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.

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

Place: All seminars in 2022 will be held remotely via Zoom. Zoom invitations will be sent out on the Cosmology seminars mailing list.

Format: 45′ + 15′ for questions

Contact: Sami Raatikainen

Scheduled Seminars

Spring Term

  • 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.