# 2016 Seminars

## Spring

• 20.01.2016 Till Sawala (Helsinki)
Cosmological simulations: a brief overview
Abstract: Numerical simulations have become an essential tool for much of theoretical astrophysics and for our understanding of the non-linear evolution of the universe. I will give a brief overview of the field, with a focus on simulations both of cosmic structure formation and of galaxy formation. I will discuss the successes and shortcomings of numerical simulations in these areas, and recent work to overcome the current limitations.
• 10.02.2016 Miguel Zumalacárregui (Nordita, Stockholm)
Gravity at the horizon: from the cosmic dawn to ultra-large scales
Abstract: Recent advances in cosmology provide both the motivation and the data to probe gravity on the largest scales available to observation. I will revise the landscape of gravitational theories, focusing on modern scalar-tensor theories and their cosmological implications. Then I will present the ongoing effort to test gravity in novel regimes such as the early universe, non-linear effects and ultra-large scales. I will also introduce the hi_class code (www.hiclass-code.net), which is central to this program.
• 18.02.2016  Shinsuke Kawai (Sungkyunkwan University)
SUSY-phenomenology-based models of Higgs inflation
Abstract: Present CMB observation tightly constrains the amplitude of the primordial tensor mode fluctuations. As phenomenologically well-motivated and observationally viable models of inflation, I discuss supersymmetric Higgs inflation type scenarios in this talk. These are natural beyond-the-Standard Model extension of the popular nonminimally coupled Higgs inflation model by Bezrukov-Shaposhnikov. In this talk I will mainly focus on a scenario based on the supersymmetric seesaw model.
• 24.02.2016 Francisco Villaescusa-Navarro (OATS, Trieste)
Massive neutrino signatures on the large-scale structure of the Universe
Abstract: Neutrinos are described as fundamental particles by the standard model of particle physics. The fact that neutrinos are massive, as demonstrated by neutrino oscillations experiments, point towards physics beyond the standard model. One of the most important questions in modern physics is: what are the neutrino masses? Current tightest constrain on the sum of the neutrino masses arise from cosmological observables. In order to extract the maximum information from current and future surveys, as well as to avoid introducing biases in the values of the cosmological parameters, it is of primordial importance to understand, both at the linear and at the fully non-linear order, the impact that massive neutrinos induce on the distribution of matter, halos and galaxies. In this seminar I will present some of the effects neutrinos induce on the Universe large-scale structure, among then the clustering of matter, the clustering of dark matter halos, the abundance of halos, the abundance of voids, their impact on the BAO peak and their effects on the spatial distribution of neutral hydrogen in the post-reionization era.
• 02.03.2016 Venus Keus (Helsinki)
Dark Matter in multi-inert doublet models
Abstract: Dark Matter (DM), arising from an Inert Higgs Doublet, may either be light, below the W mass, or heavy, above about 525 GeV. While the light region may soon be excluded, the heavy region is known to be very difficult to probe with either Direct Detection (DD) experiments or the Large Hadron Collider (LHC). In this talk I will discuss that adding a second Inert Higgs Doublet helps to make the heavy DM region accessible to both DD and the LHC, by either increasing its couplings to the observed Higgs boson, or lowering its mass to 360 GeV < $m_{DM}$, or both.
• 09.03.2016 Jonathan White (KEK, Japan)
Gravitational reheating after multi-field inflation
Abstract: It has recently been highlighted that details of the reheating process must be properly taken into account when constraining individual models of inflation with current high-precision CMB data, even in the case of single-field models.  As inflation models with a non-minimal coupling between the gravity and inflaton sectors have recently attracted much attention, in this work we discuss reheating in this class of models, allowing for multiple inflaton fields.  In such models, even in the absence of explicit interaction terms the inflaton sector can decay into matter as a result of its non-minimal coupling to gravity, thereby reheating the Universe gravitationally. Using the Bogoliubov approach we evaluate the gravitational decay rates of the inflaton sector, and analyse the reheating dynamics.
• 16.03.2016 Thomas Jacques (SISSA, Trieste)
Simplified models for DM searches at the LHC
Abstract: As the first results from Run II of the LHC are released, it is important to evaluate the ways in which we study DM at colliders. In the past we have used EFTs to constrain DM in a semi-model-independent way, but it is now clear that this approach has limitations. The community is moving full-speed into the usage of simplified models of dark matter, and it is important to approach them in a logical and consistent way so that we can learn as much as possible about the dark sector. Simplified models are designed to have fewer parameters than full, UV-complete models of dark matter, so that the full parameter space can be explored. At the same time, they are designed to still provide much the same phenomenology as full models, so that we don’t miss any potential signals. I will talk about some of the techniques and challenges we use to achieve these sometimes contradictory goals, and also how collider constraints fit into the broader search for dark matter.
• 23.03.2016 Sami Nurmi (Jyväskylä)
On Higgs physics in the early universe
Abstract: I discuss the role of the Higgs field in the very early universe. Inflationary fluctuations of the Higgs field generically form a primordial Higgs condensate which might have left observable signatures. Stability of the SM vacuum against the fluctuations implies stringent constraints on Higgs couplings to spacetime curvature, or requires physics beyond SM. I review the stability constraints which may open interesting insights in probing new physics. I also discuss the decay of the primordial Higgs condensate which determines the duration of the early out-of-equilibrium epoch. If time allows, I comment also on the observational signatures of Higgs inflation.
• 06.04.2016 Maria Archidiacono (Aachen University)
Efficient calculation of cosmological neutrino clustering
Abstract: Within the next few years cosmological structure formation will be probed in greater detail than ever before by new and very large surveys, most notably EUCLID and LSST. While this opens great possibilities for probing for example dark energy and the mass of neutrinos, it also puts very stringent requirements on theoretical calculations of cosmological observables, such as the matter power spectrum. In this regard, the treatment of massive neutrino is particularly challenging both in N-body simulations and in linear theory Boltzmann codes. In this talk I will present a new approximation to the third moment of the Boltzmann hierarchy and demonstrate that with this new approximation the neutrino power spectrum can be calculated with a precision of a few percent. Then I will discuss an extremely efficient way of calculating the neutrino power spectrum in the regime of non-linear dark matter clustering.
• 13.04.2016 Matti Herranen (Jyväskylä)
Nonequilibrium QFT approach to leptogenesis
Abstract: The observed baryon asymmetry of the Universe may have been produced dynamically in the very early Universe by a baryogenesis via leptogenesis process from the CP- and lepton number violating decays of heavy right-handed neutrinos. For reliable calculations of the baryon asymmetry generated in leptogenesis first principle methods of nonequilibrium quantum field theory are required.We construct a transport theory of leptogenesis based on the Schwinger-Keldysh or Closed Time Path (CTP) formalism of nonequilibrium QFT. The resulting kinetic equations incorporate finite density medium effects and quantum coherence effects and therefore provide a generalization to standard (quantum) Boltzmann equations. We find that the finite density corrections from loop integrals typically lead to an enhancement of the asymmetry. In the resonant regime with nearly degenerate right-handed neutrino masses, our results confirm the well-known resonant enhancement of CP-violation, however, novel effects from the flavour oscillations can be important in the dynamics.
• 20.04.2016 Adam Amara (ETH Zurich)
Cosmology and the Dark Energy Survey
Abstract: The Dark Energy Survey (DES) is a 5000 square degree survey targeting dark matter and dark energy science. This year sees the release of the first science results from the DES collaboration using data taken during science verification. I plan to present these new results, which include dark matter maps, cosmology constraints and new discoveries such as the new strong lens systems being found. I will also give an update on the progress of the main science survey, which is still ongoing.
• 27.04.2016 Steen Hansen (DARK, Copenhagen)
Pancakes in space
Abstract: The cosmological web consists of equilibrated galaxies and clusters of galaxies, one dimensional filaments, two dimensional sheets (also called Zeldovich pancakes) and three dimensional voids. I will discuss the first detection of two Zeldovich pancakes near the Coma cluster. These sheets/pancakes, which are few Mpc wide and about 10 Mpc long, are very cold structures with velocity dispersion about 100 km/sec. In comparison, groups of galaxies have dispersions around 300 km/sec. By measuring the departure from a pure Hubble flow of the galaxies belonging to the sheets/pancakes, one can determine the virial mass of the nearby galaxy cluster, and I will discuss how this method complements other methods.
• 04.05.2016 Francesca Day (University of Oxford)
Astrophysical signatures of axions
Abstract: Many extensions of the Standard Model include axions or axion-like particles. Such axions are potentially observable via their interaction with electromagnetism, leading to axion-photon conversion in an external magnetic field. I will discuss potential observational signatures of axions in galaxies and galaxy clusters. In particular, I will present predictions from a cosmic axion background propagating in galaxies. I will also describe a scenario in which the recently observed 3.5 keV photon line is caused by dark matter decay to axions, which then mix with the photon in astrophysical magnetic fields. I will motivate this scenario in terms of the observed morphology of the line, and present predictions unique to this model.
• 11.05.2016 David Wiltshire (Canterbury University)
Models of inhomogeneity and backreaction in cosmology: A status report
Abstract: The possibility of backreaction – that inhomogeneous structures on small (< 100/h Mpc) scales change average cosmic evolution relative to a homogeneous isotropic FLRW cosmology – can be tested in different ways.
1. In our own < 100/h Mpc environment the assumption of a FLRW geometry can be tested model-independently. We present very strong Bayesian evidence that the standard CMB rest frame is not the frame in which the spherically averaged variation of the Hubble expansion is minimized. Using large ray tracing simulations in exact solutions of Einstein’s equations we further show that observations are consistent with a 0.5% differential expansion of space on < 70/h Mpc scales that is not included in the standard cosmology. This has testable implications for large angle CMB anomalies.
2. On > 100/h Mpc scales detailed models of backreaction are required for cosmological tests. The Timescape Cosmology provides such a phenomenology, without dark energy. We report on the status of observational tests of this model, including fits of the acoustic peaks in the CMB anisotropy spectrum. The Timescape Cosmology passes current tests, and can be distinguished from the FLRW cosmology with future Euclid satellite data. In terms of the CMB we find that the effects of backreaction in the primordial plasma still need to accounted for to deal with systematic uncertainties of 8-13% in particular cosmological parameters.
• 18.05.2016 Stefano Foffa (University of Geneva)
Binary parameters measurement from coalescence event(s) in LIGO
Abstract: After a brief introduction about gravitational waves (GW), binary systems and data analysis, I will discuss the informations that can be extracted from the study of a GW signal produced during a coalescence. I will take as a case study the recently announced GW detection at LIGO, and discuss what can be expected from other coalescence events and/or when other detectors will join the network.
• 25.05.2016 Enea Di Dio (OATS Trieste)
Relativistic effects on LSS spectrum and bispectrum
Abstract: I will discuss the Large Scale Structure spectrum and bispectrum in a relativistic framework. To first order, a relativistic description includes terms beyond the Kaiser approximation (doppler effects and galaxy evolution), gravitational potentials and integrated terms (cosmic magnification, integrated Sachs-Wolfe and Shapiro time-delay). These terms are currently neglected, but they might play a role in future surveys which probe larger scales. I will show how they can be isolated by correlating different probes, or by using the so-called multi-tracer technique. Moreover, some relativistic effects could give a non-negligible contribution to the LSS observables, hence, by neglecting them, the analysis may lead to biased cosmological parameters.
• 15.06.2016 Eemeli Tomberg (Helsinki)
Decoherence in Inflation
Abstract: During inflation, cosmological scalar perturbations can be described by the Sasaki-Mukhanov variable. Quantizing this variable, we get a highly quantum mechanical, `squeezed’ vacuum state. However, it is common in cosmology to use classical perturbation theory to describe the perturbations after their birth. In this talk I will discuss some properties of the squeezed vacuum state, and show how decoherence can be used to understand the transition from the quantum mechanical initial conditions to the classical treatment. In decoherence, the system gets entangled with its environment, so that its reduced density matrix becomes an ensemble of classically observable states.

## Autumn

• 14.09.2016 Alexey Golovnev (St. Petersburg State University)
Conformal and disformal relations in extended models of gravity
Abstract: Conformal and disformal relations between metrics are very important for modern scalar-tensor models. As a somewhat non-trivial example, the so-called mimetic gravity can be understood from the viewpoint of (non)invariance under disformal transformations. On the other hand, recently the concept of those relations was generalised to curvature-dependent (instead of scalar-dependent) transformations in disguise of C- and D-models. The latter might be nicely related to non-local gravity models. In my talk, I will review some of these topics.
• 15.09.2016 Henrik Nersisyan (ITP Heidelberg)
Non-local cosmology
Abstract: I will give a basic introduction to non-local modifications of General Relativity with special emphasis on the cosmological consequences of the mostly discussed $R\Box^{-2}R$ Maggiore, Mancarella (MM) model. The pathologies associated with non-local tensorial extensions will be also presented.
• 28.09.2016 Matteo Viel (OATS Trieste)
Cosmology with the Intergalactic Medium
Abstract: I will review the role of intergalactic matter between galaxies as a cosmological observable to address fundamental physical questions: nature of dark matter, masses of neutrinos and the cosmic cycle of baryons.
• 05.10.2016 Fedor Bezrukov (Manchester)
Higgs boson and cosmology — can we learn anything about fundamental physics?
Abstract: I will discuss the possibility to use Standard Model with very minimal extensions to explain the whole observable Universe.  One of the key points for the bheviour of the model at large energy scales is the stability of the Electroweak vacuum.  If the current parameters of the electroweak observables lead to the metastable vacuum, this would require modifications of the high energy theory.  I’ll try to give couple examples of viable models.
• 19.10.2016 Lavinia Heisenberg (ETH Zurich)
Consistent construction of generalised Proca theory
Abstract: Learned from lessons for constructing healthy and self consistent interactions for massive gravity and scalar-tensor theories, I will construct the Lagrangian of a vector field with derivative self-interactions, that propagates only the 3 desired polarisations associated with a Proca field. If time allows, I will also discuss some cosmological implications of this generalized Proca theory.
• 02.11.2016 Marco Bruni (ICG Portsmouth)
Nonlinear GR effects in structure formation: from approximations to full numerical relativity simulations
Abstract: In this talk I will describe progresses in considering general relativistic effects in the dynamics of structure formation in cosmology. First I will briefly describe results obtained with a nonlinear post-Friedman approach, a kind of post-Newtonian formalism, results validated and extended by others. Then I will focus on recent full numerical relativity simulations in cosmology. The recent detections of gravitational waves and of binary black holes have validated the accuracy of numerical relativity codes that are now publicly available. The application of numerical relativity to cosmology is in its infancy, but in the next few years can be a fundamental tool to understand to which extent we can have reliable predictions from standard newtonian N-body simulations; it is indeed clear that, to match the precision of future cosmological measurements, we need theoretical predictions that are not only equally precise, but also accurate at the same level. I will illustrate the first results of these numerical relativity simulations, representing the fully nonlinear GR evolution  of  perturbations in a Einstein de Sitter background: 1) we find that back-reaction  effects on the overall expansion of the mode are tiny; 2) voids expansion rate is significantly higher than that of the background and close to that of an open universe; 3) over-densities can reach turn-around much earlier than predicted by the standard top-hat model.  I will conclude with an outline of future work that is needed to establish the  real significance of these results.
• 09.11.2016 Olga Cucciati (INAF-OABO Bologna)
The effects of environment on galaxy evolution. Lessons learned and current and future projects.
Abstract: We have learned a lot about how environment affects galaxy evolution, nevertheless many questions are still matter of debate, starting from the meaning of environment itself and the complex inter-relations among galaxy properties. I will present the most recent reconstructions of galaxy environment in the VIPERS survey (the largest spectroscopic galaxy sample at 0.5 < z < 1.2), its impact on galaxy evolution, and how these results can be used to improve models of galaxy evolution. I will also discuss which are the potentiality of the forthcoming Euclid Deep survey in this context.
• 16.11.2016 Savvas Nesseris (UAM Madrid)
A phenomenological approach to the properties of dark matter
Abstract: We use large-scale cosmological observations to place constraints on the dark-matter pressure, sound speed and viscosity, and infer a limit on the mass of warm-dark-matter particles. Measurements of the cosmic microwave background (CMB) anisotropies constrain the equation of state and sound speed of the dark matter at last scattering at the per mille level. Since the redshifting of collisionless particles universally implies that these quantities scale like $a^{-2}$ absent shell crossing, we infer that today $w_{\rm (DM)}< 10^{-10.0}$, $c_{\rm s,(DM)}^2 < 10^{-10.7}$ and $c_{\rm vis, (DM)}^{2} < 10^{-10.3}$ at the $99\%$ confidence level. This very general bound can be translated to model-dependent constraints on dark-matter models: for warm dark matter these constraints imply $m> 70$ eV, assuming it decoupled while relativistic around the same time as the neutrinos; for a cold relic, we show that $m>100$ eV. We separately constrain the properties of the DM fluid on linear scales at late times, and find upper bounds $c_{\rm s, (DM)}^2<10^{-5.9}$, $c_{\rm vis, (DM)}^{2} < 10^{-5.7}$, with no detection of non-dust properties for the DM.
• 30.11.2016 Nelson Christensen (Carleton College; LIGO and VIRGO Collaborations)
Searching for the stochastic gravitational-wave background with Advanced LIGO and Advanced Virgo, and then with LISA
Abstract: Observations by Advanced LIGO and Advanced Virgo in the coming years will allow for important limits to be set on on the strength of a stochastic gravitational-wave background; a detection may happen as well. Sources for the stochastic gravitational-wave background could be cosmologically or astrophysically produced. The implications of the recent observations of GW150914 and GW151226 indicate that a stochastic background produced from the superposition of binary black hole mergers throughout the history of the universe may be detectable by LIGO and Virgo in the coming years. Presented will be a summary of the current LIGO-Virgo search for such a stochastic background using data from the first observing run, and what implications those results will have on different models.The projected sensitivities of future observation runs with Advanced LIGO and Advanced Virgo will also be given. LISA will be a spaced based gravitational wave detector; LISA’s goals for observing a stochastic gravitational-wave background will be discussed. Finally, information on how the stochastic background searches by ground-based detectors and LISA can complement one another will be presented.
• 07.12.2016 Angelo Ricciardone (Stavanger)
Probing Inflation with LISA Gravitational Wave Detector
Abstract: Primordial Gravitational Waves (GWs) represent a key test of inflation and they are a unique tool to explore the physics and the microphysics of the early Universe. After the recent GW detection by the advanced LIGO (aLIGO) detector the next target of modern cosmology is the detection of primordial GWs. Even if the main probe of primordial GWs is the Cosmic Microwave Background, we will see in this talk how we can extract information about primordial GWs at  smaller scale. In particular the space-borne LISA interferometer, in addiction to detection and characterization of GWs of astrophysical origin, will give compelling information about possible sources of GWs in the early Universe. In this talk I will briefly review the physics of primordial gravitational waves and then I will mainly discuss on the promise of LISA measurements for giving information about the physics of inflationary well motivated scenarios.