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 2021 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

Autumn Term

  • 08.12.2021 Ariel Megevand (Mar del Plata U.)
    Title: TBA

    Abstract: TBA
  • 24.11.2021 Tracy Slatyer (MIT)
    Title: TBA

    Abstract: TBA
  • 17.11.2021 Jorinde van de Vis (DESY)
    Title: TBA

    Abstract: TBA
  • 27.10.2021 (at 16:15) Malte Buschmann (Princeton U.)
    Title: TBA

    Abstract: TBA
  • 20.10.2021 (at 16:15) Eleanor Hall (UC, Berkeley)
    Title: TBA

    Abstract: TBA
  • 13.10.2021 Graham Ross (Oxford U.)
    Title: R^2/Higgs inflation and the hierarchy problem

    Abstract: We analyse Starobinsky (R^2) inflation in the presence of the Brout Englert Higgs (BEH) boson with non-minimal coupling to the Ricci scalar, R. It is shown how contact terms are needed to determine the effective Lagrangian which has a new scalar field, the scaleron, associated with the R^2 term. The heavy scaleron couples to the BEH boson and radiatively generates a contribution to the BEH mass, resulting in an unacceptably high level of fine tuning, greater than 1 part in 10^10. This is significantly reduced to one part in 10^4 in a scale invariant version of the model. We analyse the consistency with electroweak vacuum stability both during and after inflation and discuss phenomenological aspects of the models.
  • 06.10.2021 (at 16:15) Itamar Allali (Tufts U., Inst. of Cosmology)
    Title: Gravitational Decoherence with Applications to Dark Matter Phenomenology [video]

    Abstract: Nontrivial quantum arrangements of matter, such as Schrodinger cat-like states, are sensitive to decoherence from their environment. However, matter that interacts only gravitationally is weakly coupled to its environment, and thus may exhibit slower rates of decoherence. Since dark matter (DM) may only interact via gravity, we explore the decoherence rate of a dark-matter-Schrodinger-cat-state (DMSCS). In the nonrelativistic approximation of gravity, we find that a superposition of distinct DM density profiles can undergo decoherence from the scattering of nearby standard model (SM) particles on observable timescales. In addition, when considering light bosonic DM like an axion, one can conceive of a superposition of the phase of oscillation of the scalar (axion) field, requiring a truly relativistic formalism of gravitational scattering. We derive such a formalism and find that for typical DM populations in the Milky Way, a DMSCS of the axion phase can maintain quantum coherence for exponentially long times, while exotic configurations including DM near a black hole and dense boson stars can experience rapid decoherence. This can have potential observable consequences for direct detection experiments that are sensitive to the axion’s phase, such as haloscopes which rely on resonant cavities to detect axions. This talk will be based on the work in Refs. 2005.12287, 2012.12903, and 2103.15892.
  • 22.09.2021 Scott Melville (Cambridge U., DAMTP)
    Title: Positivity Bounds for Cosmology [video]

    Abstract: “Positivity bounds” are a powerful tool in particle physics which use scattering amplitudes to connect large-scale phenomenology with the underlying fundamental physics on small scales. This talk will describe the recent progress in bringing this machinery to cosmology, and show how the basic requirements of unitarity, causality and locality on small scales can be used to place constraints on scalar-tensor theories of dark energy and on the non-Gaussianities in the CMB which can arise from single-field inflation.
  • 15.09.2021 Kensuke Akita (IBS, Daejeon, CTPU)
    Title: Precise Capture Rate of the Cosmic Neutrino Background [video]

    Abstract: The Cosmic Neutrino Background (CνB) is one of the most promising candidates, containing a wealth of information about the universe and neutrino physics. Although the direct detection of the CνB is still difficult, a precise understanding of the CνB and its direct observations would be important. In this talk, I will discuss the number density of the CνB and the capture rates of its direct observations, including sub-leading cosmological contributions, such as the neutrino spectral distortions from their decoupling and neutrino clustering on our Galaxy and nearby galaxies. In particular, I will focus on cosmic neutrino capture on tritium, which is the most discussed method so far.

Spring Term

  • 19.05.2021 (at 16:15) James M. Cline (McGill U.)
    Title: A little theory of everything [video]

    Abstract: I present a minimal model that attempts to address the main missing ingredients of the standard model: inflation, baryogenesis, dark matter, and the origin of neutrino masses. We introduce a complex inflaton that decays into three generations of GeV-scale heavy neutral leptons, creating a lepton asymmetry during inflation. One HNL is stable and provides (partially) asymmetric dark matter. A light scalar singlet is needed to suppress its symmetric relic density. Neutrino masses are generated by the usual seesaw mechanism, with heavy right-handed neutrinos above the inflation scale, and an MFV-like ansatz that relates neutrino masses to the HNL couplings, that are then linked to the light neutrino properties with only one adjustable parameter. The stability of dark matter implies the lightest neutrino is massless. The model is highly testable, and could explain excess events seen by KOTO.
  • 12.05.2021 Jessica Turner (Durham U.)
    Title: Towards an all-orders calculation of the electroweak bubble wall velocity [video]

    Abstract: In this talk I discuss recent work where we calculate the velocity of the Higgs condensate bubble wall during a first-order electroweak phase transition in the early Universe. The interaction of particles with the bubble wall can be accompanied by the emission of multiple soft gauge bosons. When computed at fixed order in perturbation theory, this process exhibits large logarithmic enhancements which must be resummed to all orders when the wall velocity is large. We perform this resummation both analytically and numerically at leading logarithmic accuracy. The numerical simulation is achieved by means of a particle shower in the broken phase of the electroweak theory. The two approaches agree to the 10% level. For fast-moving walls, we find the scaling of the thermal pressure exerted against the wall to be P~γ^2T^4. This is impactful for baryogenesis, gravitational wave radiation and generation of other cosmic relics.
  • 28.04.2021 Susana Cebrián (Zaragoza U.)
    Title: Dark matter annual modulation results from 3 years exposure of the ANAIS-112 experiment [video]

    Abstract: For dark matter particles in the Milky Way’s halo, an annual modulation in the interaction rate is expected by the revolution of the Earth around the Sun; an annual modulation signal compatible with expectations has been observed by the DAMA/LIBRA experiment for about twenty years, being one of the most puzzling experimental results in the field as it has not been confirmed by other dark matter direct detection experiments. The ANAIS experiment, based on the same target and technique using 112.5 kg of sodium iodide as target, is taking data smoothly since August 2017 at the Canfranc Underground Laboratory in Spain aiming at testing the observation by the DAMA/LIBRA experiment in a model independent way.

    The results on the search for modulation in the ANAIS-112 experiment were firstly derived applying a blind procedure for an exposure of 157.5 kg·y [1] and later updated from two years of data corresponding to 220.7 kg·y [2]. Before unblinding the data, the whole analysis procedure was fixed [3], the background of the experiment thoroughly studied [4] and the expected sensitivity evaluated [5]. Now, the analysis of three years of data for an expsoure of 313.95 kg·y has been presented, following the same method but improving the background modelling in the fitting of the rates in the region of interest [6]. The best fits obtained for the modulation amplitude in the [1-6] keV ([2-6] keV) energy regions are (-0.0034)±0.0042 cpd/kg/keV (0.0003±0.0037 cpd/kg/keV), supporting the absence of modulation in the data and being incompatible with the DAMA/LIBRA result at 3.3 (2.6) σ, for a sensitivity of 2.5 (2.7) σ. In addition, some complementary analyses (a phase-free annual modulation search and the exploration of the possible presence of a periodic signal at other frequencies) have been made together with several consistency checks. All the obtained results have confirmed the ANAIS-112 projection of reaching a 3σ sensitivity for the scheduled 5 years of operation.

    In this seminar, after reminding the DAMA/LIBRA conundrum and the description of the ANAIS-112 experiment including the set-up, performance and analysis methods, the obtained results from the annual modulation analysis will be shown and their implications and the future prospects will be discussed.

    [1] First results on dark matter annual modulation from ANAIS-112 experiment, J. Amaré et al, Phys. Rev. Lett. 123 (2019) 031301.
    [2] ANAIS-112 status: two years results on annual modulation, J. Amaré et al, J. Phys.: Conf. Ser. 1468 (2020) 012014.
    [3] Performance of ANAIS-112 experiment after the first year of data taking, J. Amaré et al, Eur. Phys. J. C (2019) 79:228.
    [4] Analysis of backgrounds for the ANAIS-112 dark matter experiment, J. Amaré et al, Eur. Phys. J. C (2019) 79:412
    [5] ANAIS-112 sensitivity in the search for dark matter annual modulation, I. Coarasa et al, Eur. Phys. J. C (2019) 79:223.
    [6] Annual Modulation Results from Three Years Exposure of ANAIS-112, J. Amaré et al, arXiv:2103.01175 [astro-ph.IM]

  • 21.04.2021 Deanna C. Hooper (Brussels U.)
    Title: Cracks in the CDM paradigm: NCDM as a way forward [video]

    Abstract: Despite its remarkable success, the standard cosmological paradigm has been challenged lately by a growing tension in the Hubble Constant measurements, as well as a mismatch between simulations and observations on smaller scales. Combined with the lack of detection of dark matter in any experiment, this has called into question the standard cold dark matter paradigm, and reinvigorated interest in non-cold dark matter models. Such models are characterised by modified dark matter couplings or production mechanisms. In this talk I will review two such models: dark matter interacting with an additional dark sector, and non-cold dark matter produced by primordial black hole evaporation. These scenarios leave a clear imprint in the matter power spectrum on small scales, making them an ideal target to be constrained with Lyman-alpha data. I will discuss new ways of extracting information from Lyman-alpha data for non-cold dark matter scenarios, and present up-to-date constraints on these models.
  • 14.04.2021 Constantinos Skordis (Prague, Inst. Phys.)
    Title: New gravitational degrees of freedom as a solution to the dark matter problem

    Abstract: Cosmological and astronomical observations indicate that the majority of mass and energy density of fields in the universe are in a form which interacts extremely weakly, if at all, with light. The standard interpretation is the existence of dark matter, commonly thought to be in the form of particles not part of the standard model of particle physics. At present a firm detection of such a particle is lacking, and moreover, all these observations concern a mismatch between the observed dynamics of visible matter with its gravitational influence. Hence, a less explored interpretation is that the underlying theory of gravity may not be General Relativity. A hint that this may be the case is the observation by Milgrom that discrepancies concerning galaxies are controlled by a single, seemingly universal, acceleration scale.

    In this talk, I will discuss this possibility and focus on a particular relativistic realization constructed to reproduce Milgrom’s Modified Newtonian Dynamics law at the scale of galaxies. I will show that this proposal leads to (i) correct gravitational lensing on galactic scales, (ii) tensor modes propagating at the speed of light, and (iii) cosmological evolution in line with observations of the Cosmic Microwave Background anisotropies and the large-scale structure power spectrum. I will show that propagating fluctuations around Minkowski spacetime are healthy and discuss possibilities towards creating a more fundamental theory with these properties.

  • 07.04.2021 Enrico D. Schiappacasse (Jyväskylä U., HIP)
    Title: A novel way to detect the QCD Axion [video]

    Abstract: Since the QCD axion is predicted to interact very weakly with Standard Model particles, probing the QCD axion dark matter (DM) hypothesis is quite challenging. As a result, it is the crucial importance to explore new avenues for the QCD detection including those set in the astrophysical context. We propose a new striking avenue to probe the QCD axion dark matter (DM) hypothesis via transient radio signatures coming from encounters between neutron stars (NSs) and axion minihalos around primordial black holes (dressed PBHs). Due to PBHs are local overdensities in the DM distribution, they will unavoidably acquire axion minihalos around them. Thus, we expect the presence of dressed PBHs today in the Milky Way halo. During NS-dressed PBH encounters, the resonant axion-photon conversion in the NS magnetosphere will give rise to a transient line-like emission of radio frequency photons. Based on the sensitivity of current and prospective radio telescopes, we show that this transit emission should be detectable on the Earth under suitable conditions.

    Papers: arXiv:2102.05680 [hep-ph], Phys.Lett.B 807 (2020) 135566

  • 31.03.2021 (at 13:15) Ilia Musco (Rome U.)
    Title: Primordial Black Holes: formation mechanism and cosmological impact [video]

    Abstract: Primordial black holes (PBHs) formed in the radiation dominated Universe are possible candidates for the dark matter as well as for the seeds of supermassive black holes observed in the centre of galaxies. Numerical simulations of spherically symmetric collapse shows that a PBH is formed if a cosmological perturbation amplitude is larger than a threshold value depending on the specific shape of the perturbation. Recents studies have investigated how to link the initial conditions of numerical simulations with the power spectrum of cosmological perturbations: with simple analytic prescription it is possible to compute the threshold of PBHs from the shape of the peak of the power spectrum. This will allow to compute more precisely the cosmological impact (i.e. the abundance and the mass spectrum) of these objects.
  • 17.03.2021 Yann Gouttenoire (Tel Aviv U.)
    Title: String Fragmentation in Supercooled Confinement and implications for Dark Matter [video]

    Abstract: A strongly-coupled sector can feature a supercooled confinement transition in the early universe. When fundamental quanta of the strong sector are swept into expanding bubbles of the confined phase, the distance between them is large compared to the confinement scale. The flux linking the fundamental quanta then deforms and stretches towards the wall, producing an enhanced number of composite states upon string fragmentation. The composite states are highly boosted in the plasma frame, which leads to additional particle production through the subsequent deep inelastic scattering. I will discuss the modelling of these dynamics and introduce the consequences for the abundance and energetics of particles in the universe and for bubble-wall Lorentz factors. As a case of study, I will show that the composite dark matter relic density is affected by many orders of magnitude.
  • 10.03.2021 (at 15:15) Suddhasattwa Brahma (McGill U.)
    Title: Quantum entanglement in the sky: Predictions for inflation [video]

    Abstract: Cosmic inflation is widely accepted as the standard paradigm of the early universe since it not only solves the usual cosmological puzzles but also explains observed large scale inhomogeneities as originating from quantum vacuum fluctuations. However, much of its quantum properties remain obscure, and indeed, there have been recent arguments against (some models of) inflation having a consistent quantum embedding into a UV theory. In this talk, I shall explore an often-ignored aspect of this — the crucial role that quantum entanglement (between the quantum modes of the fluctuating field) plays in the dynamics, and observable predictions, of inflation. The same dissipative effects which are at the heart of decoherence (and how quantum fluctuations turn “classical”) will be shown to be responsible for this primordial entanglement, resulting from treating inflation as an open quantum system. I will demonstrate why our conclusions are inescapable, and provide universal upper bounds on the duration of inflation similar to those coming from quantum gravity considerations.
  • 03.03.2021 (at 16:15) Jeff Dror (UC, Santa Cruz, Inst. Part. Phys.)
    Title: The Cosmic Axion Background [video]

    Abstract: Existing searches for cosmic axions relics have relied heavily on the axion being non-relativistic and making up dark matter. However, light axions can be copiously produced in the early Universe and remain relativistic today, thereby constituting a Cosmic axion Background (CaB). In this talk, I will study the production and detection of a CaB. Prototypical examples of axion sources are thermal production, dark-matter decay, parametric resonance, and topological defect decay. Each of these has a characteristic frequency spectrum that can be searched for in axion direct detection experiments. I will focus on the axion-photon coupling and study the sensitivity of current and future versions of ADMX, HAYSTAC, DMRadio, and ABRACADABRA to a CaB, finding that the data collected in search of dark matter can be repurposed to detect axion energy densities well below limits set by measurements of the energy budget of the Universe. In this way, direct detection of relativistic relics offers a powerful new opportunity to learn about the early Universe and, potentially, discover the axion.
  • 24.02.2021 (at 12:15) Kazunori Kohri (Tokyo U., IPMU and KEK, Tsukuba)
    Title: Formations of primordial black holes in the early matter-dominated Universe [video]

    Abstract: After aLIGO detected the gravitational wave (GW) produced by mergers of binary black holes (BHs), researchers have aggressively studied the origin of the BHs with masses of the order of O(10) M_sun. In addition to astrophysical origins through evolutions of Pop.III/Pop.II stars, one of the attractive candidates of those BHs should be Primordial Black Holes (PBHs). The PBHs can be produced in the early radiation and matter dominated Universe due to spherical collapses of regions which have a large curvature perturbation produced by inflation. I will explain the mechanism of the PBH formations in the early matter dominated Universe in detail. Next, I will review the current status of cosmological and astrophysical constraints on PBHs with introducing my own latest bounds on PBHs in terms of polarization of Cosmic Microwave Background photons due to cosmological accretions onto PBHs (arXiv:1707.04206 [astro-ph.CO], arXiv:2002.10771 [astro-ph.CO]), CMB distortions (arXiv:1405.5999 [astro-ph.CO]), gamma-ray/cosmic-rays/BBN bounds on evaporating PBHs (arXiv:0912.5297 [astro-ph.CO], arXiv:2002.12778 [astro-ph.CO]), Higgs phenomenology (arXiv:1708.02138 [hep-ph]), stochastic GWs (arXiv:1903.05924 [astro-ph.CO], arXiv:1904.12879 [astro-ph.CO], arXiv:2009.11853 [astro-ph.CO]), PBH dark matter (arXiv:1802.06785 [astro-ph.CO]), ultra-compact mini halo formations (arXiv:1905.04477 [astro-ph.CO]) and so on. If time permitted, I will mention the current status of concrete inflation modes based on particle physics which can successfully produce PBHs (e.g., arXiv:0711.5006 [hep-ph], arXiv:0906.1398 [astro-ph.CO], arXiv:1211.2371 [hep-ph]).
  • 17.02.2021 (at 12:15) Yuto Minami (Osaka U., Res. Ctr. Nucl. Phys.)
    Title: Search for parity-violating physics in the polarisation of the cosmic microwave background, so called “Cosmic Birefringence” [video]

    Abstract: Polarised light of the cosmic microwave background, the remnant light of the Big Bang, is sensitive to parity-violating physics, cosmic birefringence. In this presentation we report on a new measurement of cosmic birefringence from polarisation data of the European Space Agency (ESA)’s Planck satellite. The statistical significance of the measured signal is 2.4 sigma. If confirmed with higher statistical significance in future, it would have important implications for the elusive nature of dark matter and dark energy.
  • 10.02.2021 Vincent Vennin (APC, Paris)
    Title: Quantum diffusion during cosmic inflation [video]

    Abstract: When inhomogeneities are produced with sufficiently large amplitude during inflation, they may subsequently collapse into primordial black holes. I will explain why the effect of quantum diffusion during inflation needs to be taken into account in such a case, and how the abundance of primordial black holes can be predicted within the formalism of stochastic inflation. I will also show that quantum diffusion at small scales (potentially leading to primordial black holes) affects the large-scale perturbations observed in the cosmic microwave background. The cosmic microwave background measurements can thus set explicit constraints on the entire inflationary potential down to the end of inflation.
  • 03.02.2021 Alexey Koshelev (UBI, Covilha)
    Title: Analytic Infinite Derivative (AID) gravity theories [video]

    Abstract: I will explain in brief the ideas of AID gravity pointing precisely how such models arise naturally and what questions they are aimed at. Examples of AID scalar field theories will be discussed for deeper clarity. Based on an explicit model of quadratic in curvature AID gravity I will explain how Starobinsky inflation stays in this framework as an exact solution and will discuss inflationary observables in regard to available data. If time permits, I will briefly outline what happens to centrally symmetric solutions for massive objects in this class of theories.