Yukihiro Kanda, ICRR
Particle emission from semilocal string networks
Hybrid On-site: Kenkyu Honkan meeting room 321 322 Online: Zoom
Topological defects can arise as remnants of symmetry-breaking phase transitions in the early universe and are usually classified according to the homotopy groups of the vacuum manifold. However, field theories can admit stable localized configurations whose stability is ensured not only by topology but also by energetic considerations. Semilocal strings provide such an example: they are string-like defects that arise even when the fundamental group of the vacuum manifold is trivial. We present results from numerical lattice simulations of semilocal string networks, focusing for the first time on their particle-emission properties. We find that semilocal string networks efficiently emit Nambu-Goldstone bosons, with emission properties similar to axion radiation from global string networks. Motivated by this result, we discuss a scenario in which the Nambu-Goldstone bosons acquire a soft-breaking mass, and their non-thermal production from the string network accounts for the observed dark matter abundance.
Keisuke Harigaya, Univ. of Chicago
Kinetic isocurvature perturbations
hybrid On-site: Kenkyu Honkan meeting room 321 322 Online: Zoom
We formulate a new class of primordial perturbations called kinetic isocurvature perturbations, where the mass density of dark matter is constant relative to the photon number density while the kinetic energy of dark matter fluctuates in space. Such perturbations naturally arise in scenarios where a nonrelativistic heavy field decays into relativistic dark matter particles with a spatially modulated rate. As dark matter cools and becomes nonrelativistic, these fluctuations in kinetic energy leave large-scale density perturbations essentially unaffected and therefore evade the Cosmic Microwave Background bounds on isocurvature perturbations, yet survive as spatial variations in the free-streaming scale, resulting in patch-by-patch variation of the matter power spectrum.
Hiroki Matsui, Osaka Metropolitan University
Perturbative and Non-Perturbative Aspects of Lorentzian Quantum Cosmology
Hybrid On-site: Kenkyu Honkan meeting room 321 322 Online: Zoom One of the fundamental issues in quantum gravity is how to describe the initial state of the Universe. In quantum cosmology, the quantum state of the entire Universe is formulated as the “wave function of the Universe,” and its origin has been investigated using the path integral approach. However, conventional formulations based on the Euclidean path integral suffer from intrinsic problems.
One of the fundamental issues in quantum gravity is how to describe the initial state of the Universe. In quantum cosmology, the quantum state of the entire Universe is formulated as the “wave function of the Universe,” and its origin has been investigated using the path integral approach. However, conventional formulations based on the Euclidean path integral suffer from intrinsic problems.
In this talk, I will review recent developments in quantum cosmology based on the Lorentzian path integral and introduce our work applying Picard-Lefschetz theory and resurgence theory to this framework. First, I will explain how introducing an infinitesimal deformation in the parameter space fixes the Lefschetz thimble structure, naturally selecting the tunneling wave function. Second, I will demonstrate that the Borel resummation ambiguity of the perturbative series around the saddle is exactly canceled by the non-perturbative contribution from the other saddle. These results illustrate how non-perturbative aspects of quantum gravity offer new perspectives on the origin of the Universe. If time permits, I will also discuss the perturbation issues in quantum cosmology.
Jianing Wang, Kavli IPMU
PBH Formation and Its Origin in Inflation
Hybrid On-site: Kenkyu Honkan meeting room 321 322 Online: Zoom Primordial black holes (PBHs) are believed to form through the gravitational collapse of overdense regions in the early Universe. They may serve as seeds for galaxy formation and are also considered one of the important candidates for cold dark matter (DM).
Primordial black holes (PBHs) are believed to form through the gravitational collapse of overdense regions in the early Universe. They may serve as seeds for galaxy formation and are also considered one of the important candidates for cold dark matter (DM).
In particular, I will focus on several representative toy models of single-field inflation. The enhanced primordial perturbations in these models can not only produce PBHs, but also generate gravitational waves through higher-order effects. I will further extend the discussion to the possibility of a PBH-dominated era, which could leave observable signatures if PBH evaporation produces stable relics.
These studies demonstrate the significant potential of PBHs as probes of the early Universe, naturally leading to the important question of how to accurately estimate the PBH abundance. In the latter part of the talk, I will introduce a method based on peaks theory for estimating the abundance of primordial black holes. Our approach works well for arbitrary forms of the power spectrum, and by incorporating more systematic statistical methods, we expect it to provide useful cross-checks in combination with future gravitational-wave observations and related cosmological probes.
Fukutaro Miya, Osaka University
Generalizations and UV completions of Cho–Maison monopole
Hybrid On-site: Kenkyu Honkan meeting room 321 322 Online: Zoom Magnetic monopoles are important topological solitons predicted in gauge theories with spontaneous symmetry breaking. In the electroweak theory, Cho and Maison constructed a monopole configuration by allowing a singular behavior at the origin. Since its essential structure is tied to electroweak-type symmetry breaking, analogous monopoles are expected to arise in a wider class of gauge theories.
Magnetic monopoles are important topological solitons predicted in gauge theories with spontaneous symmetry breaking. In the electroweak theory, Cho and Maison constructed a monopole configuration by allowing a singular behavior at the origin. Since its essential structure is tied to electroweak-type symmetry breaking, analogous monopoles are expected to arise in a wider class of gauge theories.
In this talk, I will show that Cho-Maison-like monopole configurations can indeed be constructed in broad classes of models. I will also discuss how the electroweak Cho-Maison monopole can be embedded into a regular ‘t Hooft-Polyakov monopole as its low-energy effective description. In particular, I will show that a monopole in the Pati-Salam model reduces to the electroweak Cho-Maison monopole after heavy degrees of freedom are integrated out. This provides a possible ultraviolet origin of the Cho-Maison monopole and clarifies its generality beyond the Standard Model.
Sunao Sugiyama, Kavli IPMU
[IPNS Joint Experimental-Theoretical Cosmology Seminar] Probing Primordial Black Hole Dark Matter with Subaru HSC Microlensing Observations of M31
Hybrid On-site: Bldg 4 Seminar Hall Online: Zoom Primordial black holes (PBHs) are a well-motivated dark-matter candidate that may have formed in the early Universe from the collapse of primordial density fluctuations. If PBHs constitute even a fraction of dark matter, they can be probed through gravitational microlensing. To test this possibility, we have been conducting microlensing observations of stars in the Andromeda galaxy (M31) with the Subaru Hyper Suprime-Cam (HSC).
Primordial black holes (PBHs) are a well-motivated dark-matter candidate that may have formed in the early Universe from the collapse of primordial density fluctuations. If PBHs constitute even a fraction of dark matter, they can be probed through gravitational microlensing. To test this possibility, we have been conducting microlensing observations of stars in the Andromeda galaxy (M31) with the Subaru Hyper Suprime-Cam (HSC).
In this talk, I will present the latest results from our Subaru HSC analysis, including both a reanalysis of previous observations and the analysis of newly acquired data, based on a new event-search pipeline designed to improve the sensitivity to PBH microlensing signals. The pipeline adopts a more flexible microlensing light-curve model, including finite-source effects, enabling improved fits to candidate events. I will also discuss how these candidates should be interpreted in the context of recent independent reanalyses of the Subaru HSC data and related microlensing results from other surveys.
Matthew Baldwin, Univ. of Chicago
Upper bounds on the Parity breaking scale in WIMP dark matter models
Hybrid On-site: Kenkyu Honkan meeting room 321 322 Online: Zoom
Parity symmetric left-right models provide a viable solution to the strong CP problem. Spontaneous breaking of the Parity symmetric model results in heavy gauge bosons that are searched for at colliders, placing a lower bound on the Parity breaking scale close to 14 TeV. However, without an upper bound, the Parity breaking scale may be as large as the Planck scale. In this talk, I will discuss the minimal Parity symmetric left-right model and show that by embedding a WIMP dark matter candidate one obtains an upper bound on the Parity breaking scale that is far below the Planck scale. I will focus on three well-motivated embeddings of WIMP dark matter and demonstrate how an upper bound on the Parity breaking scale is obtained by requiring that the WIMP obtains the correct relic abundance. I will then discuss current constraints on the free parameters of the model— the Parity breaking scale and WIMP mass– and motivate future indirect detection experiments such as C.T.A. that can probe the entire parameter space for certain galactic center dark matter density profiles.
Francescopaolo Lopez, SISSA
A Quantum Tale of Primordial perturbations: From Inflation to Reheating
Online (Zoom) Primordial perturbations are born as quantum fluctuations during inflation and are believed to be the seeds of the (classical) cosmic structure observed today, such as the anisotropies in the cosmic microwave background (CMB). In order to shed light on the quantum-to-classical transition of primordial perturbations during single field inflation, we investigate the decoherence of superhorizon scalar curvature perturbations. These are considered as an open quantum system interacting with a time-dependent environment of deep subhorizon tensorial modes through the trilinear interactions predicted by General Relativity. We show that derivativeless interactions provide the dominant contribution to decoherence, while derivative interactions introduce significant non-Markovian effects that tend to slow it down. We introduce a modification to the quantum master equation to take into account the time dependence of the subhorizon environment induced by a time dependent environment of deep subhorizon tensorial modes. Finally, we compute the associated quantum corrections, due to the trilinear interactions, to cosmological correlators. We find a non-perturbative resummation of the quantum corrections to the power spectrum, and we extend this result to the bispectrum, where an analogous resummation structure emerges.
Primordial perturbations are born as quantum fluctuations during inflation and are believed to be the seeds of the (classical) cosmic structure observed today, such as the anisotropies in the cosmic microwave background (CMB). In order to shed light on the quantum-to-classical transition of primordial perturbations during single field inflation, we investigate the decoherence of superhorizon scalar curvature perturbations. These are considered as an open quantum system interacting with a time-dependent environment of deep subhorizon tensorial modes through the trilinear interactions predicted by General Relativity. We show that derivativeless interactions provide the dominant contribution to decoherence, while derivative interactions introduce significant non-Markovian effects that tend to slow it down. We introduce a modification to the quantum master equation to take into account the time dependence of the subhorizon environment induced by a time dependent environment of deep subhorizon tensorial modes. Finally, we compute the associated quantum corrections, due to the trilinear interactions, to cosmological correlators. We find a non-perturbative resummation of the quantum corrections to the power spectrum, and we extend this result to the bispectrum, where an analogous resummation structure emerges.
We then follow the evolution of primordial perturbations beyond inflation into the reheating era. Very little is known about this effectively matter dominated phase: it is only constrained to end before Big Bang Nucleosynthesis, at a temperature higher than O(1) MeV. We show that reheating can be long enough that structures, such as halos, can form, inflaton stars can condense inside the halos and eventually grow to collapse into PBH. By employing PBH constraints we are able to improve constraints on reheating temperature, depending on the scale of inflation, strengthening existing bounds by several orders of magnitude.
Ayan Chakraborty, IIT Guwahati
From Inflation to the Hot Big Bang: Nonperturbative Aspects of Reheating and Its Signatures on Cosmic Relics
Online (Zoom) The early universe is regarded as an ideal laboratory for the generation of all kinds of elementary particles (Standard Model (SM) and beyond the Standard Model (BSM)), populating the present universe. An intermediate phase, which bridges the gap in energy and time scales between the end of inflation and the beginning of the hot Big Bang, is the post-inflationary reheating phase. The absence of direct observational evidence has left this important phase of the early universe poorly constrained, both at present and in the foreseeable future. However, the distinct imprints of this phase on cosmic relics offer us a promising avenue for its indirect probe through various cosmological observables.
The early universe is regarded as an ideal laboratory for the generation of all kinds of elementary particles (Standard Model (SM) and beyond the Standard Model (BSM)), populating the present universe. An intermediate phase, which bridges the gap in energy and time scales between the end of inflation and the beginning of the hot Big Bang, is the post-inflationary reheating phase. The absence of direct observational evidence has left this important phase of the early universe poorly constrained, both at present and in the foreseeable future. However, the distinct imprints of this phase on cosmic relics offer us a promising avenue for its indirect probe through various cosmological observables.
The early inflation and post-inflationary reheating phases are an important playground for investigating various non-perturbative, non-equilibrium phenomena. Parametric resonance and tachyonic instability are some distinctive features of the non-perturbative phenomena in the early era. For instance, in the early reheating era (Preheating), plenty of elementary particles were produced within a very short period through the mechanism of parametric resonance instability, when the interaction strength between the inflaton and the daughter particles is appropriate. Another notable aspect where non-perturbative effects become instrumental is the Cosmological Gravitational Particle Production (CGPP). This CGPP is the quantum mechanical particle production in a time-dependent dynamical background. Unlike Preheating, in CGPP, the background dynamics during reheating itself causes particle production without any direct coupling to the inflaton. These particles can play an important role in cosmic history, being possible candidates for dark matter, gravitational wave radiation, dark radiation, etc. This talk sheds light on various non-perturbative signatures of gravitational particle production, both in minimalistic (no coupling to gravity) and non-minimalistic (non-zero coupling to gravity) scenarios.
We have made unprecedented progress in observational cosmology after the detection of gravitational waves. In this talk, one of the prime objectives is to decipher various non-perturbative signatures of different early universe phenomena through their discernible imprint on gravitational waves. These ideas present the main theme of my presentation.
Junseok Lee, Tohoku University
A Number-Theoretic Structure of Chiral Minicharged Sectors
Hybrid On-site: Seminar room 321, 322 Online: Zoom
