Ali Rostami

Department of Physics, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan

Black Holes as Catalysts: The Far-Reaching Effects of Black Hole Retention in Evolution of Star Clusters 

Abstract: Massive stars, upon exhausting their thermonuclear fuel, undergo gravitational collapse followed by supernova explosions, leaving behind black holes (BHs) as remnants. These explosions aren’t perfectly symmetrical, and the uneven ejection of mass imparts a “natal kick” to the newborn BH. The magnitude of these kicks remains a subject of debate, creating uncertainty about the initial population of BHs retained in star clusters. High natal kicks lead to the immediate ejection of BHs from their host clusters, while gentler kicks allow them to stay put, forming a sub-system of BHs right in the cluster’s heart. This BH sub-system (BHSub) acts like a bustling, dynamic factory at the cluster’s core, pumping energy into its surroundings and dramatically influencing its evolutionary path.

In this talk, I’ll first address the dynamical processes governing clusters harboring a BHSub, followed by an examination of the emergence and evolution of the dark cluster phase. But that’s not all – I’ll demonstrate how assuming low natal kicks for BHs and the subsequent formation of a BHSub in the cluster’s heart can provide explanations for several outstanding open questions regarding globular clusters:

1. The origin of high dark remnant fractions in Milky Way globular clusters
2. The dichotomy between metal-rich and metal-poor globular clusters in the Milky Way
3. The mysterious lack of metal-poor globular clusters in the inner regions of early-type galaxies

By the end, you’ll see just how far-reaching the effects of BH retention can be in the grand story of star cluster evolution.

یکشنبه  20 آبان 1403، ساعت 17:00

Sunday 10 November 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Saeed Pourojaghi

Department of Physics, Bu-Ali Sina University, Hamedan.

Recent Challenges to the Standard Cosmological Model 

Abstract: The standard cosmological model (ΛCDM) is considered the first and simplest model to describe the accelerated expansion of the universe. Despite its good compatibility with observational data, this model faces various theoretical and observational challenges. In this talk, I intend to discuss the latest challenges to this model. First, we will examine the redshift evolution of the free parameters of the ΛCDM model using observational data binning. This approach is significant because these parameters are mathematically and observationally considered constants, but our results show different values for them in various redshift bins. In the next part of my talk, we will present a study on the cosmological constraints of dark energy parametrizations post-DESI 2024, suggesting potential deviations from the standard ΛCDM cosmology. Our main goal is to examine the results of DESI collaborations on some extensions of the CPL approximation, known as the BA and Pade parametrizations. Finally, we will provide an analysis of the ΛCDM model using a cosmography approach, which includes recent DESI BAO observations and Type Ia supernova catalogs, including the DES-SN5YR and Pantheon+ sets.

یکشنبه  13 آبان 1403، ساعت 17:00

Sunday 3 November 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 /Physics Department – first floor – Room Physics 3   

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Samaneh Sarbaz

Department of Physics, Sharif University of Technology

Frequency shift in binary microlensing 

Abstract: Gravitational microlensing with binary lensing is one of the channels for detecting exoplanets. Due to the degeneracy of the lens parameters for the binary microlensing, additional features such as parallax and finite-size effects need to identify the lens parameters. The frequency-shift effect as the relativistic analogy of the gravity assist for the photons, is an extra observation that provides additional constraint between the lens parameters. In this work, we extend the application of the frequency shift effect to binary microlensing and derive the frequency shift during the lensing and caustic crossing. The frequency shift for the binary lens is of the order of $\Delta\nu/\nu∼10^{-12}$. We also investigate the feasibility of detecting this effect by employing Cross-Correlation methods.

یکشنبه  6 آبان 1403، ساعت 17:00

Sunday 27 October 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Hedieh Abdollahi

School of Astronomy, Institute for Research in Fundamental Sciences (IPM)

The Isaac Newton Telescope Monitoring Survey of Local Group Dwarf Galaxies VIII, Detection of the Long Period Variable Stars of Andromeda Dwarf Satellite

Abstract: We present an extensive analysis of spheroidal dwarf satellite galaxies of the Andromeda galaxy, aiming to provide a comprehensive catalog of long-period variable (LPV) stars. This study is part of an optical monitoring survey targeting 55 dwarf galaxies and four globular clusters in the Local Group. Observations were conducted using the Wide Field Camera (WFC) on the 2.5-meter Isaac Newton Telescope (INT) over nine epochs, employing the i-band (Sloan) and V-band (Harris) filters. Our focus is on detecting asymptotic giant branch (AGB) stars with magnitude amplitudes exceeding 0.2 mag to trace the evolution of these dwarf galaxies. The resulting catalog of LPV candidates in Andromeda’s satellite galaxies provides updated characteristics such as half-light radii, TRGB magnitudes, and distance moduli. By applying the Sobel filter, we determine distance moduli for these satellites, ranging from 23.57±0.08 to 25.62±0.17 mag. Given that LPV stars in dwarf galaxies span ages from 30 Myr to 10 Gyr, they serve as valuable probes for reconstructing star formation histories (SFHs). The relationship between LPV birth mass and luminosity at the end of their evolution makes them ideal for studying galaxy properties. This catalog will also facilitate the estimation of quenching times, galaxy masses, and the assessment of mass-loss and dust production in these systems.

یکشنبه  29 مهر 1403، ساعت 17:00

Sunday 20 October 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Sina Hooshangi

School of Astronomy, Institute for Research in Fundamental Sciences (IPM)

Tail Diversity from Inflation

Abstract: Primordial black holes (PBHs) hold significant importance in modern cosmology as possible candidates for dark matter and as potential explanations for the origin of some observed gravitational waves. The primary mechanism for the formation of these black holes is the large fluctuations generated during cosmic inflation. However, conventional perturbative methods used to calculate the statistics of these primordial fluctuations may lead to inaccurate estimates of PBH abundance. In this presentation, we examine a non-perturbative method for describing large fluctuations during the inflationary period. Initially, the concept of non-attractor inflation is introduced, which can potentially produce large perturbations with low but non-zero probability during cosmic inflation. Subsequently, the $\delta N$ formalism is discussed as an effective tool for non-perturbative treatment of primordial fluctuations. We then analyze a set of inflationary models in which the non-perturbative treatment of fluctuations shows significant differences from perturbative calculations. This difference is particularly evident in the tail of the probability distribution of primordial fluctuations. The results of this research indicate that the non-linear behavior of curvature perturbations in inflationary models can lead to the formation of non-Gaussian tails in the probability distribution, which may result in more accurate estimates of PBH abundance. These findings highlight the importance of employing non-perturbative methods in studying PBH formation and demonstrate that conventional linear approximations may not be sufficient for accurately predicting the abundance of these objects.

یکشنبه  22 مهر 1403، ساعت 17:00

Sunday 13 October 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 /Physics Department – first floor – Room Physics 3   

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Mohammad Hossein Jalali Kanafi

Department of Physics, Shahid Beheshti University

Imprint of Massive Neutrinos on Persistent Homology (Morphology) of Large-Scale Structure 

Abstract: The morphology of the cosmos provides valuable information about the structure of the cosmic web, which can be used for cosmological inferences. In this talk, I will discuss the capability of Persistent Homology (PH) as a morphological tool to investigate the impact of massive neutrinos on the Large-Scale Structure (LSS). PH naturally addresses the multi-scale topological characteristics of the LSS as a distribution of clusters, loops, and voids. Utilizing the Fisher forecast formalism, I will provide a quantitative assessment about the constraining power of some PH vectorizations to estimate the parameters of the $\nu \Lambda CDM$ model.

یکشنبه 15 مهر 1403، ساعت 17:00

Sunday 6 October 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3  / 

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Nima Khosravi

Department of Physics, Sharif University of Technology,

Department of Physics, Shahid Beheshti University

Random Matrix Gravity and Double Hierarchy Problem 

Abstract: Why are the cosmological constant, electroweak and Planck scales so different? This “double hierarchy” problem, where Λ≪M^2_EW≪M^2_p, is one of the most pressing in fundamental physics. We show that in a theory of N randomly coupled massive gravitons at the electroweak scale, these scales are linked precisely by such a double hierarchy for large N, with intriguing cosmological consequences. Surprisingly, in all the physical scales, only one massless graviton emerges which is also, effectively, the only one that is coupled to matter, giving rise to standard Einstein gravity, with M^2_pGμν=Tμν at large N. In addition, there is a tower of massive gravitons, the lightest of which can drive late-time acceleration. In this scenario, the observed empirical relation ΛMp^2∼M_EW^4 as well as the double hierarchy, arise naturally since since Λ ∼ M_EW^2/√(N) and Mp^2 ∼ √(N)M_EW^2.

یکشنبه  8 مهر 1403، ساعت 17:00

Sunday 29 September 2024 – 17:00 Tehran Time

Hybrid Seminar

دانشکده فیزیک – طبقه اول – کلاس فیزیک 3 Physics Department – first floor – Room Physics 3   /

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Dina Mousavi

Department of Physics, Sharif University of Technology

Nonlinear Structure Formation in Beyond Cold Dark Matter Models with Nonzero Sound Speed

Supervisor: Shant Baghram

Abstract: The cold dark matter (CDM) paradigm has been successful in explaining large-scale structure formation in the universe, yet it faces significant challenges on length scales smaller than 1 Mpc and mass scales smaller than 10^11 M_sun. So the search for alternative models is ongoing. In this work, we investigate a novel dark matter model based on the dark gravitons in the Dark Dimension scenario, inspired by the Swampland program. 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. Using the growth functions, We calculate the matter power spectrum in this model. 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. We also calculate how this model modifies dark matter halo bias.

یکشنبه 18 شهریور 1403، ساعت 10:00

Sunday 8 September – 10:00 Tehran Time

دانشکده فیزیک – طبقه پنجم – کلاس 512 Physics Department fifth floor – Room 512  / 

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.