First Nordic Cosmology Meeting

23 Oct 2023, 09:05 → 25 Oct 2023, 16:00 Europe/Stockholm

Albano Campus

Florian Niedermann (Stockholm University), Oksana Iarygina (Stockholm University), David Marsh (Stockholm University)

Venue

Nordic Institute for Theoretical Physics and Oskar Klein Centre, Stockholm, Sweden.  
 

Last minute update: The room in Albano Building 2 was changed from 17 to 18!

23. Oct.: Room 18, Albano Building 2

24.-25. Oct.: Room 4204, Conference Center, Albano Building 3, floor 4 (Nordita building)

OKC Colloquium, 24. Oct.: Oskar Kleins auditorium (FR4 / AlbaNova Building) 

Dinner, 24. Oct. 19:30: Kvarnen, Tjärhovsgatan 4   
Directions from Nordita: bus 50 or 61 to Odenplan and then the green line to Medborgarplatsen. Alternatively, 25 minutes by bike starting from the AlbaNova main entrance.

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Scope

Modern cosmology is an active interplay between quantum and classical field theories, gravity, astrophysics and particle physics. It combines theoretical constructions and observational tests in order to study the fundamental physics at very high energy scales. At the same time, the increasing precision of upcoming experiments in the next decade will probe our current understanding of cosmology, raising the prospect of finding new physics and making this an exciting time for cosmology.

The aim of this meeting is to gather researchers from Nordic countries and beyond working in cosmology and related areas to promote new collaborations and tighten scientific connections with and between Nordic universities to further advance progress in the area of cosmology. The meeting will include talks by invited speakers with an overview of research directions represented at Nordic universities, including (but not limited to) such areas as dark matter and dark energy, inflation, reheating, gravitational waves, cosmological phase transitions, axions and axion-like particles and advances in observational cosmology. Topical review lectures will be complemented by short talks by researchers at any career stage.   
 

A poster can be found here.  
 

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Topics

1. Dark matter and dark energy.
2. Inflation and the Early Universe.
3. Gravitational waves.
4. Cosmological phase transitions.
5. Axions and axion-like particles.
6. Advances in observational cosmology.

 

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Invited speakers

1. Ulf H. Danielsson (Department of Physics and Astronomy, Uppsala University, Sweden)
2. Jens Jasche (Oskar Klein Centre, Stockholm University, Sweden).
3. M.C. David Marsh (Oskar Klein Centre, Stockholm University, Sweden).
4. Martin S. Sloth (Universe-Origins, SDU, Denmark).
5. David J. Weir (Helsinki Institute of Physics, University of Helsinki, Finland).
6. Ingunn Kathrine Wehus (Institute of Theoretical Astrophysics, University of Oslo, Norway).

Special guest speaker: Ana Achúcarro (Leiden University and Basque University Bilbao, The Netherlands) 

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Accommodation

Participants are asked to organize their own accommodation and travel.

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Registration and abstract submission

Registration will close on 30 September 2023 (or earlier, subject to capacity).

Abstract submission will close on 1 September 2023. There will also be a possibility to present a poster.

Researchers at any career stage are welcome to submit an abstract for a talk. Also, registrations from non-Nordic countries are welcome.

There is no conference fee. Coffee breaks are covered by Nordita. The information about a conference dinner will be provided later.

Update: There will be no remote participation possible.

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Code of conduct

The Organization is committed to make this meeting productive,  safe, and enjoyable for everyone regardless of nationality, gender, race, sexual orientation, religion, political views, disability, and physical appearance. We will not tolerate harassment or misconduct in any form including, but not limited to, physical, verbal, and online actions. The following guidelines are expected to be followed by everyone at the meeting.

1. Treat everyone with respect and consideration, this includes being respectful and mindful in your critique of ideas, and also to be mindful of your surroundings and your fellow participants. Do not insult or put down fellow attendees or staff.
2. Communication is expected to be appropriate for a professional audience with diverse backgrounds. Harassment and sexist, racist, derogatory, or exclusionary jokes or comments will not be tolerated, neither are sexual language or imagery.
3. Harassment includes (but it is not limited to) sustained disruption of talks or other events, inappropriate physical contact, sexual attention or innuendo, deliberate intimidation, stalking of any individual, and language bashing. It also includes offensive comments related to gender, sexual orientation, disability, physical appearance, race, religion or political views.

Any violation of these guidelines may result in the immediate removal of the offender from the meeting without warning and without refund. Furthermore, the offender’s action may be reported to the corresponding national institution. This will be at the sole discretion of the local organizing committee. Participants asked to stop any kind of inappropriate behaviour are expected to comply immediately. Anyone wishing to report any violation of these guidelines, is encouraged to speak to the organizers. The organization will grant confidentiality to both parties involved and will do everything in their power to restore a professional and safe environment for all as fast as possible.

(Adapted from "A Cosmic Window to Fundamental Physics: Primordial Non-Gaussianity and Beyond" workshop).

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Sponsored by:

Map_Campus.pdf

Nordic_Cosmo_2023.pdf

Monday, 23 October

09:45 Registration and Coffee

1 Welcome

Speaker: David Marsh (Stockholm University)

Opening slides.pdf

Invited Talks Session

Convener: Oksana Iarygina (Stockholm University, Nordita)

2 Inflationary tails, and other multi-field stories

Motivated by primordial black holes as an observational window into primordial spectra on sub-CMB scales, I will discuss some interesting properties of the perturbations generated during multi-field inflation.

Among them: power spectrum features, non-gaussianity — or otherwise — of small perturbations, and the statistics of large, rare, curvature fluctuations ("non-gaussian tails").

Speaker: Ana Achúcarro

3 The dark bubble model – how to get dark energy from string theory

I review the dark bubble model, its connection to quantum cosmology and how the cosmological constant can be computed in terms of other fundamental scales.

Speaker: Ulf Danielsson

Ulf_Danielsson_Nordita231023UD.pdf

12:40 Lunch

Contributed Talks

Convener: Deanna C. Hooper (Helsinki Institute of Physics, University of Helsinki)

4 Palatini F(R,X): a new framework for inflationary attractors

Palatini F(R) gravity proved to be a powerful tool in order to realize asymptotically flat inflaton potentials. Unfortunately, it also inevitably implies higher-order inflaton kinetic terms in the Einstein frame that might jeopardize the evolution of the system out of the slow-roll regime. We prove that a F(R−X) gravity, where X is the inflaton kinetic term, solves the issue. Moreover, when F is a quadratic function such a choice easily leads to a new class of inflationary attractors, fractional attractors, that generalizes the already well-known polynomial α-attractors.

Speaker: Antonio Racioppi (NICPB, Tallinn, Estonia)

Racioppi.pdf

5 Primordial black holes as dark matter: the role of stochastic effects

Primordial black holes are a candidate for dark matter. Their seeds may be generated during inflation. I discuss the mechanism, why stochastic effects are expected to be important, and how they can be modelled consistently. I will also discuss new results on how stochastic fluctuations make the density profiles that collapse into black holes very choppy, unlike assumed so far.

Speaker: Syksy Räsänen (University of Helsinki)

rasanen.pdf

6 Dynamics of domain walls in the asymmetron model

The asymmetron dark energy model is an extension of the symmetron that aspires to resolve some of the cosmological tensions by allowing for a late-time, stochastic and large-scale spatial variation of the effective gravitational constant, in addition to the environmental variation due to screening. Whether this model actually manages to do so is complicated as it relates to questions of stability of the domain walls, how the field configuration traces the cold dark matter structure and the resultant domain sizes. To model this self-consistently requires resolving a large range in temporal and spatial scales, which we have set out to do here. I will show some of our convergence results on the field configurations, and make comments on features of the fully non-linear and dynamic model that we have gained access to. I will make comparisons between the symmetron and the asymmetron.

Speaker: Øyvind Christiansen

7 Dissecting the Primordial Signal in Large-Scale Structure Power Spectra

The primordial universe may provide a window to the highest energy densities accessible in nature. The observational detection of primordial non-Gaussianity and/or primordial features would therefore shed unique light on the laws of physics at scales far beyond those of any terrestrial experiment. In this talk, I will show how current and future large-scale structure (LSS) surveys can place interesting constraints on the degrees of freedom and dynamics of cosmic inflation (and its alternatives). I will focus on inflationary imprints on LSS power spectra, in particular the scale-dependent bias induced by any primordial non-Gaussian signal in biased tracers and oscillations induced by primordial features. The emphasis will be on the scale-dependent bias beyond the local type, and how auto- and cross-correlations of several different LSS tracers will help us to constrain the underlying primordial physics. I will also explain where the constraining power comes from and present current constraints on these general signals. This talk will be mainly based on ongoing work with Daniel Green, Jiashu Han and Yi Guo.

Speaker: Benjamin Wallisch (Stockholm University)

nordic-cosmology_wallisch.pdf

8 Electroweak sphaleron in a magnetic field

Electroweak sphaleron leads to non-conservation of baryon and lepton numbers. Thus, for many baryogenesis scenarios it is crucial to know how active the sphaleron transitions were through out the history of our universe. The Electroweak sphaleron has a magnetic dipole moment and in an external magnetic field the energy of the sphaleron can be lowered affecting the rate at which the transitions can occur. In addition the form of the electroweak phase transition is modified by the presence of an external magnetic field. In this talk I will motivate why it is interesting to look at the electroweak sphaleron in a magnetic field. Then I will present our recent results of computing the sphaleron rate in an external magnetic field through out the electroweak cross over using dynamical lattice simulations.

Speaker: Jaakko Annala

nordic_cosmo23_Annala.pdf

9 Cosmological-scale magnetic fields from galactic outflows and search for primordial magnetic field

Primordial magnetic field, when it is confidently discovered, will play an important role in cosmology as CMB or Large Scale Structure. However, its observation can be affected by the presence of the large-scale magnetic field of galactic origin, magnetic bubbles. In the series of works, we study the effects of galaxy formation physics on the magnetization of the intergalactic medium (IGM) using the IllustrisTNG simulations. We demonstrate that large-scale regions affected by the outflows from galaxies and clusters contain magnetic fields that are several orders of magnitude stronger than in unaffected regions with the same electron density. Moreover, like magnetic fields amplified inside galaxies, these magnetic fields do not depend on the primordial seed, i.e. the adopted initial conditions for magnetic field strength. We discuss the effects of these strong magnetic fields on ultra-high energy cosmic rays, gamma rays, and Faraday Rotation Measure. We show that using different observables one should be able to disentangle the effects of magnetic bubbles from the primordial magnetic field and discuss a strategy for future observations.

Speaker: Kyrylo Bondarenko

Kyrylo_Bondarenko_2023_10_23_Nordic_cosmology_meeting_Bondarenko.pdf

15:30 Coffee

Contributed Talks

Convener: Joshua Eby (Stockholm University)

10 Is gravity the only dark matter interaction that matters in the physics of galaxies?

One of the most relevant problems in Physics is to identify new particles beyond the Standard Model, which can fit the properties of the cosmological dark matter. Despite decades of searching for candidates, the nature of dark matter remains a mystery even though its role in explaining cosmic structure formation has become progressively more fundamental. Analysing the properties of the galaxy population is arguable among the most promising short-term avenues to look for clues to solve the dark matter enigma. In this talk, I will present a broad overview of the status of the allowed impact of new dark matter physics (i.e., beyond its gravity) in the physics of galaxies.

Speaker: Jesús Franco Zavala

Jesus_Nordita_2023_no_extra_slides.pdf

11 Prospects for percent-accuracy measure of tau with Tau'r'us

Of the six ΛCDM parameters, five are constrained to percent level or lower by Planck data. The constraints on the optical depth to reionisation τ, are however ten times looser. Indeed, τ is almost fully degenerate with the primordial power spectrum amplitude As. Breaking the degeneracy between As and τ is important for several cosmological challenges, including measuring the sum of neutrino masses. Thanfully, in CMB polarisation spectra, the degeneracy is lifted at very large angular scales on the sky (>10°). Accurate measurements of CMB polarisation on those scales require an experiment with a very wide sky coverage. In this talk, I will present Taurus, a stratospheric balloon experiment that should observe the polarized microwave sky for a month in four frequency bands between 150 and 350 GHz. Its launch is scheduled from New Zealand in the mid 2020s. Results from Taurus should greatly the uncertainty on τ and be complementary with other upcoming CMB experiments, like CMB-S4.

Speaker: Alexandre Adler

TaurusSlides

12 Probing Cosmological Models Through the Era of First Galaxies

The epoch of reionisation marks the emergence of the first galaxies that emit light into the intergalactic medium, leading to the ionisation and heating of the Universe. This phase is a forefront area in astrophysics. While our current understanding relies on theoretical models of early galaxy formation within the standard cold dark matter (CDM) cosmology, recent observations, including early galaxy data from the James Webb Space Telescope (JWST) and the thermal evolution of intergalactic gas from the Experiment to Detect the Global EoR Signature (EDGES), have raised questions about this conventional view. I will present our initial testing of models using JWST observations of early galaxies and discuss the potential for further insights with upcoming observations.

Speaker: Sambit Kumar Giri (Stockholm University, Nordita)

FirstNordicCosmo_2023-1.pdf

13 Glueball dark matter

I explore the possibility of dark matter being composed of stable scalar glueballs from a confining dark SU(N) gauge theory. The relic abundance of these glueballs is studied for the first time in a thermal effective theory, using an effective potential fitted by lattice simulations. The predicted relic abundance is smaller than previously believed. Moreover, this framework can be easily extended to different gauge groups and modified cosmological histories to explore strongly coupled dark sectors and their cosmological implications. Based on P.~Carenza, T.~Ferreira, R.~Pasechnik and Z.~W.~Wang, %Glueball dark matter, precisely,'' [arXiv:2306.09510 [hep-ph]]. P.~Carenza, R.~Pasechnik, G.~Salinas and Z.~W.~Wang, %Glueball Dark Matter Revisited,'' Phys. Rev. Lett. \textbf{129} (2022) no.26, 26 [arXiv:2207.13716 [hep-ph]].

Speaker: Pierluca Carenza (Stockholm University)

Pres_NordiCosmology.pdf

14 Testing gravity through the distortion of time

The distribution of galaxies provides an ideal laboratory to test deviations from General Relativity. In particular, constraints on gravity modifications are commonly obtained by measuring the growth of cosmic structures through redshift-space distortions. However, such constraints rely on the validity of the weak equivalence principle, which has never been tested for the dark matter component. In my talk, I will employ data from the Sloan Digital Sky Survey to show that dropping this restrictive assumption leads to severe degeneracies and makes it challenging to distinguish fundamental gravity modifications from interactions in the dark sector. Luckily, I will demonstrate that it is possible to break such degeneracies and recover tight constraints thanks to measurements of the distortion of time expected from upcoming galaxy surveys.

Speaker: Sveva Castello

SvevaCastello_NordicCosmo.pdf

17:30 Welcome Reception

Tuesday, 24 October

Invited Talks Session

Convener: Florian Niedermann (Stockholm University, Nordita)

15 A simultaneous solution to the H0 and the S8 tensions and the implication for inflation

I will discuss the requirements of a successful simultaneous solution to the Hubble and the S8 tension; why it requires new dark physics at the eV scale, and why New Early Dark Energy (NEDE) is a particularly simple and minimalistic framework for such solutions. I will then explain why this class of successful solutions to the tensions implies a more blue initial spectrum of primordial curvature perturbations favouring the simplest curvaton model over the Starobinsky inflation model. I will also discuss how this can be tested by future stronger constraints or measurements of local non-gaussiandity and the tensor-to-scalar ratio.

Speaker: Martin S. Sloth

new_ede-23-nordita.pdf

16 Gravitational waves from early universe phase transitions

Many extensions of the Standard Model of particle physics predict that one or more phase transitions took place in the early universe. Such phase transitions involve the nucleation, expansion, and collision of bubbles of the new phase. These collisions (and associated interactions of sound waves in the plasma) are substantial, potentially detectable, sources of gravitational waves. As a result, they can act as a probe of particle physics beyond the Standard Model. Excellent progress has been made in developing high-level modelling of the gravitational wave power spectrum from early universe phase transitions. This progress has been made possible in part by large-scale simulations of bubble collisions. However, challenges remain, particularly in understanding and modelling the development of nonlinearities, including shocks and turbulence. These become increasingly important for strong phase transitions. I will discuss features of the underlying particle physics models, as well as the latest simulation results. I will explain how the results can inform theoretical understanding and future analytical models of the resulting gravitational wave power spectra.

Speaker: David Weir (University of Helsinki)

Link to slides

17 CMB observations - mapping the sky from the Milky Way to the Big Bang

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

Speaker: Ingunn Kathrine Wehus (University of Oslo)

cosmoglobe

11:45 Lunch

18 OKC Colloquium

Title: Echos of the Early Universe in Axion Haloscopes

Abstract: The coming decade will bring dramatic improvement in the axion dark-matter program as new experimental designs move beyond the proof of principle stage. In this talk I will outline two signals beyond dark matter that these instruments could discover. The first is a population of relativistic axions that were produced in the early universe and persist as a residual Cosmic axion Background (CaB). The second is high-frequency gravitational waves; I will outline how exploiting an analogy between axion and gravitational-wave electrodynamics allows for axion haloscopes to be converted into gravitational-wave telescopes.

Speaker: Nicholas Rodd

14:00 Coffee

Coffee will be provided in connection with the OKC colloquium.

Contributed Talks

Convener: Aleksandr Chatrchyan (Stockholm University)

19 Impact of fifth force large scale screening models on the Hubble tension

Fifth forces are ubiquitous in modified theories of gravity. In this talk, I present our recent results investigating their effect on the Cepheid-calibrated cosmic distance ladder, specifically with respect to the inferred value of the Hubble constant (H0). We consider a variety of effective models where the strength of the fifth force is estimated using proxy fields related to the large-scale structure of the Universe. For all models considered, the local distance ladder and the Planck value for H0 agrees with a probability >20%, relieving the tension compared to the concordance model with data being excluded at 99% confidence. The alleviated discrepancy comes partially at the cost of an increased tension between distance estimates from Cepheids and tip of the red-giant branch stars (TRGB). Demanding also that the consistency between the Cepheid and TRGB distance estimates is not impaired, models can still accommodate the data with a probability >20%. This provides incentive for more detailed investigations of fundamental theories on which the effective fifth force models are based, and their effect on the Hubble tension.

Speaker: Marcus Högås (Stockholm University)

The Hubble tension and fifth forces [2].pdf

20 Frequentist analyses of beyond-LambdaCDM models

In response to the current cosmological tensions, a multitude of proposed extensions to the cosmological concordance model, the LambdaCDM model, have been proposed. The analysis of these models has largely been made within the Bayesian statistical paradigm, which can be prone to undesired effects arising from the prior probability distributions. In this talk, I will discuss my recent work on conducting frequentist inference, using profile likelihoods, on a variety of cosmological models. In particular, I will present the new inference code PROSPECT, which allows the user to efficiently construct profile likelihoods of their favourite models. Furthermore, I will present recent results on a series of cosmological models, such as decaying dark matter and cold new early dark energy, in which the frequentist analysis paints a substantially different picture than the corresponding Bayesian analyses, confirming the importance of including both statistical paradigms for a fully nuanced analysis of the many new LambdaCDM extensions.

Speaker: Emil Brinch Holm

nordita_2023_profiles.pdf

21 Exploring Demographic Drift of Type Ia SNe and It's Impact on Cosmological Constraints

Type Ia supernovae (SNe Ia) have played a significant role in measuring the acceleration of the Universe's expansion and the existence of dark energy. Correct characterization of systematics is crucial to accurately measuring cosmological parameters. An example of such a systematic is the potential presence of SN Ia progenitor-related sub-populations, the rates of which can change over cosmic time. In this talk, we present a novel Bayesian hierarchical two-population model of SNe Ia observables which enables us to measure the presence and properties of two sub-populations as well as the redshift evolution of their relative fractions. This allows us to investigate the impact of these elements on the precision and accuracy of constraints on cosmological parameters. Our model builds on earlier work by accounting for the potentially varying fraction of two distinct SNe Ia populations over cosmic time. By modeling the redshift dependence of the two populations, we can estimate their respective fractions at different epochs and explore the impact of these changes on cosmological constraints. We apply our model to both simulations and to observational data from Pantheon+. We show that observational data has signatures of redshift dependent fractions of the SNe populations and discuss these results in the context of current work on SNe Ia progenitors. We find that this demographic drift has potentially important implications for measuring the properties of dark energy, as it affects the derived distances and properties of Type Ia supernovae.

Speaker: Jacob Osman Hjortlund

22 Testing the relic neutrino decay solution to the EDGES anomaly and ARCADE2 excess with neutrino oscillation experiments

The EDGES anomaly and the ARCADE-2 excess are not explained within the standard cosmological model or by any astrophysical solution and so they might be an indication of new physics. The decays of relic neutrinos into sterile neutrinos provide intriguingly a simultaneous solution to both puzzles. We first build a simple model able to describe the correct values of the parameters, lifetime and mass difference, requested by this EDGES-ARCADE2 combined solution. Then we show that, within such a model, the same EDGES-ARCADE-2 combined solution implies active-sterile neutrino mixing and we show how neutrino oscillation experiments place interesting constraints on the parameters of the model.

Speaker: Rishav Roshan

Talk_Rishav.pdf

23 General relativistic bubble growth in cosmological phase transitions

We use a full general relativistic framework to study the self-similar expansion of bubbles of the stable phase into a flat Friedmann-Lemaître-Robertson-Walker Universe in a first order phase transition in the early Universe. With a simple linear barotropic equation of state in both phases, and in the limit of a phase boundary of negligible width, we find that self-similar solutions exist, which are qualitatively similar to the analogous solutions in Minkowski space, but with distinguishing features. Rarefaction waves extend to the centre of the bubble, while spatial sections near the centre of the bubble have negative curvature. Gravitational effects redistribute the kinetic energy of the fluid around the bubble and can change the kinetic energy fraction significantly. The kinetic energy fraction of the gravitating solution can be enhanced over the analogous Minkowski solution by as much as O(1), and suppressed by a factor as larger as O(10) in case of fast detonations. The amount of negative spatial curvature at the centre of the bubble is of the same order of magnitude of the naive expectation based on considerations of the energy density perturbation in Minkowski solutions, with gravitating deflagrations less negatively curved, and detonations more. We infer that general relativistic effects might have a significant impact on accurate calculations of the gravitational wave power spectrum when the bubble size becomes comparable to the cosmological Hubble radius, affecting the primary generation from the fluid shear stress, and inducing secondary generation by scalar perturbations.

Speaker: Lorenzo Giombi

Presentation_Stockholm.pdf

16:00 Coffee

Contributed Talks

Convener: Benjamin Wallisch (Stockholm University)

24 Real scalar phase transitions: bubble nucleation, nonperturbatively

In the Standard Model the electroweak phase transition is a crossover, but in many beyond the Standard Model theories the transition is of first order. Strong first order phase transitions could produce gravitational waves that might be detectable by the Laser Interferometer Space Antenna (LISA). Perturbation theory is commonly used to estimate the parameters that enter the calculation of gravitational wave spectra. However, perturbation theory is known to run into the infrared problem in the regime we are interested in and furthermore it is important to test the reliability of existing results. Here I will discuss our recent results where we studied a real singlet scalar model with a tree level potential barrier and performed nonperturbative simulations to determine the bubble nucleation rate. Our preliminary results show that higher orders in perturbation theory are necessary, and we expect our findings to allow calibration of the systematic uncertainty in perturbative results.

Speaker: Anna Kormu

Nordic Cosmo-Kormu.pdf

25 Gravitational waves from Dark Phase Transitions at Strong Coupling

In this talk, we demonstrate how to predict the gravitational wave spectra of Strongly coupled QFTs using holography and lattice data input for a pure SU(N) Yang-Mills theory with small uncertainties. We will elaborate on how we obtain an effective potential using holography with the free energy landscape approach and formulate an effective action. Once the effective action is in our grasp, we will use this to study bubble nucleation to predict the gravitational wave spectra. Furthermore, we will discuss how the bubble wall velocity computations can be made in steady-state configurations using holographic techniques by computations of the plasma friction force.

Speaker: Nicklas Ramberg

Nordic Cosmology Meet Short Talk Slides .pdf

26 Reconstructing phase transitions from future LISA data

A gravitational wave background from a first order phase transition at the electroweak scale may be observable with future detectors such as LISA. While the Standard Model does not predict a first order phase transition, these occur in many BSM scenarios. Therefore, detecting a stochastic gravitational wave background could point to new physics, while a null detection could constrain or even exclude many BSM models. However, recovering the physical parameters of an underlying phase transition from a possible signal at LISA is not straightforward, partly due to other possible sources of a stochastic gravitational wave background, such as a population of white dwarf binaries. In this talk I will present our recent advances in reconstructing the phase transition parameters from mock LISA data using parameterised templates as an approximation to a more complete physical model, which greatly speeds up the process. I will also discuss how we hope to include more realistic noise sources like white dwarf binaries in our mock LISA data.

Speaker: Deanna C. Hooper

Hooper_nordita23.pdf

27 Perturbative EFT expansions for cosmological phase transitions

Gravitational waves (GW) from cosmological phase transitions bear huge discovery potential and can be probed by planned future space-based experiments. Complementary to current and future collider experiments, such GW signatures can offer a powerful probe for beyond the Standard Model physics. Predictions for stochastic GW spectrum of a cosmological origin are often plagued by large theoretical uncertaintities related to poor understanding of phase transition thermodynamics. In this talk, I present novel reformulation to the perturbative analysis of equilibrium thermodynamics for generic cosmological phase transitions in terms of effective field theory (EFT) expansions. These EFT expansions resolve all theoretical inconsistencies that have plagued previous studies (spurious infrared divergences, imaginary parts, gauge dependence and renormalisation scale dependence). Moreover, EFT expansions provide a numerically inexpensive method to determine thermodynamics, and significantly improve agreement with the non-perturbative lattice simulations.

Speaker: Tuomas Tenkanen

tenkanen-first-nordic-cosmology-meeting-2023.pdf

19:30 Dinner

Wednesday, 25 October

Invited Talks Session

Convener: Pierluca Carenza (Stockholm University)

28 Recent developments in axion physics.

Speaker: David Marsh (Stockholm University)

Nordic Cosmology - Axions.pdf

29 Bayesian Forward Modelling of Galaxy Surveys

The standard model of cosmology predicts a rich phenomenology to test the fundamental physics of the origin of cosmic structure, the accelerating cosmic expansion, and dark matter with next-generation galaxy surveys. However, traditional data analysis methods focus on limited statistical summaries and overlook important information in the complex filamentary distribution of cosmic matter in three-dimensional space. Currently, Physics-informed field-level inference is emerging as a viable alternative to studying galaxy surveys. This approach utilizes nonlinear structure formation models to jointly infer cosmic initial conditions, map nonlinear density and velocity fields, and provide dynamic structure formation histories with a detailed treatment of uncertainties. Besides promising improved cosmological parameter constraints, it also offers novel opportunities to study fundamental physics using cosmic structures. In this presentation, I will give a conceptual introduction to field-level inference and Bayesian physical forward modeling and illustrate the approach through diverse data applications. Besides mapping nonlinear dark matter density and velocity fields I will discuss examples of using these results to test fundamental physics, such as the particle nature of dark matter or the weak equivalence principle. Finally, I will present a detailed analysis of the Nearby Universe via the SIBELIUS project, yielding one of the most extensive data-constrained simulations, including a Milky-Way Andromeda galaxy pair at its center.

Speaker: Jens Jasche (Stockholm University)

First Nordic Cosmology Meeting.pdf

11:00 Coffee

Contributed Talks

Convener: Kyrylo Bondarenko

30 Gravitational wave emission from a cosmic string loop

Cosmic strings are one-dimensional topological defects expected to form in the early-universe by many extensions of the Standar Model. In the case of axion models, the resulting strings, called global strings, then decay via the emission of particles and gravitational waves. In this talk, I will present a recent work in which we studied the simultaneous decay of isolated global string loops into scalar particles and GWs using field-theory lattice simulations. For any reasonable value of the string tension, we found the production of GW to be highly suppressed compared to decay into particle radiation. I will also briefly comment on the possible impacts of discretization and finite-volume effects.

Speaker: Jorge Baeza-Ballesteros (University of Valencia/IFIC)

Talk_JorgeBaezaBallesteros.pdf

31 Gauge invariant and self-consistent phase transition calculations

Standard approaches taken in theoretical calculations of phase transition quantities, such as the critical temperature or the bubble nucleation rate, suffer from a wide range of ambiguities: gauge dependence, strong renormalization scale dependence, IR divergences, imaginary potentials… Though a daunting list, these problems can be resolved with the proper methods. The issue is that phase transitions are often influenced by several scales, and that an effective field theory (EFT) approach is needed to separate them. Dimensional reduction can be used to factorize contributions from modes with momenta ~$ \pi T$. There might be further intermediate scales below $ \pi T$ but above the nucleation scale, which also need to be integrated out. A power-counting approach should be undertaken in order to verify that the considered EFT accurately describes the dynamics of bubble nucleation. In this talk, I will demonstrate the use of these methods for calculating observables in a gauge theory with a radiative barrier by comparing calculations of equilibrium quantities in SU(2) + Higgs to lattice data. I will also explain how gauge invariance of the nucleation rate is established. This analysis is generic and can be applied to various cosmological phase transitions, and holds bearing on observables such as the rate of gravitational wave production from the phase transition.

Speaker: Johan Löfgren (Uppsala University)

Nordita-Cosmology-JL-V2.pdf

32 Reliable dynamics in a hot Universe

The discovery of gravitational waves by the LIGO-Virgo collaboration was a watershed moment for fundamental physics. Now, with the hint gravitational waves from pulsar timing arrays, and with the anticipated LISA experiment on the horizon, particle physics, too, turns to studying these waves. Nevertheless, a link between gravitational-waves and fundamental physics can not be formed without understanding the dynamics of hot field-theories in detail. In this talk I will discuss recent progress in describing quantum fields at high temperatures in the early Universe. In particular I will discuss how effective-field theory techniques can be used to calculate gravitational-wave production from phase transition, and how dissipative effects can be included in curved backgrounds.

Speaker: Andreas Ekstedt (Uppsala University)

AndreasEkstedt.pdf

12:30 Closing Remarks

12:40 Lunch

14:00 Lab Tour