Cosmology Seminars

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 in autumn 2022.

Time: Wednesdays 14:15-15:15 Helsinki time, unless otherwise noted.

Remote: Zoom invitations will be sent out on the Cosmology seminars mailing list.

On-site:  Physicum A315    (will be streamed in Zoom)

Format: 45′ + 15′ for questions

Contact: Jaakko Annala

Scheduled Seminars

Spring Term

  • 18.1.2023 Giorgio Torrieri (Campinas State U.), (On-site)
    Title: TBA

    Abstract: TBA

Autumn Term

  • 14.12.2022 Jiamin Hou (U. Florida), (Remote), Note: at 15:15 – 16:15
    Title: Recent Developments in Clustering Analysis of Large-Scale Structure

    Abstract: Large-scale structure provides insights into the Universe’s expansion history and the growth rate of structure. While the 2-point statistics can capture all information in a Gaussian random field, higher-order statistics encode crucial information about gravitationally-induced nonlinearity and deviation from a Gaussian initial condition. Higher-order N-point correlation function or polyspectra as its Fourier counterpart thus allow us more precise knowledge about the energy density content of the Universe and provides unique windows into the primordial Universe. Recently, we showed that higher-order statistics could be used to test parity violation at cosmological scales. In this talk, I will present our analysis and the implication of the measurement. Finally, I will also present our recent analysis of the galaxy skew spectra as a compressed bispectrum estimator and a forecast of its cosmological information.
  • 2.12.2022 Andrew Gow (Portsmouth U., ICG), (On-site) Note: Friday at 14:15
    Title: Primordial black holes: rapid transitions and non-Gaussianity [Video]

    Abstract: Primordial black holes may form in the early universe and could explain some or all of the dark matter. In this talk, I will discuss aspects of PBH formation from inflation. In particular, I will demonstrate the effects from rapid transitions between inflationary phases and discuss whether these are realistic. I will also discuss a non-perturbative method of incorporating primordial non-Gaussianity into the PBH calculation.
  • 30.11.2022 Ivan Rybak (Porto U., Astron. Dept.), (On-site)
    Title: Superconducting cosmic strings with zero thickness approximation [Video]

    Abstract: Some early universe scenarios predict the formation of cosmic strings. Moreover, it is generic to anticipate that these one-dimensional topological defects are superconducting. We consider an effective action to describe the dynamics and evolution of current-carrying cosmic strings. We demonstrate how the presence of a current on cosmic strings can change observational predictions, particularly CMB anisotropies and gravitational wave radiation.
  • 23.11.2022 Oksana Iarygina (Nordita), (On-site)
    Title: Multi-field effects in the early universe [Video]

    Abstract: Nowadays, the search for primordial gravitational waves is mainly focused on the parity-odd polarization pattern in the CMB the B-modes. A correct interpretation of B-mode measurements strongly relies on understanding their production mechanism. In the first part of my talk I will describe the viability of inflation with a spectator sector comprised of non-Abelian gauge fields coupled through a higher order operator. I will discuss the theoretical restrictions for the amplitude and tensor tilt for chiral gravitational waves as well as the maximum possible enhancement of the gravitational wave background with respect to the one coming from vacuum fluctuations. Gravitational waves can also be produced during reheating. The reheating era, that connects inflation and big-bang nucleosynthesis, is still very weakly constrained. Inefficient preheating can lead to a prolonged matter-dominated phase after inflation, changing the time during inflation when the Cosmic Microwave Background (CMB) modes exit the horizon. This shifts CMB predictions and thus can break the degeneracy of otherwise indistinguishable inflation models. In the second part of the talk I will demonstrate how physical mass scales of multi-field preheating control the dynamics and observable predictions of models of inflation with hyperbolic manifolds.
  • 16.11.2022 Volodymyr Takhistov (KEK, Tsukuba), (Remote)
    Title: Marvelous Manifestations of Primordial Black Holes: from Multiverse to Neutron Star Explosions

    Abstract: Primordial black holes from the early Universe constitute an attractive non-particle dark matter candidate and appear in a multitude of theoretical models. I will discuss novel observational signatures for primordial black holes spanning orders of magnitude in mass-range, connecting them to recent breakthroughs in gravitational wave and multi-messenger astronomy as well as long-standing astrophysical puzzles such as the origin of heavy elements. Primordial black holes from intriguing scenario of bubble multiverse might have already been seen by Subaru HSC.
  • 9.11.2022 Sebastian von Hausegger (Oxford U.), (Remote)
    Title: The cosmic matter dipole [Video]

    Abstract: The ever-growing quantity and quality of astronomical data today has underpinned both the LCDM model of cosmology and general relativity as effective working theories. Yet also tensions between theoretical prediction and observation, and even between different observations arose. I will centre my presentation on one such discrepancy — the excess amplitude of the cosmic matter dipole in the angular number counts of high-redshift galaxies. We established this discrepancy at high significance across frequencies by compiling high-redshift galaxy catalogues in the mid-IR and the radio. In particular, these observations represent a significant challenge to the Cosmological Principle, upon which the LCDM model is built, evocative of inhomogeneous and anisotropic cosmological models. I will present details of this analysis, and also provide an overview of the relevant literature and elaborate on consequences as well as future research directions. This talk is based on https://arxiv.org/abs/2009.14826 and https://arxiv.org/abs/2206.05624.
  • 26.10.2022 George Zahariade , (Remote)
    Title: Quantum Mechanics of Gravitational Waves [Video]

    Abstract: We study the effect of a quantized gravitational wave on a LIGO-type gravitational wave detector. We find that the arm-length is subject to a stochastic tidal force whose properties depend on the exact quantum state of the gravitational field, if the gravitational field is quantized. The quantum nature of gravity may thus be detectable as an additional noise source at gravitational wave detectors.
  • 19.10.2022 Sarif Khan (Gottingen U.), (On-site)
    Title: WIMP and FIMP Dark Matter in Singlet-Triplet Fermionic Model [Video]

    Abstract: In this talk, I will present an extension of the SM involving three triplet fermions, one triplet scalar and one singlet fermion, which can explain both neutrino masses and dark matter. One triplet of fermions and the singlet are odd under a $Z_2$ symmetry, thus the model features two possible dark matter candidates. The two remaining $Z_2$-even triplet fermions can reproduce the neutrino masses and oscillation parameters consistent with observations. We consider the case where the singlet has feeble couplings while the triplet is weakly interacting and investigate the different possibilities for reproducing the observed dark matter relic density. This includes production of the triplet WIMP from freeze-out and from decay of the singlet as well as freeze-in production of the singlet from decay of particles that belong to the thermal bath or are thermally decoupled. While freeze-in production is usually dominated by decay processes, we also show cases where the annihilation of bath particles give substantial contribution to the final relic density. This occurs when the new scalars are below the TeV scale, thus in the reach of the LHC. The next-to-lightest odd particle can be long-lived and can alter the successful BBN predictions for the abundance of light elements, these constraints are relevant in both the scenarios where the singlet or the triplet are the long-lived particle.In the case where the triplet is the DM, the model is subject to constraints from ongoing direct, indirect and collider experiments. When the singlet is the DM, the triplet which is the next-to-lightest odd particle can be long-lived and can be probed at the proposed MATHUSLA detector. Finally we also address the detection prospects of triplet fermions and scalars at the LHC.
  • 12.10.2022 Alexey Boyarsky (Leiden U. and CERN) , (Remote) (Cancelled)
  • 28.09.2022 Enrico Pajer (Cambridge U., DAMTP), (Remote)
    Title: Bootstrapping large graviton non-Gaussianities [Video]

    Abstract: Gravitational interferometers and cosmological observations of the cosmic microwave background offer us the prospect to probe the laws of gravity in the primordial universe. To study and interpret these datasets we need to know the possible graviton non-Gaussianities. To this end, I’ll derive the most general tree-level three-point functions (bispectra) for a massless graviton to all orders in derivatives, assuming scale invariance. Instead of working with explicit Lagrangians, I’ll take a bootstrap approach and obtain our results using the recently derived constraints from unitarity, locality and the choice of vacuum, which will be reviewed during the talk. Since I’ll make no assumptions about de Sitter boosts, the results capture the phenomenology of large classes of models such as the effective field theory of inflation and solid inflation. I’ll present formulae for the infinite number of parity-even bispectra as well. Remarkably, for parity-odd bispectra, unitarity allows for only a handful of possible shapes: three for graviton-graviton-graviton, three for scalar-graviton-graviton and one for scalar-scalar-graviton, which we bootstrap explicitly. These parity-odd non-Gaussianities can be large, for example in solid inflation, and therefore constitute a concrete and well-motivated target for future observations.
  • 21.09.2022 Takashi Toma (Kanazawa U.), (On-site)
    Title: Ultra-violet completion of a pseudo-Nambu-Goldstone dark matter model [video]

    Abstract: Dark matter direct detection experiments impose strong bounds on the elastic scattering cross section with nuclei. A pseudo-Nambu-Goldstone dark matter with a soft explicit breaking mass term suppresses the amplitude for the elastic scattering in non-relativistic limit, and thus can naturally avoid the strong bound of the dark matter direct detection experiments. In this talk, we build an ultra-violet complete model of the pseudo-Nambu-Goldstone dark matter based on a gauged U(1) B-L symmetry. Namely, the soft explicit breaking term can be derived from high energy. In addition, the gauge coupling unification is achieved by further embedding the model in SO(10).
  • 14.09.2022 Archie Cable (Imperial Coll., London), (On-site)
    Title: Second-order stochastic theory for self-interacting scalar fields in de Sitter spacetime [Slides]

    Abstract: The study of scalar field theory in de Sitter spacetime is of great importance in many facets of inflationary cosmology; however current perturbative techniques in QFT contain IR divergences that seriously limit its computational power. Instead, we turn to effective theories. One such theory is the stochastic approach, where one utilises the inflationary expansion to write the scalar field equations in a stochastic framework. In this talk, I will introduce a second-order stochastic effective theory of spectator scalar fields in de Sitter in the light field limit, extending the stochastic approach beyond the massless limit and demonstrating how it can be used to compute long-distance correlation functions non-perturbatively.
  • 7.09.2022 Mikhail M. Ivanov (IAS, Princeton), (Remote) 
    Title: Fundamental Physics with Galaxy Surveys [video]

    Abstract: The distribution of galaxies on large scales is a sensitive probe of cosmological physics. In particular, the structure of this distribution depends on properties of dark matter and the dynamics of the early universe. Understanding this dependence, however, is a challenging task because the observed galaxy distribution is modulated by a variety of non-linear effects. I will present the effective field theory of large-scale structure (EFT) – an innovative theoretical tool that has allowed for a systematic analytic description of these effects. The EFT plays a central role in a new program of extracting cosmological information from galaxy surveys. I will share some results of this program from my independent analysis of the public data from the Baryon acoustic Oscillation Spectroscopic Survey. These results include the measurement of the Hubble constant as well as constraints on new physics and the early Universe.

Spring Term 2022

  • 11.05.2022 Daniel Carney (LBL, Berkeley)
    Title: Dark matter searches with mechanical quantum sensors [video]

    Abstract: Mechanical objects read out by light in the quantum regime have recently enabled the detection of gravitational waves with LIGO. While the LIGO mirrors are huge, roughly 40kg, similar measurement techniques can be applied to devices at a wide variety of mass scales, from macro-scale objects down to single ions and electrons. I will outline a nascent set of experimental programs utilizing these opto-mechanical devices to search for a wide variety of potential dark matter candidates. In particular, I’ll discuss the long-term prospect of designing a detector that could detect heavy (m ~ m_planck) dark matter candidates directly, purely via their gravitational coupling to the visible sector.
  • 27.04.2022 Dražen Glavan (Prague, Inst. Phys.)
    Title: Nonlinear dissipation and how to describe it for long times [video]

    Abstract: Relaxation of systems to equilibrium is often described by linear physics. However, this need not be so and the relaxations does not need to be exponential. Furthermore, there are other transient phases where the system is not close to equilibrium. Reheating is one such example, where an oscillating inflaton is dissipating by producing other particles it couples to. Different kinds of interactions lead to qualitatively different ways in which the inflaton dissipates. These are missed by the time dependent perturbation theory that suffers from secular divergences and cannot describe the long time behaviour. I will present efficient and conceptually simple resummation methods appropriate to quantify the long time behaviour of the oscillating scalar condensate and compare them to the more known approaches. I will also draw some parallels to the dynamical renormalization group often utilized in inflation and point out its limitations.
  • 20.04.2022 Glenn D. Starkman (Case Western Reserve U. and Imperial Coll., London)
    Title: Macroscopic dark matter, and how we might find it [video]

    Abstract: To be dark, matter doesn’t have to have a low scattering cross-section, it can just be massive, and so have a very low abundance. Primordial black holes exploit this fact, but they are probably at least 10^15g, and so extremely rare — best searched for astrophysically/cosmologically. Dark matter more massive than about 50g can be much less dense and so we can hope that terrestrial processes will allow us to find them. Most models for such macroscopic dark matter envisions it being of approximately nuclear density. I will discuss what we know about the mass versus cross-section parameter space for these objects, and how we might make progress on searching it.
  • 06.04.2022 Gongjun Choi (CERN)
    Title: Cosmic Birefringence and Electroweak Axion Dark Energy [video]

    Abstract: In this talk, we discuss the electroweak (EW) axion quintessence dark energy in light of the recently reported non-zero rotation angle of the cosmic microwave background (CMB) linear polarization β=0.35±0.14deg. We further discuss the model’s prediction on the equation of state of EW axion dark energy with the aid of the cosmic birefringence.
  • 23.03.2022 Mustafa A. Amin (Rice U.)
    Title: A Spin on Wave Dark Matter [video]

    Abstract: Can the intrinsic spin of light dark matter manifest itself on macroscopic/astrophysical scales? I will first show that in the non-relativistic limit, bosonic spin-s dark fields are described by a 2s+1 component Schrodinger-Poisson (SP) system. I will then show that in this multi-component SP system, there exist solitons with macrosopic spin. Using 3+1 dimensional simulations of the SP system, I will discuss how the small scale structure in spin-1 (vector) dark matter can differ from the same in spin-0 (scalar) dark matter. Halo cores in vector dark matter are less dense and more extended than their scalar counterparts, they can have macroscopic spin, and the interference patterns in the halo are less pronounced — all leading to potential observational avenues to distinguish the two.
  • 02.03.2022 Christian G. Boehmer (University Coll. London)
    Title: Modified Theories of Gravity — Foundations and Models [video]

    Abstract: In the first part of the talk I am discussing General Relativity, in particular its basic ingredients and its mathematical structure. This will naturally lead the way to consider various modifications or extensions of General Relativity, many of which have been studied recently. Next I will discuss modified gravity models based on generalised geometries and on actions no longer linear in curvature. The main part of the talk will discuss how these many different theories can be studied using a single unified approach which also shows the equivalence of some of these models. Boundary terms in the action will play a crucial role in establishing the equivalence between different theories. The final part discusses the study of cosmological models using dynamical systems techniques.
  • 16.02.2022 Philip J. Bull (Queen Mary, U. of London and Western Cape U.)
    Title: 21cm intensity mapping and the limits of cosmography [video]

    Abstract: Line intensity mapping is a relatively new observational technique that seeks to map out the 3D structure of the Universe using low angular resolution spectroscopic instruments. Intensity mapping with the 21cm line of neutral hydrogen is a particularly popular approach, as it allows such maps to be made over a truly vast redshift range, spanning from the local Universe all the way back to the cosmic Dark Ages. In this talk I will try to appease both observers and theorists, by reviewing some of the exciting applications of forthcoming large 21cm intensity mapping surveys while also giving an overview of the systematics challenges they face. I will particularly focus on an ongoing “autocorrelation” survey with MeerKAT, a 64-dish radio telescope in South Africa that is a precursor to the Square Kilometre Array, before turning to future prospects for constraining dark energy models with gigantic arrays (10,000’s of dishes).
  • 09.02.2022 Lukas Witkowski (Paris, Inst. Astrophys.)
    Title: Detecting primordial features with gravitational waves [video]

    Abstract: The nascent field of gravitational wave astronomy can give us new insights about inflation where Cosmic Microwave Background data is not available. In particular, it allows for experimentally testing models of inflation that at some point depart from the single-field slow-roll paradigm, as motivated by ultraviolet completions of inflation, which can lead to a characteristic oscillation in the stochastic gravitational wave background (see also the previous seminar by Jacopo Fumagalli). I will explain what a detection of these oscillations can teach us about inflation and comment on the prospects of observing this oscillatory signal with upcoming gravitational wave observatories such as LISA.
  • 02.02.2022 Jacopo Fumagalli (Madrid, IFT and Madrid, Autonoma U.)
    Title: Lighting the dark ages of inflation: features in the stochastic gravitational wave background [video]

    Abstract: Observational constraints on inflation have so far concentrated on the CMB and Large Scale Structure surveys. After a broad introduction, I will show how phenomena arising during unconstrained stages of inflation, and motivated by high energy embeddings, lead to characteristic oscillatory patterns in the stochastic gravitational wave background. Probing these features could, for instance, establish the existence of heavy particles beyond the reach of terrestrial experiments, and even test the inflationary paradigm or point to alternatives to it. This provides a clear target for gravitational wave observatories as well as a challenge for developing dedicated data analysis techniques to look for this unique insight into the physics of the early universe.
  • 26.01.2022 (at 16:15) Dillon Brout (Harvard-Smithsonian Ctr. Astrophys.)
    Title: The Recent Results From The Pantheon+ Sample and SH0ES Analysis [video]

    Abstract: It has now been 5 years since the last main analyses of the Hubble constant and cosmic acceleration by the SH0ES and Pantheon teams. Since then, both the datasets of cepheid calibrators and Type Ia supernovae have more than doubled in size. Meanwhile there have also been critical leaps in our understanding of systematics and improvements to calibration. In this talk I will detail the progress made towards a 1km/s/Mpc measurement of the Hubble constant and measurements of dark energy.
  • 19.01.2022 Pierre Sikivie (Florida U.)
    Title: Cold dark matter caustics [video]

    Abstract: On a sunny breezy day, sharp lines of light dance on the bottom of a swimming pool. They are due to folds – sometimes called ‘caustics’, or ‘catastrophes’ – in the wavefront of light from the Sun. Caustics also arise naturally in the distribution of dark matter in space. The dark matter density is very large at the location of a caustic. I’ll show that the late infall of cold dark matter onto isolated galaxies, such as our own, produces discrete flows throughout the galactic halo, and associated caustics. One set of caustics are topological spheres surrounding the galaxy. Another set are rings in the galactic plane. Caustic rings are closed tubes whose cross-section is a D_{-4} catastrophe. I’ll argue on theoretical and observational grounds that the caustic ring radii a_n (n=1,2,3..) obey the approximate law: a_n goes like 1/n. There is evidence for these rings in the distribution of bumps in the rotation curves of spiral galaxies, including our own Milky Way. The implications for dark matter searches will be discussed.