Behnam Pirayesh
Department of Physics, Sharif University of Technology
Seminar 1: A Novel Mechanism for Early Dark Energy: Bifurcation Theory and Interacting Dark Matter-Dark Energy model to Resolve the H0 Tension
Rohina Hassan
Department of Physics, Sharif University of Technology
Seminar 2: Revisiting the step-like features on the Vanilla Inflationary potentials
Dina Mousavi
Department of Physics, Sharif University of Technology
Seminar 3: Dark matter from the Dark Dimension scenario and structure formation
Mahbod Khordbin
Department of Physics, Sharif University of Technology
Seminar 4: Simulation of magneto hydrodynamic confinement in magnetic mirrors using Python code
یکشنبه 31 تیر 1403، ساعت 10:00
Sunday 21 July 2024 – 10:00 Tehran Time
Hybrid Seminar
دانشکده فیزیک – طبقه پنجم – کلاس 512 Physics Department fifth floor – Room 512 /
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Abstract of the Seminar 1: The Hubble constant (H0) represents the rate at which the universe is expanding. Over the past decade, two primary methods of measuring H0 have resulted in significantly different values, leading to what is known as the “H0 tension.”
1. Direct Measurements (Local Universe): Observations of supernovae and Cepheid variables in the local universe suggest a higher H0 value, around 73-74 km/s/Mpc.
2. Indirect Measurements (Early Universe): Data from the Cosmic Microwave Background (CMB) measured by the Planck satellite, combined with the standard ΛCDM model, yield a lower H0 value of about 67-68 km/s/Mpc.
This discrepancy has led to the exploration of new physics beyond the standard ΛCDM model to reconcile these differences. One proposed solution is Early Dark Energy (EDE). EDE posits that an additional component of dark energy was present in the early universe, especially around the time of recombination (when the CMB was formed). This early dark energy would have temporarily contributed a significant fraction of the total energy density of the universe, altering the expansion rate and leading to a higher inferred value of H0 from CMB data. While EDE models can potentially resolve the H0 tension, they come with their own set of challenges:
1. Fine-Tuning: EDE models often require fine-tuning of parameters to match observations, which can make them less attractive from a theoretical standpoint.
2. Consistency with Other Observations: Any new model must not only resolve the H0 tension but also remain consistent with a wide range of other cosmological observations, including large-scale structure, baryon acoustic oscillations (BAO), and Big Bang Nucleosynthesis (BBN).
3. Physical Motivation: The physical origin of EDE is not well understood. The introduction of a new component necessitates a compelling theoretical framework that explains its properties and behavior.
4. Impact on the CMB and LSS: EDE models modify the dynamics of the early universe, which can impact the CMB anisotropies and the formation of large-scale structures. Ensuring that these modifications do not contradict existing data is a significant challenge.
We introduce a new mechanism for EDE via interaction between Dark Matter and Dark Energy (DE-DM interacting model) by using Bifurcation Theory. This theory is a mathematical framework used to study changes in the qualitative or topological structure of a given family of dynamical systems. Bifurcation occurs when a small change in the system parameters causes a qualitative change in its behavior. In our DE-DM interacting model, the number and stability of the equilibrium points are changed due to the Bifurcation phenomenon, and EDE arises naturally in this model.
Abstract of the Seminar 2: Phase transitions in cosmic Inflation have always been under scrutiny due to their natural occurrence in an Inflationary landscape and their ability to produce primordial black holes and gravitational waves.
These transitions can often be translated as local features of inflation potential. However various realizations that can be imagined for such features do not lead to the same results. This may affect the predictions of both large and small scales and recent loop corrections debates.
In the initial steps of this project, we tried to categorize these models by limiting ourselves to local step-like features and identifying the parameters related to the shape of the final power spectrum.
Abstract of the Seminar 3: The cold dark matter (CDM) paradigm has been successful in explaining large-scale structure formation in the universe, yet it faces significant challenges on small scales, including the missing satellite problem. This discrepancy arises from the overprediction of small satellite galaxies around larger galaxies like the Milky Way, as simulations predict more satellites than are observed. In this talk, we investigate a novel dark matter model based on the dark gravitons in the Dark Dimension scenario, inspired by the Swampland program, which offers a promising resolution to this small-scale issue.
We begin by discussing the theoretical underpinnings of the dark dimension scenario and how it modifies the properties of dark matter. We then calculate the growth function of matter perturbations in the linear regime, establishing the groundwork for understanding the evolution of density fluctuations. Extending our analysis to the nonlinear regime, we employ excursion set theory to determine the number density of dark matter halos. By analyzing the resulting halo mass function within this dark dimension framework, we evaluate its impact on the missing satellite problem. Our results indicate that the dark dimension graviton model can significantly alter the predicted abundance of smaller dark matter halos, providing a potential solution to the small-scale challenges faced by the CDM paradigm.
Abstract of the Seminar 4: We know that most of the material world is composed of plasma. Plasma, or the fourth state of matter, is a quasi-neutral gas that consists of charged and neutral particles and exhibits collective behavior. Magnetic mirrors are a method of plasma confinement in which the amount of magnetic field increases and decreases in the direction of the field. Due to the complexity and multiplicity of equations required to investigate plasma, computer simulation is one of our most important tools to study it, which we can study by simulating magnetic mirrors. For simulation, we use pencil code. The pencil code is a modular simulation code with MPI capability for solving partial differential equations and particles. In this research, the shape and behavior of the flow in β is approximately equal to one and the frozen field regime has been investigated and plotted. Also, the rate of changes in the mass of the fluid due to its exit from the two ends of the magnetic mirrors in different regimes has been obtained and analyzed.