# Selected topics in theory and observations of Dark Energy (ITP Cosmology Tunch) + Journal Club

#### Thursday, PW 12, Room 106

##### 13:00 - 15:00

### Seminar calendar

##### 19th July 2018: Journal Club

Collective Journal Club

##### 11th July 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Akshay Rana (University of Delhi, India)- “Novel ways to constrain Graviton mass and spatial curvature”

In cosmology, it always remains a topic of great interest to constrain cosmological parameters by using different approaches and observational probes to confirm their consistency. In the same direction, I will present two novel ways that can be used to limit the mass of the Graviton and the spatial curvature of the space. My talk will be focused on two different aspects of present precision cosmology,

1) Firstly, I will discuss about the graviton, which is a massless particle in GR while considered to be massive in some alternative models of GR like; Massive gravity theories. I will present a novel approach that can be used to put stringent constraints on its mass using galaxy clusters.

2) Similarly, the estimation of spatial curvature of the Universe is also one of the most fundamental issue of modern cosmology. I will discuss a model-independent approach to test the curvature of space by using statistical features of strong gravitational lensing.

##### 05th July 2018: Journal Club

Collective Journal Club

##### 27th June 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Davi C. Rodrigues(Federal University of Espirito Santo, Brazil)-“Absence of a fundamental acceleration scale in galaxies”

The Radial Acceleration Relation confirms that a nontrivial acceleration scale a0 can be found from the internal dynamics of several galaxies. The existence of such a scale is not obvious as far as the standard cosmological model is concerned, and it has been interpreted as a possible sign of modified gravity. We consider 193 high-quality disk galaxies and, using Bayesian inference, we show that the probability of existence of a fundamental acceleration is essentially zero: the null hypothesis is rejected at more than 10σ. We conclude that a0 is of emergent nature. In particular, the MOND theory, a well-known alternative to dark matter that is based on the existence of a fundamental acceleration scale, or any other theory that behaves like it at galactic scales, is ruled out as a fundamental theory for galaxies at more than 10σ.

##### 21st June 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Elena Sellentin (Geneva University)- “Objective Bayesian analysis of neutrino masses and hierarchy”

Given the precision of current neutrino data, priors still impact noticeably the constraints on neutrino masses and their hierarchy. To avoid our understanding of neutrinos being driven by prior assumptions, we construct a prior that is mathematically minimally informative. Using the constructed uninformative prior, we find that the normal hierarchy is favoured but with inconclusive posterior odds of 5.1:1. Better data is hence needed before the neutrino masses and their hierarchy can be well constrained. We find that the next decade of cosmological data should provide conclusive evidence if the normal hierarchy with negligible minimum mass is correct, and if the uncertainty in the sum of neutrino masses drops below 0.025 eV. On the other hand, if neutrinos obey the inverted hierarchy, achieving strong evidence will be difficult with the same uncertainties. Our uninformative prior was constructed from principles of the Objective Bayesian approach. The prior is called a reference prior and is minimally informative in the specific sense that the information gain after collection of data is maximized. The prior is computed for the combination of neutrino oscillation data and cosmological data and still applies if the data improve.

##### 14th June 2018: Journal Club

Collective Journal Club

##### 07th June 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Gerasimos Belegrinos (ITP, Universität Heidelberg)-“Why not two scalars? ”

Scalar-Tensor theories are arguably the largest known and most widely used models of modified gravity due to their combination of simplicity and extensive range of applications. The recent gravitational wave/ gamma ray bursts detection seems to indicate that a subclass of simplest Horndeski theories respects the constraints set on the gravitational waves speed. One way to go further is to take into account one additional scalar degree of freedom. In my work I consider to this purpose linear pertubation theory for a Biscalar-Tensor theory. The modified gravity quantities (effective gravitational constant, anisotropic stress-slip and weak lensing potential) are derived using the quasi-static approximation. In my talk I will present some of the results achieved that might have theoretical and observational interest. Finally I will present a way for testing an approximation performed while solving the modified growth rate.

##### 31st May 2018: NO Journal Club/Tunch

Public Holiday

##### 24th May 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Caroline Heneka (SCUOLA NORMALE SUPERIORE, Pisa)-“The nature of modifications to gravity: 21cm to the rescue?”

For this tunch I will briefly review some basics of both the global 21cm signal and 21cm intensity mapping at high redshifts (of reionization and beyond). I will continue to discuss the recent measurement of the global 21cm signal by EDGES, and conclude with the prospects of measuring general modifications of gravity with intensity mapping.

##### 17th May 2018: Journal Club

Collective Journal Club

##### 10th May 2018: NO Journal Club/Tunch

Public Holiday

##### 3rd May 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Group Discussion

##### 26th April 2018: Journal Club

Collective Journal Club

arXiv 1804.04320

arXiv 1804.09206

##### 27th March 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Group Discussion

##### 20th March 2018: NO Journal Club

##### 13th March 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Jenny Wagner (ITA, Heidelberg University)-“A model-independent approach to gravitational lensing”

Strong gravitational lenses can map an extended background source to several highly distorted and magnified images. Analysing the properties of those images yields important information about the distribution of the lensing mass and the background source. Common approaches to reconstruct the source or the lensing mass distribution model the global properties of the source and the lens. They obtain a consistent description of the entire configuration by refining the model until it matches the observation to a predefined precision. We develop a new approach to infer local properties of the gravitational lens and to reconstruct the source using only the properties of the multiple images without assuming a lens or a source model. In the talk, I will introduce the method and its calibration by simulated lensing configurations, show its application to the galaxy-cluster-scale gravitational lens CL0024, and compare the resulting local lens properties to those obtained by two different lens modeling methods. As our approach relies on fewer assumptions and takes less than a second to yield results that agree to the model-based ones, it is an efficient tool for extracting local lensing properties from large data sets of forthcoming sky surveys.

##### 6th March 2018: Journal Club

Collective Journal Club

##### 27th February 2018: Selected topics in theory and observations of Dark Energy (Tunch)

##### 20th February 2018: Selected topics in theory and observations of Dark Energy (Tunch)

seminar by Alessandra Silvestri (Instituut Lorentz - Leiden)

Testing gravity with Large Scale Structure: the theoretical side of the challenge

See Seminars page for abstract

##### 13th February 2018: Journal Club

Collective Journal Club

##### 6th February 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Group Discussion

##### 30th January 2018: Journal Club

Collective Journal Club

##### 23rd January 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Alefe Almeida(ITP, Heidelberg University)

##### 16th January 2018: Journal Club

Martin Pauly, Santiago Casas

Higgs-Dilaton Cosmology: An inflation - dark energy connection constrained by future galaxy surveys

e-Print: arXiv:1712.04956

Alessio Spurio Mancini

Testing (modified) gravity with 3D and tomographic cosmic shear

e-Print: arXiv:1801.04251

##### 19th December 2017: Selected topics in theory and observations of Dark Energy (Tunch)

Group Discussion

##### 5th December 2017: Journal Club

Henrik Nersisyan

Quantum equivalence of f(R)-gravity and scalar-tensor-theories

Michael S. Ruf, Christian F. Steinwachs

e-Print: arXiv:1711.07486

Dario Bettoni

Vainshtein mechanism after GW170817

Marco Crisostomi, Kazuya Koyama

e-Print: arXiv:1711.06661

##### 28th November 2017: Selected topics in theory and observations of Dark Energy (Tunch)

Adalto R. Gomes - UFMA (Brazil)- “Some results in kink-antikink scattering”

After introduced some motivations related to this topic (see for instance possible connections to bubble collisions [1]), I review some known results from kink-antikink scattering in nonintegrable models, focusing on the main aspects of the scalar field after the collision process: the formation of a bion state or inelastic scattering (1-bounce collision). The nonlinearity leads to the intriguing effect of 2-bounce collisions, observed for a range of initial velocities of the pair kink-antikink. This means that depending on the model, we can have the possibility of the formation of a structure of 2-bounce windows in velocity. Then I discuss a deformed $\phi^4$ model in (1,1) dimensions [2]. Stability analysis leads to a Schroedinger-like equation with a zero-mode and at least one vibrational (shape) mode. For small deformation and for one or two vibrational modes, the observed two-bounce windows in velocity are explained by the standard mechanism of a resonant effect between the first vibrational and the translational modes. With the increase of the deformation, the effect of the appearance of more than one vibrational mode is the gradual disappearance of the initial two-bounce windows. The total suppression of two-bounce windows even with the presence of a vibrational mode offers a counter-example of what expected from the standard mechanism [3]. I end presenting some recent results from kink-antikink scattering in a degenerate vacuum to vacuumless model [4].

[1] J. Braden, J. R. Bonda, L.Mersini-Houghtonc, “Cosmic bubble and domain wall instabilities I: parametric amplification of linear fluctuations”, JCAP03 (2015) 007.

[2] F. C. Simas, Adalto R. Gomes, K. Z. Nobrega, J.C.R.E. Oliveira, ”Suppression of two-bounce win- dows in kink-antikink collisions”, JHEP 1609 (2016) 104.

[3] D.K. Campbell, J.S. shonfeld, C.A. Wingate, “Resonance structure in kink-antikink interaction in $\phi^4$ theory”, Physica D 9 (1983) 1.

[4] F. C. Simas, Adalto R. Gomes, K. Z. Nobrega, “Degenerate vacua to vacuumless model and kink-antikink collisions”, Phys. Lett. B775 (2017) 290.

##### 21st November 2017: Journal Club

Cornelius Rampf

Solving the Vlasov equation in two spatial dimensions with the Schrödinger method

arXiv:1711.00140

Guillem Domenech

Stability of Geodesically Complete Cosmologies

arXiv:1610.04207

##### 16th November 2017: Selected topics in theory and observations of Dark Energy (Tunch)

Lorenzo Pizzuti- “Modified gravity with galaxy cluster mass profiles: from data to simulations”

Abstract: I will provide a brief overview of my work concerning constraints on modified gravity models obtained using galaxy cluster mass profile determinations. In particular, I will present the results of a paper in which we combined the information given by the kinematics of galaxies in clusters with the information provided by lensing analyses for 2 galaxy clusters of the CLASH-CLASH\VLT collaboration to get constraints on f(R) models. In order to discuss the applicability of the proposed method in view of future imaging and spectroscopic surveys, I will further introduce my current study of cosmological simulations, aiming at estimating and calibrating the impact of systematics.

##### 7th November 2017: Journal Club

Adria Gomez-Valent

An alternative to the LCDM model: the case of scale invariance

arXiv arXiv:1701.03964

##### 26th October 2017, 13:30, Ground Floor of Philosophenweg 16: Journal Club (NOTE UNUSUAL DATE AND TIME)

Special JC on Gravitational Waves detection

Caroline Heneka - Strong constraints on cosmological gravity from GW170817 and GRB 170817A - 1710.06394

Martin Pauly - GW170817 Falsifies Dark Matter Emulators - 1710.06168

Ana Marta Pinho - Welcome to the multi-messenger Era! Lessons from a neutron star and the landscape ahead - 1710.05931

Alessio Spurio Mancini - Dark Energy after GW170817 - 1710.05901

##### 18th July : Journal Club

##### 11th July : Selected topics in theory and observations of Dark Energy (Tunch)

Martin Pauly (ITP, Heidelberg) - “Predictions of renormalized Higgs inflation”

Abstract: Higgs inflation is a model, that allows to explain inflation with the standard model degrees of freedom by introducing an additional coupling between the Higgs field and gravity. In this talk, I am going to explore the influence of the running of the standard model couplings on the observational predictions of Higgs inflation. In particular, the running of the Higgs-self-coupling and sudden changes in that coupling will be important. These sudden changes can be induced by the effective-field-theory nature of Higgs inflation. I will show that as long as the coupling stays sufficiently large in the inflationary region the predictions are insensitive to the running and become sensitive once the coupling approaches the critical point.

##### 4th July : Journal Club

Caroline Heneka

Safely smoothing spacetime: backreaction in relativistic cosmological simulations

arXiv:1706.09309

Javier Rubio

Primordial Black Hole production in Critical Higgs Inflation

arXiv:1705.04861

##### 27th June : Selected topics in theory and observations of Dark Energy (Tunch)

Cornelius Rampf (ITP, Heidelberg University ) - “Shell-crossing in quasi one-dimensional flow ”

Abstract: So far exact analytic solutions to the cosmological fluid equations existed for initial data that only depend on one space variable. Exact solutions (until shell-crossing) play an important role in cosmology, not only because they are simple but because the breakdown of smooth 3D solution through the development of infinite density caustics begins generically as an almost 1D phenomenon with the formation of pancakes. I present recent work on quasi-one-dimensional (Q1D) flow that depends on all three coordinates but differs only slightly from a strictly 1D flow, thereby allowing a perturbative treatment of shell-crossing using the Euler-Poisson equations written in Lagrangian coordinates. The signature of shell-crossing is then just the vanishing of the Jacobian of the Lagrangian map, a regular perturbation problem. In essence, the problem of the first shell-crossing, which is highly singular in Eulerian coordinates, has been desingularized by switching to Lagrangian coordinates, and can then be handled by perturbation theory. All-order recursion relations are obtained for the time-Taylor coefficients of the displacement field, and it is shown that the Taylor series has an infinite radius of convergence. This allows the determination of the time and location of the first shell-crossing, which is generically shown to be taking place earlier than for the unperturbed 1D flow.

##### 20th June : Journal Club

Caroline Heneka

Improving constraints on the growth rate of structure by modelling the density-velocity cross-correlation in the 6dF Galaxy Survey

arXiv:1706.05205

##### 13th June : Selected topics in theory and observations of Dark Energy (Tunch)

Santiago Casas (ITP, Heidelberg University) - “Non-linear structure formation”

##### 6th June : NO Journal Club on this day

##### 30th May : Selected topics in theory and observations of Dark Energy (Tunch)

Javier Rubio (ITP, Heidelberg University) - “Scale invariance: connecting inflation and dark energy”

Abstract: Inflation and dark energy share many essential properties. I will show that these two eras can be accommodated into a common framework based on scale invariance. I will discuss the phenomenological consequences of two scenarios based on i) exact scale symmetry and ii) broken scale invariance with symmetry resurgence at UV and IR fixed points.

##### 23rd May : Journal Club

Henrik Nersisyan

A Nonlocal Approach to the Cosmological Constant Problem

arXiv:1703.09715

Martin Pauly

Primordial black hole constraints for extended mass functions

arXiv:1705.05567

##### 16th May : Selected topics in theory and observations of Dark Energy (Tunch)

Alefe Almeida(ITP, Heidelberg University) - ” A method for evaluating models that use galaxy rotation curves to derive matter density profiles”

Abstract: There are some approaches, either based on GR or modified gravity that use galaxy rotation to derive the matter density of the corresponding galaxy. In this work, we proposed a test for evaluating them.

##### 9th May Journal Club

Kevin Wolz

Ana Marta Pinho

Stability of fundamental couplings: a global analysis

arXiv:1701.08724

##### 25th April : Selected topics in theory and observations of Dark Energy (Tunch)

Yves Dirian (Geneva University ) - “A numerical relativity scheme for cosmological simulations”

Abstract: I present a new 3+1 integration scheme which allows one to pass an adaptation of the robustness test to the cosmological context, at least in the case of General Relativity with a pressureless perfect fluid field. As an interesting by-product of this construction, a novel constraint-damping method is obtained.

##### 18th April : Journal Club

Viviana Niro

Dario Bettoni

Gravitational Waves in Doubly Coupled Bigravity

arXiv:1703.08016