1618 September 2021
Prague, Czech Republic
The workshop aims at fostering collaboration among the cosmology teams at CEICO in Prague (Czech Republic), the Institute of Theoretical Astrophysics in Oslo (Norway) and the Centre for Astrophysics and Cosmology of the University of Iceland in Reykjavik (Iceland), and is open to external participants upon registration (see below). The workshop focusses on the properties and tests of theoretical models that lie at the interface between dark matter and modified gravity, including developing better computational tools to simulate the local structure of the Universe in such models.
The list of topics includes the theory, observations and structureformation codes for alternative dark matter models, modified gravity, screening mechanisms, dark matter interactions and fifth forces.
The workshop is part of a broader transfer of knowledge initiative within the context of the EEA and Norway grants for bilateral cooperation, which includes a total of eight oneweek scientific stays for researchers at the three participating Institutes, to be held between August and October 2021. The scientific stays will feature specialised talks, lectures, training and twoway transferofknowledge activities for the visitors and host teams.
SCHEDULE  Thursday 16/09/2021 
Friday 17/09/2021 
Saturday 18/09/2021 

09:0009:45  OPENING  Meshveliani  Urban 
09:4510:30  The EEA and Norway grants  Burger  Christiansen 
10:3011:30  BREAK/DISCUSSION  BREAK/DISCUSSION  BREAK/DISCUSSION 
11:3012:15  Samanta  Zavala  Vikman 
12:1514:00  LUNCH  LUNCH  CLOSING 
14:0014:45  Ferrero  Ramazanov  FREE AFTERNOON 
14:4515:30  Tanidis  Özsoy  
15:3016:30  BREAK/DISCUSSION  BREAK/DISCUSSION  
16:3017:15  Hartman  MaulandHus  
17:1518:00  BREAK/DISCUSSION  BREAK/DISCUSSION  
RECEPTION 18:30 
SOCIAL DINNER 19:00 
SPEAKER  INSTITUTE  TOPIC 

Jan Burger  University of Iceland, Reykjavik 
Breaking the degeneracy between adiabatic and impulsive cuspcore transformation mechanisms
One of the most persevering challenges to the LCDM paradigm concerns the inner density profiles of DM haloes that host dwarf galaxies. Reconciling the success of LCDM on large scales with observational hints for cored DM haloes at the scale of dwarf galaxies requires a physical mechanism to flatten the ubiquitous cusps predicted by CDM Nbody simulations. Several feasible cuspcore transformation mechanisms have been proposed and, by construction, the formed DM cores are nearly indistinguishable. However, the timescale on which the gravitational potential changes as the DM halo forms a core differs greatly between core formation mechanisms. In this talk, I demonstrate that this difference leads to divergent kinematic properties of the stars that orbit in the evolving potential of the DM halo. Adiabatic core formation, caused for instance by selfinteractions between the DM particles, leads to the formation of a stellar core. Impulsive core formation, caused for instance by supernova feedback, can result in large stellar age gradients. Moreover, impulsive supernova feedback can generate characteristic shelllike features in the phase space density of monoage stars. Such shells are related to earlystage phase mixing and are direct evidence for prior impulsive evolution of the gravitational potential. Identifying characteristic signatures of either adiabatic or impulsive core formation will enable us to differentiate between core formation mechanisms as more precise spectroscopic and photometric data become available for stars in nearby dwarf galaxies.

Øyvind Christiansen  Institute of Theoretical Astrophysics, Oslo  
Ismael Ferrero  Institute of Theoretical Astrophysics, Oslo 
Fast mocks for cosmological surveys
The calibration and replication of scientific analysis in simulations is a fundamental tool to ensure unbiased and robust results. In particular, mock galaxy catalogues are a crucial resource to achieve these goals in the measurements of galaxy clustering. Here we present a set of 488 ICECOLA fast Nbody simulations that are used to produce 1952 galaxy mock catalogues designed to mimic the Dark Energy Survey (DES) Year 3 Baryon Acoustic Oscillations (BAO) sample. This work can be extended to other surveys.

Stian Hartman  Institute of Theoretical Astrophysics, Oslo 
Scalarfield dark matter from CDM, and constraints from cosmological data
Despite the success of LCDM in explaining cosmological observables on scales spanning many orders of magnitude in both time and space, there are ingredients central to the model that are completely unknown, such as dark matter (DM), the dominant matter component of our universe. A wide range of DM candidates have therefore been proposed over the years, motivated by fundamental theories beyond the Standard Model, and discrepancies between predictions of LCDM and observations. In this talk I will focus on scalar field candidates of DM, in particular the phenomenology of selfinteracting scalar field DM that have been produced from an initial standard cold DM (CDM)like phase. By solving the linear equations for such a DM model using the Boltzmann code CLASS, largescale observables, such as the cosmic microwave background and the matter power spectrum, can be computed and used to place constraints on the phenomenology of the transition between the CDM and scalar field DM phases.

Tamari Meshveliani  University of Iceland, Reykjavik  
Renate MaulandHus  Institute of Theoretical Astrophysics, Oslo 
RAMSES with massive neutrinos and f(R) gravity
A wellknown effect of massive neutrinos is their suppression of structure growth on scales smaller than the neutrino freestreaming length. Combining this with the enhancement of structure growth from f(R) gravity, it has been demonstrated that the effect of massive neutrinos and f(R) gravity on structure formation is degenerate and that the suppression and enhancement can cancel out, yielding results similar to LCDM with close to massless neutrinos. To further investigate this cosmology and ways to break the degeneracy, we have added neutrinos to the Nbody code RAMSES and combined it with an already existing RAMSES version which includes HuSawicki f(R) modified gravity. We are currently running simulations with the intent to analyse and compare LCDM, LCDM + massive neutrinos, f(R) and f(R) + massive neutrinos for different values of the total neutrino mass and various levels of fifth force effects.

Ogan Özsoy  CEICO, FZU, Prague  
Sabir Ramazanov  CEICO, FZU, Prague 
Caustic free completion of kessence
Generically kessence models develop caustic singularities at finite time. I will discuss caustic free completion of shiftsymmetric kessence (P(X)theory) by means of the canonical complex scalar field. At the background level, the selfinteracting complex scalar reproduces dynamics of P(X)theory provided that its amplitude is frozen modulo the Hubble drag. The dispersion relation for the complex scalar has two branches. In the small momentum limit, one of these branches coincides with the dispersion relation of the P(X)theory. Hence, the low momentum phase velocity agrees with the sound speed in the corresponding P(X)theory. The behavior of high frequency modes associated with the second branch of the dispersion relation depends on the value of the sound speed. In the subluminal case, the second branch has a mass gap. On the contrary, in the superluminal case, this branch is vulnerable to a tachyonic instability. We also discuss the special case of the P(X)theories with an imaginary sound speed leading to the catastrophic gradient instability. The complex field models provide with a cutoff on the momenta involved in the instability.

Rome Samanta  CEICO, FZU, Prague 
Early Universe filled with ultralight primordial black holes
Common practice is to study the physics of heavy or super heavy primordial black holes (PBH) with mass in excess of 1e15 g, particularly due to their potential to serve as a dark matter candidate. However, ultralight PBHs with mass less than 1e8 g, though they completely evaporate before BBN, are not less interesting. They can be responsible for the production of dark matter and the baryon asymmetry of the Universe as well as they can be prominent sources of strong stochastic gravitational waves, e.g., due to their inhomogeneous distribution in space, their mergers, etc. I will give a brief review of the stateoftheart works in this direction and discuss how these ultralight PBHs could be a potential source of super heavy dark matter that can be tested in with gravitational waves signals.

Konstantinos Tanidis  CEICO, FZU, Prague 
Modelindependent constraints on clustering and growth of cosmic structures from BOSS DR12 galaxies in harmonic space
We present a new, modelindependent measurement of the clustering amplitude of galaxies and the growth of cosmic largescale structures from the Baryon Oscillation Spectroscopic Survey (BOSS) 12th data release (DR12). This is achieved by generalising harmonicspace power spectra for galaxy clustering to measure separately the magnitudes of the density and of the redshiftspace distortion terms, which are respectively related to the clustering amplitude, bσ8(z), and the growth, fσ8(z). We adopt a tomographic approach with 15 redshift bins in the range z∈[0.15,0.67]. We restrict our analysis to strictly linear scales, implementing a redshiftdependent maximum multipole for each of the tomographic bins. Thus, we obtain 30 data points in total, 15 for each of the quantities bσ8(z) and fσ8(z). The measurements do not appear to suffer from any apparent systematic effect and show excellent agreement with the theoretical prediction from a concordance cosmology as from the Planck satellite. Our results also agree with previous analyses by the BOSS collaboration. Although each single datum has, in general, a larger error bar than that obtained in configuration or Fourierspace analyses, our study provides the community with a larger number of tomographic data points that allow for a complementary tracking in redshift of the evolution of fundamental cosmological quantities.

Federico Urban  CEICO, FZU, Prague 
Searching for spin2 ULDM with gravitational waves interferometers
The detection of gravitational waves from merging binaries has ushered in the era of gravitational wave interferometer astronomy. Besides these strong, transient, calamitous events, much weaker signals can be detected if the oscillations are nearly monochromatic and continuous, that is, coherent over a long time. In this work we show that ultralight dark matter of spin two, owing to its universal coupling α to Standard Model fields, generates a signal that is akin to but distinct from a continuous gravitational wave. We show that this signal could be detected with current and planned gravitational wave interferometers. In the event of a null detection, current facilities could constrain the coupling to be below α∼1e7 for frequencies of tens of Hz, corresponding to dark matter masses around the 1e13 eV mark. Future facilities could further lower these upper limits and extend them to smaller masses down to 1e18 eV. These limits would be the most stringent bounds on the spin2 Yukawa fifth force strength, parametrised by α, in the frequency ranges accessible by gravitational wave interferometers. The implementation of this type of searches for gravitational wave interferometers would therefore further our grasp of both dark matter and gravity.

Alex Vikman  CEICO, FZU, Prague 
Imprints of Primordial Magnetic Fields in Dark Matter
Primordial magnetic fields (PMF) are often thought to be the early Universe seeds that have bloomed into what we observe today as galactic and extragalactic magnetic fields. Owing to their minuscule strength, PMF are very hard to detect in cosmological and astrophysical observations. I will discuss our proposal from arXiv:2010.03383 where we showed how this changes provided a part of neutral Dark Matter has a magnetic susceptibility. In this way, by studying Dark Matter one can obtain information about the properties of PMF, even if the latter have a comoving amplitude 0.01 nG. In our model Dark Matter is a stable neutral scalar field, which interacts with electromagnetism through the socalled Rayleigh operator. For large PMF present in the early Universe this interaction breaks the symmetry of the model, so that the scalar field develops an adiabatically changing vacuum expectation value. Later, when the PMF redshift below a critical value, the symmetry is restored through an “inverse phase transition”. At that point the scalar field begins to oscillate and acts as a “magnetomorphic” Dark Matter component, inheriting the properties of the PMF space distribution. In particular, for a nearly flat spectrum of magnetic field fluctuations, the scalar field carries a statistically anisotropic isocurvature mode which could be observed in future observations.

Jesus Zavala  University of Iceland, Reykjavik 
PARTICIPANT  INSTITUTE 

Jan Burger  University of Iceland, Reykjavik 
Øyvind Christiansen  Institute of Theoretical Astrophysics, Oslo 
Antoine Depasse  UC Louvain, Louvain 
Amel Durakovic  CEICO, FZU, Prague 
William Emond  CEICO, FZU, Prague 
Ismael Ferrero  Institute of Theoretical Astrophysics, Oslo 
Stian Hartman  Institute of Theoretical Astrophysics, Oslo 
Farbod Hassani  Institute of Theoretical Astrophysics, Oslo 
Pavel Kůs  CEICO, FZU, Prague 
Renate MaulandHus  Institute of Theoretical Astrophysics, Oslo 
Tamari Meshveliani  University of Iceland, Reykjavik 
David Mota  Institute of Theoretical Astrophysics, Oslo 
Ogan Özsoy  CEICO, FZU, Prague 
Tanmoy Paul  University of Burdwan, Purba Bardhaman 
Ornella Juliana Piccinni  INFN, Rome 
Sabir Ramazanov  CEICO, FZU, Prague 
Vincenzo Rella  University of Rome La Sapienza, Rome 
Rome Samanta  CEICO, FZU, Prague 
Iggy Sawicki  CEICO, FZU, Prague 
Constantinos Skordis  CEICO, FZU, Prague 
Konstantinos Tanidis  CEICO, FZU, Prague 
Georg Trenkler  CEICO, FZU, Prague 
Leonardo Trombetta  CEICO, FZU, Prague 
Federico Urban  CEICO, FZU, Prague 
Özgen Tunç Türker  Sorbonne University, Paris 
Alex Vikman  CEICO, FZU, Prague 
Jesus Zavala  University of Iceland, Reykjavik 
Health regulations
The workshop will take place in person and following all current regulations as indicated by the Ministry of Health of the Czech Republic. For further information please visit its dedicated portal. All information about travelling to the Czech Republic can be found here. Another very useful resource that can help you plan your travels to and from Prague is the Reopen EU website and app.
SOCIAL EVENTS
There will be a spontaneous welcome reception on Thursday, September 16, to be held at the workshop venue Pracovna from 18:30 (6:30pm).
The workshop social dinner will be held on Friday, September 17, along the Vltava river near the Old Town of Prague, at Loď Pivovar from 19:00 (7pm). The boat / brewery / restaurant is a 15 minutes' walk from the Hotel Belvedere, or a two stops' tram ride from Strossmayerovo Náměstí to Dlouhá Třída (hop on the 6, 8 or 26). Please keep in mind that the social dinner will be at the participant's expense.
Around the Pracovna one can find several restaurants, bistros and cafes, either at walking distance or few minutes away by public transport. Some hot spots are in and around the park Riegrovy Sady, the streets on the way to and around Jiřího z Poděbrad Square, all the way out to Náměstí Míru. A bit further away on the other side of the Vítkov hill, several small bistros and cafes are located around Karlínské Square.