Location: Guildhall Studio

I1 - Monday 14:00-15:40 (Lucas Lombriser)

Dark matter signatures in laser-interferometric gravitational-wave detectors


Hartmut Grote (Cardiff University)

The nature of dark matter is one of the most important questions
in contemporary fundamental physics. While WIMPS (Weakly Interacting Massice Particles)
have been a prime dark matter candidate for decades, in recent years lighter particles
have come more into focus.
Laser-interferometric gravitational-wave detectors, but also laser interferometers
in general, with their excellent sensitivities can be used to search for different
dark matter candidates, as for example so-called scalar fields, topological defects,
axions, and dark photons.
I will highlight a number of connections between dark matter,
gravitational waves, and laser interferometry, pointing out in particluar
new limits that can be set for scalar fields and topological defect dark matter using
gravitational-wave detectors.

Strongly lensed Fast Radio Bursts as precision cosmological probes


Li Zhengxiang (Department of Astronomy, Beijing Normal University)

Fast Radio bursts (FRBs), bright transients with millisecond durations at $sim$ GHz and typical redshifts probably $>0.8$, are likely to be gravitationally lensed by intervening galaxies. Since the time delay between images of strongly lensed FRB can be measured to extremely high precision because of the large ratio $sim10^9$ between the typical galaxy-lensing delay time $simmathcal{O}$(10 days) and the width of bursts $simmathcal{O}$(ms), we propose strongly lensed FRBs as precision probes of the universe. We show that, within the flat $Lambda$CDM model, the Hubble constant $H_0$ can be constrained with a $sim0.91%$ uncertainty from 10 such systems probably observed with the Square Kilometer Array (SKA) in $<$ 30 years. More importantly, the cosmic curvature can be model-independently constrained to a precision of $sim0.076$. This constraint can directly test the validity of the cosmological principle and break the intractable degeneracy between the cosmic curvature and dark energy.

The Local Universe and the Cosmological and Copernican principles within Lambda CDM.


Mohamed Rameez (Niels Bohr Institute, University of Copenhagen)

The dipolar anisotropy of the CMB is believed to be due to our motion with respect to the CMB rest frame at 369 km/s. This should cause a dipolar modulation in the number counts of distant sources, through special relativistic aberration and Doppler boosting effects. We test this with various all-sky catalogues: NVSS and SUMSS radio galaxies, WISE galaxies and Quasars as well as GAIA-unWISE AGNs and consistently find a significantly larger dipole than expected, implying velocities > 1000 km s^-1 at conservative statistical significances as high as 3.3 sigma. These and other observations hint at a bulk displacement between the matter and radiation rest frames in the local Universe, extending out to scales larger than is typical in Lambda CDM N-body simulations. An observational effect of such a bulk flow would be a scale dependent dipolar modulation in the deceleration parameter. We look for this in the SDSS-II/SNLS-III Joint lightcurve analysis catalogue of SN1a curves and find such a modulation at >3.4 sigma statistical significance, while the evidence for any isotropic acceleration of the Universe simultaneously drops to <1.4 sigma. These observations and other curious features of SN1a data, as well as a significant body of literature from the classical general relativistic picture of cosmology suggest that Lambda CDM may merely be a toy model of the inhomogeneous Universe.

A Cosmology-Independent Calibration of Gamma-Ray Burst using observational Hubble parameter data


Ariadna Montiel (National Autonomous University of Mexico)

A cosmology-independent calibration of 88 Gamma-Ray Bursts (GRBs) observed by Fermi-LAT is presented. To overcome the circularity problem affecting the use of GRBs as distance indicators, the well-known Amati relation is calibrated. Specifically, the latest observational Hubble parameter data are used to approximate the cosmic evolution through a Bezier parametric curve and then, obtain the distance moduli of the GRBs which can be used to constrain dark energy models.
The constraints for the standard cosmological model LCDM and for the CPL parametrization through a Bayesian analysis using the latest observational data (the Pantheon SNIa dataset, Planck Compressed 2018 data, the latest BOSS release of BAO data and the GRBs of this work) are presented. We also show the advantages of our study with respect to the results coming from a recent calibrated dataset of GRBs.

I2 - Monday 16:10-17:50 (Lucas Lombriser)

Cosmology with galaxy clusters and the Dark Energy Survey


Paul Giles (University of Sussex)

Constraining cosmology with galaxy clusters is one of the fundamental goals of the Dark Energy Survey (DES). Tens of thousands of clusters out to redshift 0.65 have been identified in DES data. Weak lensing and multi-wavelength studies with X-ray data and cosmic microwave background are performed to provide inputs into the cosmology analysis. Furthermore, simulations are being utilised to study systematic effects. A cosmology pipeline that incorporates systematic effect modeling is being used to derive constraints on cosmology parameters. In this talk, I will present progress on the DES galaxy cluster cosmological analyses, point out the current systematic effect limits, and discuss future improvements.

The cluster counts tension : calibration issue or new physics ?


Stéphane Ilic (Observatoire de Paris – LERMA)

The ΛCDM concordance model is very successful at describing our Universe with high accuracy and only a few parameters. Despite its successes, a few tensions persist; most notably, the best-fit ΛCDM model, as derived from the Planck CMB data, largely overpredicts the abundance of Sunyaev–Zel’dovich clusters when using their standard mass calibration. Whether this is the sign of an incorrect calibration or the need for new physics remains a matter of debate. In this talk, I will present the results from our exploration of two simple extensions of the standard model — massive neutrinos and a simple modified gravity model — and their ability to release the aforementioned tension, pitted against a potential bias in the mass calibration of clusters. Under certain assumptions, we find that our conclusions may call for new physics in the gravitational sector.

Momentum transfer in interacting dark energy models.


Mark Linton (ICG, University of Portsmouth )

The nature of dark energy and dark matter remains a largely open question in cosmology and it is necessary to investigate if they could interact. I will discuss the possible nature of these interactions before focusing in on a particularly noteworthy sub category; momentum only interactions. I will explore the interesting features of these models before discussing the constraints on them using both high and low redshift data.

Cosmological Models with Rip Cosmology in Modified Gravity


Bivudutta Mishra (Mathematics, BITS-Pilani, Hyderabad Campus)

In this paper, we have constructed phantom cosmological models in modified theory of gravity. The geometrical part of the action is considered as a coupled linear function of Ricci scalar and trace of energy momentum tensor. Little rip, Pseudo rip models have been investigated where the equation of state parameter evolves asymptotically and sufficiently rapidly to -1. The effect of coupling constant of the modified gravity theory on the Dynamics has been discussed. Possible wormhole solutions for the phantom models are discussed. The possibility of Big Trip in Wormholes are discussed for the models.

I3 - Tuesday 14:00-15:40 (Lucas Lombriser)

Forward Modelling the Universe: Application to cosmic shear


Adam Amara (ICG, University of Portsmouth)

Observational cosmology is going through a golden age. In particular, we are in the midst of an influx of data from on-going experiments, such as the Dark Energy Survey (DES). In the coming five years, the volume and quality of data will rapidly increase as Stage IV surveys, Euclid, LSST and WFIRST, come online. Processing this data will require new algorithms and methods to maximise our science reach and to control for systematic errors. In this talk, I will present a method that we have developed called Monte-Carlo-Control-Loops that relies heavily on forward modelling the observed data by simulating all the processes from cosmology theory to images. Given the complexities of the late-time Universe, these forward models need to capture the important properties of galaxy populations and key features imprinted on the data from the experiments themselves. By bringing together all these elements with advanced statistical methods and new machine learning algorithms, we can build a process for extracting maximal information from the new data, which will allow us to extensively test the physics of the dark sector.

Investigating the Hubble Constant tension with CosmoEJS



It has been well documented that different observations measuring the value of the current expansion rate of the universe, or the Hubble Constant, can have tension with each other. This tension can be lessened by some particular dark energy and modified gravity models. We explore modified gravity and dark energy cosmological models by fitting their parameters to some of the latest observational data sets. We use various combinations of data sets including cosmic microwave background radiation, supernovae type Ia, baryon acoustic oscillations, strong lensing, cosmic chronometers, redshift space distortions, and various Hubble Constant data sets. CosmoEJS constructs dynamical plots of the model’s evolutionary history from the fits obtained. The CosmoEJS packages allow for interactive simultaneous plotting and comparing of different cosmological models to actual observational data sets.

Measuring the Hubble constant in the era of Euclid and SKA


Carlos Bengaly (Department of Theoretical Physics, University of Geneva)

The Hubble Constant (H_0) tension between early and late Universe probes is one of the most intriguing problems in Cosmology nowadays, given that it may open up a window for physics beyond the standard cosmological model. It is therefore essential to determine how well can we measure this quantity with upcoming cosmological data. By simulating H(z) data points reproducing SKA- and Euclid-like specifications, we find that we can measure H_0 to almost 1% precision. Hence, we can rule out the early or late Universe H_0 value at a ~5$sigma$ level.

Evidence of deviation from LCDM in the interacting vacuum scenario? The state of the art.


Natalie Hogg (ICG, University of Portsmouth)

It is well-known that there are problems with the standard model of cosmology that hint at possible physics beyond the LCDM paradigm. In particular, the 4 sigma tension in the values of H0 coming from CMB and supernovae measurements is motivation enough to consider alternative cosmological models.

In this talk, I will introduce one such alternative, the interacting vacuum scenario. Beginning with the covariant theory of the interaction, focusing on a specific “geodesic” class, I will show the linear theory of perturbations in the scenario before specialising to the spatially flat FLRW background. I will describe how the parametrisation of the interaction can be constrained using MCMC methods and show the results of our investigation, focusing on the effects of the interaction on the cosmological tensions and the results of our model comparison with LCDM.

I4 - Tuesday 16:10-17:50 (Lucas Lombriser)

A simple model of space structure


Manasse Mbonye (Physics, University of Rwanda)

Observetionally, space appears isotropic and homogeneous at large length scales while appearing foamy and possibly discrete at small length scales. This is a well-known long standing puzzle. There are equally several other puzzles, such as what exactly curves in bare space when it interacts with gravity? These and other unexplained characteristics suggest that bare physical space may have a hidden structure. In this work a simple model of a space structure that reproduces these characteristics is presented. Starting off from simple basic assumptions we reproduce a space that is grainy and discrete at small length scales, and which becomes homogeneous and isotropic at large length, scales consistent with observations of physical space. In the model, bare space is energy-neutral. Yet the model indicates a non-trivial space-energy relationship in which the former could source the latter, including dark energy. Finally the model suggests a potential route toward a resolution of the Cosmological Constant Problem.

Compact structures featuring negative pressure equations of state


PHILIP BELTRACCHI (Physics and Astronomy, University of Utah)

I present new exact solutions for static/collapsing/expanding general relativistic systems with negative pressure. Compact structures with negative pressure were first seriously considered in the 1960s as a counterexample to the Hawking/Penrose singularity theorems. There has been a renewed interest in the topic since the expansion of the universe indicates negative pressure “dark energy” exists in nature. Unlike a cosmological constant, compact objects containing dark energy also contain anisotropic stress. Our new negative pressure anisotropic systems include: a time dependent generalization to the Schwarzschild fluid solution, a model formation process for an anisotropic gravastar, and an everywhere anisotropic case featuring many unusual properties including the ability to mimic the lensing signature of a singular isothermal sphere.

Why the potentials in scalar field dark energy models are arbitrary?


Nandan Roy (The Institute for Fundamental Study, Naresuan University)

Tough we have entered an era of high precision cosmology the true nature of the dark energy is till now unknown. The cosmological constant is the most favorable and observationally consistent candidate of the dark energy but it suffers from both theoretical and observational challenges. Consideration of a scalar field as an extra component of the universe can explain the accelerated expansion and tackle the challenges faced by the cosmological constant but one of the challenges from which the scalar field models suffer is the problem of arbitrariness of the form of its potentials. In our recent research, we propose a new parameterization of quintessence potentials for dark energy based directly upon the dynamical properties of the equations of motion. Such parameterization arises naturally once the equations of motion are written as a dynamical system in terms of properly defined polar variables. We have identified two different classes of parameters, and we dubbed them as dynamical and passive parameters. The dynamical parameters appear explicitly in the equations of motion, but the passive parameters play just a secondary role in their solutions. The new approach is applied to the so-called thawing potentials and it is argued that only three dynamical parameters are sufficient to capture the evolution of the quintessence fields at late times. This work sheds new lights on the cause of arbitrariness of the quintessence potentials by trying to constrain the dynamical parameters using recent cosmological observations.