Non-local gravity

• Michele Maggiore (Universite de Geneve (CH))

Room: 503/1-001 - Council Chamber "Even if the fundamental action of gravity is local, the corresponding quantum effective action, that includes the effect of quantum fluctuations, is a nonlocal object. These nonlocalities are well understood in the ultraviolet regime but much less in the infrared, where they could in principle give rise to important cosmological effects. We discuss work of our group, in which it is assumed that a mass scale $\Lambda$ is dynamically generated in the infrared, giving rise to nonlocal terms in the quantum effective action of gravity. The requirement of providing a viable cosmological evolution severely restricts the form of the nonlocal terms, and selects a model (the so-called RR model) that corresponds to a dynamical mass generation for the conformal mode. For such a model: (1) there is a FRW background evolution, where the nonlocal term acts as an effective dark energy with a phantom equation of state, providing accelerated expansion without a cosmological constant. (2) Cosmological perturbations are well behaved. (3) Implementing the model in a Boltzmann code and comparing with observations we find that the RR model fits the CMB, BAO, SNa and structure formation data at a level statistically equivalent to $\Lambda$CDM. (4) Bayesian parameter estimation shows that the value of $H_0$ obtained in the RR model is higher than in $\Lambda$CDM, reducing to $2.0\sigma$ the tension with the value from local measurements. (5) The RR model provides a prediction for the sum of neutrino masses that falls within the limits set by oscillation and terrestrial experiments (in contrast to $\Lambda$CDM, where letting the sum of neutrino masses vary as a free parameter within these limits, one hits the lower bound). (6) Gravitational waves propagate at the speed of light, complying with the limit from GW170817/GRB 170817A.}"

Cosmological gravity after GW170817

• Johannes Noller (ETH Zurich)

The near simultaneous detections of the gravitational-wave signal GW170817 and its electromagnetic counterpart GRB 170817A place stringent constraints on modified gravity theories, significantly reducing the viable range of cosmological models that have been proposed as alternatives to general relativistic cosmology. I will show how these constraints arise and what scalar-tensor, vector-tensor and tensor-tensor (bimetric) theories remain promising candidates for present-day cosmology.

Probing gravity on cosmological scales through the GW luminosity distance

• Enis Belgacem (University of Geneva)

I will discuss some recent results for modified gravity theories that we have obtained in our group led by Prof. Maggiore in Geneva. In particular I will show that it is possible to introduce a general notion of luminosity distance for gravitational waves in modified gravity theories, which is different from the usual electromagnetic one and could be used to test GR. In a specific nonlocal model that we have studied, the relative difference between this GW luminosity distance in modified gravity and the luminosity distance in LambdaCDM is enhanced by one order of magnitude compared to the same quantity for electromagnetic luminosity distance and I will argue for the validity of this statement in generic modifications of GR. This is an effect that cannot be taken into account by only using the standard dark energy equation of state parametrization. Finally I will focus on the possibility to use future GW detectors like the Einstein Telescope to distinguish nonlocal modifications of gravity from LambdaCDM.

Cosmic Self-Acceleration from Modified Gravity before/after GW170817

• Lucas Lombriser (Eidgenoessische Technische Hochschule Zuerich (CH))

Scalar-tensor modifications of gravity have long been considered as an alternative explanation for the late-time accelerated expansion of our Universe. I will first show that a rigorous discrimination between acceleration from modified gravity and from a cosmological constant or dark energy was not possible with observations of the large-scale structure alone. I will then demonstrate how the measurement of the cosmological speed of gravitational waves with GW170817 breaks this dark degeneracy and how the combination of the two challenges the concept of cosmic acceleration from one of the most general scalar-tensor modifications of gravity. Even more general theories, however, reintroduce the dark degeneracy and I will show how a more conclusive result will only be possible with a large number of Standard Sirens. (Refs: 1509.08458; 1602.07670; https://arstechnica.com/science/2017/02/theoretical-battle-dark-energy-vs-modified-gravity).

Inflationary Perturbations in No-Scale Theories

• Alberto Salvio (CERN)

I discuss the inflationary perturbations in general (classically) scale-invariant theories. Such scenario is motivated by the hierarchy problem and provides natural inflationary potentials and dark matter candidates. All sectors (the scalar, vector and tensor perturbations) are discussed giving general formulae for the potentially observable power spectra, as well as for the curvature spectral index ns and the tensor-to-scalar ratio r. The conserved Hamiltonian for all perturbations does not feature negative energies even in the presence of the Weyl-squared term if the appropriate quantization is performed and this term does not lead to phenomenological problems at least in some relevant setups. The general formulae are applied to a concrete no-scale model, which includes the higgs and a scalar, ``the planckion", whose vacuum expectation value generates the Planck mass. Inflation can be triggered by a combination of the planckion and the Starobinsky scalar leading to a viable scenario. In general, even quadratic inflation is allowed. Moreover, the Weyl-squared term leads to an isocurvature mode, which currently satisfies the observational bounds, but may be detectable with future experiments.

What can we learn from the stochastic gravitational wave background produced by oscillons?

• Francesco Cefalà (University of Basel)

The stochastic gravitational wave (GW) background provides a fascinating window to the physics of the very early universe. Beyond the nearly scale-invariant primordial GW spectrum produced during inflation, a spectrum with a much richer structure is typically generated during the preheating phase after inflation (or after some other phase transition at lower energies). This raises the question of what one can learn from a future observation of the stochastic gravitational wave background spectrum about the underlying physics during preheating. Recently, it has been shown that during preheating non-perturbative quasi-stable objects like oscillons can act as strong sources for GW, leading to characteristic features such as distinct peaks in the spectrum. In this talk, I will discuss how the GW spectrum is affected by the parameters that characterise a given oscillon system, e.g. by the background cosmology, the asymmetry of the oscillons and the evolution of the number density of the oscillons. In addition, a comparison between semi-analytical results and results from numerical lattice simulations for a hilltop inflation model and a KKLT scenario, which differ strongly in some of these characteristics will be shown.

Advances in Lagrangian perturbation theory in LSS

• Vlah Zvonimir (CERN)

Fast Generation of Covariance Matrices for Weak Lensing

• Raphael Sgier (ETH Zurich)

Euclid Emulator

• Mischa Knabenhans (University of Zurich)

Upcoming weak lensing large scale structure surveys such as Euclid, DES, LSST, WFIRST will measure the result of billions of years of evolution from the linear fluctuations observed in the CMB. On the very largest scales these fluctuations are still evolving linearly, but most of the leverage on cosmological parameters from these surveys comes from smaller, highly non-linear scales. While the number of modes of information available to such surveys is in principle much larger than for CMB experiments, there is no analytic theory for how matter clusters non-linearly under the influence of gravity. Only numerical N-body simulations have the required precision. These are, however, far too expensive to be used for maximum likelihood searches over the fundamental cosmological input parameters. Highly precise and rapid matter power spectrum emulation is of paramount importance to these surveys. In my talk I will present a new tool to emulate the non-linear correction (called „boost factor“) of the dark matter power spectrum. A Matlab-based software called UQLab allows us to use state-of-the-art uncertainty quantification techniques to perform sensitivity analyses and error predictions on the cosmological observables and their dependence on the cosmological parameters. This emulator is based on a suite of 100 N-body simulations carried out with pkdgrav3 on the small UZH-based supercomputer zBox4 using 200'000 node hours of computation. To reproduce the matter power spectrum for any set of cosmological parameters requires only fractions of a second, making rapid forward modelling of the observations possible.

Probing the large-scale structure of the universe with phase correlations

• Felipe Oliveira Franco (University of Geneva)

Probing the large-scale structure of the universe with phase correlations

Cosmic vorticity from N-body simulations

• Francesca Lepori (University of Geneva)

Cosmic velocities can be decomposed into a scalar part, which is irrotational, and a divergenceless vector part. Nevertheless, the rotational part of the velocity field is commonly neglected. Although this approximation works very well on linear scales, on small scales it must break down: we know that most galaxies rotate and a coherent rotational motion have been observed on even larger scales, of the order of 20 Mpc. In this talk I will discuss the generation of vorticity in N-body simulations for cold dark matter particles. Even if in the initial condition the velocity field is purely potential, velocity dispersion and vorticity are generated during the evolution due to orbit crossing of the particles and finite grid resolution. In order to study the vorticity induced by shell-crossing, we implemented the computation of the velocity field in gevolution, a relativistic code for N-body simulations. I will show the tests that we performed to assess the reliability of our method and some preliminary results on the estimation of the vorticity power spectrum and its redshift evolution. This work is relevant for improving our theoretical understanding of the rotational content of the cosmic flow and for modelling its impact on galaxy clustering in redshift space.

Lensing of 21 cm intensity mapping

• Mona Jalilvand (University of Geneva)

I'll talk about our work on weak lensing of 21cm intensity mapping. 21cm intensity mapping, like the CMB, undergoes second order lensing; However unlike the CMB, here, we do not neglect the cross-spectrum between 21cm density fluctuations and the lensing potential, since they come from similar redshifts and the cross spectrum is not negligible. We show that lensing is a small effect on the intensity mapping, smaller than the CMB; However, we can observe intensity mapping for a large number of redshifts, as well as use cross-spectra, neither of which is possible with the CMB. Then using Fisher matrix forecast, we study the possibility of detecting the lensing signal.

Chiral dynamos and the origin of cosmic magnetic fields

• Jennifer Schober (EPFL)

Observations of blazar emission suggest that the intergalactic medium is permeated by large-scale magnetic fields. Most probably these are relics of primordial fields, the origin and evolution of which is still a mystery. In this talk, I will present a modified theory of magnetohydrodynamics, which describes a relativistic plasma like the one in the early Universe. We include additional terms and equations in order to follow the dynamics of the chiral chemical potential, i.e. the asymmetry between left- and right-handed fermions. This asymmetry can give rise to a new electric current along the magnetic field, an effect known as the chiral anomaly. Using high-resolution numerical simulations we study the amplification of weak magnetic seed fields shortly after the Big Bang. I will present different new dynamos which can operate in a relativistic plasma for both laminar and turbulent flows. These results, which are constrained by present-day observations of the intergalactic medium, can help us to better understand the role of magnetic fields in the early Universe.

Discussion

General Relativistic corrections in density-shear correlations

• Basundhara Ghosh (University of Geneva)

Room: 4/3-006 - TH Conference Room We investigate the corrections which relativistic light-cone computations induce on the correlation of the tangential shear with galaxy number counts, also known as galaxy-galaxy lensing. The standard-approach to galaxy-galaxy lensing treats the number density of sources in a foreground bin as observable, whereas it is in reality unobservable due to the presence of relativistic corrections. We find that already in the redshift range covered by the DES first year data, these currently neglected relativistic terms lead to a systematic correction of up to 30% in the density-shear correlation function for the highest redshift bins. This correction is dominated by the the fact that a redshift bin of number counts does not only lens sources in a background bin, but is itself again lensed by all masses between the observer and the counted source population. Relativistic corrections are currently ignored in the standard galaxy-galaxy analyses, and the additional lensing of a counted source populations is only included in the error budget (via the covariance matrix). At increasingly higher redshifts and larger scales, these relativistic and lensing corrections become however increasingly more important, and we here argue that it is then more efficient, and also cleaner, to account for these corrections in the density-shear correlations.

Jacobi Mapping Approach for a precise Cosmological Weak Lensing Formalism

• Nastassia Grimm (University of Zurich)

Upcoming cosmological weak lensing surveys have the potential to map the large-scale matter distribution up to high redshifts with unprecedented precision. Thus, they will play a major role in understanding mysteries of the universe such as the nature of dark energy. However, to realize the full potential of the upcoming surveys we need to develop a theoretical framework for the lensing observables with accuracy demanded by the data set. As I will discuss in this talk, the standard weak lensing formalism yields gauge-dependent results for the weak lensing observables. Hence, it does not capture all the physical effects contributing to the distortion of galaxy shapes. I will present a gauge-invariant formalism based on the Jacobi map which resolves this issue.

Integrated approach to cosmology

• Andrina Nicola (ETH Zurich)

Recent progress in observational cosmology and the establishment of ΛCDM have relied on the combination of different cosmological probes. These probes are not independent, since they all measure the same physical fields. The resulting cross-correlations allow for a robust test of the cosmological model through the consistency of different physical tracers and for the identification of systematics. Integrated analyses, taking into account both the auto- as well as the cross-correlations between cosmological probes, therefore present a promising analysis method for both current as well as future data. In this talk, I will present an integrated analysis of CMB temperature anisotropies, CMB lensing, galaxy clustering and weak lensing as well as background probes. I will describe the cosmological probe combination framework, the obtained results and illustrate how this analysis has provided a confirmation of ΛCDM through the consistency of different probes. Furthermore, I will discuss possible tensions between the derived constraints on cosmological parameters and existing ones.

A weak lensing measurement of the external convergence in the field of the lensed quasar HE 0435-1223

• Olga Tihhonova (EPFL - EPF Lausanne)

Time delays in gravitationally lensed quasar systems have become a key probe in our attempts to measure the expansion of the Universe. They provide a new way to obtain independent estimate of the Hubble constant, which is complementary to and competitive with other probes such as the Cosmic Microwave Background or type Ia supernovae. In order to recover the cosmological information from an individual lens system with high precision and accuracy, modelling of the environment and the line of sight of the lens is required. In this talk I will present a weak gravitational lensing measurement of the external convergence along the line of sight to the quadruply lensed quasar HE 0435-1223 (Tihhonova et al. 2017, arXiv:1711.08804). This quasar is used together with other lensed systems by the H0LiCOW collaboration (H0 Lenses in COSMOGRAIL’s Wellspring) to measure the Hubble constant alongside with other cosmological parameters. I will show that the estimated convergence, consistent with other independent studies of this lens, confirms that the intervening mass along the line of sight to HE 0435-1223 does not affect significantly the cosmological results inferred from the time delay measurements of that specific object, making HE 0435-1223 a valuable system for cosmography.

The dwarf galaxy velocity function -- a new probe for dark matter and cosmology

• Aurel Schneider (ETH Zurich)

I will show how it is possible to do cosmology using new data from dwarf galaxies of the local universe.

Primordial Black Holes as Dark Matter

• Juan Garcia-Bellido (CERN)

I will review the scenario of PBH as DM and their rich phenomenology

Massive Black Holes from Dissipative Dark Matter

• Guido D'Amico (CERN)

We show that a subdominant component of dissipative dark matter resembling the Standard Model can form many intermediate-mass black hole seeds during the first structure formation epoch. We also observe that, in the presence of this matter sector, the black holes will grow at a much faster rate with respect to the ordinary case. These facts can explain the observed abundance of supermassive black holes feeding high-redshift quasars. The scenario will have interesting observational consequences for dark substructures and gravitational wave production.

How do stars affect ψDM?

• James Chan (EPFL)

Wave dark matter, ψDM, has recently been gaining attention as a dark matter candidate. This is due in part to the model’s ability to predict, thus a falsifiable model. Noticeably it predicts a central prominent core, dubbed as the soliton, in every galaxy and large-amplitude density fluctuations in galaxy halos dubbed as halo granules. In this work we conduct the first ψDM halo simulations that include stars in the inner halo. The results show that the soliton becomes ever stronger in the presence of stars, and the soliton slowly grows by absorbing mass from its host halo to maintain the soliton scaling relation. Moreover, halo granules become “non-isothermal” after stars have thoroughly interacted with the inner halo, a situation unlike isothermal granules in halos of pure ψDM cosmological simulations. We find that the composite (dark matter+stars) mass density can locally follow the isothermal profile near the stellar half-light radius in most cases. Phase separation that segregates a uniform population of stars into two distinct populations of high and very low velocity dispersions stars is also found in some situations. While halo stars have high velocity dispersion, the cold component is bound inside the soliton resembling stars residing in faint dwarf spheroidal galaxies. The most striking result of this study is that the velocity dispersion of halo stars increases rapidly toward the galactic center by a factor of at least 2 or so inside the half-light radius caused by the deepened soliton potential, a result that compares favorably with observations of elliptical galaxies and bulges in spiral galaxies. Finally, we find that the inner halo follows the same self-similar relation as the soliton does within the mass range of dwarf to small galaxies.

Searches for and properties of the sources of cosmic reionization

• Daniel Schaerer (University of Geneva)

I will present an update and overview on searches for sources of cosmic reionization and their analogs at lower redshift from numerous recent observational efforts with the HST and large ground-based telescopes. Recent breakthroughs have allowed us to find fairly strong Lyman continuum emitters (with escape fractions ~5-40%), and to significantly increase the number of such confirmed sources. We discuss the physical properties of these sources and the physical mechanisms allowing the escape of ionizing radiation. The new results also allow us to define several new indirect diagnostics (using e.g. Lyman-alpha, UV absorption lines, rest-optical emission lines), which can be used up to the highest redshift and which will be crucial to identify and study these sources with upcoming facilities and instruments, such as the JWST and the ELTs.

The discovery of an ultra metal-poor star

• Pascale Jablonka (EPFL)

The early Universe presents a star formation environment almost devoid of heavy elements. Stars observed with the lowest total metallicities provide very important constraints for studying these conditions, however these stars are exceedingly rare in the present-day Galaxy and difficult to uncover. Here, I will present the discovery and subsequent analysis of such an outstanding and rare case.

Illuminating the Dark Universe with fluorescent Lyman-alpha emission

• Sebastiano Cantalupo (ETH Zurich)

Gravitational collapse during the Universe's first billion years transformed a nearly homogeneous matter distribution into a network of filaments - the Cosmic Web - where galaxies form and evolve. In this talk, I will present the results of a new program to directly detect and study the baryonic component of the Cosmic Web in emission using bright quasars and galaxies as external "sources of illumination’’. In particular, I will show results from ultra-deep narrow-band imaging and recent integral-field-spectroscopy as a part of the MUSE Guaranteed Time of Observation program that revealed numerous giant Lyman-alpha emitting filaments extending up to several hundred kpc around quasars and bright galaxies. I will discuss how the unexpectedly high luminosities of these systems represent a challenge for our current understanding of cosmological structure formation. In particular, I will show that current observations suggest that a large amount of "cold" and dense gaseous "clumps" should be present around high-redshift galaxies and I will present our first attempts to understand the origin and nature of these structures.

Stellar versus total mass in lensing galaxies from CFHTLS

• Philipp Denzel (University of Zurich)

"We will present mass maps of lensing galaxies recently discovered by the Space Warps project from CFHTLS, as well as reconstructions of their stellar mass distributions. This enables us to estimate the star formation efficiency at different radii in the same lensing galaxy and of galaxies of different masses and redshift. The results will be compared with galaxy formation simulations. Denzel P (1) , Saha P (1) , Küng R (1) , Ferreras I (2). (1) University Of Zurich, Zurich, Switzerland. (2) Mullard Space Science Laboratory University College London, London, UK"

Galaxy Evolution in Cosmic Voids

• Elena Ricciardelli (EPFL)

Cosmic voids are large underdense regions that constitute one of the most prominent aspect of the Cosmic Web and occupy most of the volume in the Universe. Despite their extreme low density, voids contain a tenuous network of filaments and low-mass galaxies, providing a unique and pristine environment for studying the formation and evolution of galaxies. In this talk I will present recents results showing the impact of the low density environment of cosmic voids on the properties of galaxies living within them.

General Discussion