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

#### Thursday, PW 12, Room 106

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

### Seminar calendar

**Summer term 20:**

##### 8th July 2020: Selected topics in theory and observations of Dark Energy (Tunch)

Adrià Gómez-Valent - Boosting MonteCarlo sampling with a non-Gaussian fit

We propose a new method, called MonteCarlo Posterior Fit, to boost the MonteCarlo sampling of likelihood (posterior) functions. The idea is to approximate the posterior function by an analytical multidimensional non-Gaussian fit. The many free parameters of this fit can be obtained by a smaller sampling than is needed to derive the full numerical posterior. In the examples that we consider, based on supernovae and cosmic microwave background data, we find that one needs an order of magnitude smaller sampling than in the standard algorithms to achieve comparable precision. This method can be applied to a variety of situations and is expected to significantly improve the performance of the MonteCarlo routines in all the cases in which sampling is very time-consuming. Finally, it can also be applied to Fisher matrix forecasts, and can help solve various limitations of the standard approach.

This discussion is based view arXiv

**Winter term 19/20:**

##### 20th February 2020: Selected topics in theory and observations of Dark Energy (Tunch)

Cancelled.

##### 13th February 2020: Journal Club

Christof Wetterich - The great emptiness at the beginning of the Universe

This informal discussion concerns the proposal of the great emptiness as a possible beginning of the Universe in the infinite past of physical time. In the beginning, particles are very rare and effectively massless. Only expectation values of fields and average fluctuations characterize the lightlike vacuum of this empty Universe. Our observed inhomogeneous Universe can be extrapolated backwards to the lightlike vacuum in the infinite past, and therefore can have lasted eternally. There is no physical big bang singularity. Standard inflation models admit a primordial flat frame for which spacetime is flat in the infinite past.

This discussion is based view arXiv

##### 6th February 2020: Selected topics in theory and observations of Dark Energy (Tunch)

David Camarena - Exploring the cosmic distance ladder

The cosmic distance ladder is one of the most powerful ways to constraint the Hubble constant at local scales. In this tunch, we will explore two novel approximations to the cosmic distance ladder assessing mainly the impact of the deceleration parameter and the role that BAO distances plays in the Hubble tension problem.

##### 30th January 2020: Journal Club

David Camarena

Cosmological model insensitivity of local H0 from the Cepheid distance ladder

S. Dhawan, D. Brout, D. Scolnic, A. Goobar, A.G. Riess, V. Miranda

view arXiv

##### 23rd January 2020: Selected topics in theory and observations of Dark Energy (Tunch)

Cancelled due to overlap with the Cosmology Seminar.

##### 16th January 2020: Journal Club

Adrià Goméz-Valent

Concerns regarding the use of black hole shadows as standard rulers

S. Vagnozzi, C. Bambi and L. Visinelli

view arXiv

##### 9th January 2020: Selected topics in theory and observations of Dark Energy (Tunch)

Raquel Fazolo - TBA

##### 2nd January 2020: Journal Club

Christmas break

##### 26th December 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Christmas break

##### 19th December 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Arvid Weyrauch - TBA (Christmas Tunch)

##### 12th December 2019: Journal Club

Raquel Fazolo

Is the Hubble diagram of quasars in tension with concordance cosmology?

Hermano Velten, Syrios Gomes

view arXiv

Luca Amendola

Challenging a Newtonian prediction through Gaia wide binaries

X. Hernandez, R. A. M. Cortés, C. Allen, R. Scarpa

view arXiv

Ana Marta Pinho

Wrapping up the decade - the discoveries and the tensions

##### 5th December 2019: Journal Club

Cancelled due to overlap with the Cosmology Seminar.

##### 28th November 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Josue Motoa Manzano (Universidad de Valle, Cali, Colombia; visitor at ITP, Universität Heidelberg) - “De Sitter symmetries and inflationary correlators in parity violating scalar-vector models”

We use conformal symmetry to constrain the shape of inflationary correlators in the presence of long-lived vector field perturbations. Applying conformal Ward identities, we derive general expressions, up to amplitudes and normalization factors, for the two and three-point correlators in the presence of vector fields mediated by the interaction f(\phi)\left(F_{\mu \nu}F^{\mu \nu}+\alpha\tilde{F}_{\mu \nu}F^{\mu \nu}\right), where f(\phi) is a suitable coupling function between the scalar and the vector field. This interaction allows for isotropy and parity symmetry breaking and is consistent with super horizon conformal symmetry.

##### 21st November 2019: Journal Club

Manuel Wittner

Trans-Planckian Censorship and Inflationary Cosmology

Alek Bedroya, Robert Brandenberger, Marilena Loverde, Cumrun Vafa

view arXiv

##### 14th November 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Leonardo Giani (UFES, Brazil; visitor at ITP, Universität Heidelberg) - “Late-times asymptotic equation of state for a class of nonlocal theories of gravity”

We investigate the behavior of the asymptotic late-times effective equation of state for a class of nonlocal theories of gravity. These theories modify the Einstein-Hilbert Lagrangian introducing terms containing negative powers of the d'Alembert operator acting on the Ricci scalar. We find that imposing vanishing initial conditions for the nonlocal content during the radiation-dominated epoch implies the same asymptotic late-times behavior for most of these models. In terms of the effective equation of state of the universe, we find that asymptotically ωeff→−1, approaching the value given by a cosmological constant. On the other hand, unlike in the case of ΛCDM, the Hubble factor is a monotonic growing function that diverges asymptotically. We argue that this behavior is not a coincidence and discuss under which conditions this is to be expected in these nonlocal models.

Based on view arXiv

##### 7th November 2019: Journal Club

Ana Marta Pinho

Planck evidence for a closed Universe and a possible crisis for cosmology

Eleonora Di Valentino, Alessandro Melchiorri and Joseph Silk

##### 31st October 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Manuel Wittner (ITP, Universität Heidelberg) - “Challenges of Stringy Quintessence: the F-term Problem”

In light of the recently proposed de Sitter-swampland conjecture, string theory seems to favor dynamic dark energy models like quintessence over a cosmological constant to drive the late time acceleration of the universe. In this talk I argue, however, that stringy quintessence models are difficult to realize as well. I elaborate on the light volume problem and identify a so called “F-term problem”: The positive energy density of standard-model SUSY breaking is higher than the depth of all known negative contributions. Resolving this issue would require new, unknown contributions to the vacuum energy and therefore necessitates further analysis.

(Based on the recent paper view arXiv )

##### 24th October 2019: Journal Club

Arvid Weyrauch

Obtaining Precision Constraints on Modified Gravity with Helioseismology

Ippocratis Saltas, Ilídio Lopes

view arXiv

**Summer term 19:**

##### 25th July 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Giorgio Laverda (ITP, Universität Heidelberg) - “The entropic theory of gravity”

Since its first appearance in 2010, the theory of entropic gravity has been widely debated. The interpretation of gravity as an entropic force appears to be capable of describing Newtonian gravity and general relativity. In this talk I will introduce the basic principles of the theory while highlighting the controversial points.

##### 18th July 2019: Journal Club

Canceled due to overlap with cosmology seminar.

##### 11th July 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Jakob Henrichs (ITP, Universität Heidelberg) - “Milky Way rotation curve in modified gravity”

Rotation curves of galaxies provide a strong evidence for dark matter and can be used to test modified gravity theories. In this talk I show the results of an analysis of the Milky Way rotation curve in modified gravity. I use the Horndeski Theory to derive the gravitational potential and two different dark matter models for the analysis.

##### 4th July 2019: Journal Club

Oliver Fabio Piattella

Can the quantum vacuum fluctuations really solve the cosmological constant problem?

Gabriel R. Bengochea, Gabriel León, Elias Okon, Daniel Sudarsky

view arXiv

##### 25th June 2019, PW 12, Room 060 (NOTE UNUSUAL DATE AND LOCATION): Selected topics in theory and observations of Dark Energy (Tunch)

Adrià Gòmez-Valent (ITP, Universität Heidelberg) - “Quantifying the evidence for the current speed-up of the Universe with low and intermediate-redshift data. A more model-independent approach”

According to cosmological low and intermediate-redshift data, what is the statistical evidence in favor of the current speed-up of the Universe? Although this question seems to be kind of outdated, a quick review to the many papers that address this pivotal question in the literature tells us that the answer is not obvious at all. Determining the value of the deceleration parameter in the context of particular cosmological models, concrete parametrizations of the cosmographical quantities, or even using truncated cosmographical expansions in which the truncation order is set in an ad hoc way can lead to biased estimations of the value of the deceleration parameter at present, q0, and its uncertainty. In this talk, which is based on arXiv:1810.02278, JCAP 05 (2019) 026, I present a new determination of q0 obtained with data from the Pantheon+MCT compilation of supernovae of Type Ia, cosmic chronometers and baryon acoustic oscillations. I apply the so-called Weighted Function Regression method to reconstruct q(z) more model-independently than many analyses in the literature, improving in this way the usual cosmograhical approach. We will see, for instance, that using only the first two data sets the level of evidence for the current positive acceleration of the universe is only moderate (using Jeffreys' scale and jargon), contrary to the more than 17\sigma-evidence found in the framework of the standard flat LCDM model. The level of evidence grows up to the very strong one when BAO data are also considered, giving rise to q0= -0.60 +- 0.10, with a deceleration-acceleration transition redshift at z_t = 0.80 +- 0.10. The jerk is also reconstructed, although unfortunately current data is unable to set stringent constraints to this cosmographical parameter.

##### 19th June 2019, PW 12, Room 060 (NOTE UNUSUAL DATE AND LOCATION): Journal Club

Giorgio Laverda

Strong gravity signatures in the polarization of gravitational waves

S. Shankaranarayanan

view arXiv

Julius Wons

Gravitational wave standard sirens and cosmological parameter measurement

Xin Zhang

view arXiv

##### 13th June 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Arvid Weyrauch & Manuel Wittner (ITP, Universität Heidelberg) - “Cosmology, philosophically speaking” (Collective Discussions)

As researchers we tend to find answers using the scientific method. However, some questions may not have an answer but still spark a lot of different thoughts and perspectives. In this tunch, we will address some of these questions, from the nature of time to its beginning.

##### 6th June 2019: Journal Club

Leonardo Giani

Is Gravity Actually the Curvature of Spacetime?

Sebastian Bahamonde, Mir Faizal

view arXiv

Ana Marta Pinho

Black hole shadow as a standard ruler in cosmology

Oleg Yu. Tsupko, Zuhui Fan, Gennady S. Bisnovatyi-Kogan

view arXiv

##### 29th May 2019, PW 12, Room 060 (NOTE UNUSUAL DATE AND LOCATION): Selected topics in theory and observations of Dark Energy (Tunch)

Oliver Piattella (UFES, Brazil; visitor at ITP, Universität Heidelberg) - “Neutron stars masses in R^2 gravity”

We investigate the mass-radius (M-R) relation for neutron stars in f(R) = R + R^2 gravity. We adopt a spherically symmetric metric and a SLy equation of state. We stress how in f(R) gravity it is possible to define mass in many inequivalent ways which are nonetheless all equivalent in General Relativity. We investigate the role of the extra scalar degree of freedom in the definition of the masses and in the features of the M-R relation.

##### 23rd May 2019: Journal Club

Adrià Gomez-Valent

Recasting H0 tension as OmegaM tension at low z

Eoin Ó Colgáin

view arXiv

Leonardo Giani

How to Hide a Cosmological Constant

S. Carlip

view arXiv

##### 16th May 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Luca Amendola (ITP, Universität Heidelberg) - “Fisher matrix for multiple tracers (1904.00673)”

I show how to obtain constraints on β, the ratio of the matter growth rate and the bias that quantifies the linear redshift-space distortions, that are independent of the cosmological model, using multiple tracers of large-scale structure. For a single tracer the uncertainties on β are constrained by the uncertainties in the amplitude and shape of the power spectrum, which is limited by cosmic variance. However, for two or more tracers this limit does not apply, since taking the ratio of power spectra cosmic variance cancels out, and in the linear (Kaiser) approximation one measures directly the quantity (1+β1μ2)2/(1+β2μ2)2, where μ is the angle of a given mode with the line of sight. I provide analytic formulae for the Fisher matrix for one and two tracers (up to quadrature) and quantify the signal-to-noise ratio needed to make effective use of the multiple-tracer technique. I also forecast the errors on β for a survey like Euclid.

##### 9th May 2019: Journal Club

Xuewen Liu

21-cm observations and warm dark matter models

Alexey Boyarsky, Dmytro Iakubovskyi, Oleg Ruchayskiy, Anton Rudakovskyi, Wessel Valkenburg

view arXiv

##### 2nd May 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Ana Marta Pinho (ITP, Universität Heidelberg) - “Cosmological Information Flow”

The current state of Cosmology is a strong statistical preference for the LCDM model although with some unsolved problems. The recent data tensions raise now the doubt about the validity of the data pipeline and/or the LCDM model. Information theory provides some very useful tools that can measure the amount of information given by the data. Also these tools can be related to what it is commonly used in the literature as uncertainty measurements. In this talk I will present this on going work that tries to understand how information flows in the cosmological framework.

##### 25th April 2019: Journal Club

Ana Marta Pinho

Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the Determination of the Hubble Constant and Stronger Evidence for Physics Beyond LambdaCDM

Adam G. Riess, Stefano Casertano, Wenlong Yuan, Lucas M. Macri, Dan Scolnic

view arXiv

Manuel Wittner

Probing modified gravity in cosmic filaments

Alex Ho, Max Gronke, Bridget Falck, David F. Mota

view arXiv

**Winter term 18/19:**

##### 7th March 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Collective Discussions

##### 28th February 2019: Journal Club

Jiaming Zhao

The local and distant Universe: stellar ages and $H_0$

Raul Jimenez, Andrea Cimatti, Licia Verde, Michele Moresco, Benjamin Wandelt

ePrint: arXiv: 1902.07081

Ana Marta Pinho

The observable $E_g$ statistics

Basundhara Ghosh, Ruth Durrer

ePrint: arXiv: 1812.09546

##### 21st February 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Ana Marta Pinho & Manuel Wittner (ITP, Universität Heidelberg) - “Cosmology, philosophically speaking” (Collective Discussions)

As researchers we tend to find answers using the scientific method. However, some questions may not have an answer but still spark a lot of different thoughts and perspectives. In this tunch, we will address some of these questions, from infinity to the beginning of the universe as well as the anthropic principle or the nature of time and space.

##### 14th February 2019: Journal Club

Julius Wons

Using a black hole to weigh light: can the Event Horizon Telescope yield new information about the photon rest mass?

Robert P. Cameron

ePrint: arxiv 1902.02209

Jiaming Zhao

Cosmological constrains from the Hubble diagram of quasars at high redshifts

Guido Risaliti and Elizabeta Lusso

ePrint: arXiv: 1811.02590

Oliver Piattella

On the cosmological problem

Lucas Lombrasier

ePrint: arXiv: 1901.08588

##### 7th February 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Manuel Wittner (ITP, Universität Heidelberg) - “Introduction to flux compactifications and moduli stabilization”

Kaluza-Klein compactifications are the bridge between 10D string theory and our 4D world. Through the large number of possibilities of this compactified internal space, a vast landscape of string vacua arises, one of which might represent our Universe. In this talk I will introduce basic knowledge about flux compactifications and the stabilization of moduli, which are necessary steps in order to obtain realistic 4D models.

##### 31st January 2019: Journal Club

Ana Marta Pinho

Could Solar Radiation Pressure Explain 'Oumuamua's Peculiar Acceleration?

Shmuel Bialy, Abraham Loeb

ePrint: arXiv: 1810.11490

Xuewen Liu

On the road to per-cent accuracy: nonlinear reaction of the matter power spectrum to dark energy and modified gravity

Matteo Cataneo, Lucas Lombriser, Catherine Heymans, Alexander Mead, Alexandre Barreira, Sownak Bose, Baojiu Li

ePrint: arXiv: 1812.05594

##### 24th January 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Salvador Ramirez (ITP, Universität Heidelberg) - “Deser-Woodard Model and its Reconstruction Problem”

Nonlocal modifications of gravity arise when one attempts to see the effects of radiative corrections at the classical level. These corrections are often impractical, they are difficult to calculate and depend on the method used and assumptions about the source of the corrections. To overcome this we can take a top-down approach, and construct an action with nonlocal terms based on their properties. Deser-Woodard model follows this approach and constructs an action with one of the simplest nonlocal terms and an arbitrary function. We will discuss the reconstruction problem of this model and provide a method to reproduce any given background. Since the model differs with $\Lambda$CDM at the perturbation level, we can test the model with observational data.

##### 17th January 2019: Journal Club

Jiaming Zhao

KiDS+GAMA: Constraints on Horndeski gravity from combined large-scale structure probes

Alessio Spurio Mancini, Fabian Köhlinger, Benjamin Joachimi, Valeria Pettorino, Björn Malte Schäfer, Robert Reischke, Samuel Brieden, Maria Archidiacono et al.

ePrint: arXiv: 1901.03686

Julius Wons

Melanopogenesis: Dark Matter of (almost) any Mass and Baryonic Matter from the Evaporation of Primordial Black Holes weighing a Ton (or less)

Logan Morrison, Stefano Profumo, Yan Yu

ePrint: arXiv: 1812.10606

##### 10th January 2019: Selected topics in theory and observations of Dark Energy (Tunch)

Xuewen Liu (Institute of Theoretical Physics, CAS, Peking) - Cultural Tunch

##### 3rd January 2019: Journal Club

Christmas break

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

Christmas break

##### 20th December 2018: Journal Club

Collective Discussions

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

Collective Discussions

##### 6th December 2018: Journal Club

Jiaming Zhao

Cosmological parameter constraints for Horndeski scalar-tensor gravity

Johannes Noller, Andrina Nicola

ePrint: arXiv: 1811.12928

Guillem Domenèch

Early Dark Energy Can Resolve The Hubble Tension

Vivian Poulin, Tristan L. Smith, Tanvi Karwal, Marc Kamionkowski

ePrint: arXiv: 1811.04083

##### 29th November 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Collective Discussions

##### 22nd November 2018: Journal Club

Julius Wons

Primordial Black Holes With Multi-Modal Mass Spectra

Bernard Carr, Florian Kuhnel

ePrint: arXiv: 1811.06532

Salvador Ramirez

Current Signs of Dynamical Dark Energy

Joan Sola Peracaula, Adria Gomez-Valent, Javier de Cruz Perez

ePrint: arXiv: 1811.03505.pdf

##### 15th November 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Julius Wons (ITP, Universität Heidelberg) - “Are primordial standard clocks really standard?”

Excited heavy fields during the creation of the primordial curvature perturbation leave oscillatory patterns in the power spectrum. These patterns can be used to distinguish between different primordial scenarios (inflation, bounce, …) and are therefore called primordial standard clocks. I will show that this is in general not true if the mass of the heavy field is time dependent or when having non-canonical kinetic terms.

##### 8th November 2018: Journal Club

Manuel Wittner

On the Cosmological Implications of the String Swampland

Prateek Agrawal, Georges Obied, Paul J. Steinhardt, Cumrun Vafa

ePrint: arXiv: 1806.09718

Stefano Savastano

The Primordial Black Hole Dark Matter - LISA Serendipity

N. Bartolo, V. De Luca, G. Franciolini, M. Peloso, A. Riotto

ePrint: arXiv: 1810.12218

Xue-Wen Liu

The End of Cosmic Growth

Eric V. Linder, David Polarski

ePrint: arXiv: 1810.10547

##### 1st November 2018: NO Journal Club/Tunch

Public Holiday

##### 25th October 2018: Journal Club

Dario Bettoni

Dark energy after GW170817, revisited

Edmund J. Copeland, Michael Kopp, Antonio Padilla, Paul M. Saffin, Constantinos Skordis

e-Print: arXiv:1810.08239

Ana Marta Pinho

H0 Tension: Response to Riess et al arXiv:1810.03526

Tom Shanks, Lucy Hogarth, Nigel Metcalfe

e-Print: arXiv:1810.07628

##### 18th October 2018: Selected topics in theory and observations of Dark Energy (Tunch)

Manuel Wittner (ITP, Universität Heidelberg) - Cultural Tunch

**Summer term 18:**

##### 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

**Winter term 17/18:**

##### 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

**Summer term 17:**

##### 18th July 2017: Journal Club

##### 11th July 2017: 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 2017: 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 2017: 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 2017: 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 2017: Selected topics in theory and observations of Dark Energy (Tunch)

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

##### 6th June 2017: NO Journal Club on this day

##### 30th May 2017: 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 2017: 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 2017: 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 2017: Journal Club

Kevin Wolz

Ana Marta Pinho

Stability of fundamental couplings: a global analysis

arXiv:1701.08724

##### 25th April 2017: 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 2017: Journal Club

Viviana Niro

Dario Bettoni

Gravitational Waves in Doubly Coupled Bigravity

arXiv:1703.08016