<\/p>\n\n\n
The Cosmology seminars are weekly seminars dedicated to Cosmology and Astroparticle Physics. Please write to the contact below to join the mailing list to be updated on upcoming seminars.<\/p>\n
We have both remote and on-site seminars going forward.<\/p>\n
Time:<\/strong> Wednesdays 14:15-15:15 Helsinki time, unless otherwise noted.<\/p>\n Remote:<\/strong> Zoom invitations will be sent out on the Cosmology seminars mailing list.<\/p>\n On-site:\u00a0 <\/strong>Physicum A315 (will be streamed in Zoom)<\/p>\n Format:<\/strong> 45′ + 15′<\/p>\n Contact:<\/strong> Tushar Gupta (tushar.gupta@helsinki.fi)<\/p>\n<\/div>\n <\/p>\n","protected":false},"excerpt":{"rendered":" The Cosmology seminars are weekly seminars dedicated to Cosmology and Astroparticle Physics. Please write to the contact below to join the mailing list to be updated on upcoming seminars. We have both remote and on-site seminars going forward. Time: Wednesdays 14:15-15:15 Helsinki time, unless otherwise noted. Remote: Zoom invitations will be sent out on the […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-4","page","type-page","status-publish","hentry","post"],"_links":{"self":[{"href":"https:\/\/www.hip.fi\/cosmoseminars\/wp-json\/wp\/v2\/pages\/4","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.hip.fi\/cosmoseminars\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.hip.fi\/cosmoseminars\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.hip.fi\/cosmoseminars\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.hip.fi\/cosmoseminars\/wp-json\/wp\/v2\/comments?post=4"}],"version-history":[{"count":287,"href":"https:\/\/www.hip.fi\/cosmoseminars\/wp-json\/wp\/v2\/pages\/4\/revisions"}],"predecessor-version":[{"id":985,"href":"https:\/\/www.hip.fi\/cosmoseminars\/wp-json\/wp\/v2\/pages\/4\/revisions\/985"}],"wp:attachment":[{"href":"https:\/\/www.hip.fi\/cosmoseminars\/wp-json\/wp\/v2\/media?parent=4"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Scheduled Seminars<\/h2>\n
\n
Title: TBA<\/em>
Abstract: TBA<\/li>\n
Title: Systematic approaches to new physics in cosmology<\/span><\/em>
Abstract: The standard model of cosmology, which combines the standard model of particle physics with general relativity and the added ingredients of dark matter and dark energy, is remarkably successful at explaining our observations of the cosmos. Despite this success, the nature of dark matter and dark energy remains a mystery. Meanwhile, subtle and shifting tensions are often used as a pretext for developing new physics models. We outline an approach for the systematic exploration of new physics models in the dark sector. Rather than developing specific models, it is possible to survey the space of possible theories using numerical and symbolic techniques, combined with modern sampling methods and high-performance computing. The current system constrains models based on the stability of the free theory, and we will discuss simple phenomenological constraints against observations of black holes, gravitational waves, pulsars and the distributions of galaxies. We will also discuss ongoing extensions to the automatic reconstruction of interactions and the quantum theory.<\/li>\n
Title: Fluid perturbations from expanding bubbles in first-order phase transitions<\/em>
Abstract: In this talk I am going to focus on the power spectrum of the velocity field induced in the primordial plasma by expanding scalar-field bubbles during a first-order phase transition occurring in the radiation-dominated era. Contrary to previous expectations, we find that the breaks in the velocity spectrum are not associated to the bubble size and the sound shell thickness, but to the position of the discontinuities of the velocity profiles. This distinction is particularly relevant for supersonic deflagrations, as it implies that the intermediate slope is more pronounced and the two breaks are more separated when the wall velocity approaches the Chapman-Jouget speed, instead of the sound speed. Moreover, we find that the asymptotic branches of the velocity power spectrum are determined by an integral over the velocity profiles at large scales, and by the discontinuities of the profiles at small scales. Furthermore, the position of the two breaks and the intermediate slope depend on the distribution function of the times of bubble nucleation. The main result is a refined template for the velocity spectrum at the beginning of the sound-wave phase, which can be used for studying the resulting anisotropic stresses and gravitational wave production.<\/li>\n
Title: Cosmological Gravitational Waves from Phase Transitions and Simulation-Based Inference for LISA<\/em>
Abstract: A stochastic gravitational-wave (GW) background of cosmological origin is a prime target for present and forthcoming GW detectors, ranging from pulsar timing arrays to LISA. In this seminar, I will present two complementary perspectives on cosmological GW signals. First, I introduce an efficient numerical framework to model GW production from first-order cosmological phase transitions using the Higgsless approach. Using these simulations, I construct a set of GW templates within a generalized modeling scheme that extends the standard sound-wave description to include damped sources, allowing us to capture the saturation of the GW amplitude at long source durations. I will present new results for strong phase transitions, demonstrating the saturation of the GW spectrum in the limit of long-duration simulations. Second, I will briefly introduce a modern parameter-inference framework known as simulation-based inference (SBI). As a demonstration of its capabilities, I apply this approach to cosmic-string gravitational-wave signals and assess their detectability with LISA in the presence of astrophysical foregrounds.<\/li>\n
Title: TBA<\/em>
Abstract: TBA<\/li>\n
Title: TBA<\/em>
Abstract: TBA<\/li>\n
Title: TBA<\/em>
Abstract: TBA<\/li>\n
Title: Stochastic simulation of reheating and\/or warm inflation<\/em>
Abstract: The late stage of inflation\/reheating\u00a0may be captured by a two-component approach, in which a self-interacting plasma has already attained local equilibrium, while the inflaton field is still far from equilibrium. This\u00a0 should be particularly suitable if the plasma contains non-Abelian gauge bosons, which are believed to equilibrate fast. We describe the foundations of\u00a0such an approach and present a gauge-invariant numerical implementation in the linear regime (2507.12849). As a particular example, we apply the formalism to the Standard Model scenario (originally proposed in 2503.18829).<\/li>\n<\/ul>\nSpring Term 2026<\/h3>\n
\n
Title: Gravitational waves from strong first order phase transitions [video<\/a>]<\/span><\/em>
Abstract: Multiple extensions of the Standard Model of particle physics predict the existence of first order phase transitions occurring in the early Universe, leading to an imprint in the stochastic background of gravitational waves. When the transition occurs at the electroweak scale, this imprint will be in the expected range of LISA. In this talk we explore the gravitational wave production of strong first order phase transitions, seeking to understand the role of fluid non-linearities and their impact on the expected signal. To do so, we employ large scale simulations of two transitions: one preceded by a detonation, another by a deflagration. We then study the evolution of vortical and compressional modes, how they are intrinsically related and what their respective impacts are on the expected gravitational wave background signal. We also study the relationship of the rate of emission of gravitational wave power with non-linear decay of flow.<\/li>\n
Title: The expansion rate and the geometry of the Universe through the time delays of time-varying sources strongly lensed by galaxy clusters [slides<\/a>]<\/span><\/em>
Abstract: Current and upcoming wide-field surveys will discover thousands\u00a0of\u00a0new\u00a0multiply imaged quasars\u00a0and\u00a0supernovae, several\u00a0of\u00a0which strongly lensed by galaxy clusters. This will\u00a0offer\u00a0a\u00a0unique opportunity\u00a0to\u00a0transform\u00a0time\u00a0delays\u00a0in lens galaxy clusters into a fundamental alternative tool for measuring the expansion rate and the geometry of the Universe. Time delay cosmography is based on well-known physics (General Relativity) and is a single-step technique. I will present the results of\u00a0high-quality spectro-photometric (HST\u00a0and\u00a0VLT) data\u00a0and\u00a0high-precision strong lensing modelling in the core of the Hubble Frontier Fields galaxy cluster MACS J1149.6+2223, where the first magnified and spatially-resolved images of\u00a0supernova\u00a0(SN) \u2018Refsdal\u2019 at redshift 1.489 were detected. These results were exploited for a wide variety of science topics: 1) the successful\u00a0prediction\u00a0of\u00a0the reappearance of SN \u2018Refsdal\u2019 at\u00a0a\u00a0specific sky position and time; 2) competitive measurements of the cosmic expansion rate and geometry, completely independent from\u00a0other cosmological probes; 3) the reconstruction of the kinematics of the SN host, a regular, star-forming, rotation-dominated spiral galaxy in a 4-Gyr-old Universe; 4) a detailed study of the environment of the SN, showing a high degree of ionisation with low metallicity. By considering\u00a0the\u00a0independent estimates from ~10 similar cluster strong lensing studies, we will achieve the ambitious goal of a complementary measurement of the value of H0 with a 2% uncertainty. This will help clarify whether\u00a0the\u00a0current, hotly debated tension on\u00a0the\u00a0value\u00a0of\u00a0H0\u00a0must be\u00a0ascribed\u00a0to\u00a0intriguing\u00a0new\u00a0physics or\u00a0to\u00a0significant systematic effects.<\/span><\/li>\n
Title: Skyrmions at finite temperature [video<\/a>]<\/span><\/em>
Abstract: Skyrmions are stable and topologically non-trivial field configurations that behave like localized particles. They appear in the chiral effective theory for pions, where they correspond to the baryon states. In this talk, focusing on toy models that capture different limits of the electroweak sector of the SM, I will show that skyrmions not classically stable at zero temperature can be stabilized by thermal effects. I will also speculate with the possibility that quantum effects make these particles exist also at zero temperature, potentially implying the existence of dark matter without new physics.<\/li>\n
Title: Solving Einstein Field Equations on a quantum computer: a quantum algorithm for numerical general relativity [video<\/a>]<\/span><\/em>
Abstract: In this talk, I will present a quantum algorithm for numerical general relativity to show how some computationally expensive simulations performed on classical computers can be more efficiently tackled by quantum algorithms for solving systems of partial differential equations (pdes). I will cover the theoretical and practical aspects of this interdisciplinary project: from the identification of a suitable numerical general relativity formalism (the WEBB hyperbolic tetrad formalism), to the translation of the system of pdes into a set of quantum computing operations in the form of a quantum walk, the programming of the algorithm components for a gate-based digital quantum computer using the Qiskit software, the choice of a system to test it (Schwarzschild black hole gravitational quasinormal modes), and the tests run on both classical simulators and physical quantum computers. I will conclude by discussing on how this project fits in the landscape of quantum information applications for gravity studies and its possible future extensions and applications.<\/li>\n
Title: Supermassive Dark Stars and their remnants as a possible solution to cosmic dawn puzzles [video<\/a>]<\/em>
Abstract: Dark Stars would have been the first stars to form in the history of the Universe; they would have been powered by dark matter heating rather than by fusion. Dark Stars are made (almost entirely) of hydrogen and helium from the Big Bang, with 0.1% of the mass in the form of Dark Matter. The relevant types of dark matter include Weakly Interacting Massive Particles (WIMPs) and Self Interacting Dark Matter (SIDM). Although dark matter constitutes only \u22720.1% of the stellar mass, this amount is sufficient to power the star for millions to billions of years. They are very bright diffuse puffy objects (10 AU in radius) and grow to be very massive. In fact, they can grow to become ten million times as massive as the Sun and ten billion times as bright. We have found strong candidates for Supermassive Dark Stars in James Webb Space Telescope data: the spectra are a good match, and in the future with better spectra the detection of a HeII 1640 absorption line would be a smoking gun for a Dark Star vs a galaxy. JWST has made surprising discoveries: too many and too massive early galaxies, potentially in conflict with standard LCDM cosmology; unexplained gigantic black holes; and little red dots. Supermassive Dark Stars and their remnants offer solutions to all of these cosmic dawn puzzles. Once the dark matter fuel for a dark star runs out, the dark star dies, and it collapses to a giant black hole. Thus dark stars may provide seeds for they many unexplained supermassive black holes observed throughout the Universe and at early times.<\/li>\n
Title: Limits of EFTs at finite temperature for strong phase transitions [video<\/a>]<\/span><\/em>
Abstract: Phase transitions are violent and interesting phenomena that could have occurred in the early universe. Possible techniques to study these phenomena can be used in the presence of a hierarchy of scales, leading to the construction of finite temperature Effective Field Theories by integrating out heavier scales. These EFTs are reliable when the dynamics are mainly encoded in the most relevant operators. I will discuss the limits of such EFTs, showing how higher-dimensional operators affect the prediction of stronger transitions, including those detectable by LISA. These considerations impact the applicability of effective theory techniques, including their use in lattice studies. I will then discuss how subtleties associated with these operators, such as gauge and renormalization group dependence, can emerge and how they are addressed only when higher loop computations are taken into account. Finally, I will compare the impact of these higher order corrections with that of higher dimensional operators on the predicted strength of the transition.<\/li>\n
Title: Dark matter probes in astrophysical scenarios: compact stars and blazars [video<\/a>]<\/em>
Abstract: Dark matter\u00a0is one of the most compelling pieces of evidence for physics beyond the Standard Model, yet its particle nature remains unknown. While direct detection, collider, and indirect searches probe complementary regions of parameter space, large areas\u2014particularly in the GeV and sub-GeV mass range\u2014remain unexplored. This seminar\u00a0shows how\u00a0highly energetic astrophysical environments are\u00a0natural laboratories to probe particle dark matter beyond the reach of terrestrial experiments. We will focus in two kinds of relativistic systems: compact stars (neutron stars, white dwarfs),\u00a0and blazars, to show how we may find visible signatures of dark matter\u00a0from these scenarios.\u00a0This approach connects particle physics, astrophysics, and cosmology to open new discovery channels for dark matter.<\/li>\n
Title: Tunneling rate at finite temperature [slides<\/a>]<\/span><\/em>
Abstract: Cosmological phase transitions, especially first order phase transitions, have attracted renewed interest as potential sources of gravitational waves. Accurately predicting the resulting gravitational wave signal requires a reliable estimate of the transition rate, which is governed by a saddle-point configuration known as the bounce solution. The seminal work of Coleman, Glaser, and Martin established that at zero temperature, any nontrivial bounce solution to the equations of motion that minimizes the Euclidean action is O(D)-symmetric in D-dimensional spacetime. At finite temperature, however, it has not been proven that an O(D-1)-symmetric bounce solution in the spatial directions indeed yields the minimal Euclidean action, despite this assumption being widely used in the literature. In this talk, we extend the Coleman\u2013Glaser\u2013Martin analysis to finite temperature. We rigorously prove that for a broad class of scalar potentials, any saddle-point configuration with the least action is necessarily O(D-1)-symmetric and monotonic in the spatial directions. This result provides a firm mathematical foundation for the symmetry properties widely assumed in studies of thermal vacuum decay and cosmological phase transitions.<\/span><\/li>\n<\/ul>\n
\n\n\n\n
\n
Title:<\/em>
Abstract: <\/li>\n<\/ul>\n<\/blockquote>\n\n\n\n