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, during term time.

Place: All seminars in the autumn term 2020 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

  • 12.02.2020 Dimitrios Karamitros (NCBJ, Warsaw)
    Title: Recalculating freeze-in
    Abstract: The freeze-in mechanism has become very popular recently, due to its ability to explain the dark matter relic abundance with suppressed couplings, and thus (usually) evading all experimental constraints. However, the production of dark matter often is not treated carefully, and so some aspects of frozen-in relics are not pointed-out. In this talk, I will try to summarize some of the ways freeze-in can be different than one expects. In sort, I will show as generally as possible the effects of thermal masses, quantum statistics, and potential non-standard cosmological expansion on freeze-in.
  • 04.03.2020 Valentina De Romeri (IFIC)
    Title:  Searches for new physics with neutrino experiments
    Abstract: In this talk I will discuss the possibility of unveiling new physics with neutrino experiments. I will focus on two different cases. In the first part of the talk I will use the recent observation of coherent elastic neutrino-nucleus scattering (CEvNS) to probe neutrino generalised interactions. Then, I will investigate opportunities for detecting light dark matter at DUNE, a next generation long baseline neutrino experiment.
  • 01.04.2020 Jonathan Gair (Max Planck Institute)

    Cancelled

  • 22.04.2020 Saul Ramos-Sanchez (Technical University Munich and UNAM, Mexico)

    Cancelled

  • Autumn Term

  • 02.09.2020 Daniel Litim (University of Sussex)
    Title: Asymptotic safety and physics beyond the Standard Model
    Abstract: Ultraviolet fixed points are key for a fundamental definition of quantum field theory. Fixed points can be free such as in asymptotic freedom, or interacting such as in asymptotic safety. Today, I discuss the status and prospects for asymptotic safety in particle physics. This includes an overview of rigorous results for interacting fixed points in general 4d quantum field theories at weak coupling, necessary conditions, no-go theorems, and links with conformal field theory. These results are then used to find asymptotically safe extensions of the Standard Model which can explain the muon and electron g-2 anomalies. If time permits, I will also comment on the status for asymptotic safety of gravity.
  • 09.09.2020 Lisa Glaser (University of Vienna)
    Title: Spectral dimension and other ways to recover geometric information from spectral triples
    Abstract: A compact manifold can be described through a spectral triple, consisting of a Hilbert space H, an algebra of functions A and a Dirac operator D. But what if we are given a spectral triple? Then the situation is more complicated, it is not clear how a spectral triple, in particular one with a non-commutative algebra A, or a finite hilbert space H, relates back to manifolds, or geometry in a more general way. But these are questions one would like to ask if trying to use spectral triples to possibly quantize gravity. In this talk I will explore the spectral dimension of random spectral triples as a possible way to characterize them, and also show how we can recover metric information from a truncation of a spectral triple.
  • 16.09.2020 Chris Shepherd (University of Manchester)
    Title: A heatwave affair: mixed Higgs-R^2 preheating on the lattice
    Abstract: Nonminimal Higgs inflation occupies a special position amongst inflationary theories, as it solves the horizon and flatness problems without introducing new physics between the electroweak and Planck scales. However, the theory becomes strongly-coupled during reheating, which cannot be studied using conventional methods. This issue may be remedied by including an R^2 term in the model Lagrangian, and for certain parameters the theory is perturbative up to the Planck scale. We use a semiclassical lattice approach to perform the first nonlinear study of preheating in the R^2-healed theory, in the regime where the curvature-squared coupling β and nonminimal coupling ξ of the Higgs field contribute similarly to the CMB scalar perturbations. Preheating occurs first through tachyonic production of Higgs bosons, and later scattering off the homogeneous inflaton field. We generalise our results for the parameter range 1.1 × 10^9 < β < 1.8 × 10^9 to “Higgs-like” parameters with smaller β, where observables saturate the bound of instantaneous preheating. All predictions for the spectral index and tensor-to-scalar ratio lie within the 1σ region of measurements by the Planck satellite, but a future ground-based experiment optimised for 21~cm tomography may be able to discriminate the mixed Higgs-curvature inflation from the pure Higgs and R^2 theories.
  • 23.09.2020 Juan S. Cruz (Technical University of Munich)
    Title: Gradient effects on false vacuum decay in gauge theory
    Abstract: We study false vacuum decay for a gauged complex scalar field in a polynomial potential with nearly degenerate minima. Radiative corrections to the profile of the nucleated bubble as well as the full decay rate are computed in the planar thin-wall approximation using the effective action. This allows to account for the inhomogeneity of the bounce background and the radiative corrections in a self-consistent manner. In contrast to scalar or fermion loops, for gauge fields one must deal with a coupled system that mixes the Goldstone boson and the gauge fields, which considerably complicates the numerical calculation of Green’s functions. In addition to the renormalization of couplings, we employ a covariant gradient expansion in order to systematically construct the counterterm for the wave-function renormalization. The result for the full decay rate however does not rely on such an expansion and accounts for all gradient corrections at the chosen truncation of the loop expansion. The ensuing gradient effects are shown to be of the same order of magnitude as non-derivative one-loop corrections.
  • 07.10.2020 Clare Burrage (University of Nottingham)
    Title: Testing dark energy models with atom interferometry
    Abstract: The accelerated expansion of the universe motivates a wide class of scalar field theories that modify gravity on large scales. In regions where the General Relativity has been confirmed by experiment, such theories need a screening mechanism to suppress the new force. I will describe how theories with screening mechanisms can be tested in the laboratory, in particular with atom-interferometry experiments. I will describe the results of a recent experiment in which we measured the acceleration of an atom toward a macroscopic test mass inside a high vacuum chamber, where the new force is unscreened in some theories. Our measurement shows that the attraction between atoms and the test mass does not differ appreciably from Newtonian gravity. This result places stringent limits on the free parameters in chameleon and symmetron theories of modified gravity.
  • 21.10.2020 Marco Drewes (University of Louvain, CP3)
    Title: Sterile neutrinos as Dark Matter candidates
    Abstract: Massive sterile neutrinos appear in many extensions of the Standard Model of particle physics. For sufficiently small mixing angles with ordinary neutrinos, their lifetime can exceed the age of the universe, and they are viable Dark Matter candidates. We give a pedagogical introduction and review recent progress in the study of sterile neutrino Dark Matter. We mostly focus on the minimal, highly testable scenario known as nuMSM, and briefly comment on alternative production mechanisms, such as the decay of heavy particles.
  • 28.10.2020 Tessa Baker (Queen Mary University of London)
    Title: Testing gravity with gravitational waves
    Abstract: Gravitational waves (GWs) have already proved immensely powerful for constraining cosmological extensions of GR, both from data-driven and theoretical perspectives. However, GWs really come into their own when used in combination with complementary electromagnetic data. I’ll start by reviewing some of the bounds on extended gravity theories from GW detections to date. I’ll introduce the formalism, the phenomenology, and the astrophysical pitfalls of these tests. Finally, we’ll explore the impact of future experiments like LISA and accompanying galaxy surveys on the remaining parameter space of modified gravity theories.
  • 29.10.2020 Neil Cornish (Montana State University)
    Title: On the road to low frequency gravitational wave detection: NANOGrav at mile marker 12.5
    Abstract: The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) monitors a large array of milli-second pulsars that together form a galactic scale gravitational wave detector. Analysis of 12.5 years of data collected between July 2004 and June 2017 show strong evidence for a common red-noise process. This could be the harbinger of a stochastic background of gravitational waves, or something more mundane such as a heretofore unknown type of low frequency noise. The current analysis shows scant evidence for the tell-tale quadrupolar correlations expected for a gravitational wave signal, but if the red-noise is indeed due to gravitational waves, these correlations should become clearly visible with the addition of another 3-5 years of data. Moreover, forecasts show that when the correlations are detected, measurements of the amplitude and spectral slope will be sufficiently precise to distinguish between potential sources such as cosmic strings and supermassive black holes, and to constrain the underlying astrophysical models.
  • 04.11.2020 Giorgio Arcadi (Roma Tre University)
    Title: Collider prospects for vector Dark Matter coupled with the Higgs
    Abstract: Searches of invisible decays of the SM-like Higgs represent one of the most interesting strategies for collider detection of potential Dark Matter candidates. We provide a reappraisal of some theoretical and phenomenological aspects related to the case of a vectorial Dark Matter candidate.
  • 11.11.2020 Hayley Macpherson (University of Cambridge)
    Title: An improved calculation of cosmological backreaction in simulations with numerical relativity
    Abstract: High-precision cosmological surveys are due to deliver measurements accurate to the percent level. In order to ensure we correctly interpret these data, we need to be sure that our cosmological model is accurate. The current standard model assumes that the Universe is homogeneous and isotropic. These assumptions are often justified by the fact that our Universe is homogeneous and isotropic on large scales, however, we are therefore smoothing over highly nonlinear structures on small scales. This smoothing feeds back onto the large-scale evolution of the Universe in the context of nonlinear General Relativity. The true size of this backreaction of small-scale structures has been debated for decades. I will present our cosmological simulations of large-scale structure formation using numerical relativity. These simulations allow us to directly calculate cosmological backreaction in a background-free, symmetry-free framework. I will present new results using a recent improved formalism for averaging in general fluid cosmologies (Buchert et. al 2020), providing a calculation of backreaction with minimal foliation dependence.
  • 18.11.2020 Amel Durakovic (Technical University of Denmark and NBI)
    Title: A Map Between Primordial Power Spectra and the Effective Field Theory of Inflation
    Abstract: We have developed a precise dictionary between the spectrum of primordial density fluctuations and the parameters of the effective field theory (EFT) of inflation that determine the primordial power spectrum (PPS). At lowest order the EFT contains two parameters: the slow-roll parameter, which acts as an order parameter, and the speed of sound. Applying second-order perturbation theory, we provide maps from the PPS to the EFT parameters that are precise up to the cube of the fractional change in the PPS, or less than 1% for spectral features that modulate the PPS by 20%. While such features are not required when the underlying cosmological model is assumed to be ΛCDM they are necessary for alternative models that have no cosmological constant/dark energy. We verify the dictionary numerically and find those excursions in the slow-roll parameter that reproduce the PPS needed to fit Planck data for cosmological models with and without a cosmological constant.
  • 25.11.2020 Mairi Sakellariadou (King’s Coll. London)
    Title: Gravitational Waves: the theorist’s swiss knife

    Abstract: After a short introduction to the stochastic GW background I will highlight how one uses currently available LIGO/Virgo data not only to learn about compact binaries and the large-scale-structure of our universe, but also to constrain particle physics models beyond the Standard Model, modified gravity proposals, and even quantum gravity theories.
  • 02.12.2020 Anastasiia Filimonova (Heidelberg University/Nikhef)
    TBA.
  • 09.12.2020 Mindaugas Karciauskas (Complutense, Madrid)
    TBA.