yang-Mills gauge bosons. As these rapidly thermalize, the friction felt by the inflaton field is increased, leading to a self-amplifying process. If the gauge bosons of the thermal bath represent a dark sector, the reheating of the Standard Model is then realised through portal interactions. The dark relic abundance in this scenario depends mostly on the departure from equilibrium after the universe cools down below the critical temperature, when the dark vectors confine into composite states. This constrains the confinement scale of the dark sector. Moreover, indirect detection and Big-Bang nucleosynthesis set bounds on the portal interactions, revealing a predictive parameter space.

Title: Gravitational Waves from Stochastic Scalar Fluctuations

Abstract: In this talk, we will present a novel mechanism for gravitational wave generation in the early Universe. Light spectator scalar fields during inflation can acquire a blue-tilted power spectrum due to stochastic effects. We show that this effect can lead to large curvature perturbations at small scales (induced by the spectator field fluctuations) while maintaining the observed, slightly red-tilted curvature perturbations at large cosmological scales (induced by inflaton fluctuations). Along with other observational signatures, such as enhanced dark matter substructure, large curvature perturbations can induce a stochastic gravitational wave background (SGWB). We will discuss this mechanism in detail and present several benchmark scenarios that result in gravitational wave strengths visible to near-future detectors.

Title: Scalar perturbations from inflationary magnetogenesis

Abstract: Primordial non-Gaussianities, though yet unobserved, remain an important observable since they can help differentiate various models of inflation. This necessitates a deep understanding of the various processes that could contribute to these non-Gaussianities, with inflationary magnetogenesis being one of them. Often, the spectrum and the bispectrum of the perturbations produced during inflation are studied under the assumption that the metric perturbations can be neglected and that all the relevant physics resides in the coupling of the inflaton and the gauge fields. Here, we present a full set of equations self-consistently accounting for the perturbations of the inflaton and the gauge field along with the scalar perturbations of the metric. In this talk, I will specifically consider the case of axion inflation with purely axial coupling of the inflation to the gauge fields. I will compare the scalar power spectrum and bispectrum derived from our equations to those reported in earlier literature. In particular, by comparing the amplitude of the scalar bispectrum with modern observational constraints for primordial non-Gaussianity, I will reconcile the existing constraints for the axion-vector coupling during inflation.

Title: Bulk Reconstruction in de Sitter 

Abstract: In this talk, I will present recent work on construction of boundary representations of de Sitter fields. I will start by introducing the HKLL construction of boundary representations in AdS. Then, we will review previous work on construction of boundary representations on de Sitter and finally present our results.

Title: Domain walls and their gravitational waves

Abstract: I will report results of evolution of domain walls and their gravitational waves obtained with the publicly available code CosmoLattice. First, I will talk about conventional domain walls arising in models with spontaneous breaking of a discrete symmetry due to 

 non-zero constant expectation value of a scalar field. In the scaling regime, the domain wall network is dominated by one large wall stretching throughout the simulation box, while the total area of closed domain walls is negligibly small. Obtained spectrum of gravitational waves agrees well with past results at low frequencies, but there are significant differences in the ultraviolet part of the spectrum, which can have important implications in view of the future LISA and TianQin experiments. Conventional domain walls overclose the Universe, unless one assumes some mechanism of their dissolution in the primordial plasma. 

This motivates to introduce a different class of domain walls arising in models where the expectation value responsible for symmetry breaking is proportional to the Universe temperature. While the network of such ``melting" domain walls is still dominated by one long wall, closed walls have a comparable total area. Another crucial difference is the gravitational spectrum characterised by the spectral index n=1.6 in the infrared part compared to n=3 in the standard case. This is remarkably close to the central value 1.8 obtained in the recent measurements of gravitational wave background by pulsar timing arrays. 

Title: The Role of the Curvaton Post-Planck

Abstract: Curvatons are light unstable scalar fields that, upon decay, contribute to the curvature perturbation post-inflation. As a result, they can alter the CMB observables ($n_s$, r) and generate non-Gaussianities. When a curvaton is added to an inflation model, it is then crucial to investigate its parameter space facing the Planck constraints. 

In this talk, I address the following question: What are the masses and decay rates of a curvaton to (1) resurrect an inflation model which was excluded due to incompatible ($n_s$, r), or (2) not ruin a successful inflation model. An example for each category will be presented.

This talk is based on 2409.08279.

Title: Fingerprints of a Non-Inflationary Universe from Massive Fields

Abstract: Precision cosmology has allowed us to learn a great deal about the very early universe through correlations in the primordial fluctuations. New data will abound in the next decade, from which we forecast the potential appearance of features in the correlations that could be due to new high-energy particles, otherwise inaccessible in particle accelerators on Earth. Moreover, the distinctive form of these features could inform us about the evolution of the very early universe, e.g., whether it was inflating or contracting before a bounce. Hardly any other single observable could properly distinguish whole scenarios in a model-independent way. Discriminating evidence for the paradigm (inflation) or an alternative (such as a bounce) would significantly advance our knowledge of primordial cosmology and high-energy physics. This talk will review the latest developments of this program. 

Title: Phenomenology of Primordial Black Hole Hot Spots

Abstract: Primordial Black Holes (PBHs) are not only fascinating candidates for the Dark Matter (DM) relic density, but also act as volcanoes in the vacuum, violently producing high energy particles in what Stephen Hawking referred to as "Black hole explosions" in 1974. Spanning over 20 orders of magnitude in their initial mass, PBHs in the so called "asteroid mass" range may still constitute all of the relic DM, while lighter PBHs could have evaporated entirely by the present. For initial mass $M_{\rm PBH}^{\rm ini} \lesssim 10^9$g, such ultralight PBHs would evaporate before Big Bang Nucleosynthesis in the early universe making them both extremely difficult to probe and contemporaneous with key cosmological processes such as the generation of DM and baryon asymmetry. Recently studies have revealed how PBHs heat their surroundings locally, producing hot spots in the universe with potentially extreme temperature gradients. Previous treatments of ultralight PBHs in the early universe na\"ively assumed that Hawking radiation would heat the entire universe homogeneously. Properly accounting for the presence of hot spots reveals a rich phenomenology which is explored in this seminar. The paradigm of PBH-produced DM is revisited, where it is found that hot spots efficiently absorb DM. Furthermore, hot spots are shown to be able to support successful leptogenesis even after the freeze-out of sphalerons. The enterprise of hot spot phenomenology is truly in its infancy, so an overview of the field and details of recent advances will be followed by discussion of future directions and open questions.

Title: Gauged Global Strings

Abstract: I will present the string solutions and cosmological implications of the gauge U(1)Z × global U(1)PQ model. With two hierarchical symmetry-breaking scales, the model exhibits three distinct string solutions: a conventional global string, a global string with a heavy core, and a gauge string as a bound state of the two global strings. This model reveals rich phenomenological implications in cosmology. When incorporating this model with the QCD axion framework, the heavy-core global strings emit more axion particles due to their large tension. This radiation significantly enhances the QCD axion dark matter abundance, thereby opening up the QCD axion mass window. Furthermore, in contrast to conventional gauge strings, the gauge strings in this model exhibit a distinctive behavior, radiating axions.

Title: de Sitter as an Axion Detector

Abstract: Axions, scalar fields with compact field spaces, are some of the most well-motivated candidates for physics beyond the Standard Model. In this talk, I will explain how inflationary correlations are uniquely sensitive to the topology of a scalar's field space, and can thus be used to distinguish axions from other light scalar fields even if they share the exact same action. As a proof of concept, I will show that axions can have a qualitatively distinct impact on a heavy field's cosmological collider signal. 

Title: Neutron decay anomaly, dark matter and neutron stars

Abstract: The discrepancies on measurements of the lifetime of the neutrons could be resolved considering an extra neutron decay channel into dark matter, with a branching ratio of the order of O(1%). Although the decay channel into a dark fermion plus visible matter has been already experimentally excluded, a dark decay into a dark matter fermion plus either a scalar or dark photon remains still a possibility. In particular, a model with a fermion mass of the order of 1  GeV and a scalar with a mass of the order of 1-2 MeV could provide not only the required branching ratio to explain the anomaly but also a good dark matter candidate with the right thermal abundance today, and being consistent with neutron stars physics. In addition, the dark matter self-interactions mediated by the light scalar could help to resolve some of the small-scale structure problems found when comparing cold dark matter N-body simulations with observations.

Title: Decoding the physics of the early universe through primordial black holes, dark matter, and gravitational waves

Abstract: I will start my talk with a brief overview of the standard reheating scenario. Then, I will discuss reheating through the evaporation of primordial black holes (PBHs) if one assumes PBHs are formed during the reheating phase. Depending on their initial mass, abundance, and inflaton coupling with the radiation, I discuss two physically distinct possibilities of reheating the universe. In one possibility, the thermal bath is solely obtained from the decay of PBHs, while inflaton plays the dominant energy component in the entire process. In the other possibility, PBHs dominate the total energy budget of the universe during evolution, and then their subsequent evaporation leads to a radiation-dominated universe. I will discuss the impact of monochromatic and extended PBH mass functions and estimate the detailed parameter ranges for realizing those distinct reheating histories. The evaporation of PBHs is also responsible for the production of DM. I will show its parameters in the background of reheating obtained from two chief systems in the early universe: the inflaton and the primordial black holes (PBHs). Then, I will move my discussion towards decoding the very early universe physics through GWs, where initially, I will focus on the primordial gravitational waves (PGWs), the tensor perturbation without any source term. If PBHs are produced due to the enhancement of the primordial scalar power spectrum on small scales, such primordial spectra also inevitably lead to strong amplification of the scalar-induced secondary gravitational waves (GWs) at higher frequencies. I will show how the recent detection of the stochastic gravitational wave background (SGWB) by the pulsar timing arrays (PTAs) has opened up the possibility of directly probing the very early universe through the scalar-induced secondary gravitational waves. Finally, I will conclude my talk by elaborating on the effect of quantum correction on the Hawking radiation for ultra-light PBHs and its observational signature through dark matter and gravitational waves. 

Title: Cosmic Magnetic Shear of Blazars

Abstract: I will show that the presence of a stochastic intergalactic magnetic field in the Universe induces correlated quadrupolar distortions on the gamma-ray images of blazars in the sky. Using an E- and B-mode decomposition of an all-sky gamma-ray shear map, I will then quantify the strength of the resulting two-point signal and estimate Fermi-LAT's sensitivity to it. Such a decomposition also allows one to distinguish parity-even and -odd magnetic fields. I will also comment on the relation of this technique to other blazar-based methods of probing intergalactic magnetic fields. 

Title: Primordial black holes in the string axiverse

Abstract: I will discuss the evaporation of primordial black holes (PBHs) in the presence of a large number of light scalars, motivated mainly by the string axiverse. In particular, I will show that this prevents spin down and may even spin up a PBH, and that the present mass-spin distribution of PBH remnants could be used to probe the whole string axiverse. I will also discuss other potential consequences, such as superradiant instabilities triggered by evaporation and the possibility of probing new physics beyond the TeV scale by tracking the final stages of PBH evaporation, as well as methods to infer a PBH’s mass and spin from its Hawking emission spectrum.

Title: Discriminating properties of self-interacting dark matter through gravitational waves

Abstract: Dark matter (DM) environments around black holes can influence the evolution of their merger due to the dynamical friction provided by the dense DM region. In this work, we study the inspiral of black hole binaries in the presence of collision-less dark matter (CDM) and self-interacting dark matter (SIDM) environments with the aid of simulation tools. We focus on the gravitational wave (GW) signatures emitted during the inspiral phase, where the presence of DM environments results in a dephasing of the gravitational waveform. We find that this dephasing depends on the density of the SIDM profile and the strength of the self-interactions. Finally, we determine the prospects future GW experiments, such as the Laser Interferometer Space Antenna (LISA), have of distinguishing the properties of DM environments based on an analysis of the uncertainties on observables related to the GW signature.

Title: Seeing particles beyond the inflationary Hubble scale

Abstract: Cosmological production and propagation of heavy particles during inflation can leave a distinctive signature in the non-Gaussianity of primordial density fluctuations. This “cosmological collider signature” allows us to detect particles at the inflationary Hubble scale, which can be as high as Hinf ≲ 10^13 GeV. While this technique presents a unique opportunity to probe particle physics well beyond any terrestrial collider, its reach is often limited to a narrow window around Hinf due to the exponential suppression suffered by particles heavier than Hinf. However, a class of ‘chemical potential’ mechanisms can alleviate this suppression and extend the observational reach up to masses ~ 60 Hinf. In this talk, I will discuss our realization of this mechanism for a pair of scalars, and its generalization to vectors. I will then show how the vector chemical potential can allow us to target heavy gauge bosons of the GUTs at ~ 10^15 GeV.

Title: Mass Generation and Asymptotic Freedom for Large N Abelian Higgs Model 

Abstract: Solving scalar field theory using large N expansions allows for a systematically improvable non-perturbative handle on quantum field theories. Often, results from this method are surprising, such as finding that contrary to popular belief scalar field theory in 4d can be asymptotically free. Applying this novel large N formalism to the Abelian Higgs model one finds mass generation without spontaneous symmetry breaking, with additional predictions for experiment. If time allows, I will discuss current limitations on the large N program, and how our group intends to make further progress.

Title: Signatures of domain wall networks: from gravitational waves and primordial black holes to cosmic birefringence

Abstract: In this talk, I will present recent progress in the study of domain wall networks. First, in terms of their gravitational relics - gravitational waves (GWs) and primordial black holes - that might be behind the recent signals observed at Pulsar Timing Arrays. Second, I will discuss the isotropic birefringence effect that domain wall networks coupled to photons cause on the polarization of the CMB, with striking connections to the recent evidence found in the CMB data.  

Title: Gauged Global Strings

Abstract: I will present the string solutions and cosmological implications of the gauge U(1)Z × global U(1)PQ model. With two hierarchical symmetry-breaking scales, the model exhibits three distinct string solutions: a conventional global string, a global string with a heavy core, and a gauge string as a bound state of the two global strings. This model reveals rich phenomenological implications in cosmology. When incorporating this model with the QCD axion framework, the heavy-core global strings emit more axion particles due to their large tension. This radiation significantly enhances the QCD axion dark matter abundance, thereby opening up the QCD axion mass window. Furthermore, in contrast to conventional gauge strings, the gauge strings in this model exhibit a distinctive behavior, radiating axions.

Title: Cold-atom analogues for vacuum decay

Abstract: False vacuum decay plays a pivotal role in many models of particle physics and the early Universe. However, we lack a satisfying theoretical understanding of this process, with existing approaches working only in imaginary (Euclidean) time, and relying on assumptions that have yet to be empirically tested. A promising route forward is to use cold-atom systems which undergo first-order phase transitions that are analogous to vacuum decay. In this talk, I will present recent theoretical work to understand this analogy using semiclassical lattice simulations, and will discuss possibilities and challenges for realising these analogues in the laboratory.

Title: HEFTy new physics effects at colliders 

Abstract: Using the $\kappa$ framework, the constraints on the quartic interactions of Higgs with gauge bosons give a qualitative picture of consistency with the SM when the statistical yield is low. However, increasing statistics necessitates a more theoretically consistent framework to constrain such couplings. Therefore, using the framework of the non-linear Higgs Effective Field Theory (HEFT), I will talk about the radiative corrections to Higgs decays and discuss the current and future sensitivity to quartic Higgs gauge couplings using the single Higgs data. In the subsequent part of the talk, using the off-shell Higgs boson measurements in massive gauge boson pair production, I will study these processes in the context of the HEFT framework and discuss the constraints on the relevant Higgs boson non-linear interactions.

Title: Solving Cosmological Mysteries with Axion Rotations

Abstract: We present a paradigm where the (QCD) axion’s novel evolution, a rotation in field space, can address cosmological mysteries of the Universe. This dynamics may naturally arise as a result of quantum gravity effects and cosmic inflation but was overlooked in the extensive literature. This talk will explore the example where axion rotations contribute to axion dark matter through kinetic misalignment and can generate the observed baryon asymmetry of the Universe via axiogenesis. Remarkably, rich phenomenology automatically arises with sharp, distinct, and correlated predictions. These include specific axion properties more experimentally accessible than the conventional scenarios, unique gravitational wave signals, and correlated mass scales of supersymmetry and neutrinos. Thus far, axion rotations have added fuel to experimental efforts and paved new theory research avenues, opening up resolutions to the deepest cosmological mysteries with discoverable signatures.

Title: Searching for Cosmological Collider in the Planck CMB Data

Abstract: New heavy particles during inflation can leave imprints in the primordial perturbations and subsequently in the observed cosmic microwave background (CMB) anisotropies. This remarkable detection channel allows us to probe new physics at extremely high energies. In this talk, I will present our recent work on the first extensive search for cosmological collider signals with the CMB data. We utilise the publicly available CMB bispectrum estimator code CMB-BEST to study various analytic templates guided by the cosmological bootstrap, providing the most stringent constraints to cosmological collider signals to date.

Title: Electroweak phase transition dynamics and its cosmological implications

Abstract: We discuss the precise calculations of the electroweak phase transition dynamics. Based on the bubble wall velocity from the phase transition dynamics, we further discuss its implications in baryogenesis, new dark matter mechanism,  and the corresponding phase transition gravitational wave signals.

Title: A spin on wave dark matter

Abstract: What can we learn about the intrinsic spin of an ultralight dark matter field from astrophysical observations? I will argue that the imprint of spin, ie. whether it is a scalar, vector or a tensor field, can be seen via (i) interference patterns in the density field inside dark matter halos, and through, (ii) (polarized) solitons with macroscopic intrinsic spin. With increasing intrinsic spin, interference patterns in halos are reduced (and the inner shapes of halos modified) -- which can be probed by lensing and dynamical heating of stars. After introducing polarized solitons, I will show that the time-scale of emergence of solitons (within halos) increases with increasing spin, and briefly discuss electromagnetic and gravitational wave signatures from such polarized solitons. Time permitting, I might discuss production mechanisms (which lead to bounds on DM mass), direct detection prospects, or connections to "spinor" BECs in the laboratory.

Title: Gravitational wave spectrum from expanding string loops on domain walls

Abstract: We analytically calculate the spectrum of stochastic gravitational waves (GWs) emitted by expanding string loops on domain walls in the scenario where domain walls decay by nucleation of string loops. By introducing macroscopic parameters characterizing the nucleation of the loops, the stochastic GW spectrum is derived in a way that is independent of the details of particle physics models. In contrast to GWs emitted from bubble collisions of the false vacuum decay, the string loops do radiate GWs even when they are perfectly circular before their collisions, resulting in that more and more contribution to the spectrum comes from the smaller and smaller loops compared to the typical size of the collided loops. Consequently, the spectrum is linearly proportional to the frequency at the high-frequency region, which is peculiar to this GW source. Furthermore, the results are compared with the recent nano-Hertz pulsar timing array signal, as well as the projected sensitivity curves of future gravitational wave observatories.

Title: Novel Searches at Neutrino Facilities

Abstract: Over decades now, our understanding of neutrinos has improved rapidly, largely thanks to continuous improvement in experimental facilities. With a new generation poised to take data, I will demonstrate how these facilities can serve as beyond-the-Standard-Model facilities in tandem with their neutrino-oriented goals. This includes the possibility of searching for new particles (e.g. dark matter) lurking amidst neutrino data.

Title: Testability of GUTs in neutrino and GW experiments

Abstract: Grand Unified Theories (GUTs) aim to unify three fundamental particle forces including electromagnetic, strong and weak forces. Thanks to the recent fast progress in neutrino precision measurements and gravitational wave (GW) observations, two complementary tests becomes important in addition to the traditional proton decay measurements. One is to test correlations of masses and mixing in the quark sector and lepton sector. Another is to measure spectrum of cosmic GW background from cosmic strings, which is predicted in most GUT framework. I will address all these phenomenological constraints on GUTs and further comment on the influence of the recent Pulsar Timing Array measurements. 

Title: Beginning cosmic inflation from inhomogeneous initial conditions

Abstract: The original purpose of cosmic inflation was to explain the large-scale homogeneity and flatness of our Universe. However, it has long been debated whether scalar field inflation can begin with generic initial configurations in the presence of significant inhomogeneities in the scalar and/or gravitational sectors. In this work, I will present the latest insights from numerical relativity investigations aiming to provide an answer to the initial condition problem of inflation.

Title: How to simulate ALPs and cosmologies

Abstract: I will talk about MiMeS and NSC++, two libraries that can simulate axions/ALPs and the energy density of fluids, respectively, in the Universe. I will argue for their necessity, I will explain how they work and how to use them, and I will share my near-future vision.

Title: Can higher derivatives resolve the black hole singularity?

Abstract: Higher-derivative terms are relevant in several approaches to quantum gravity. For instance, they occur in the perturbative quantization of Einstein gravity, and they can be used to construct (super-)renormalizable quantum gravity models. In this talk, we discuss the problem of whether higher derivatives could already resolve black hole singularities at classical level. In the first part, we review some results regarding regularization of singularities in general linearized higher-derivative models. The non-linear regime is considered in the second part, when we present spherically symmetric static solutions of the most general sixth-derivative gravity using series expansions. We prove that the only solutions of the complete theory, i.e., with generic coupling constants, that possess a Frobenius expansion around the origin (r=0) are necessarily regular. Moreover, by expanding around a nonzero r we identify solutions with black-hole horizons. Families of singular solutions emerge only for specific branches of the theory (i.e., imposing particular constraints on the coupling constants). These results suggest that there is an important difference between higher-derivative gravity models with 4 and 6 derivatives in what concerns the space of spherically symmetric static solutions.

Title: Probing the Electroweak Phase Transition through Colliders, Gravitational Waves, and Microlensing experiments

Abstract: The knowledge of the Higgs potential is crucial for understanding the origin of mass and the thermal history of our Universe. In this talk, we demonstrate how collider measurements, observations of stochastic gravitational wave signals, and microlensing experiments can complement each other to explore the multifaceted scalar potential. This will be illustrated using two theoretical frameworks: xSM and 2HDM. Furthermore, using the complex incarnation of the 2HDM, we will discuss the impact of the tunneling profile in the calculation of the baryon asymmetry associated with electroweak baryogengesis.

Title: Cosmic birefringence and its implications

Abstract: Cosmic birefringence is a parity-violating phenomenon that rotates the plane of linear polarization of the CMB photons. Recently, a nonzero isotropic cosmic birefringence (ICB), its overall rotation angle from the last scattering surface to the present, with a statistical significance of 3.6 sigma has been reported for the latest joint analysis of Planck/WMAP data. In this talk, we will present some theoretical implications of ICB for new physics beyond the Standard Model, such as axions. Or, we will show why new physics is necessary to explain the measured ICB angle.

Title: Revisiting the fermion-field nontopological solitons

Abstract: Nontopological fermionic solitons exist across a diverse range of particle physics models and have rich cosmological implications. This study establishes a general frame- work for calculating fermionic soliton profiles under arbitrary scalar potentials, utilizing relativistic mean field theory to accurately depict the interaction between the fermion condensate and the background scalar field. Within this framework, the conventional “fermion bound states” are revealed as a subset of fermionic solitons. In addition, we demonstrate how the analytical formulae in previous studies are derived as special cases of our algorithm, discussing the validity of such approximations. Furthermore, we explore the phenomenology of fermionic solitons, highlighting new formation mechanisms and evolution paths, and reconsidering the possibility of collapse into primordial black holes.

Title: Applications of the Tunneling Potential Formalism

Abstract: The Tunneling Potential formalism offers an alternative to the Euclidean bounce approach for calculating tunneling actions, which govern the exponential suppression of metastable vacuum decay in quantum field theory. In this talk, after introducing the new formalism, I will discuss some general appealing properties of the approach and present two applications: to pseudo-bounce decays and to bubbles of nothing.

Title: Back to the fEAtureS: signatures of primordial physics in the cosmic microwave background

Abstract: Primordial features are scale-dependent departures from the leading order scale-invariant spectra of the primordial density perturbations produced by Inflation. Such unique traces contain a high degree of informativity and their detection can be used to identify non-standard inflationary trajectories. In this talk, I discuss recent advances in comparing feature models to cosmological data, focusing on Cosmic Microwave Background (CMB) anisotropies, and present primordial feature signals that provide interesting fits to anomalies in the data. While the models remain statistically indistinguishable from the standard slow-roll scenario, future measurements of the polarization of the CMB offer optimistic prospects for detecting such best fit candidates and shed light on their primordial origin.

Title: Axion inflation and abelian gauge fields: phenomenology and the transition to the strong backreaction regime

Abstract: I will talk about a model in which the rolling inflaton, through an axionic coupling to a U(1) gauge field, causes the amplification of the modes of the gauge field. I will start by reviewing the rich phenomenology of this setup. The main focus of the talk will be the analytical study of an instability of the system that emerges when the quanta of the gauge field strongly backreact on evolution of the inflaton. This study, which treats perturbatively the deviation of the inflaton velocity from its mean-field value and ignores the inhomogeneities in the inflation, confirms previous numerical results. I will also discuss how, in this regime, peculiar observable features emerge  in the spectrum of gravitational waves produced by the modes of the gauge field.

Title: Magnetic fields in Cosmic String wakes 

Abstract: Topological defects are a natural consequence of several symmetry breaking phase transitions in the early universe. In this talk I will concentrate on one of these defects, the cosmic string defect and give an overview of the generation of magnetic fields by these defects. I will then discuss the evolution of the magnetic fields in the wakes of cosmic strings. Though density fluctuations have predicted quite a few signatures of the string defects in the Cosmic Microwave background, the magnetic field generated in these wakes can open up a whole new field for detection of these exotic objects through eletromagnetic radiation.

Title: PTA Bounds on Graviton Mass

Abstract: We will introduce the graviton mass bounds from PTAs and further discuss an inevitable uncertainty due to cosmic variance.

Title: Primordial Inflation: Known Knowns, Known Unknowns, and Unknown Unknowns  

Abstract: Thanks to the rapid progress in observational cosmology, we have started to understand the very early stage of the Universe, namely primordial inflation. But our knowledge is still far from complete. In this talk, I will review what we know about inflation and what it explains. Then I will discuss some recent topics of inflation that may improve our understanding of the early Universe. Among other topics, I will discuss primordial black hole formation and its implications for gravitational wave cosmology.

Title: Gravitational waves from cosmic superstrings and gauge strings

Abstract: We perform a phenomenological comparison of the gravitational wave (GW) spectrum expected from cosmic gauge string networks and superstring networks comprised of multiple string types. We show how violations of scaling behavior and the evolution of the number of relativistic degrees of freedom in the early Universe affect the GW spectrum. We derive simple analytical expressions for the GW spectrum from superstrings and gauge strings that are valid for all frequencies relevant to pulsar timing arrays (PTAs) and laser interferometers. We analyze the latest data from PTAs, and study correlations between GW signals at PTAs and laser interferometers.

Title: Early dark energy beyond slow-roll: Implications for cosmic tensions

Abstract: In this work, we explore the possibility that Early Dark Energy (EDE) is dynamical in nature and study its effect on cosmological observables. We introduce a parameterization of the equation of state allowing for an equation of state $w$ differing considerably from cosmological constant (cc, $w={-1}$) and vary both the initial $w_i$ as well final $w_f$  equation of state of the EDE fluid. This idea is motivated by the fact that in many models of EDE, the scalar field may have some kinetic energy when it starts to behave like EDE before the CMB decoupling. We find that the present data have a mild preference for non-cc early dark energy $( w_i= -0.78)$ using Planck+BAO+Pantheon+S$H_0$ES data sets, leading to $\Delta \chi^2_{\rm min}$ improvement of -2.5 at the expense of one more parameter. However, $w_i$ is only weakly constrained, with $w_i < -0.56$ at $1\sigma$. We argue that allowing for $w_i\neq -1$ can play a role in decreasing the $\sigma_8$ parameter. Yet, in practice the decrease is only $\sim0.4\sigma$ and $\sigma_8$ is still larger than weak lensing measurements. We conclude that while promising, a dynamical EDE cannot resolve both $H_0$ and $\sigma_8$ tensions simultaneously.

Title: Thermal radiation exchange in primordial gravitational waves

Abstract: The radiation-dominated universe is a key component of standard Big Bang cosmology. Radiation comprises numerous quantum elementary particles, and its macroscopic behavior is described by taking the quantum thermal average of its constituents. The dynamics of gravitational waves are considered in this smooth fluid. While interactions between individual particles and gravitational waves are often neglected in this context, it raises the question of whether such a hydrodynamical approximation is reasonable. To address this question, we explored the quantum mechanical aspects of gravitational waves in a universe dominated by a massless scalar field, whose averaged energy-momentum tensor serves as background radiation. We computed thermal loop corrections for the gravitational wave power spectrum using the Schwinger-Keldysh formalism. Interestingly, we found that the loop effect enhances the super-horizon primordial gravitational wave spectrum, indicating that the inflationary spectrum is not conserved, contrary to conventional wisdom. These findings have significant implications for our understanding of the early universe. In this talk, I will begin with the basics of cosmology and explain the significance of these results and their relevant observational consequences.

Title: Precision searches for Heavy Neutral Leptons

Abstract: Heavy Neutral Leptons (HNLs) constitute a simple yet powerful extension of the Standard Model that can simultaneously explain neutrino oscillations, baryon asymmetry of the Universe, and dark matter. These requirements severely constrain the properties of the new particles. I will discuss the parameter space of viable HNL models and how to probe it in future collider experiments that can potentially observe many events, focusing on the recently approved SHiP experiment.

Title: Predictions of m_ee and neutrino mass from a consistent Froggatt-Nielsen model

Abstract: The seesaw mechanism is the most attractive mechanism to explain the small neutrino masses, which predicts the neutrinoless double beta decay (0νββ) of the nucleus. Thus the discovery of 0νββ is extremely important for future particle physics. However, the present data on the neutrino oscillation is not sufficient to predict the value of m_ee as well as the neutrino mass m^i_ν. In this talk, by adopting a simple and consistent Froggatt-Nielsen model, which can well explain the observed masses and mixing angles of quark and lepton sectors, we calculate the distribution of m_ee and m^i_ν. Interestingly, a relatively large part of the preferred parameter space can be detected in the near future.

Title: Solving Standard Model Puzzles with Line-Intensity Mapping

Abstract: It has now been over 50 and 25 years since the establishment of the Standard Models of particle physics and cosmology, respectively. Over this time, both have withstood a wide range of experimental tests. Still, several key questions about their ingredients remain, such as the model of inflation, the nature of dark matter, the form of dark energy and the masses of the neutrino species. We will show that line-intensity mapping is uniquely poised to probe all of these questions and also shed light on some of the tensions in current data.  Line-intensity mapping (LIM) is an emerging approach to survey the Universe, using relatively low-aperture instruments to scan large portions of the sky and collect the total spectral-line emission from galaxies and the intergalactic medium. Mapping the intensity fluctuations of an array of atomic and molecular emission lines offers a unique opportunity to probe redshifts well beyond the reach of other cosmological observations, access regimes that cannot be explored otherwise, and exploit the enormous potential of cross-correlations with other measurements. Over the next decade, LIM will transition from a pathfinder era of first detections to an early-science era where data from more than a dozen missions will be harvested to yield new insights and discoveries.I will review the primary target lines for these missions, describe the different approaches to modeling their intensities and fluctuations, motivate the opportunities for synergy with other observables and survey how each of the Standard Model puzzles above can be uniquely probed with LIM.  

Title: Axion dark matter from inflation-driven quantum phase transition

Abstract: We propose a new mechanism to produce axion dark matter from inflationary fluctuations. Quantum fluctuations during inflation are strengthened by a coupling of the axion kinetic term to the inflaton. This coupling acts as an order parameter, breaking the scale invariance of the axion power spectrum and driving a quantum phase transition. A red-tilted spectrum leads to an exponential enhancement of the axion abundance as the comoving horizon shrinks during inflation. This enhancement allows sufficient axion production to comprise the entire dark matter abundance, despite the ultralight mass. Our mechanism predicts a significantly different parameter space from the usual misalignment mechanism, allowing much larger couplings to Standard Model particles, which can be probed by future experiments including haloscopes, nuclear clocks, CASPEr, and CMB-S4. It is applicable to both QCD axion and axion-like particles.

Title: Dark Radiation Isocurvature from Cosmological Phase Transitions

Abstract: Cosmological first order phase transitions are typically associated with physics beyond the Standard Model, and thus of great theoretical and observational interest. Models of phase transitions where the energy is mostly converted to dark radiation can be constrained through limits on the dark radiation energy density (parameterized by $\Delta N_{\rm eff}$). However, the current constraint ($\Delta N_{\rm eff} < 0.3$) assumes the perturbations are adiabatic. We point out that a broad class of non-thermal first order phase transitions that start during inflation but do not complete until after reheating leave a distinct imprint in the scalar field from bubble nucleation. Dark radiation inherits the perturbation from the scalar field  when the phase transition completes, leading to large-scale isocurvature that would be observable in the CMB. We perform a detailed calculation of the isocurvature power spectrum and derive constraints on $\Delta N_{\rm eff}$ based on CMB+BAO data. For a reheating temperature of $T_{\rm rh}$ and a nucleation temperature $T_*$, the constraint is approximately $\Delta N_{\rm eff}\lesssim 10^{-5} (T_*/T_{\rm rh})^{-4}$, which can be much stronger than the adiabatic result. We also point out that since perturbations of dark radiation have a non-Gaussian origin, searches for non-Gaussianity in the CMB could place a stringent bound on $\Delta N_{\rm eff}$ as well.

Title: Dark Interactions in the Cosmic Microwave Background

Abstract: It is now established that about 95% of the energy density of the Universe is made up of Dark Matter (DM) and Dark Energy (DE). Given our limited understanding of their physical nature, an intriguing possibility is to test interactions involving the dark components of the model. In this talk, I will focus on two different categories of interactions: scatter-like interactions between DM and neutrinos, and interacting DE. I will show that both offer interesting perspectives from a theoretical and observational standpoint. I will argue that small-scale Cosmic Microwave Background (CMB) observations can open novel observational windows to accurately test interactions, possibly revealing unique signatures that would be challenging to detect on larger angular scales. Interestingly, several independent CMB observations seem to hint at a mild preference for an interacting dark sector that might help with cosmological tensions.

Title: Inflation models with the Higgs and baryogenesis

Abstract: Although the ΛCDM model is the most complete model of modern cosmology to date, it still lacks a compelling description of the baryonic matter origin and of the cosmological inflationary period. In this talk, I will present the implications of the inflaton coupling to the Chern-Simons density in various scenarios, focusing on the role played by the Higgs sector in the baryogenesis and preheating capabilities. 

Title: What can modular flavour symmetries do for you?

Abstract: In recent years, modular invariance has been applied to the SM flavour puzzle, yielding compelling results. In this string-inspired paradigm, one does not require a multitude of scalar fields (flavons) with aligned VEVs and complicated potentials. Taking a bottom-up approach, one may instead rely on a single complex field -- the modulus. Yukawa couplings and mass matrices are obtained from functions of its VEV, which can be the only source of flavour symmetry breaking and of CP violation. Such predictive modular setups may, among other things, shed light on the patterns of fermion mixing, the origin of fermion mass hierarchies and the strong CP problem.

Title: Ultraheavy atomic dark matter freeze-out through rearrangement

Abstract: Atomic dark matter is usually considered to be produced asymmetrically in the early Universe. In this work, we first propose that the symmetric atomic dark matter can be thermally produced through the freeze-out mechanism. The dominant atom antiatom annihilation channel is the atomic rearrangement. It has a geometrical cross section much larger than that of elementary fermions. After the atomic formation, this annihilation process further depletes dark matter particles and finally freezes out. To give the observed dark matter relic, the dark atoms are naturally ultraheavy, ranging from $10^6$ to $10^{10}$ GeV.

Title: Neutrino masses from new Weinberg-like operators

Abstract: The unique dimension-5 effective operator, LLHH, known as the Weinberg operator, generates tiny Majorana masses for neutrinos after electroweak spontaneous symmetry breaking. If there are new scalar multiplets that take vacuum expectation values (VEVs), they should not be far from the electroweak scale. Consequently, they may generate new dimension-5 Weinberg-like operators which in turn also contribute to Majorana neutrino masses. In this talk, I will present scenarios with one or two new scalars up to quintuplet SU(2) representations and their possible UV completions. I'll also discuss general limits on the new scalar multiplets from direct searches at colliders, loop corrections to electroweak precision tests and the W-boson mass.

Title: Sub-GeV dark matter search at ILC beam dumps

Abstract: Light dark matter particles may be produced in electron and positron beam dumps of the International Linear Collider (ILC). We propose an experimental setup to search for such events, the Beam-Dump eXperiment at the ILC (ILC-BDX). The setup consists of a muon shield placed behind the beam dump, followed by a multi- layer tracker and an electromagnetic calorimeter. The calorimeter can detect electron recoils due to elastic scattering of dark matter particles produced in the dump, while the tracker is sensitive to decays of excited dark-sector states into the dark matter particle. We study the production, decay and scattering of sub-GeV dark matter particles in this setup in several models with a dark photon mediator. Taking into account beam-related backgrounds due to neutrinos produced in the beam dump as well as the cosmic-ray background, we evaluate the sensitivity reach of the ILC-BDX experiment. We find that the ILC-BDX will be able to probe interesting regions of the model parameter space and, in many cases, reach well below the relic target.

Title: Primordial black holes and curvature perturbations from general first-order phase transitions

Abstract: Recently, much attention has been focused on the false vacuum islands that are flooded by an expanding ocean of true-vacuum bubbles slightly later than most of the other parts of the world. These delayed decay regions will accumulate locally larger vacuum energy density by staying in the false vacuum longer than those already transited into the true vacuum. A false vacuum island with thus acquired density contrast of a super-horizon size will evolve locally from radiation dominance to vacuum dominance, creating a local baby Universe that can be regarded effectively as a local closed Universe. If such density contrasts of super-horizon sizes can ever grow large enough to exceed the threshold of gravitational collapse, primordial black holes will form similar to those collapsing curvature perturbations on super-horizon scales induced by small-scale enhancements during inflation. If not, such density contrasts can still induce curvature perturbations potentially observable today. In this talk, we will revisit and elaborate on the generations of primordial black holes and curvature perturbations from delayed-decayed false vacuum islands during asynchronous first-order phase transitions with fitting formulas convenient for future model-independent studies.

Title: Primordial non-Gaussianity in CMB distortions and PBH constraints

Abstract: Distortions in the blackbody spectrum of Cosmic Microwave Background (CMB) could have been induced by multiple astrophysical and cosmological processes and their detection or non-detection gives a better understanding of these processes. One such cosmological distortion sourcing mechanism is the acoustic dissipation of inflationary fluctuations. Owing to the possibility of large enhancement in the primordial power spectrum at small scales, the plausibility of Gaussian initial conditions is highly questionable. In this talk, I will explain how the spectral distortions vary when one incorporates the effects of primordial non-Gaussianity. Furthermore, I will discuss how this picture has direct implications to the constraints on Supermassive Black Holes if they were to be of primordial origin.

Title: Finite Statistics Constraints on Late Cosmological Phase Transitions

Abstract: I will discuss the super-horizon (iso)curvature perturbations arising from statistical fluctuations during a first-order phase transition (FOPT). Finite statistics in various Hubble patches during an FOPT result in perturbations that follow a generic power-law scaling in FOPT parameters. Even if the FOPT occurs in a secluded dark sector, the resulting curvature perturbation is constrained by cosmological data. Utilizing Cosmic Microwave Background and Large Scale Structure measurements, I present the primary constraint on energy released in a dark-sector FOPT, expressed as a ratio of dark to Standard Model radiation. For cosmological FOPTs between photon temperatures of 0.1 eV to 1 keV, this constraint already surpasses the ∆Neff bound typically cited for PT in a dark sector. Future constraints on the primordial curvature perturbation may further extend the bound to the 100 MeV scale.

Title: Probing Dark Matter with Gravitational-Wave Interferometers in Space

Abstract: The discovery of gravitational waves has opened a new window for exploring the universe. The talk will discuss the use of gravitational wave observations for searching and detecting dark matter. It will focus on two types of dark matter candidates, WIMPs and ultra-light dark matter. The former can form dark matter spikes around black holes, affecting the motion of compact astrophysical objects through dynamical friction effects, thereby altering the gravitational waveforms emitted by the system, potentially observable by space-based gravitational wave experiments. Ultra-light dark matter can directly interact with detectors, influencing detection signals, and can also be probed through gravitational wave experiments.

Title: Kramers-Kronig relation in gravitational lensing

Abstract: The Kramers-Kronig relation is a well-known relation, especially in the field of optics. The key to this relation is the causality that output comes only after input. We first show that gravitational lensing obeys the causality in the sense that (electromagnetic/gravitational) waves emitted from the source arrive at an observer only after the arrival of the signal in geometrical optics. We then explain the Kramers-Kronig relation in gravitational lensing, as the relation between real and imaginary parts of the amplification factor, which is the amplitude ratio of the lensed wave to the unlensed wave. Finally, we argue that an incorrect separation of the observed gravitational waveform into the amplification factor and the unlensed waveform generically leads to the violation of the Kramers-Kronig relation. This suggests that examining the violation of the Kramers-Kronig relation may be used for correctly extracting the lensing signal in the gravitational wave observations.

Title: Emergent Metric Space-Time and Early Universe Cosmology from the BFSS Matrix Model

Abstract: The BFSS matrix model is a proposed non-perturbative definition of superstring theory. Considering this matrix model in a high temperature state, I will discuss a proposal for how a metric space-time can emerge. The emergent space-time has exactly three large spatial dimensions, and the thermal fluctuations in the emergent background give rise to scale-invariant spectra of curvature fluctuations and gravitational waves, as what happens in ``String Gas Cosmology".

Title: Effect of self-interacting dark matter on supermassive black hole mergers and pulsar timing array signals

Abstract: Mergers of supermassive black holes are a favored process to explain stochastic gravitational waves seen by pulsar timing arrays, but they suffer from a long-standing "final parsec problem," which causes them to stall before reaching the stage of GW emission.  We show that dynamical friction with dark matter spikes can solve this problem, and produce softening of the GW spectrum at low frequencies, if the DM is self-interacting, with cross sections previously identified as ameliorating small scale structure problems of CDM.

Title: Probing cosmological transition redshift through gravitational wave memory

Abstract: Despite numerous attempts over the last two decades to explain the accelerated expansion of our universe, its cause still remains elusive. However, with the advent of gravitational wave (GW) astronomy, there exists promising avenues for exploring unresolved cosmological mysteries. In this presentation we will demonstrate how one such feature of GWs known as GW memory offers a yardstick for understanding the acceleration of the universe. Initially, we will provide a brief overview of the formulation of a master equation describing GW memory within a broad class of spacetimes. Subsequently, we will study its application in FLRW spacetimes and show how GW memory shows observable signatures between accelerated and decelerated universes, potentially enabling the identification of the transition redshift from a matter-dominated to a dark-energy-dominated universe. 

Title: Cosmological Correlators Through the Looking Glass: Reality, Parity, and Factorisation

Abstract: While parity violation has become a familiar notion at microscopic scales of particle physics, whether the universe is parity symmetric at cosmic distances remains unclear. Recent discoveries seem to suggest this possibility may indeed exist. In the scalar sector, such information is encoded in the primordial curvature trispectrum, which contains a parity-odd sector that is purely imaginary. In this talk, I will focus on discovering certain universal analytic structures of tree-level inflationary wavefunctions and parity-odd correlators, and show how these universal structures lead to simplified computations for the privileged parity-odd correlators. With this simplification, I will then present exact solutions for the parity-odd curvature trispectrum from the exchange of massive fields of any mass, spin, sound speed and chemical potential.

Title: Primordial black holes from slow phase transitions: a model-building perspective

Abstract: We show the relation between the Higgs potential structure and primordial black hole (PBH) formations. Recently, it has been often discussed that PBHs can be formed by first-order phase transition at the early Universe. In this talk, we consider the PBH formation mechanism throught delayed first-order phase transitions at the early Universe. If the phase transition is delayed, the large energy density fluctuation can be realized between symmetry broken and unbroken regions. If the density fluctuation can be larger thant a certain criterion, the overdensity region may collapse to PBHs. We discuss the form of the Higgs potential needed to realize this PBH formation. In addition, we show that the commonly used exponential approximation of the bubble nucleation rate fails to capture such PBH formation.

Title: Probing new physics effects via observations of gravitational waves from first-order phase transition

Abstract: There are some phenomena that the standard model (SM) cannot explain, such as the baryon asymmetry of the universe. Thus the new physics effects beyond the SM are necessary to explain them. The new effects could potentially act as the cause of first-order phase transition in the early universe. The first-order phase transition generates gravitational waves (GWs). Since the GWs travel through the universe virtually unimpeded, it gives us information about the potential of the early universe. In this talk, we discuss the possibility of probing new physics effects via observations of GWs from first-order phase transition.

Title: Lessons for axion dark matter from nonstandard cosmologies

Abstract: Most studies of the early Universe assume a simple standard thermal scenario of radiation domination between the end of cosmic inflation and the start of Big Bang Nucleosynthesis. However, many well-motivated deviations from the standard scenario exist. In such nonstandard cosmologies many processes, including dark matter production, can be significantly affected. I will present a simple model of early matter domination and discuss how the properties of a dark matter axion change relative to the standard thermal model. This will be followed with some more involved, and more realistic, scenarios. Ensuing implications for discovery prospects will also be presented.

Title: Darkest Before The Dawn: Exploring the Very Early Universe

Abstract: The visible universe is 60 orders of magnitude larger than the Planck length, offering a rough lower bound on its overall expansion since the Big Bang. In typical theories, inflation accounts for 30 of these 60 “decades" and during the next 15 decades typical interaction energies are above the TeV scale.  In contrast to the final 15 decades of growth, which take roughly 13.8 billion years, the middle phase lasts just a trillionth of a second and occurs at energy scales which are experimentally untested. Despite its brevity, this epoch can host a complex range of nonlinear phenomena including gravitationally-driven “structure formation” from the collapse of self-gravitating quantum matter and gravitational wave production and I will discuss possible fingerprints from this era that might be visible today.

Title: Dark matter minihalos from primordial magnetic fields

Abstract: Primordial magnetic fields (PMFs) offer a simple explanation for the origin of galactic magnetic fields as well as of the purportedly detected magnetic fields in cosmic voids. A unique signature of the presence of PMFs is how they enhance baryon perturbations on small scales. However, on scales below the baryon thermal jeans scale, PMFs cannot enhance baryon density perturbations. In this talk, I show that on such small scales, PMFs instead enhance dark matter power perturbation purely by gravitational influence. I conclude by arguing how the search for 10^−11 to 10^3 solar mass dark matter minihalos can potentially provide the most sensitive probe for PMFs. 

Title: Neutrinos as an indirect new physics probe: oscillations, scattering, and thermal decoupling in the context of the SMEFT

Abstract: Neutrinos have been playing a significant role in our understanding of nature. Examples include the debate on momentum and energy conservation originated from nuclear beta decay in the 1930s, and the discovery of the weak neutral current following the pioneering Gargamelle experiment at CERN in the 1970s. While the modern particle physics can be well described by the standard model (SM), with a recent milestone in the discovery of the Higgs particle in 2012 at the LHC, other well-established phenomena such as neutrino oscillations, dark matter and the matter-antimatter asymmetry, require an extension of the SM. Tremendous efforts have been put in this direction in the past decades. In this talk, I will utilize neutrinos as an indirect new physics probe in the SM Effective Field Theory (SMEFT) framework, with a special focus on neutrino oscillations, neutrino scattering experiments like CEvNS, and neutrino decoupling in the early Universe.

Title: Detecting Rare Species of Dark Matter with Terrestrial Detectors

Abstract: A sub-component of dark matter with a short collision length compared to a planetary size leads to efficient accumulation of dark matter in astrophysical bodies. Such particles represent an interesting physics target since they can evade existing bounds from direct detection due to their rapid thermalization in high-density environments. In this talk, I will demonstrate that terrestrial probes, such as, large-volume neutrino telescopes as well as commercial/research nuclear reactors, can provide novel ways to constrain or discover such particles.

Title: Regularization and renormalization on cosmological backgrounds, primordial gravitational waves, and N_{eff} bounds revisited

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.

Title: Second leptogenesis: a source of large lepton asymmetry

Abstract: Right-handed neutrinos play an important role in explaining various BSM phenomena, for example, tiny neutrino masses and baryon asymmetry of the Universe. Recently, a scenario was proposed where they have time-(and temperature-)dependent masses due to their coupling with wave dark matter, a coherently oscillating scalar field, and their effect has been studied variously. In this talk, we discuss the effect of the temperature-dependent mass in the early Universe, in particular, that in the lepton number production via their decay (leptogenesis). We suggested that such masses can cause the decoupling of the heavy neutrinos twice; in other words, it leads to two-fold leptogenesis. We consider a scenario where the lepton number produced at the first leptogenesis is partly converted to the baryon number via the sphaleron process as in the normal leptogenesis scenario, while the second leptogenesis occurs after the decoupling of the sphaleron process, and the produced lepton number remains until the current Universe. This extra production of the lepton number may cause fairly large lepton asymmetry compared with baryon asymmetry, which is suggested by the latest 4-Helium observation. We will show that the second leptogenesis significantly amplifies the lepton asymmetry and potentially explains this discrepancy. We will also discuss distinctive phenomenological predictions of the scenario. This talk is based on JHEP 03 (2024) 003.

Title:  Gravitational waves from quasi-stable cosmic strings and PTA data

Abstract: We discuss the stochastic gravitational wave background emitted from a network of 'quasi-stable' strings (QSS) and its realization in grand unified theories. A symmetry breaking in the early universe produces monopoles that suffer partial inflation. A subsequent symmetry breaking at a lower energy scale creates cosmic strings that are effectively stable against the breaking via Schwinger monopole-pair creation. As the monopoles reenter the horizon, we will have monopole-antimonopoles connected by strings, and further loop formation essentially ceases. Consequently, the lower frequency part of the gravitational wave spectrum will be suppressed compared to that of topologically stable cosmic strings. The gravitational radiation emitted in the early universe by QSS with a dimensionless string tension $G\mu\sim 10^{-6}$, is compatible with the exciting evidence of low-frequency gravitational background in PTA data, as well as the recent LIGO-VIRGO constraints, provided the superheavy strings and monopoles experience a certain amount of inflation.

Title: Generic Predictions for Primordial Perturbations and their implications

Abstract: We introduce a novel framework for studying small-scale primordial perturbations and

their cosmological implications. The framework uses a deep reinforcement learning to

generate scalar power spectrum profiles that are consistent with current observational

constraints. The framework is shown to predict the abundance of primordial black

holes and the production of secondary induced gravitational waves. We demonstrate

that the set up under consideration is capable of generating predictions that are

beyond the traditional model-based approaches.

Title: Analytical Results on Dark Matter Velocity Distributions

Abstract: We review recent work on determining dark matter velocity distributions from analytical methods  of classical mechanics, and the potential impact of these results on dark matter indirect detection  strategies.  In particular, we discuss velocity-dependent dark matter annihilation in subhalos and in the Galactic Center.  We compare the results of these analytic methods to those obtained in large N  numerical simulations.

Title:  Parity Solution to the Strong CP Problem and its Experimental Tests

Abstract:  In theories where Parity (P) is a spontaneously broken symmetry, the strong CP problem can be solved by P symmetry alone, without the need for an axion.  In this talk I shall elaborate on this proposal and present left-right symmetric theories which offer such a solution to the strong CP problem.  These theories have a natural embedding in SU(5) x SU(5) grand unification, which will be discussed. Phenomenological implications of this class of models will be discussed, focusing on neutrino oscillations and certain flavor anomalies.  Neutrinos are predicted to be naturally light Dirac fermions in these models.

Title: TBA

Abstract: TBA

Title: Neutrinos masses, ultra-light dark matter and cosmology

Abstract:  Can the neutrino mass be a function of time? In this talk, I 

will present a consistent scenario of time-varying neutrino masses, and 

discuss its impact on cosmology, beta decay, and neutrino oscillation 

experiments. Such time-varying masses are assumed to be generated by the 

coupling between a neutrino and an ultralight scalar dark matter 

candidate. I will discuss how such a model fares in the light of various 

cosmological bounds, such as those coming from Big Bang Nucleosynthesis, 

the cosmic microwave background, as well as large-scale structures. This 

scenario can be further constrained using neutrino oscillation 

experiments, beta-decay experiments like KATRIN, as well as reactor and 

gallium experiments. Finally, I will also touch upon some recent ideas 

of whether the neutrino mass can be entirely generated by neutrino-dark matter interactions.

Title: Signatures of Primordial Black Holes: Correlation of various measurements

Abstract: Primordial black holes (PBH) can explain the observed dark matter abundance while being consistent with various constraints. In this talk, I will explore the correlation of various types of signals that can arise from the PBH and its formation history. For example, PBHs which give rise to observable gamma-ray signals from Hawking radiation at AMEGO, e-ASTROGAM can have a companion, stochastic gravitational wave (GW) background. Additionally, the PBH superradiance emerging from axion-like particles can produce complementary signals. In this talk, I will investigate the PBH as a dark matter candidate utilizing various complementary searches.

Title: Flavor Matters, but Matter Flavors: Matter Effects on Flavor Composition of Astrophysical Neutrinos

Abstract: We show that high-energy astrophysical neutrinos produced in the  

cores of heavily obscured active

galactic nuclei (AGNs) can undergo strong matter effects, thus  

significantly influencing their source

flavor ratios. In particular, matter effects can completely modify the  

standard interpretation of the

flavor ratio measurements in terms of the physical processes occurring  

in the sources (e.g., pp versus

pγ, full pion-decay chain versus muon-damped pion decay). We contrast  

our results with the existing

flavor ratio measurements at IceCube, as well as with projections for  

next-generation neutrino

telescopes like IceCube-Gen2. Signatures of these matter effects in  

neutrino flavor composition

would not only bring more evidence for neutrino production in central  

AGN regions, but would

also be a powerful probe of heavily Compton-thick AGNs, which escape  

conventional observation in X-rays and other electromagnetic wavelengths

Title: Dark Matter and Matter-Antimatter Asymmetry from Long-Lived Particles 

Abstract: In this, I will first discuss how sub-TeV long-lived particles (LLPs) in the visible sector may drive an epoch of early matter domination (EMD). I will then present a minimal extension of the standard model that includes a weak-scale LLP and a light dark matter (DM) candidate with O(GeV) mass. The LLP decay at the end of an EMD period yields the correct DM abundance and generates the observed baryon asymmetry of the universe. The parameter space of this model can be probed by proposed LLP searches as well as next-generation neutron-antineutron oscillation experiments.

Title: Back-reaction in the early universe

Abstract: Models of inflation might lead to enhanced scalar fluctuations on scales much smaller than those seeding the large-scale structure formation. In these scenarios, it is possible that the spike of power at high wavenumber might induce large corrections to the scalar power spectrum, e.g. in the form of loop corrections, potentially endangering the perturbativity of the underlying models. In this talk we discuss recent developments in the calculation of the 1-loop correction.

Title: Nordic Walking: A Dynamical Route to Hierarchies, and its Application to 3D Wess-Zumino-Witten theory

Abstract: A hierarchy in QFT refers to the existence of two scales with a large separation between them. If the coupling is dimensionful, the running between these scales is power-law and leads to a fine-tuning problem; if it is dimensionless, the running is logarithmic and only needs mild tuning. The fine-tuning problem for dimensionful couplings can be alleviated entirely if the coupling's beta function has a pair of complex conjugate fixed points close to the real line; this is called Walking, which I shall review briefly. I shall then discuss in detail our main result: a dimensionful coupling can be rendered (nearly) marginal dynamically, softening the fine-tuning problem to a mild one. It is an intermediate between walking and running, and christened hence 'Nordic Walking'. As an example for its emergence, I shall present our efforts to study 3D WZW theory on 4-sphere target manifold, which is an effective theory for deconfined quantum criticality, a proposed mechanism for certain (putative) order-to-order transitions beyond the Ginzburg-Landau paradigm in statistical/condensed matter physics. Time permitting, I shall also highlight some details of the calculational procedure, which entails a very instructive (and rather entertaining) application of higher-order regulators in functional RG. This may be useful beyond the present context in constructing non-perturbative gauge/diffeomorphism invariant RG flows.

(Based on 2312.11614.)