2017 Seminars

Please click on the talk title to download the presentation (if available).

Spring

  • 25.01.2017 Massimiliano Rinaldi (University of Trento)
    The scale-invariant inflationary Universe.
    Abstract: Cosmological observations seem to reveal that the Universe evolved through a highly symmetric state during its inflationary phase. One possibility is that the inflationary Universe is fundamentally scale-invariant. In this talk, I will entertain the idea that the inflationary Universe is indeed described by a classically and manifestly scale-invariant scalar-tensor theory of gravitation. As I will show, there are ways to naturally break the symmetry and to generate physical fundamental masses. I will also show how these theories can be connected to other popular models, such as \alpha-attractors and Higgs inflation.
  • 01.02.2017 Guillermo Ballesteros (IPhT, Saclay)
    Unifying inflation with the axion, dark matter, baryogenesis and the seesaw mechanism.
    Abstract: I will present a minimal extension of the SM that solves the strong CP problem and explains the smallness of the neutrino masses. This simple model provides a dark matter candidate (the axion), explains the matter-antimatter asymmetry of the Universe and leads to predictive primordial inflation in agreement with the CMB.
  • 21.02.2017 Anupam Mazumdar (Lancaster University)
    Infinite derivative ghost free and singularity free theory of gravity.
    Abstract: I will discuss how to construct ghost free and singularity free theory of gravity which can also ameliorate quantum aspects of ultraviolet gravity.
  • 22.02.2017 Ruth Durrer (University of Geneva)
    Testing General Relativity with the Large Scale Structure of the Universe.
    Abstract: In my talk I shall explain how relativistic effects are relevant in the observations of the large scale structure of the Universe. This renders the analysis of observations more challenging but it also allows us to measure not only the density field but also the velocity field and the metric from large scale structure observations. In my talk I shall show with a few examples how future observations, e.g. with ESA’s Euclid satellite in preparation for launch in 2020, can be used not only to measure the large scale matter distribution but also to test General Relativity on cosmological scales.
  • 01.03.2017 Alexandre Barreira (MPA Garching)
    Observational signatures of non-GR theories of gravity in cosmology.
    Abstract: In this talk, I will use cosmological simulations to look into a few of the imprints that departures from GR can leave on cosmological observables. By using the simulation results we can construct controlled galaxy mock catalogues, which we can/should use to validate current data analysis methods, as well as to investigate the merits of new ways to constrain gravity. Specifically, I will test the validity of current redshift space distortion analysis pipelines in estimating the growth rate of structure in non-GR cosmologies. I will also show how departures from GR affect the lensing signal along underdense lines-of-sight (called troughs), for which the Dark Energy Survey has already provided the first measurements.
  • 08.03.2017 Tommi Tenkanen (Queen Mary U. London)
    The many faces of the inflaton field.
    Abstract: The minimal requirements for every inflationary model are that the inflaton field gives a long enough period of exponential expansion and subsequently reheats the universe, so that thermal equilibrium is attained prior to the Big Bang Nucleosynthesis. But could the inflaton be responsible for something else as well, namely the generation of dark matter or the electroweak phase transition? As a representative model example, we study these aspects in a scenario where the inflaton is a real singlet scalar. Requiring the model to describe inflation successfully, be compatible with the LHC data, and yield a) the correct dark matter abundance or b) a strong first order electroweak phase transition, we identify the regions of the parameter space where the model is viable.
  • 15.03.2017 Jose Beltrán (CPT Marseille)
    Dusty inflation in Born-Infeldized gravity.
    Abstract: In order to regularize the energy of point-like charged particles, Born and Infeld introduced a modification of the Maxwell Lagrangian that naturally imposes an upper bound on electromagnetic fields. Similar ideas can be applied to gravity to resolve the GR singularities. I will discuss a class of theories based on this scheme and present a scenario where inflation can be supported by a dust component within the context of these theories.
  • 29.03.2017 Thomas Tram (University of Aarhus)
    Structure formation under the spell of General Relativity.
    Abstract: Upcoming large scale structure surveys will map the structure of the Universe on very large scales where General Relativistic effects become important. In order to take full advantage of upcoming Large Scale Structure datasets, we must be able to make accurate theoretical predictions for all observational scales. This is well known on small scales where the precision of Newtonian N-body codes is constantly being pushed, but it is equally true at large scales. In this talk I will first present numerical and analytic computations of the matter bispectrum generated from Gaussian initial conditions at second order in perturbation theory, and I will then discuss a set of recently discovered methods for including GR effects and radiation in Newtonian N-body simulations.
  • 05.04.2017 Jens Chluba (Jodrell Bank Centre for Astrophysics, Manchester)
    What CMB spectral distortions can teach us about early-universe and particle physics.
    Abstract: Since the measurements with COBE/FIRAS in the mid-90’s we know that the CMB spectrum is extremely close to a perfect blackbody. There are, however, a number of processes in the early Universe that should create spectral distortions at a level within reach of present day technology. I will give an overview of recent theoretical and experimental developments, explaining why future measurements of the CMB spectrum will open up an unexplored new window to early-universe and particle physics, with possible non-standard surprises and several guaranteed signals awaiting us. I will highlight the complementarity of the distortion signals and the CMB anisotropies, illustrating how future distortions measurements could shed new light on different inflation models. I will also briefly discuss some of the limitations caused by CMB foregrounds.
  • 12.04.2017 Vivian Poulin (LAPTH Annecy)
    Cosmological signatures of decaying Dark Matter.
    Abstract: Although the existence of Dark Matter (DM) is by now well-established thanks to a variety of observations on many different scales, its nature is still unknown and so are many of its most basics properties, such as its lifetime. Even if obvious arguments require that most of the DM is stable on timescales of (at least) the lifetime of the universe, nothing prevent a priori that a fraction of it could be in the form of unstable exotic particles that are free to decay at much shorter times. In the literature, numerous relics from the early universe have been proposed in many extensions of the standard model of particle physics, in some cases unstable to processes injecting electromagnetic (e.m.) forms of energy (e.g. ‘superWIMP’, R-parity breaking SUSY models, Sterile neutrinos, Primordial black holes …), but also non-e.m. ones (e.g. the Majoron scenario).  In this talk, I would like to review how CMB temperature and polarization anisotropies can be used as powerful probes of the abundance of such electromagnetically and non-electromagnetically decaying exotic particles, depending on their lifetime. I will then emphasize the synergy with CMB spectral distortions and Big Bang Nucleosynthesis studies when e.m. channels are switched on, and illustrate how the 21cm signal, one of the main target of future experiments, could be used in order to improve (but not always !) over current sensitivity.
  • 26.04.2017 Christian Byrnes (University of Sussex)
    Searching for spectator fields during inflation.
    Abstract: Even if the observed primordial perturbations were generated during inflation, it remains an open question whether these perturbations are due to the inflaton field, or a subdominant spectator field. A Bayesian model comparison between these two scenarios critically depends on the expected field value of the spectator field, and I will present recent work on calculating the vacuum expectation value of spectator fields using the stochastic formalism. The Planck non-Gaussianity constraints do not discriminate between these two classes of models, but probing fNL~1 is an important target.
  • 03.05.2017 Silvia Galli (IAP Paris)
    Reviewing tensions between Planck and other data.
    In 2015, the Planck collaboration published results on cosmological parameters based on the Planck second data release.  The Planck preferred best fit model is consistent but somewhat different from the one obtained using the WMAP satellite data, and presents interesting mild tensions with a few astrophysical datasets (e.g. direct measurements of H0), while being in very good agreement with others (e.g. the latest BAO measurements from BOSS). It has been advocated that some of these differences might be driven by inconsistent results of the Planck measurements at small scales.  In this talk, I will review some of the aforementioned differences and tensions and present our recent work to assess the internal consistency of the Planck data. I will then discuss the features in the Planck power spectra that lead to shifts in cosmological parameters, arguing that these are consistent with statistical fluctuations.
  • 24.05.2017 Stefano Camera (University of Turin)
    Synergistic cosmology across the spectrum.
    Abstract: ‘Synergy’ means ‘the interaction of two or more agents to obtain a combined effect greater than the sum of their separate effects’.  With this in mind, in this talk I shall present my current lines of research, all relying on developing novel combinations of astrophysical and cosmological observables to the aim of testing the foundations of the concordance cosmological model.  Specifically, I shall discuss cosmological tests of gravity and inflation on the largest cosmic scales, indirect detection of particle dark matter signatures, and studies of dark energy and modified gravity models.  All, with a view on the current and oncoming generation of cosmological experiments and large-scale surveys.
  • 31.05.2017 David Marsh (King’s College London)
    Searching for the QCD Axion with Gravitational Microlensing.
    Abstract: The phase transition responsible for axion dark matter production can create large amplitude isocurvature perturbations which collapse into dense objects known as axion miniclusters. We use microlensing data from the Subaru Hyper Suprime Cam to place constraints on the minicluster scenario. While miniclusters must form, the fraction of DM in miniclusters, fMC, is not known from theory. If fMC is of order unity, then axion direct detection is severely limited. We compute the microlensing event rate for miniclusters treating them as spatially extended objects with an extended mass function. For the QCD axion we find the bound fMC<0.083(ma/100μeV)^0.12, which represents the first observational constraint on fMC. We also constraint axion-like particles. I will discuss the many approximations in the theoretical calculation. However, I will argue that the ongoing renaissance in microlensing observations will render these irrelevant, and allow constraint and disocovery prospects for all possibilities. If accurate theoretical predictions for fMC can be made, then microlensing can be used to exclude, or discover, the QCD axion. I will briefly describe efforts by simualtion groups to make this prediction.
  • 06.06.2017 Qaisar Shafi (Delaware University)
    Quest for Unification.
    Abstract: Grand unification of the strong, electromagnetic and weak forces, especially in conjunction with Einstein’s general relativity, yields a number of striking testable predictions. We consider both supersymmetric and non-supersymmetric models, and highlight predictions related to LHC physics including dark matter, magnetic monopoles and primordial gravity waves.
  • 29.06.2017 Robert Caldwell (Dartmouth College)
    Cosmological Gravitational Waves and Gauge Fields.
    Abstract: The dynamics of a gravitational wave propagating through a cosmic gauge field are dramatically different than in vacuum. We show that a gravitational wave acquires an effective mass, is birefringent, and its normal modes are a linear combination of gravitational waves and gauge field excitations, leading to the phenomenon of gravitational wave–gauge field oscillations. In the cosmological context, a cosmic gauge field leaves a novel imprint on a spectrum of gravitational waves. As illustration, we present a toy model of axion-gauge field inflation. This scenario predicts a blue-tilted gravitational wave spectrum that is dominated by one circular polarization, resulting in unique observational targets for cosmic microwave background and gravitational wave experiments.

 

Autumn

 

  • 07.09.2017 Andi Hektor (NICPB, Tallinn)
    Thermal loophole in the Higgs Portal.
    Abstract: The standard Higgs portal Dark Matter (DM) — fermion (or scalar or vector) DM that interacts with the Standard Model (SM) sector via a scalar mediator that interacts with the SM Higgs — is a minimal scenario relate the SM particles and dark sector. However, the direct detection and collider constraints restrict the models strongly in the most interesting parameter region. We show how a minimal extension of the usual scenario, assuming a complex mediator, can relax the constraints due to the thermal effects at the freeze-out. Interestingly, the thermal effects modify strongly the masses and vevs of the scalars and Higgs at the freeze-out and so it decouples the direct detection and freeze-out physics of the model. The extension could provide for the baryogenesis and a gravitational wave signal from the early Universe.