The Journal Club (JC) sessions aim to provide a safe space for early-career researchers working in the field of cosmology to freely interact, present their work and exchange ideas.
The friendly environment of the sessions provides a medium for participants to ask the questions they want. The JC sessions are designed to foster a culture of communication, collaboration and learning, empowering early-career researchers to advance their knowledge and skills in cosmology interactively.
The sessions will be held on a monthly basis and will feature 2 talks, each lasting for 20 minutes,
followed by 10 minutes of discussion and Q&A.
Upcoming Sessions
Ziyang Zheng
The theory of gravity holds significant importance in our understanding of the large-scale structure and dynamics of the Universe. We first demonstrate how one can measure the gravitational slip, η, in a model-independent way by combining observations from galaxy clustering and weak lensing. Additionally, we propose a method to test the cosmological Poisson equation model-independently while maintaining independence from specific models for the background expansion, the power spectrum shape, and the non-linear corrections. We show that one can only measure the combination M≡Ω_{m,0}μ, where μ quantifies the deviation of the Poisson equation from the standard one and Ω_{m,0} is the present matter density fraction. We also obtain constraints on M for a survey that approximates a combination of the Dark Energy Spectroscopic Instrument and Euclid by employing a recent model-independent forecast for the growth rate f(z) and the expansion rate E(z).
Reference papers: 1210.0439; 1902.06978; astro-ph/0307460; 2107.12990; 2302.09777; 2305.02863; 2307.02117; 2312.07436
David Benisty
The dynamics of the Local Group (LG), especially concerning the contributions of the Milky Way (MW) and Andromeda (M31) galaxies, is sensitive to the presence of dark energy. This work compares the evolution of the LG by considering it as a two-body problem in a homogeneous and isotropic expanding spacetime, i.e. the Mc-Vitte spacetime (McV) versus the spherically symmetric metric for LG dynamics with the Cosmological Constant, i.e. the De Sitter-Schwarzschild spacetime (DsS). Using the Timing Argument (which links LG dynamics to LG mass) we find that the McV spacetime predicts a lower mass for the LG for the McV spacetime vs. the DsS spacetime. By the calibration of the IllustrisTNG simulations, the TA mass is shown to be biased high, and modified to be $~3 10^12$. With the Large Magellanic Cloud (LMC) the center of mass of the MW is shifted and reduces the mass the toal mass to $2.4 10^{12}$. The mass is compatible with other estimations in the literature, such as the Hubble flow and the Virial Theorem with the other dwarf galaxies in the LG and the full sum of the MW and M31 estimated mass.
Past Sessions
Sveva Castello
In this epoch of cosmological tensions, it is essential to question the fundamental assumptions underlying our standard picture of cosmology, including the theory of gravity. The standard approach to test for deviations from General Relativity on cosmological scales is to combine measurements of the growth rate of structure with gravitational lensing. In my talk, I will demonstrate that this method suffers from an important limitation, since models of dark matter with additional interactions can lead to exactly the same signatures as modified gravity in these two observables. Luckily, I will show that the coming generation of large-scale structure surveys, like the Square Kilometer Array, will allow us to break this degeneracy through measurements of the distortion of time.
Reference paper: arXiv:2404.09379
Julia Ziegler
At high energies, such as during inflation, the quartic coupling of the Standard Model (SM) Higgs potential runs negative, according to current measurements. This can lead the potential into a tachyonic regime, where the square of the mass of the SM Higgs becomes negative. This tachyonic instability can exponentially enhance Higgs particle production via Hubble-induced effects and via the dynamics of the Higgs field itself. Furthermore the enhanced Higgs particle production can draw energy out of the Higgs field and produce stabilizing thermal corrections. The early produced Higgs particles would then modify the curvature perturbations of the early universe which in turn can cause hot or cold spots on the cosmic microwave background (CMB). The aim of our work is to look into this enhanced Higgs particle production and calculate the temperature of the CMB hotspots, as well as looking into CMB hotspots from other sources such as primordial black holes.
A few reference papers: arXiv:2301.08754; 2107.09061
Miguel Pinto
Although General Relativity remains an impressive description of gravitational phenomena, it suffers from specific issues, such as the incapacity of producing an accelerated expansion without introducing a cosmological constant, non-renormalizability, and lack of uniqueness. These problems could hint that new physics is needed, and one possible avenue is to assume that a more general action describes gravity, which is the core idea behind Modified Gravity Theories (MTGs). In this talk, I provide an overview of a particular class of MTGs, the non-minimal geometry-matter coupling theories. More specifically, I offer my perspective on why this class of theories remains a valid option to put our knowledge of gravity to the test. Moreover, I highlight some projects I have been working on in this line of research, such as gravitationally induced particle production, late-time cosmic evolution, and regular black hole solutions. Finally, I plan to briefly mention a work-in-progress Hubble tension research program involving a theory of this type.
arXiv:2205.12545, 2309.15497, 2310.15018
Masoume Reyhani
In modern cosmology, cosmological tensions are a leading topic in the field. One of the most controversial subjects in the wake of these discrepancies is the difference between the direct and indirect measurements of the Hubble constant. This has led to alternative scenarios becoming the subject of study in modern cosmology, with dynamical dark energy being one of the most promising. Studies have shown that the key to distinguishing between different dynamical dark energy models lies in their effects on large scales. Therefore, the Integrated Sachs-Wolfe (ISW) effect could be a hint for finding the most convincing scenarios for dark energy. In this work, we have investigated the effect of dynamical dark energy models (CPL, JBP and BA) in comparison with constant dark energy models (LCDM and wCDM) on structure formation in late time by evaluating the matter power spectrum and ISW effect. Our study confirms that the matter power spectrum and the ISW effect are suitable probes for studying dark energy.
Marcin Postolak
We found general solutions of matter stress-energy (non-)conservation in scalar-tensor FLRW-type cosmological models by extending the logotropic formalism to the case of non-minimal coupling between the scalar field and new dark fluid candidates. The energy conditions expressed by the generating function are introduced. Next, we investigate the possibility of separating baryonic from dark matter and explain their ratio as a chameleon effect in the presence of non-minimal coupling. To answer the question affirmatively we analyze simple extensions of the Λ-CDM model by adding a non-minimally coupled scalar field in the Einstein frame. Two scenarios involving either a scalaron (quintessence) or a phantom (ghost) are numerically solved and compared. As a result, it is shown that in both cases LCDM model can be reproduced with a high accuracy in the region covered by observations. As expected, in the case of the phantom (ghost) field the Big-Bang scenario is replaced by the (matter) Bounce.
Yashi Tiwari
The Hubble tension refers to the discrepancy in the value of the Hubble constant H0 inferred from the cosmic microwave background observations, assuming the concordance ΛCDM model of the Universe, and that from the distance ladder and other direct measurements. In order to alleviate this tension, we construct a plausible dark energy scenario, within the framework of Horndeski gravity which is one of the most general scalar-tensor theories yielding second-order equations. In our set-up, we include the self-interactions and nonminimal coupling of the dynamical dark energy scalar field which enable very interesting dynamics leading to a phantom behaviour at low redshifts along with negative dark energy densities at high redshifts. These two features together make this model a promising scenario to alleviate the Hubble tension for appropriate choices of the model parameters. Towards a consistent model building, we show that this set-up is also free from both the gradient and ghost instabilities. Finally, we confront the predictions of the model with low redshift observations from Pantheon, SH0ES, cosmic chronometers and BAO, to obtain best fit constraints on model parameters.