Group Seminar
Topics on Cosmology and Particle Physics. The format is open, participation beyond T70 and active discussions are encouraged.
Time: Wednesdays, 4 pm
Location: Physik Department 3344.
Schedule
Date  Who  Topic  Abstract/Info 

17.06.15  Maxime Guilleux (APC Université Paris 7)  Renormalisation group flow for scalar fields in De Sitter space  Quantum field theory on curved spacetime is most relevant to several open questions in cosmology. In relation with inflation, we study O(N) scalar field theories in de Sitter space. For light fields in units of the spacetime curvature these feature nontrivial quantum effects on superhorizon scales that require resummation techniques. Using nonperturbative renormalization group methods, we derive a flow equation for the (quantum) effective action which allows us to study the flow of the effective potential from sub to superhorizon scales. This efficiently captures several nontrivial effects induced by the curvature, such as effective dimensional reduction, symmetry restoration, or mass generation. Finally, we show that our approach provides an alternative derivation of the stochastic description of Starobinsky and Yokoyama and provides a unified description of the latter and of Euclidean de Sitter space. 
03.06.15 2pm (!)  Jean Alexandre (King's)  Nonperturbative convexity of the effective potential and relevance to ultralight scalar Dark Matter  Joint event with TPP seminar. 
28.01.15, 14:00 3344 Physik Department  Gerasimos Rigopoulos (Heidelberg)  The adhesion model as a field theory for cosmological clustering  The adhesion model has been proposed in the past as an improvement of the Zel'dovich approximation, providing a good description of the formation of the cosmic web. We recast the model as a field theory for cosmological large scale structure. The dynamics of this Stochastic Adhesion Model (SAM) is reminiscent of the well known KardarParisiZhang equation. We comment on the suitability of this model for describing the nonlinear regime of the CDM power spectrum and its utility as a relatively simple approach to cosmological clustering. 
07.01.15, 15:00 Room 1121
 Yi Cai (Melbourne)  Beyond the EFT Approach to Dark Matter  Despite being a simple and elegant approach to dark matter physics, the validity of the effective operator analysis is under siege. Beyond the effective operator approach of the dark matter, we considered two simplified models of dark matter with $Z^\prime$s, which have richer physics, consistent with all current constraints and present interesting signatures at the LHC. 
11.11.14  Daniele Teresi (Manchester)  Symmetryimproved CJT Effective Action  The formalism introduced by Cornwall, Jackiw and Tomboulis (CJT) provides a systematic approach to consistently resumming nonperturbative effects in Quantum Field Theory (QFT). The CJT effective action has proven to be an extremely useful theoretical tool in thermal QFT, providing, among other things, the starting point for consistent descriptions of nonequilibrium phenomena. One major limitation of the CJT effective action is that its loopwise expansion introduces residual violations of possible global symmetries, thus giving rise to massive Goldstone bosons in the spontaneously broken phase of the theory. We develop a novel symmetryimproved CJT formalism for consistently encoding global symmetries in a loopwise expansion. In our formalism, the extremal solutions of the fields and propagators are subject to additional constraints given by the Ward Identities. We show that, unlike the existing approaches, our formalism satisfies a number of important fieldtheoretic properties, concerning phase transitions, absorptive effects and the effective potential. As an application of the symmetryimproved CJT formalism, we study the issue of infrared divergences of the Standard Model (SM) effective potential due to Goldstone bosons, which may affect the stability analyses of the SM, presenting some preliminary comparisons with partial diagrammatic resummations recently developed to address this problem. 
09.09.14  Yuki Watanabe (Tokyo)  Gravitational reheating after inflation  We show that reheating of the universe occurs spontaneously in a broad class of inflation models with nonminimal gravitational coupling. The model does not require explicit couplings between the inflaton and bosonic or fermionic matter fields. The couplings arise spontaneously when the inflaton settles in the vacuum expectation value (vev) and oscillates, with coupling constants given by the vev and the mass of resulting bosonic or fermionic fields. This mechanism allows inflaton quanta to decay into any fields which are not conformally invariant in nonminimal gravity theories. We also show the rate of an anomalous channel that the inflaton field decays into a pair of gauge fields. Comparing this new decay channel via the anomaly with the channels from the treelevel analysis, we find that the branching ratio crucially depends on masses and the internal multiplicities (flavor quantum number) of decay product particles. While the inflaton decays exclusively into light fields, heavy fields still play a role in quantum loops. We argue that this process in principle allows us to constrain the effects of arbitrary heavy particles in the reheating. Finally, we mention a possibility of gravitational modulated reheating if the inflaton mass is much heavier than that during inflation. 
02.09.14  Juraj Klaric (TUM)  Resonant Leptogenesis in the Strong Washout Regime  
08.07.14  Kohei Kamada (EPFL Lausanne)  Generalized Ginflation: as the most general framework of the standard model Higgs inflation  The question what drives primordial inflation is the longstanding question for inflationary cosmology. The discovery of the SM Higgs boson made it a concrete question, "is it possible for the SM Higgs to drive inflation?", and there arise several proposals of "Higgs inflation". In this talk, I will explain the most general framework of Higgs inflation, based on the "Generalized Ginflation", which is the most general single scalar field inflation. I will show how to distinguish these possibilities observationally and their potential problems. I also give the implication from the BICEP2 result. 
06.05.14  Jens Chluba (Johns Hopkins U.)  Science with CMB Spectral Distortions: a New Window to EarlyUniverse Physics  Since COBE/FIRAS 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 that is 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 window to earlyuniverse and particle physics, with possible nonstandard surprises but also guaranteed signals awaiting us. 
29.11.13 special date: Friday, 2pm in 3344  Tomasz Rembiasz  Numerical Studies of the Magentorotational Instabiltiy in Core Collapse Supernovae  
06.11.13  Ignacio Izaguirre  On Leptogenesis from mixing Lepton doublets  
29.10.13 18:00 special time  Peter Millington (Manchester)  Perturbative NonEquilibrium Thermal Field Theory  We present a fully perturbative approach to nonequilibrium thermal field theory, applicable to the description of ultrarelativistic manybody systems across a range of disciplines. The resulting diagrammatic perturbation series are built from nonhomogeneous free propagators and explicitly timedependent vertices, which together encode the absolute spacetime dependence of the statistical background. The perturbative expansion is free of socalled pinch singularities, without the need for resummation of finitewidth effects. Thus, we do not suffer the mathematical pathologies previously thought to spoil truly perturbative treatments of nonequilibrium field theory. Arriving at a definition of particle number density that does not rely on quasiparticle Ansätze, we derive general master time evolution equations for statistical distribution functions. Subsequently truncating these transport equations in a perturbative loopwise sense, without the need for gradient expansion, we successfully capture dynamics on all timescales, including the emergence of threshold effects. Finally, within the context of a scalar model, we show how transient earlytime behaviour is dominated by nonMarkovian energyviolating processes, which result from the systematic inclusion of finitetime effects. 
21.10.13 (special date, time 5:30pm)  Azadeh Maleknejad (IPM Tehran)  The role of gauge fields during inflation  The isotropy and homogeneity of the cosmic microwave background (CMB) favors "scalar driven" early Universe inflationary models. Nonscalar fields, and in particular gauge fields, are on the other hand commonplace in all high energy particle physics models proposed to be at work at the upper bound on energy scale of inflation set by the current CMB observations. In this review we consider the role and consequences, theoretical and observational, that gauge fields can have during inflationary era. Gauge fields may be turned on in the background during inflation, or may become relevant at the level of cosmic perturbations. There have been two main class of models with gauge fields in the background, models which show violation of cosmic nohair theorem and those which lead to isotropic FLRW cosmology, respecting the cosmic nohair theorem. Models in which gauge fields are only turned on at the cosmic perturbation level, may source primordial magnetic fields. We also review specific observational features of these models on the CMB and/or the primordial cosmic magnetic fields. Our discussions will be mainly focused on the inflation period, with only a brief discussion on the post inflationary (p)reheating era. 
17.09.13  Atsushi Naruko (Paris)  Primordial perturbations from anisotropic inflation  In this talk, we study perturbations from an anisotropic inflation where inflation field couples with a vector field. In this model, vector field has the vacuum expectation value at background level which is homogeneous in space and hence the background spacetime becomes homogeneous but anisotropic different from usual homogeneous and isotropic background, FLRW universe. Interestingly, this model predicts statistically anisotropic power spectrum and it is customarily parametrized by g* where g* is defined by P (\vec{k}) = P (k) [1 + g* (\hat{k} \cdot \hat{v})] and \hat{v} denotes a preferred direction in space. By revisiting fluctuation on top of this anisotropic background, we find that the former prediction for g* will be revised and the constraint on background anisotropy during inflation from current observations can be much more loose.
The latter part of talk will be devoted to the study of the behavior of long wavelength (nonlinear) fluctuations based on spatial gradient expansion approach. To begin with, by carefully looking at linear perturbation equations, we find that perturbation equations are exactly same as those for background under particular gauge (coordinate) choice. This fact indicates long wavelength fluctuations can be evaluated just by solving background equation as in \delta N formalism for FLRW universe. Finally we extend the analysis to nonlinear order and investigate the behavior of nonlinear fluctuation by constructing general solution. 
03.09.13  Kaz Sugimura (Kyoto)  Bispectrum from open inflation  We calculate the bispectrum of quantum fluctuations in open inflation, where quantum tunneling of an inflaton field from a false vacuum triggers slowroll inflation inside the nucleated bubble. Assuming that the false vacuum inflation before the quantum tunneling is long enough for the quantum state in such a period to be approximated as the BunchDavies vacuum state, we compute the evolution of the state, based on the inin formalism. Since the state inside the nucleated bubble is different from the one naturally defined for the observer in the bubble, we can study the effect of socalled "nonBunchDavies initial state" on the bispectrum in a model where such a state is naturally generated. 
18.06.13  Tomislav Prokopec (Utrecht)  The final state on de Sitter Special date & time: 14:00 in 3344  I will begin by reviewing de Sitter space, the problems with perturbative quantizations on de Sitter and Starobinsky's stochastic inflation. Next, I will present a nonperturbative technique based on a Legendre transform and show how to construct an effective action based on which one can determine the quantum state of fields at the asymptotic future infinity, which allows for understanding deep infrared (IR) correlators. As an important corollary I will present a proof that an O(N) symmetric scalar theory, that is broken in the UV, undergoes a symmetry restoration in the deep IR. Namely, the strong quantum fluctuations on de Sitter generate a large amount of vacuum energy, thus restoring the symmetry. 
14.05.13  Michael J. RamseyMusolf  The Higgs Portal: Phase Transitions, Dark Matter, & the LHC  
08.05.13  Rishi Khatri MPA  CMB Spectral Distortions  
24.04.13  No meeting > Kosmologietag  
17.04.13  Frank Glowna & Yi Zhu  Practice Talks for Komsologietag  
10.04.13  Sébastien Clesse  Report from Planck Meeting  
03.04.13  no seminar  
21.03.13  Planck press conference webcast. Special Time: 10:00  
06.03.13  Marco Drewes  Kinematics of  We calculate the relaxation rate of a scalar field in a plasma of fermions with gauge interactions and other scalars in thermal quantum field theory. It yields the rate of cosmic reheating and thereby determines the temperature of the ``hot big bang'' in inflationary cosmology. We in particular study the effect of finite temperature corrections to quasiparticle dispersion relations on the efficiency of reheating.

27.02.13  Sébastien Clesse  Nongaussianities and Curvature Perturbations for (SUSY) Hybrid Inflation  I will first explain how to use the delta N formalism and the theory of multifield perturbations to calculate the power spectrum of curvature perturbations in the case of multifield models of inflation. Then I will apply these methods to hybrid models, in the specific case of a mild waterfall phase. 
20.02.13  Björn Garbrecht  Soft and Collinear Divergences in Thermal Relaxation Rates  On the example of righthanded neutrinos in the Early Universe, I discuss the cancellation of soft and collinear infrared divergences in processes involving the radiation of gauge bosons. 
13.02.13  Gerasimos Rigopoulos  Gradient expansion for cosmological perturbations  I will discuss the use of the gradient expansion as a means of studying (subHubble) nonlinear cosmological perturbations, highlighting the relation to Lagrangian Perturbation Theory and the Zeldovich approximation and mapping out possible future directions. 